Mad idea for DHW retrofit based on Mixergy?

[ReedRichards]

24 minutes ago, JamesPa said:

How can we collectively help?

 

Finding a way to retrofit a longer coil (or something with an equivalent heat transfer capability) to an existing hot water cylinder would reduce the cost of a heat pump installation.  I had to fit new radiators but that was relatively cheap and painless, it seems more of an obstacle than it really is.

[JamesPa]

9 minutes ago, ReedRichards said:

 

Finding a way to retrofit a longer coil (or something with an equivalent heat transfer capability) to an existing hot water cylinder would reduce the cost of a heat pump installation.  I had to fit new radiators but that was relatively cheap and painless, it seems more of an obstacle than it really is.

Completely agree on all counts.   There are some cases where replacing a cylinder is easy, but many, like mine, where the knock on effects are huge.  At least we need an option to consider, even if its sub-optimal.

 

That's how I started this thread (with an idea to solve just this problem).  We ended up with two, either of which could probably be made to work.  Will the industry do anything, doubt it but hope so.

[SteamyTea]

6 hours ago, JamesPa said:

How can we collectively help?

The Guardian is trying.

https://www.theguardian.com/environment/2023/apr/11/blending-hydrogen-gas-heating-add-almost-200-uk-bills

[markocosic]

21 hours ago, JamesPa said:

How can we collectively help?

 

- Call out hydrogen-in-boilers as the most ridiculous thing you could possibly do at every opportunity 

 

- Campaign for welfare taxes/subsidies to be part of general taxation rather than on electricity; and for cost reflexive pricing on electricity including transparency on their make-up (standing charges are way too low vs reality; unit rates are way too high vs reality; all of which conspire to make heat pumps less economic than they actually are for UK plc)

 

- Prepare for electrified heating as the house is renovated generally (induction hobs, electric ovens, sacking off gas fires, recreating space for hot water storage in many cases etc)

 

- Decide when end of life is for your existing heating system so that the upgrades can be a thought through exercise rather than an almighty panic whereby the easiest thing to do is replace has with gas

 

 

e.g.

 

Newbuild gets heat pump. That's a no brainer.

 

I don't have an air to water heat pump at the existing place in Cambridge though. Uneconomic at the time the boiler went in back in 2017 and still uneconomic/undesirable to install a hot water cylinder where none exists.

 

I do have a little 3.5 kW air to air unit downstairs though; which has electrified ~80% of space heating load at a sCOP of ~5 if datasheet are to be trusted; for a peanuts installed cost and disruption.

 

The combi is retained for instantaneous hot water and top up space heating. Radiators can heat the place with a 55C supply/47C return and the connections are all under the plinths; as is a 32A supply for a heat pump. Cooking is all electric already.

 

When propane splits become available a tall kitchen unit gets swapped for the all in one tank. Hopefully they become available before gas standing charges skyrocket and/or disconnections cost real money. (next 2-3 years) Or when propane monoblocs ceae with their stupid restrictions on placement near doors etc.

 

Prefer the split though to keep the size of the outdoor unit down, keep all water indoors, and keep the valuable half of the system indoors. (EV chargers are already starting to go missing; monoblocs won't be too far behind once the pikeys realise that two hoses and an a cable is all that secures a couple thousand £s outside the house and all you need to operate it is a new thermostat etc)

 

Gas boiler is therefore end of lifed based on availability of alternatives / roughly in next 3 years rather than on it expiring in this example. There's been a few years of "whilst at it" preparation beforehand and a quick win of an air to air unit in the interim.

 

(which given the low install cost, PV, and recent price hikes has probably already washed its face economically in addition to providing the welcome sanctuary when it's chuffing hot out)

 

Alternative option is a standalone DHW cylinder with heat pump is and a slightly more powerful standalone air to air for heating. (probably a two head multi split) It's the winter DHW that's hard though.

 

[JohnMo]

Just started a new thread. My cylinder is equipped as per mixergy concept, the heat transfer efficiency doesn't look very good.

 

https://forum.buildhub.org.uk/topic/33175-heating-cylinder-via-heat-pump/

[sharpener]

Just to round this topic out, I have tried to press Vaillant's tech people about fitting a secondary circulating pump and this is what I received today. Perhaps not surprisingly they do not like to think out of their comfort zone:

 

<I have looked around but cannot find a definitive answer on the coil size for that
[OSO 210l] cylinder. The rule of thumb for sizing a coil surface area for a heat pump is 'for
every 4kw of heat you need 1m2 of surface area'. This would indicate a 12kw unit
would need a 3m2 surface area coil within the cylinder. 

For the same cylinder to be used and the water recirculated we would be unable to
comment on how this would effect the running of the unit when in DHW mode. The unit
will be looking for a minimum of 5k delta T when in DHW mode and may cause reduced
DHW efficiency or may even go to fault if this is not achieved.>

 

[JamesPa]

39 minutes ago, sharpener said:

Just to round this topic out, I have tried to press Vaillant's tech people about fitting a secondary circulating pump and this is what I received today. Perhaps not surprisingly they do not like to think out of their comfort zone:

 

<I have looked around but cannot find a definitive answer on the coil size for that
[OSO 210l] cylinder. The rule of thumb for sizing a coil surface area for a heat pump is 'for
every 4kw of heat you need 1m2 of surface area'. This would indicate a 12kw unit
would need a 3m2 surface area coil within the cylinder. 

For the same cylinder to be used and the water recirculated we would be unable to
comment on how this would effect the running of the unit when in DHW mode. The unit
will be looking for a minimum of 5k delta T when in DHW mode and may cause reduced
DHW efficiency or may even go to fault if this is not achieved.>

 

My vote, based a bit on logic and a bit on gut feel, is still for recirculation with a PHE instead of a cylinder coil.  Hopefully I will get to try that soon.

[sharpener]

Yes in the abstract I agree, you can then achieve whatever surface area you want. But as posted elsethread, in my situation I am concerned about corrosion from the rainwater, even with treatment.

 

Let us know what PHE you fit and how you get on. Do we infer you are close to installing your HP now, I have lost track?

 

I have just applied for full P/P, no idea if I will get it in time to install before this winter, or whether I will still want to. Might however fit my own rad upgrades over the summer, there are some that are undersized even for the boiler so the effort will not be wasted.

 

Edited July 7, 2023 by sharpener

[JamesPa]

8 hours ago, sharpener said:

Let us know what PHE you fit and how you get on. Do we infer you are close to installing your HP now, I have lost track?

Hopefully, but Im not counting chickens yet.

[sharpener]

On 08/07/2023 at 08:26, JamesPa said:

Hopefully, but Im not counting chickens yet.

 

How is yr planning application going @JamesPa?

 

I have heard today mine is not valid for various technical reasons to do with the maps and plans. Like a missing N arrow on one plan (of several) fgs. Even though I believe the additional info required would not meet the following test from the LA's own web site.

 

"The statutory tests are as follows:

• reasonable having regard, in particular, to the nature and scale of the proposed development; and
• about a matter which it is reasonable to think will be a material consideration in the determination of the application."

But they give themselves 8 weeks to respond to complaints of that nature so it is not a useful way forward, and Installer A confirms they are very picky and have caused him grief in the past

 

So I guess the simple way out is to pay for them to be produced professionally, has anyone got any experience of online mapping services e.g. these people?

 

 

[JamesPa]

8 hours ago, sharpener said:

 

How is yr planning application going @JamesPa?

 

I have heard today mine is not valid for various technical reasons to do with the maps and plans. Like a missing N arrow on one plan (of several) fgs. Even though I believe the additional info required would not meet the following test from the LA's own web site.

 

"The statutory tests are as follows:

• reasonable having regard, in particular, to the nature and scale of the proposed development; and
• about a matter which it is reasonable to think will be a material consideration in the determination of the application."

But they give themselves 8 weeks to respond to complaints of that nature so it is not a useful way forward, and Installer A confirms they are very picky and have caused him grief in the past

 

So I guess the simple way out is to pay for them to be produced professionally, has anyone got any experience of online mapping services e.g. these people?

 

 

Mine is on hold at my request while I go through all the design options with two prospective installers.  I'm not optimistic though, they seem determined to insist on 27dB(A) at the most affected assessment point which is nigh on unachievable unless I put it 20m down the garden at a trenching cost of 3-5k.  Political pressure on the ruling green party has so far failed, their exec member in charge of planning takes the view that greening up should be permitted subject to the condition that it doesn't inconvenience anyone at all, particularly someone like her 'a keen gardener'.  

 

Mine is equally picky about plans.  I created  mine by downloading (from the LPA website) and modifying plans of the neighbours extensions and a survey of my house I had done.  Libre office draw is remarkably good at importing and has quite powerful drawing tools.  It's no CAD package but it works.

 

 

[sharpener]

7 hours ago, JamesPa said:

Mine is equally picky about plans.  I created  mine by downloading (from the LPA website) and modifying plans of the neighbours extensions and a survey of my house I had done.  Libre office draw is remarkably good at importing and has quite powerful drawing tools.  It's no CAD package but it works.

 

Takes some ingenuity. Did a bit of playing around last night with a sample file as I was worried about preserving the scaling which LA seems to think v important.

 

Can take their sample pdf from the above link, open with IrfanView, convert to bmp, edit with Paint, save as bmp, and convert back to pdf with IV. Resulting image is indistinguishable except where edited and to exactly the same scale, phew!

[SteamyTea]

3 hours ago, sharpener said:

open with IrfanView, convert to bmp, edit with Paint

In Irfanview you can edit by using the F12 key.

You do only get the 1 undo though.

[JamesPa]

5 hours ago, sharpener said:

I was worried about preserving the scaling which LA seems to think v important

Just rescale after importing, using whatever known measurement is available.

[Beelbeebub]

Very interesting topic 

 

Tomget back to the original but about a PHE retrofit to a vented cylinder.

 

Has it been suggested that you ditch the circulator pump and just fit a big arse PHE between the cold inlet at the bottom and the top and rely on thermosiphon to circulate the water?

 

With a big enough PHE this should get around all.of the capacity/modulation/dT issues?

 

My only concern would be if cold water might be drawn via that external circuit during hot water use. That may need some careful design of the plumbing connections top and bottom.

 

My second point is about encouraging HP adoption in new build prior to the.offical ending of gas (which will forever be punted down the road)

 

Just change the building regs so the house has to be designed for a max CH flow temp of 40C.  It doesn't matter if you're planning a boiler, before you get building approval you need to show the emitters, losses etc are all capable of working at 40C. At that point  HP retrofits become.much easier.  It's mad we are.still.building houses today that will require extensive rework to fit HPs in a few years

 

A new build estate near me went up with heatpumps. But because the developers cheaped out and stuck crappy tiny rads in that drove the flow trmpsmup and COPs through the floor, the majority of owners have ripped them out for oil boilers, and then advocate against HPs socially because "they don't work and cost a fortune to run"

 

One of the things the heating industry like about boilers is, because they provide high powers in a compact package at high flows they can be lazy about speccing and installing.  If you balls up the calculations and the client complains they are a bit cold because your rads or pipes are too small, you can just nip back, crank the flow temp up a.few degrees and done. Sure it costs.a.bit more to run but the customer won't notice and you've got a happy customer.

 

HPs don't give installers that margin. I've estimate demand, and you've spent alot more on a bigger unit that takes up more space and short cycles ruining efficiency etc. Underestimate and your client is cold and you have no headroom to compensate.

[JamesPa]

1 hour ago, Beelbeebub said:

Just change the building regs so the house has to be designed for a max CH flow temp of 40C.  It doesn't matter if you're planning a boiler, before you get building approval you need to show the emitters, losses etc are all capable of working at 40C. At that point  HP retrofits become.much easier.  It's mad we are.still.building houses today that will require extensive rework to fit HPs in a few years

The latest version of part L makes it mandatory to size for 55C.  I guess that won't change again for a few years.

 

The latest R290 heat pumps (eg Vaillant) have a scop of between 3 and 3.6 at 55C (the larger ones have a higher scop).  SCOP=3 makes it about the same cost as gas even at todays gas/electricity price ratio, so is good enough.

 

Put a 5 or 7kW Vaillant in a new house insulated to modern standards and you are good to go in most cases I would think.  Possibly you need a small volumiser, (not a 4 port buffer, because its bound to be connected wrongly and/or undersized) if the house is small or particularly well insulated.  Bear in mind most new houses are built to standard designs so it's not that difficult to get it right if you can be bothered.

 

It's all doable now.  But builders won't do it until they are forced to do so.

 

Obviously this is an urgent must-do, but by far the majority of our housing stock is the more difficult retrofit job.  Here we need to sort out the regulation, and in particular MCS and planning rules, and the system design industry (part of installation if you will, but actually there are plenty of competent plumbers and electricians around, just not many people who can do a practical system design for retrofit).  This industry barely exists in any competent form, albeit that there is a flourishing grant harvesting industry. 

Edited July 15, 2023 by JamesPa

[JamesPa]

1 hour ago, Beelbeebub said:

Has it been suggested that you ditch the circulator pump and just fit a big arse PHE between the cold inlet at the bottom and the top and rely on thermosiphon to circulate the water?

 

Would this work, I thought that the pressure differential across a PHW was quite high?

[JohnMo]

I have an external PHE connected to heat a vented thermal store. It takes water from the bottom of the store and it is pumped through the PHE to the top of the store.  Heat is provided by a 6kW ASHP. The PHE works in a similar/same principle as a mixergy one does.

 

The thermostat located about 40% up from the bottom of cylinder is set for about 42 degs.

 

Heating cycle goes something like this.

 

Cylinder PHE pump starts, and ASHP starts DHW.

 

Cylinder becomes unstratified, overall cylinder temp drops as water from bottom and top of cylinder mix.

 

Heat pump gradually ramps up temperature of its output to maintain a 6 deg dT. When it gets to about 55 deg the thermostat is made in the cylinder. The heat pump stops

 

Over the next 20 mins or so the cylinder re-stratifies with a temp of around 50 at the thermostat.

 

If I reduce the PHE pump speed heat transfer is not as effective and the ASHP cannot heat the cylinder to satisfy the thermostat set temperature.

[Beelbeebub]

37 minutes ago, JamesPa said:

The latest version of part L makes it mandatory to size for 55C.  I guess that won't change again for a few years.

 

The latest R290 heat pumps (eg Vaillant) have a scop of between 3 and 3.6 at 55C (the larger ones have a higher scop).  SCOP=3 makes it about the same cost as gas even at todays gas/electricity price ratio, so is good enough.

 

Put a 5 or 7kW Vaillant in a new house insulated to modern standards and you are good to go in most cases I would think.  Possibly you need a small volumiser, (not a 4 port buffer, because its bound to be connected wrongly and/or undersized) if the house is small or particularly well insulated.  Bear in mind most new houses are built to standard designs so it's not that difficult to get it right if you can be bothered.

 

It's all doable now.  But builders won't do it until they are forced to do so.

 

Obviously this is an urgent must-do, but by far the majority of our housing stock is the more difficult retrofit job.  Here we need to sort out the regulation, and in particular MCS and planning rules, and the system design industry (part of installation if you will, but actually there are plenty of competent plumbers and electricians around, just not many people who can do a practical system design for retrofit).  This industry barely exists in any competent form, albeit that there is a flourishing grant harvesting industry. 

As you say the real win is in the retro fit.  I think you are right that we need to stop clinging to ultimate efficency and just go for good enough.(¹)

 

One idea is a subsidy for the electricity used by your HP so the cost of heating is pegged to the cost of gas. IE it will not cost you any more than a gas boiler to run for the duration of the scheme (say 10 years)

 

This could be done by fitting heat meters to the HPs so they can measure the heat delivered. These meters would be linked to your smart meter so the energy co get the data. The company eatimates out how much electricity you used by taking your actual heat delivered and dividing by an assumed SCOP (say 2.5). It then works out the price difference between the p/kWh you paid for electricity and what you would have paid for the same kWh of gas. That is knocked off your bill.  

 

If your system is more efficient than the assumed 2.5, you actually end up "scamming" the scheme. This would be an incentive for people to boost their SCOP by upgrading the system (eg better emmiters)

 

the scheme is paid for by a levy on gas. This means the price of gas increases which also drives adoption by makeing the payback time shorter kand also makes the scheme sort of self regulating as the price difference narrows as the schem grows)

 

alongside this would be a big effort to help manufacturers develop "drop in" HPs (tank aside). An effort to develop compact hot water tanks (phase change?) Optimised for HP use so combi swaps would be easier. A boiler sized DHW unit and diverter valve inside, replacing the boiler with the pipes and cables to the outside unit exiting via the old flue hole to a unit outside would be a step forward.

 

as an aside if the HPs could provide 70C flow, they wouldn't need any electric immersion backups. That means your DHW system could *never* go above 70C. This means the risk of boiling and catastrophic overpressure on your invented cylinder is zero. Thus you could ditch the safety regs (g3) around install and inspect reducing cost and faff for install.

 

So you could have the boiler replacement system (inside unit goes where your boiler is, pipes go to external box, maybe via flue route) at plumbers merchants for 2-3k installable by any plumber to install, you register with the subsidy scheme and away you go.

 

At that point retro fit would take off.

 

 

(¹)

I think the magic COP needs to be about 2.5 at the design lowest outside temp. The reason for me picking that number, is the efficiency of a gas fired power station producing electricity is about 40% (Inc transmission losses).  So with a COP of 2.5 the heat delivered to your house from the source gas is the same as if you burned that gas in your house.  

 

If you drop below this figure you risk potentially burning more gas than you do do with gas heating. Granted the actual break even COP might be lower at 2.0 or something due to boiler and gas network inefficiency etc. But the 2.5 figure kills the naysayers "but we'll actually burn *more* gas!" argument dead and also ensures we don't accidentally do that!

 

 

[Beelbeebub]

41 minutes ago, JamesPa said:

Would this work, I thought that the pressure differential across a PHW was quite high?

I'm not sure!  

 

Ultimately it's a question of PHE size. You'll get heat transfer, but it's a question of how much Vs the modulation of the HP.

[Beelbeebub]

14 minutes ago, JohnMo said:

I have an external PHE connected to heat a vented thermal store. It takes water from the bottom of the store and it is pumped through the PHE to the top of the store.  Heat is provided by a 6kW ASHP. The PHE works in a similar/same principle as a mixergy one does.

 

The thermostat located about 40% up from the bottom of cylinder is set for about 42 degs.

 

Heating cycle goes something like this.

 

Cylinder PHE pump starts, and ASHP starts DHW.

 

Cylinder becomes unstratified, overall cylinder temp drops as water from bottom and top of cylinder mix.

 

Heat pump gradually ramps up temperature of its output to maintain a 6 deg dT. When it gets to about 55 deg the thermostat is made in the cylinder. The heat pump stops

 

Over the next 20 mins or so the cylinder re-stratifies with a temp of around 50 at the thermostat.

 

If I reduce the PHE pump speed heat transfer is not as effective and the ASHP cannot heat the cylinder to satisfy the thermostat set temperature.

Seems.lije a good system, are you happy with it?

 

Strikes me that could be packaged in a pretty small, neat unit.

 

Also strikes me, it could be used as a defrost volumiser. If heat was pulled from the PHE. You have a huge reserve of heat for the defrost cycle. Ideally the system would simply work via back thermosyphon. the cold water sinking tgrougbt the PHE to the bottom of the tank and fresh warm water coming from the top.

[JohnMo]

It's ok, I really just used what I already had. Used really as a proof of concept, the thermal store is only 180l so there isn't a big reserve of heat for DHW. If it was an UVC the concept would be better.

 

PHE is really the wrong side of the diverter valve to use for defrost.

 

38 minutes ago, Beelbeebub said:

Ideally the system would simply work via back thermosyphon. the cold water sinking tgrougbt the PHE to the bottom of the tank and fresh warm water coming from the top.

Trouble with the heat source it just wants to make dT correct all the time. If the dT is low it just ups the discharge temp to compensate, so you soon get to max discharge temp and the heating cycle stops. Then you are into short cycling. Without a pump in the system the water transfer via thermosyphon is not controlled and likely to be too slow.

[JamesPa]

2 hours ago, Beelbeebub said:

As you say the real win is in the retro fit.  I think you are right that we need to stop clinging to ultimate efficency and just go for good enough.(¹)

 

Yup, good enough is good enough.  Better is still better, but can be sold as a business case led upgrade.

 

2 hours ago, Beelbeebub said:

as an aside if the HPs could provide 70C flow, they wouldn't need any electric immersion backups. That means your DHW system could *never* go above 70C. This means the risk of boiling and catastrophic overpressure on your invented cylinder is zero. Thus you could ditch the safety regs (g3) around install and inspect reducing cost and faff for install.

 

They already can.  Check out the Vaillant R290 units - flow temp goes to 75C.  One wouldn't necessarily design for this, its almost certainly preferable is to retrofit a PHE to the existing cylinder to increase the 'coil' area without having to rip out the cylinder yet still allow the flow temp to be 55/60.

 

The technology for mass retrofit is here now; we need

• fit-for-purpose regulation (or better still scrap MCS altogether)
• fit for purpose planning rules (PD requires MCS and excludes most if not all 2-fan units.  MCS-rules-based over sizing drives installs for larger retrofits to a 2 fan unit, LPAs then object to the noise even though it meets the PD noise rules so legal install becomes impossible or impossibly difficult
• a fit for purpose system design industry 

By the time the UK has those, the technology will have advanced even further, perhaps to the point where our under-educated construction industry can cope with it without completely messing it up.

 

There are no excuses left, its all within UK control.

Edited July 15, 2023 by JamesPa

[JamesPa]

1 hour ago, JamesPa said:

Yup, good enough is good enough.  Better is still better, but can be sold as a business case led upgrade.

 

They already can.  Check out the Vaillant R290 units - flow temp goes to 75C.  One wouldn't necessarily design for this, its almost certainly preferable is to retrofit a PHE to the existing cylinder to increase the 'coil' area without having to rip out the cylinder yet still allow the flow temp to be 55/60.

 

The technology for mass retrofit is here now; we need

• fit-for-purpose regulation (or better still scrap MCS altogether)
• fit for purpose planning rules (PD requires MCS and excludes most if not all 2-fan units.  MCS-rules-based over sizing drives installs for larger retrofits to a 2 fan unit, LPAs then object to the noise even though it meets the PD noise rules so legal install becomes impossible or impossibly difficult
• a fit for purpose system design industry 

By the time the UK has those, the technology will have advanced even further, perhaps to the point where our under-educated construction industry can cope with it without completely messing it up.

 

There are no excuses left, its all within UK control.

I just checked the current gas and electricity prices (average).  Gas is now 7.5p and electricity 30.1p so somehow what was, prior to the energy crisis, a 3:1 ratio between the prices has become 4:1.  Not sure if thats a seasonal effect, obviously the demand for gas increases in winter disproportionately to the demand for electricity.

 

However its (hopefully temporary) bad news for heat pumps, at 4:1 most retrofits wont achieve cost parity.  the EU average ratio remains about 3 (10c vs 26c) figures from Eurostat; how did the government get us to this point?

 

[markocosic]

1 hour ago, JamesPa said:

a 3:1 ratio between the prices has become 4:1. 

 

Nope.

 

You're too optimistic in the cost of gas. 😉

 

Multiply the gas unit rate by 1.25 (80% boiler efficiency), so base price is actually 9.4p/kWh for gas.

 

Then add the standing charge in that you would avoid buying sacking off the gas supply (say £100 divide by 10,000 kWh for 1p/kWh)

 

And you're at 3:1 or a sCOP of 3 for cost neutrality.