Mad idea for DHW retrofit based on Mixergy?

[markocosic]

16 hours ago, sharpener said:

I am not sure where all this concern about 28mm pipes comes from, I have not seen it raised as an issue before, either by installers or manufacturers.

 

Heat pump efficiency is fundamentally / intrinsically linked to the supply temperature from the heat pump.

 

You minimise the flow temperature needed (for heating a space, or for heating hot water) by reducing the "deltaT" between supply and return that is going to the heating system or heat exchanger for hot water.

 

5C is common. 3C yields improved efficiency in most scenarios. 8C is a tolerable compromise. 10C is about the upper limit and only tolerated by some heat pumps.

 

Go do the math on pipe velocities and pressure drops. You'll find that you need 28 mm or even 35 mm primary pipework when operating at > 6 kW or so.

 

You can't run 22 mm primaries on anything but the smallest heat pumps. You'll therefore need to replace the primaries running to the cylinders if you're wanting to heat the cylinder at any respectable power level.

 

 

16 hours ago, sharpener said:

I think better turn-down ratios will come in time just as they have for boilers, there is no intrinsic problem with the thermodynamics.

 

There is absolutely a fundamental / intrinsic problem with turndown.

 

Compressors are available in various flavours. Rotaries and scrolls are the most common in resi sized units.

 

1) Compressors leak. If you run them at their rated outputs / rated speeds the gases don't have time to leak before they are compressed and shoved on their way. If you run them too slow then too much gas leaks past the internals of the compressor during compression.

 

2) Compressors need to last. If you run them at their rated outputs then the bearing loads will be balanced/neutral. If you run them below their rated outputs/speeds then the bearing loads (due to gas pressure, centripetal forces, etc) change and you'll increase load on bearings / shorten life

 

3) Motors are not efficient throughout their entire operating range. Nor can they cool themselves when running at low speeds etc.

 

So in the real world you're limited to say 30 revs/sec to 120 revs/sec on a compressor before either the efficiency falls off too far for it to be worthwhile running at those speeds OR the bearing loads all go to heck and you destroy the compressor.

 

So whilst you can turn down the units, even with today's tech, you'd be daft to do so. 

 

 

16 hours ago, sharpener said:

can you provide an authoritative link to the issues surrounding R290 as a flammable gas?

 

Nope. You won't find it written down because it stinks.

 

DuPont have patents on the environmentally nasty gases. DuPont support the compressor vendors designing their compressors to use the nasty gases. DuPont lobby the regulators to make using noddy basic hydrocarbons difficult. So do the unions. (the F-Gas registration bodies etc) in much the same way as the natural gas lobby.

 

Greenpeace had great success with fridges in the 90s; banishing F-Gas in favour of butane by doing some great demonstrators. DuPont didn't care because fridges were cheap junk.

 

Is R290 a problem? Not in the grand scheme of things. The EU is about to allow 0.9 kg of R290 in split systems in the coming years. At that point expect to see a bunch of R290 splits not just the (low capacity) air to air units of today. 

 

For now it's just monoblocs (R290 outdoors is fine) and low-charge water to water units indoors (where volume of R290 is already below the limit where room size matters)

 

16 hours ago, sharpener said:

I also wonder where is the equivalent of the combi boiler to provide DHW on demand?

 

The refrigerant cycle doesn't respond as quickly as dialling a flame up and down so cannot load follow. You also need to consider defrosting in cooler weather. A 25 kW+ unit wouldn't be able to turn down far enough to run a small basin tap. And efficiency would be pants if running at high delivery temperatures all the time Not viable. You run with a tank.

 

6 hours ago, sharpener said:

have you read about Vattenfall and supercritical CO2

 

CO2 is a very different beast. CO2 needs a high temperature drop (e.g. 60/30C) to work effectively; where a typical heat pump would be running 47.5/42.5 to deliver the same heat output from a radiator.

 

It has potential for DHW heating. It has potential in district heating schemes too small to be worth the hassle of ammonia. Propane kicks it to the kerb in domestic applications.

 

8 hours ago, JamesPa said:

Not sure I understand this, can you explain how it works and/or provide a reference.

 

Also what is the modulation ratio of  Samsung hps , I can't find any specs (not that Samsung are alone in this respect, only Mitsubishi seem to provide really comprehensive specs)

 

It is probably difficult to understand because it's such a dumb thing to do.

 

Normal heat pump

Step 1: compress gas

Step 2: take off some heat (condense)

Step 3: expand to a lower pressure (and temperature)

Step 4: take some heat from outside (evaporate)

Step 5: go back to Step 1

 

Samsung:

Step 1a: compress all the gas

Step 1b: "throw away" all the work that you put into compressing some of the gas by skipping to step 3

Step 2: take off some heat from the gas that is left (condense) 

Step 3: expand to a lower pressure (and temperature)

Step 4: take some heat from outside (evaporate)

Step 5: go back to Step 1

 

Say your compressor runs at COP 4 to produce 4-8 kW for arguments take; using 1-2 kW of electricity

 

Samsung still put 1 kW into the compressor, but then waste that work by skipping the condense stage using hot gas bypass, so that they can give less than 4 kW output

 

It's a dumb thing to be doing. They used to do this on old, fixed speed, compressors when they needed to do precise control of air temperature regardless of energy efficiency.

 

Samsung have implemented in their new heat pumps so that idiots can plumb them into heating systems with a flowrate that's too low for the heat pump without tripping the unit out. Before "you're being bumd; increase the flowrate else I refuse to operate" and Now "sure I'll operate with a terrible COP at below the minimum compressor output if you want me to"

 

Samsung do not provide modulation information OR part load performance information for their heat pumps. There's a reason that they don't want you to know.

 

Misti provide some information. Also Vaillant. (example for 7 kW attached - and you can see the compressor led dropoff in COP at lower frequencies in spite of the oversized evaporator/condenser at lower outputs)

 

A 2:1 ratio between "max output at minimum ambient" and "min output at max ambient" is par for the course / what to plan for. I think the 8 kW Samsung starts to do hot gas bypass at ~3.5 kW. Just below the minimum output is that the EU efficiency test uses to calculate the unit's performance. 😉

 

 

58 minutes ago, sharpener said:

Proposal is small (15 litre) intermediate store combined with inline boost heater as per discussion above

 

Defrost pretty much kills that proposal. You need 100+ litres to stand a chance of supplying hot water through a defrost cycle.

 

 

The good ways to do this are:

 

- Replumb and new cylinder (folks to stop being sissies about replastering/redecoration it really isn't that hard)

 

- Split system and new cylinder (easier once the EU changes rules and we have almost 1 kg of propane to play with and reduces need to lift floors)

 

- Separates (one heat pump for space heat, plus A N Other solution for hot water including a standalone cylinder with it's own heat pump pulling heat from cold roof etc plus option for solar diversion)

- You can get A2W and W2W separates (the latter use a low grade input from your space heating circuit aka the existing primaries) easily - e.g. https://www.electricpoint.com/dimplex-edel-hot-water-cylinder-heat-pump-270l-edl270uk-630rf.html

- There are also combined heat pump / cylinder / mvhr assemblies from Europe if you've got a wall spare and a low energy build:

 

 

 

- Indoor phase change (sunamp etc) to avoid many of the installation constraints surrounding unvented cylinders and eat the reduced efficiency caused by running at max temperature all the time

- Barge pole not touch would not I etc though on that.

 

 

Clagging a plate heat exchanger onto an existing cylinder to avoid some paint/plastering; given that you're then still ripping up all your floorboards etc to run dirty great primaries to serve it and indeed the heating system; does not appear to achieve anything whatsoever sorry.

HeatPumpPerformanceData.pdf

[sharpener]

22 minutes ago, markocosic said:

There is absolutely a fundamental / intrinsic problem with turndown.

 

Compressors are available in various flavours. Rotaries and scrolls are the most common in resi sized units.

 

1) Compressors leak. If you run them at their rated outputs / rated speeds the gases don't have time to leak before they are compressed and shoved on their way. If you run them too slow then too much gas leaks past the internals of the compressor during compression.

 

2) Compressors need to last. If you run them at their rated outputs then the bearing loads will be balanced/neutral. If you run them below their rated outputs/speeds then the bearing loads (due to gas pressure, centripetal forces, etc) change and you'll increase load on bearings / shorten life

 

3) Motors are not efficient throughout their entire operating range. Nor can they cool themselves when running at low speeds etc.

 

These are not fundamental problems like trying to defeat the 2nd law of thermodynamics. They are engineering design problems like the ones I have been solving all my professional life. When I started in wind turbines 2MW was at the limits of what was feasible. Now 10MW is standard. If you want difficult conditions for rotating machinery try an ICE turbocharger, once they were extremely temperamental and fitted only to high end expensive cars, now they are commonplace.

 

By contintually quoting current practice as though it was the unquestionable truth you are doomed to repeat it.

 

FYI 18 litres/min with a delta T of 5 deg will transfer 6 kW of heat. That is 50% of a 12kW HP, so more than the minimum output and minimum flow. And it is a reasonable rate to heat a DHW tank. This flow will have a velocity of 0.9 m/s in a 22mm Cu pipe so is not noisy. Over a 20m run it will require delta P of 110mbar or a head of 1.1m which is unremarkable.

 

This is not even 'A' level physics. If it does not demonstrate that you don't need 28mm pipework for a DHW tank then I don't know what will.

[markocosic]

23 minutes ago, sharpener said:

 

These are not fundamental problems like trying to defeat the 2nd law of thermodynamics.

 

Fair. Too strong a word.

 

They're real issues to optimise around not imaginary ones though.

 

Real limits to turndown at practical price points and ones that are not actually worth solving to boot.

 

 

25 minutes ago, sharpener said:

FYI 18 litres/min with a delta T of 5 deg will transfer 6 kW of heat. That is 50% of a 12kW HP, so more than the minimum output and minimum flow. And it is a reasonable rate to heat a DHW tank. This flow will have a velocity of 0.9 m/s in a 22mm Cu pipe so is not noisy. Over a 20m run it will require delta P of 110mbar or a head of 1.1m which is unremarkable

 

Yep.

 

I agree 6 kW is achievable on 22 and that's indeed the example I used.

 

Going beyond 6 kW will want 28 mm primaries.

 

But if you're fitting a 12 kW heat pump (for space heat) and have 22 mm primaries then you'll be making a ruddy mess of the house anyway to get your 28 mm primaries in.

 

What disruption have you really saved by plate-loading a janky old cylinder that'll likely be undersized anyway and moreso at modest storage temperatures?

 

For the smaller heat pumps it's easier to recycle the primary pipework. Then it's briefly worth thinking about retaining the cylinder to minimise disruption. But you probably still swap it out for one of a more appropriate size and heat loss; at which point you can fit one that has an appropriately sized coil in it and be unvented-ready when the time comes that you're prepared to migrate your potable pipework/fixtures over.

[JamesPa]

34 minutes ago, markocosic said:

But if you're fitting a 12 kW heat pump (for space heat) and have 22 mm primaries then you'll be making a ruddy mess of the house anyway to get your 28 mm primaries in.

 

What disruption have you really saved by plate-loading a janky old cylinder that'll likely be undersized anyway and moreso at modest storage temperatures?

Not necessarily.  My house, like many, has two stories.  So 6kW to each on 22mm is just fine for my 7.5kW(measured) or 11kW (calculated using MCS assumptions) demand.  6kW to the tank is also fine, throughout the summer and much of spring and autumn my dhw is heated from solar PV using a 3kW immersion.  It's only a few years old and plenty large enough thanks.  I don't have a problem with reheat times even if I turn my boiler flow temp down to 55 (or even 50).

 

No need at all to make a ruddy mess unless some idealist insists on running 28mm to my tank, upgrading the CW flow to the tank from 15mm to 22mm, or can't find a way to make do with the existing coil.

 

Let's consider one fact we know.  With current practice we are installing way too few heat pumps even with a whacking govt subsidy to hit anything like what is necessary to mitigate climate change..  So we need to change practice unless the technology is going to change radically and bail us out.  Of course the industry is probably quite happy to continue in high price low volume mode for as long as it can.  The planet isn't.

[SteamyTea]

I have, after a long 12 hours driving, rather lost the plot in this thread.

With current technology, I think that @markocosic us right. 

The main thing is to not let the enemy if the good be the better, I am still waiting for my sustainable electricity from Hinckley Point C.

The one that I was told in 2005 would supply 8% of the UK's electric.

I was told, in 2006, by David Elliot, that it would be over 20 years until it happened.

So 3 more to go.

[JamesPa]

3 hours ago, markocosic said:

1) Compressors leak. If you run them at their rated outputs / rated speeds the gases don't have time to leak before they are compressed and shoved on their way. If you run them too slow then too much gas leaks past the internals of the compressor during compression.

Thanks for this explanation, since its very plausible I'm happy to believe it.  However the leak reduces at reduced flow temps (ie compression ratio), which is where you are operating (assuming WC) when you need to reduce output.  So at least the physics works to assist the engineering for once.

 

3 hours ago, markocosic said:

Samsung still put 1 kW into the compressor, but then waste that work by skipping the condense stage using hot gas bypass, so that they can give less than 4 kW output

I grant this is ugly and arguably deceptive, but if its only done to extend the modulation range which is otherwise achievable with a reasonably state of the art compressor, is it so bad?  If the condition in the prior sentence is true then

 

a) you don't have to go that far and the pump would still perform as well as any other

b) (assuming you have got your HP sizing about right - unlikely with the MCS over-specification approach I grant) then you are only doing this when ambient is high, flow temp is low, and thus COP is pretty high anyway and in the periods when a minority of the total annual energy consumption occurs.   In this case does it really matter in the grand scheme of things if, as a result,  more heat pumps are sold to replace gas boilers*.  I'm not condoning deception, but its perfectly legitimate (I would argue essential in some cases) to provide people with (declared) sub optimal options which might just make the package viable whilst not degrading overall performance too much.  Obviously if others use a more state of the art compressor that natively achieves a better modulation ratio that that is to be preferred, but you seem to suggest above that this is not the case.

 

Engineering is about making sensible compromises within the parameters of available technology.  Sometimes a few percent loss of efficiency (or whatever metric you are dealing with) is worthwhile in the overall scheme of things and necessary to find an acceptable solution.

 

* to clarify Im not advocating 'dodgy tricks' to sell more heat pumps for any given manufacturer.  However I do regard the overriding objective as being to sell as many reasonably performant heat pumps in total as possible to displace as many gas boilers as possible in the shortest possible timeframe.  Even a lousy CoP is better for the planet than a gas boiler, and every one sold reduces the political pressure to continue artificially inflating the price of electricity relative to gas, which is a major disincentive to making the switch.

 

Edited April 9, 2023 by JamesPa

[sharpener]

1 hour ago, markocosic said:

But if you're fitting a 12 kW heat pump (for space heat) and have 22 mm primaries then you'll be making a ruddy mess of the house anyway to get your 28 mm primaries in.

 

What disruption have you really saved by plate-loading a janky old cylinder that'll likely be undersized anyway and moreso at modest storage temperatures?

 

For the smaller heat pumps it's easier to recycle the primary pipework. Then it's briefly worth thinking about retaining the cylinder to minimise disruption. But you probably still swap it out for one of a more appropriate size and heat loss; at which point you can fit one that has an appropriately sized coil in it and be unvented-ready when the time comes that you're prepared to migrate your potable pipework/fixtures over.

 

You're making a lot of assumptions here and confusing two different houses owned by two different people.

 

1. I won't be "making a mess anyway" or taking any floors up, there is a straight uncomplicated run of 5m at ceiling height from the outside to where the oil boiler currently is. All the 22mm circuits (UFH downstairs, rads upstairs, cylinder coil) branch off there, just as @JamesPa describes above. Same in last house, could run 6m up the outside to where boiler is on 2nd floor. In both cases I would put buffer/thermal store where boiler is now.

 

2. I don't plan to fit a PHE, it was a thought experiment. As per earlier calcs the HP will get tank to 45 or more with existing coil, then if/when it shuts down the immersion takes over for last bit. Or a recirculating pump might improve the heat transfer without the PHE. Elec is free anyway as I have surplus PV most days even at this time of year.

 

3. I already have an unvented cylinder, and it was installed before the airing cupboard was built so that would need to be partly dismantled to fit a replacement. It is @JamesPa who has the vented cyliinder, and I sympathise with his unwillingness to pay good money to scrap off a working tank unnecessarily.

[markocosic]

1 hour ago, JamesPa said:

if its only done to extend the modulation range which is otherwise achievable with a reasonably state of the art compressor, is it so bad? 

 

If the system has enough flow/radiators for 8kW at say 45/40C, it'll also do 4kW at say 35/30C, and then it'll cycle below this.

 

Running at say 50% of the time to achieve a 2 kW average output is the right thing to do here. Running at poor efficiency 100% of the time is the wrong thing to do here.

 

Why would you ever need the option to reduce at reduced output?

 

You wouldn't IMO. Unless you'd oversized the heat pump and was unable to run above minimum output. In which case you'll be bouncing along in that low efficiency zone the entire time. Avoids support calls for "heat pump won't run" though.

 

[JamesPa]

2 minutes ago, markocosic said:

 

Running at say 50% of the time to achieve a 2 kW average output is the right thing to do here. Running at poor efficiency 100% of the time is the wrong thing to do here. Why would you ever need the option to reduce at reduced output?

I agree, Hopefully I wouldn't. 

 

3 minutes ago, markocosic said:

You wouldn't IMO. Unless you'd oversized the heat pump and was unable to run above minimum output. In which case you'll be bouncing along in that low efficiency zone the entire time. Avoids support calls for "heat pump won't run" though.

Sadly I have the impression (but no solid evidence) that oversizing the heat pump may be all to common.  MCS contributes directly to thus by insisting that the hp must be sized to meet 100% of the heat demand, leaving no room for downside error.

 

Sadly I also have the impression that 'avoiding support calls' is the dominant system design criterion, both for the products and more so still for installers.

 

42 minutes ago, sharpener said:

I sympathise with his unwillingness to pay good money to scrap off a working tank unnecessarily

The thing is this.  I am fortunate enough to be able to afford to scrap a working tank unnecessarily.  Many aren't, and many more won't (I may, but not if I can avoid it).  Yet they are told by the installation industry that 'there is no alternative'. 

 

So they find their own alternative, and fit a replacement gas boiler instead,  which will continue to exacerbate climate change for the next 15 years or more.

 

Thus the whole heat pump installation industry, by being rigid, idealistic, uninventive and having the cheek to presume they know what the customer wants, is directly contributing to climate change.  Shame on them!

 

Thus I come back to needing a greater diversity of solutions and a more flexible approach. 

[markocosic]

12 hours ago, JamesPa said:

Sadly I have the impression (but no solid evidence) that oversizing the heat pump may be all to common.  MCS contributes directly to thus by insisting that the hp must be sized to meet 100% of the heat demand, leaving no room for downside error.

 

Sadly I also have the impression that 'avoiding support calls' is the dominant system design criterion, both for the products and more so still for installers.

 

Absolutely spot on.

 

 

12 hours ago, JamesPa said:

Thus I come back to needing a greater diversity of solutions and a more flexible approach. 

 

 

On the flipside of this:

 

- You'll find that most of the installations where the end client took advice, installed a proven packaged solution, and kept it simple by following a known recipe or a known heat pump with a known space heating delivery/control strategy and a known hot water provision and control strategy.

 

- You'll find that most of the installations "that decided to do it their way" wind up with unexpected headaches and / or are overly complex and overly fragile. The grand designs type customers can be the absolute worst. 

 

The industry needs...to get good at walking before it tries to run...to deliver turnkey off the peg suits before it goes custom designed saville row etc.

 

 

 

 

I think entertaining vented cylinders will be a headache not worth the risk given:

 

- Vented is lousy from a user experience perspective anyway/half the value of the heat pump upgrade is decent potable water delivery

- Vented is likely to be too small for "modern life" anyway and even moreso at the lower storage temperatures useful to heat pumps

- Vented is an increased biological risk at the lower storage temperatures useful to heat pumps (header tanks are disgusting lukewarm buckets of rat piss; cylinder full of scale from being heated at high temperatures are never heated all the way down to the bottom because it's impossible; hence trying to pasteurise them within the hour by use of 60C+)

- Buying and then plumbing and warrantying a plate, pump, and controls to legacy pipework and cylinders (time = money; individual tasks on site = risk; something that tinkerers usually neglect) is not attractive vs bolting in a new factory made piece of kit

- You're probably tearing up the house for other reasons too

 

I just don't see this ever being worthwhile.

 

 

I buy the argument for "making it work" with an existing cylinder:

 

If you have a modest property with a modest space heat load (particularly if it's had insulated attic / blown cavities / double glaze retrofitted since the heating circuit was designed) then your heat pump perhaps stands a chance of turning down low enough to work with a low-area coil as found in most unvented cylinders.

 

You're probably talking 6 kW or smaller here. It's possible that you're going to have a headache configuring your heat pump to work with a heat transfer area smaller than is intended. It's probable that you take a lifetime performance hit. It's also probably that you make the heat pump work harder than it otherwise ought to.

 

 

Ye olde control?

 

I have a fixie ground source unit here. Control strategy? If hot water cylinder drops below 40C (measured 1/3rd of way up cylinder) start the compressor at the only speed it runs at. When refrigerant pressure hits 28 bar stop the compressor. With the OEM cylinder that'll correspond to a tank temperature of the order 50-60C depending on what the brine temperature is at the time/what the temperature profile in the cylinder was at the time. Plus a couple of safety trips if the compressor hot gas outlet exceeds 130C or the pressure exceeds 30 bar.

 

It doesn't care what you hook it up to though. It runs until it can do no more and the temperature will be what the temperature will be.

 

 

A Vaillant Arotherm?

 

- Normal mode allows 120 revs/sec on compressor (

- ECO mode allows 40 or 50 revs/sec on compressor (unless very old outside)

- Balanced mode runs at full output when the return from cylinder is <45C (i.e. there's no "usable" hot water") then drops back off for final heating (where trimming back the compressor frequency and flow temperature matters most for efficiency and longevity)

 

It'll again start a reheat cycle when the cylinder temperature 1/3rd of the way up drops below setpoint X; then it aims for a flow temperature of Y degC above the measured cylinder temperature; stopping when the stat 1/3rd of the way up hits Z degC.

 

Here's an example of the "balanced" mode in operation paired with an OEM cylinder (and coil area)

 

 

From:

https://heatpumpmonitor.org/

https://emoncms.org/app/view?name=MyHeatpump2&readkey=0c28f25dea9e38f983d9c83a6dd455c0

 

(the "Telford" property with "5 kW" nameplate unit in a pre-1900, but insulated, 140 m2 property - definitely not one calculated with standard MCS assumptions on ventilation rates and net heat losses!)

 

DHW is set to "49C constant" for that unit so it's just reheating the cylinder as soon as the target temperature drop has been reached. The bottom is properly cold by that point hence starting with returns in the mid 20s.

 

It's not turning down below ~3.5 kW. With, say, a 2.4 m2 coil the coil deltaT is 3 degC flow:return (55/52) and 4.5degC meancoil:tanktemp (53.5:49).

 

With an 0.8 m2 coil tat might be 9degC flow:return on the coil and 13-15 degC meancoil:tank temp.

 

It could be made to work (the unit will achieve 70C flow temp; so you won't even be limited to 45C tank contents) but you'll have a poorer COP, you'll be wearing it harder (higher temps = higher pressures), and you'll be making more of a racket outside in the process. 

 

Doable with a *small* heat pump. Much above a nominal 6 kW though and it'll be unable to turn down far enough / start bouncing off high limit stats and safeties if you can't dump enough heat into the piddly little coil in the standard unvented cylinder. 

 

You could stretch that with a de-stratificaiton pump; but now as soon as the hot water reheat starts you have zero usable hot water; or you could go with an external plate and pump and control assembly to load it again with zero usable hot water once that kicks in)

 

Or you could...just...put a ruddy cylinder in that has a large coil from top to bottom that offers a good compromise between reheating efficiently/quickly from the heat pump vs operational/maintenance complexity (pretty hard to break a cylinder that just has a coil inside) vs hot water availability (having a bit of stratification at the top of the cylinder doesn't in fact hurt you) vs install hassle/risk/cost.

 

It doesn't' cost much to try it with the existing initially. I wouldn't try it with a sizeable heat pump though. And I wouldn't spend more than five minutes fiddling with it if it didn't work out ok. At that point you might as well just swap the cylinder IMO. (though you pay 20% VAT if doing upgrades after the fact now)

 

 

 

Potential interesting aside:

 

Another tweak that I think you'll see when propane splits become more common is de-superheating.

 

Heat pumps can produce two grades of heat simultaneously. A little bit of high grade heat (high temperature heat) and a lot of low grade (low temperature) heat.

 

https://www.swep.net/refrigerant-handbook/10.-systems/asdf7/

 

In air source monoblocs these all get mixed together. It would be a pain to run three pipes through the wall.

 

In packaged ground source units desuperheating is viable. All the time that space heat is running (at say 35C supply temp) you also get a little bit of very high temperature heat going into the hot water (up to 95C before they cut the desuperheat to avoid boiling the tank); but without any penalty on COP (which is the same as if you were running at 35C) Works best for properties that need a lot of space heat vs ht water. (i.e. cold climate/old build)

 

The same trick is vaguely practical (not quite as easy / you need to be careful not to cook the lines running from outside to inside / you will lose more of that superheat en route) on air source splits where you're brining the refrigerant into a combined unit indoors. Perhaps we'll see it on those too.

 

 

[JamesPa]

 

 

 

1 hour ago, markocosic said:

It doesn't' cost much to try it with the existing initially. I wouldn't try it with a sizeable heat pump though. And I wouldn't spend more than five minutes fiddling with it if it didn't work out ok. At that point you might as well just swap the cylinder IMO. (though you pay 20% VAT if doing upgrades after the fact now)

 

Thanks for the comments, most helpful.  As its my house I can decide (or at least should be able to decide) the level of risk I am prepared to accept.  Im minded to try it and retrofit if it doesn't work, the 20% VAT is a factor, but the retrofit can be done by any old (or young) plumber so will probably be cheaper than if I were getting an MCS plumber to do the job anyway. 

 

My biggest design concern, to be honest, is whether to go with the consumption actually measured during a 5 day consistently cold spell (7.5kW), and thus fit say an 8.5kW heat pump, the calculated consumption using MCS assumptions but known insulation levels (10.5-11kW) and thus fit a 11-12kW heat pump, or the consumption calculated by an MCS surveyor (14kW), and thus fit a 14-16kW heat pump.  My gut tells me the middle of these.  

 

1 hour ago, markocosic said:

Heat pumps can produce two grades of heat simultaneously. A little bit of high grade heat (high temperature heat) and a lot of low grade (low temperature) heat.

 

https://www.swep.net/refrigerant-handbook/10.-systems/asdf7/

 

In air source monoblocs these all get mixed together. It would be a pain to run three pipes through the wall.

That's very interesting and clearly potentially useful.  Not sure what the problem is with running three pipes through a wall, a lot easier than the work associated with fitting a replacement cylinder, at least in my house!

 

1 hour ago, markocosic said:

The industry needs...to get good at walking before it tries to run...to deliver turnkey off the peg suits before it goes custom designed saville row etc.

Fair enough, but if this is the case then perhaps the installers should be up front about it, as in 'at the moment we are only doing certain types of property and if yours doesn't fit then we will consider it, but may have to put in a solution that requires more disruption/cost than may be the case in a couple of years time (as opposed to what most have said to me namely 'its my way or the highway'.  Being upfront is exactly what Octopus are doing, to their credit.

 

 

Edited April 10, 2023 by JamesPa

[markocosic]

28 minutes ago, JamesPa said:

My biggest design concern, to be honest, is whether to go with the consumption actually measured during a 5 day consistently cold spell (7.5kW), and thus fit say an 8.5kW heat pump, the calculated consumption using MCS assumptions but known insulation levels (10.5-11kW) and thus fit a 11-12kW heat pump, or the consumption calculated by an MCS surveyor (14kW), and thus fit a 14-16kW heat pump.  My gut tells me the middle of these

 

Smallest heat pump; biggest radiators.

 

You can change the default MCS assumptions on heat loss and ventilation rates and room temperatures and still qualify as an MCS installation/for your £5k.

 

An assumed room temperature of 18C at design condition.is perfectly acceptable too. If that's the expectation

 

Else insulate or add supplementary heating if you find that you were too optimistic. An air oto air unit that kicks in 2.5 kw of heat or cool works wonders for avoiding radiator changes etc.

[markocosic]

33 minutes ago, JamesPa said:

perhaps the installers should be up front about it, as in 'at the moment we are only doing certain types of property and if yours doesn't fit then we will consider it, but may have to put in a solution that requires more disruption/cost than may be the case in a couple of years time (as opposed to what most have said to me namely 'its my way or the highway'. 

 

They're offering what they offer today.

 

Most won't know what might be coming or care to build up projects for the future if they're not interested today.

 

Many won't have a clue how to size up what they're fitting today either.

 

(if your measured heat loss is 7.5 kW there's something very amiss if it comes out at 14 kW)

 

Tesco won't let you know what's on offer next week either.

 

In a market where demand outstrips supply if it ain't interesting that's perfectly ok you're welcome to jog on. Not what the consumer is used to of course; or what they'd like; but it's probably the reality for the next few years and entirely reasonable where there's loads of work and time spent quoting the non committed is wasted time.

 

At least material prices are back down to not entirely insane lead times and prices. That includes the heat pumps themselves.

 

35 minutes ago, JamesPa said:

Being upfront is exactly what Octopus are doing, to their credit

 

My experience was the opposite fwiw.

 

Initially utterly full of excrement when challenged as to why a property running at 55/47C on a gas boiler couldn't be heated at 55/47C by a heat pump - and only reluctantly admitted "we're just not touching anything with microbore right now" when called out on it.

 

Or people without his and hers EVs and large houses that are good buyers in an electricity market where the marginal rates charged exceed actual coats but the overheads charged are below actual costs.

[JamesPa]

1 hour ago, markocosic said:

(if your measured heat loss is 7.5 kW there's something very amiss if it comes out at 14 kW)

Incorrect assumptions about insulation accounts for 3kW.  Basically they ignored me when I told them which walls had been upgraded or were of a different construction because they were a later extension, and assumed they were all solid brick, which is far from the case.  Also pessimistic assumptions about windows which have also been upgraded.  Their calculation actually came out at 15kW (and they based their quote on this) bu,t when I asked for the detail, it transpired that they had had also counted room losses in the house total, but not room gains, thus effectively double counting 1kW.  I paid £300 for this particular survey.

 

I'm struggling to explain the difference between the 10.5kW I calculate with the correct insulation values, and 7.5kW measured.  Part of it is certainly temperature, we tend to heat to 20 max.  I suspect however that  the bulk is air changes, I don't think the house is particularly well ventilated.  Solar gain, people and appliances are a further contribution, our baseload leccy consumption is 400W, and I guess most of that eventually appears as heat.  Two people there most of the time is another 600W, so that's a kilowatt accounted for!

 

Perhaps, given this experience, you can understand why I am suspicious when told that something seemingly sensible and entirely within the bounds of physics is impossible!

 

1 hour ago, markocosic said:

You can change the default MCS assumptions on heat loss and ventilation rates and room temperatures and still qualify as an MCS installation/for your £5k.

 

 

1 hour ago, markocosic said:

Else insulate or add supplementary heating if you find that you were too optimistic. An air oto air unit that kicks in 2.5 kw of heat or cool works wonders for avoiding radiator changes etc.

Helpful/good points.  Doubt I will convince any MCS installers though (at least based on my experience to date)!  Its going to end up as a DiY job I fear and still be cheaper than if I get the grant.  Not the outcome I'm hoping for, just the one I'm now expecting.

[Miek]

1 hour ago, JamesPa said:

Two people there most of the time is another 600W, so that's a kilowatt accounted for!

 

Not sure what your doing to generate that level of heat!

 

More like 100W per adult. 

[JamesPa]

19 minutes ago, Miek said:

Not sure what your doing to generate that level of heat!

 

More like 100W per adult. 

Fair enough.  For some reason I had always had 300W in mind, but on the basis of 2000Kcal/day (which is high) thats 140W, so I will go with 100W.   So only 600W accounted for with baseload leccy consumption and two adults.  Still significant though. 

Edited April 10, 2023 by JamesPa

[SteamyTea]

2 hours ago, JamesPa said:

Perhaps, given this experience, you can understand why I am suspicious when told that something seemingly sensible and entirely within the bounds of physics is impossible

It is because, as you have shown, a lot more inputs and outputs.

I always reduce the ∆T by 3K as that is where my house seems to sit above OAT when left empty.

[markocosic]

2 hours ago, JamesPa said:

Incorrect assumptions about insulation accounts for 3kW.  Basically they ignored me when I told them which walls had been upgraded or were of a different construction because they were a later extension, and assumed they were all solid brick, which is far from the case.  Also pessimistic assumptions about windows which have also been upgraded.  Their calculation actually came out at 15kW (and they based their quote on this) bu,t when I asked for the detail, it transpired that they had had also counted room losses in the house total, but not room gains, thus effectively double counting 1kW.  I paid £300 for this particular survey.

 

Name of the firm, for reference?

 

If you can show they double counted that's ground for not paying a penny for the garbage.

 

 

2 hours ago, JamesPa said:

I'm struggling to explain the difference between the 10.5kW I calculate with the correct insulation values, and 7.5kW measured.

 

- Use the MCS worksheet directly.

- Split it out by fabric vs ventilation

- Make the ventilation more reaslistic

- Subtract your incidental gains (not just hotel load electricity which all equals heat but also your incidental gains from cooking / laundry etc much of which end up as internal gains

 

The rest may be be due to it not being as cold as you think during cold snaps (AVERAGE of -2C for example; not just -2C overnight) and your comfort criteria being relaxed in cold snaps (ok ok 18C is ok rather than 20C)

 

 

The heat loss spreadsheet provided by Jeremy via this forum is as good as any for a baseline. MCS worksheet is only useful for seeing the approximate distribution internally / sanity checking radiator sizes. Their ventilation assumptions are insanity though.

 

[JamesPa]

3 hours ago, markocosic said:

comfort criteria being relaxed in cold snaps (ok ok 18C is ok rather than 20C)

That's actually a particularly interesting one.  I am pretty certain that my comfort temperature is higher at +4 than it is at -2.  I assume this is humidity related.  I wonder if this is common.

 

The late November cold snap was -2 day and night here just north of London, for 4 days in a row.  Great weather for experimental work on central heating. 

3 hours ago, markocosic said:

Their ventilation assumptions are insanity though.

I had pretty much concluded that but, short of making measurements (which some on here do) don't know what to substitute.  My own calcs already split fabric and ventilation which is part of the reason why I am pretty confident that is a large part of the difference.

[markocosic]

7 minutes ago, JamesPa said:

That's actually a particularly interesting one.  I am pretty certain that my comfort temperature is higher at +4 than it is at -2.  I assume this is humidity related.  I wonder if this is common.

 

I suspect it's common.

 

Also in reverse. 24C and dehumidified slightly by having been actively chilled from 30C and 100% humidity to 24C by a coil at say 15C feels materially cooler than 24C and 100% humidity otherwise would.

 

And in a related way folks with silver hair and cash to spare actually like it to be 23C or 25C in winter.

 

That (1) increases heat loss AND (2) reduces radiator:room temperature difference and therefore output.

 

So flow temperatures end up materially higher (~3C per degree of room temperature increase) and either the heat pumps can't do it (kW output drops as flow temperature increases) or they can but the COP plummets.

 

You can bet the MCS tickbox brigade and indeed Octopus don't check that assumption either. They ought to. It's as material as stair lifts for some.

[markocosic]

13 minutes ago, JamesPa said:

I had pretty much concluded that but, short of making measurements (which some on here do) don't know what to substitute

 

- Actual electricity and gas use

 

- Reduce flow temps from boiler and if it works ok you know the maximum possible output from the rads at that

 

- Crossed fingers with the "insulate or supplement air to air" get out of jail free card up your sleeve

[sharpener]

36 minutes ago, markocosic said:

Crossed fingers with the "insulate or supplement air to air" get out of jail free card up your sleeve

 

Not quite. As I understand it is only the first HP that comes under Permitted Development in the UK. A second or subsequent heat pump requires full Planning Permission. So starting now, if you are lucky enough to get an HP installed over the summer but it turns out in October it isn't quite big enough you won't be able to fix it with an A2A this side of Christmas.

 

Edited April 10, 2023 by sharpener

[markocosic]

Is planning permission similar to planning forgiveness? 😉

 

[ReedRichards]

11 hours ago, markocosic said:

Is planning permission similar to planning forgiveness? 😉

 

Planning forgiveness is called "retrospective planning permission" - and you cannot assume that you will be forgiven.

[JamesPa]

22 hours ago, markocosic said:

The industry needs...to get good at walking before it tries to run...to deliver turnkey off the peg suits before it goes custom designed saville row etc.

A final(?) comment/question on this. 

 

The HP industry, including the lobbying side, needs to get its skates on big time.  Almost every day I see a headline somewhere telling me (erroneously) that heat pumps are going to destroy Britain (that was in the Telegraph recently) and hydrogen is the solution.  The hydrogen-ready brigade are coming for you, and will surely find a way to continue to sell gas boilers on the promise that one day they can be powered by 'clean hydrogen' from the same pipes. 

 

They wont care about retaining a vented cylinder. however crappy it might be.  They wont want to rip up flooring unnecessarily to fit 22mm cw feeds to the DHW in a house where the mains pressure is 9 bar (!), and they wont need to swap radiators out.  Nor will they need planning consent, or MCS, or government subsidy at point of sale, or a complete redesign of a heating system that may not be perfect, but which the customer has got used to.

 

But they will be (doubtless already are) lobbying government in the background to allow gas boilers to be fitted so long as they can, at some future date, be run off of 'green' hydrogen.  Then, a few years down the line, it will transpire that hydrogen is at least 30% more expensive than electricity and, because gas has been retained, there will be pressure not to make the switch 'because people cant afford it'.   Furthermore, in the intervening years (because more gas installations have been retained than are necessary), the political pressure to maintain the artificial relationship between gas and electricity prices, which today disadvantages heat pumps, will remain, and proposals to fix this will be watered down.

 

You can see this all coming.  The Telegraph recently ran another article 'Heat pumps won't work in old homes, warns Bosch' where Bosch was represented as a heat pump manufacturer, thus making it more plausible.  My cynical suspicion is that Bosch are, in reality, scared about loss of boiler sales so need to find a way to perpetuate the unconscionable.

 

Currently perfection is, to my perception anyway, getting in the way of the good, with a very real risk that the result is a 'victory' for the disaster that is climate change.   This is not intended as a criticism, just a comment on how things may need to change very fast if the battle is not to be lost.  I am a believer (because I have a background in science so trust facts about climate change not opinions), but haven't yet put in a heat pump, and am really frustrated by the industry for always presenting problems not solutions.  I will now almost certainly end up doing it myself, most wouldn't bother.  I understand the need to walk before running, but there is a runner right behind you ready to take your lunch and our children's futures.

 

How can we collectively help?