Mad idea for DHW retrofit based on Mixergy?

[markocosic]

2 hours ago, JamesPa said:

MCS-rules-based over sizing

 

You can mitigate this by finding a competent installer willing to use non default insulation and ventilation requirements.

 

The latest wheeze by the self serving pigs at the MCS trough, however, is persuading mortgage companies to start asking for MCS certifications for heat pumps.

 

Almost like they're planning for new reasons to exist after their role as the sole pathway to government gravy ends...

[Beelbeebub]

I suspect there is a seasonality element. 

 

The price of gas fluctuates lower in the summer

 

The price of electricity will often go higher as it is generator constrained and southern Europe is guzzling electricity for cooling. 

 

I had a look at what makes up unit prices.  Both gas.and electrify have a "per unit green tax"

 

If you look at the absolute amounts 

Gas - about 0.2p/kWh

Elec - about 1p/kwh

 

You can see elec has a much higher level of taxation

 

We could move almost all the cost of green taxes to gas.

 

That would help.

 

But this is where my subsidy pegged to the gas.price.wouid come in. It would effectively make the swap risk free.  Even if electricity Soares away from gas your bill.woiod be capped at what it would have been if you remained. You may (if your setup beats the official COP) make a little.from it (winks). Human nature being human nature that fact may motivate people to increase efficiency. People will often work extra hard to get "free" stuff.

 

This will remove one of the barriers to switching "what if is spend all this money and my bills go up?"

 

[JamesPa]

2 minutes ago, markocosic said:

 

You can mitigate this by finding a competent installer willing to use non default insulation and ventilation requirements.

 

The latest wheeze by the self serving pigs at the MCS trough, however, is persuading mortgage companies to start asking for MCS certifications for heat pumps.

 

Almost like they're planning for new reasons to exist after their role as the sole pathway to government gravy ends...

First point noted and the route I'm still trying to take.  Second point (mortgages), you are surely kidding.  No I guess you aren't.  This should be outlawed immediately.

[markocosic]

4 hours ago, JohnMo said:

it just wants to make dT correct all the time. If the dT is low it just ups the discharge temp to compensate

 

Heat pumps have little choice in this respect.

 

Their only meaningful output variable is compressor frequency.

 

There's a sweetspot for this to maximise performance at a given source/sink temperature.

 

There are also min/max limits. (with the sweetspot often near the minimum but not quite)

 

You need to pull heat away at the rate the compressor is pumping it. Dropping flowrates through the plate just forces up the dT until you trip out on high temperature/pressure.

 

You want to be running flow as quickly as you can through the plate before (a) it has to operate above the sweet spot output to maintain a minimum dT (usually 3C or so;. tunable on some units in DHW mode) or (b) the pumping power increase exceeds the COP benefit of a lower dT. (could happen I guess, if primary runs in a retrofit scenario are large?)

 

Thermosyphon is going to be insufficient to generate the turbulent flow in the HX needed for efficient heat transfer. On the space heating plates in district heating schemes we have issues at low space heating outputs where the flow through the exchangers is so low that it stratifies and transfer is at high DT rather than being turbulent over the whole plate

[JohnMo]

So high flow either side of the PHE is key

[markocosic]

3 minutes ago, JamesPa said:

This should be outlawed immediately

 

No; but it should be a fair term in a consumer contract.

 

If they are using the MCS accreditation of the heat pump installation in order to claim carbon reduction credits that reduce their scope three emissions footprint...then this should be explicit and result in a discounted interest rate.

 

It could help subsidise energy efficiency etc.

 

Else; yes; they ought to be prevented for doing this in much the same way as they should be prevented from allowing lodgers etc. 

 

But MCS are the darlings of government who give them the warm, fuzzy, fictional figures to report on their spreadsheets...

[markocosic]

Just now, JohnMo said:

So high flow either side of the PHE is key

 

Appropriate flow.

 

A 3, 5 or 7 degC dT (probably the same dT both sides unlesss primaries are particularly long?) at the target power output ought to give optimal COP improvement for minimal pumping power increase.

 

You can't run at part part part load though. And not do you necessarily want to run at 11/10ths either.

[Beelbeebub]

From reading @markocosic it seems that heatpumps (despite recent advances) are rather inflexible beasts.

 

The combo of compressor characteristics and working fluid charge mean there is a narrow range of efficency power/temp pairings for any given external air temp.

 

If you stray from this zone your efficency starts to drop away.

 

Would this be correct?

 

[JohnMo]

14 minutes ago, Beelbeebub said:

From reading @markocosic it seems that heatpumps (despite recent advances) are rather inflexible beasts.

 

The combo of compressor characteristics and working fluid charge mean there is a narrow range of efficency power/temp pairings for any given external air temp.

 

If you stray from this zone your efficiency starts to drop away.

 

Would this be correct?

 

Our heat pump, will follow a heat up curve, it's a matter of being able to take away the heat effectively, otherwise the you move heat around a system and it doesn't get to where it needs to be.

 

That's why it's best to limit zones etc. in the heating system.

 

But boilers are the same, it's just that gas has been traditionally very cheap, so really bad installs go unnoticed, by the owner. Another thread today mentioned oil boilers are 93% efficiency, but in most installs with loads of thermostats and an oil boiler limited or none existent modulation, short cycling is the order of the day unless running against a large thermal store. Would expect in this installs 50 to 60% efficiency if your lucky.

[Beelbeebub]

28 minutes ago, JohnMo said:

Our heat pump, will follow a heat up curve, it's a matter of being able to take away the heat effectively, otherwise the you move heat around a system and it doesn't get to where it needs to be.

 

That's why it's best to limit zones etc. in the heating system.

 

But boilers are the same, it's just that gas has been traditionally very cheap, so really bad installs go unnoticed, by the owner. Another thread today mentioned oil boilers are 93% efficiency, but in most installs with loads of thermostats and an oil boiler limited or none existent modulation, short cycling is the order of the day unless running against a large thermal store. Would expect in this installs 50 to 60% efficiency if your lucky.

Yeah, tge cheapness of gas combined with the very high power density mean you can pack way more power in to a boiler package than any house is likely to use. This has allowed installers to get away with sloppy installs, eyeballing the rad sizing, knowing they can get round any cockups by just turning the wick up on the boiler.

 

Rhe same approach comes unstuck with Heqtpumps.

 

Dumping a load of cash into R&D to develop HPs with a broader operating range would be useful. If hearing installers and retro fitters just need to go and grab a white label "3-15kw" unit and know it will work pretty well for most homes, it would be very beneficial.

 

What would enable that? Twin compressor models? 

 

Maybe smaller units (like 2-3kw) that can be installed in parallel to boost power? Just keep adding units until you get the right power and gives much more "turndown" than an individual unit can offer?

[markocosic]

1 hour ago, Beelbeebub said:

Dumping a load of cash into R&D to develop HPs with a broader operating range would be useful

 

Not necessary - buffer intelligently instead if you must.

 

 

[Beelbeebub]

1 hour ago, markocosic said:

 

Not necessary - buffer intelligently instead if you must.

 

 

The flexibility of the system is the important bit. Where.we.get that from, fancy HP with wide operating range or a buffer to provide that flex instead, is not important.

 

I'd note that any buffer needs to be compact for retrofit.

 

I can see milage in combining the DHW tank with a buffer and even the expansion - this is more or less what some of the indoor modules are.  But the big issue (from a retro fit pov) is the size. They are fridge sized. What we really need is a unit about the size of a boiler, somkitchen cupboard. At an absolute pinch washing machine sized.

[JohnMo]

Or just need to get people used to operating the heating system as one or possibly 2 zones only. Then there is no need for a buffer or any form of indoor unit, just a 3 way diverter, to the heating system and the DHW system. Run on weather compensation or a modified curve to use almost a batch heating style in the shoulder months to overcome short coming in the modulating range of the heat pump.

 

 

[Beelbeebub]

3 minutes ago, JohnMo said:

Or just need to get people used to operating the heating system as one or possibly 2 zones only. Then there is no need for a buffer or any form of indoor unit, just a 3 way diverter, to the heating system and the DHW system. Run on weather compensation or a modified curve to use almost a batch heating style in the shoulder months to overcome short coming in the modulating range of the heat pump.

 

 

Higher thermal mass housing construction would help. I've got a UFH concrete slab, about 20 tons IIRC. During the winter, when I operate my woodstoce, it dumps heat into the slab at about 7kw which then rides us through the night and next day no problem. When I eventually get round to fitting a HP, I imagine it will do the same, run for a few hours then shut off for a day or so.

[SteamyTea]

6 hours ago, JamesPa said:

how did the government get us to this point?

 

It is about CO_2e reduction, not price.

 

People overestimate their boiler efficiencies and never trust a properly set up heat pump efficiency.

Edited July 15, 2023 by SteamyTea

[JamesPa]

26 minutes ago, SteamyTea said:

It is about CO_2e reduction, not price.

 

People overestimate their boiler efficiencies and never trust a properly set up heat pump efficiency.

I agree but...for many it's about price.  Particularly for those who campaign against change.

[SteamyTea]

2 minutes ago, JamesPa said:

I agree but...for many it's about price.  Particularly for those who campaign against change.

It is why I very really mention price when talking about renewable/low carbon generation.

 

But if you take the drastic costs reduction that has happened with PV and Wind Power, and how it has happened in a decade.  Heat generation won't be far behind.

 

I hope so, only go tot look at all the countries that are suffering from the exceptionally hot Northern Hemisphere summer this year.

USA, Southern Europe, Central Asia and China.

If we think it is expensive to fit a heat pump, wait till the costs of crop failures and migration take hold.

[JamesPa]

2 minutes ago, SteamyTea said:

If we think it is expensive to fit a heat pump, wait till the costs of crop failures and migration take hold.

You and I know that.  The Daily Mail and the Torygraph probably also know that, but it doesn't suit them to tell people.  Nor does it suit most politicians.  And many if not most are happy to put their head in the sand if it saves them a few pounds.

 

Like it or not price at the point of delivery does matter.

[JamesPa]

1 hour ago, Beelbeebub said:

I can see milage in combining the DHW tank with a buffer and even the expansion - this is more or less what some of the indoor modules are.  But the big issue (from a retro fit pov) is the size. They are fridge sized. What we really need is a unit about the size of a boiler, somkitchen cupboard. At an absolute pinch washing machine sized.

Begs the question, why not just run both dhw and Ch at 55 and use the dhw tank, as you imply, intelligently as a buffer.  

 

The volume of a washing machine, subtracting 50mm all round for insulation, is about 170l which should be plenty of dhw/buffer if people could get used to showering for a sensible amount of time.

[Beelbeebub]

3 hours ago, markocosic said:

28 minutes ago, JamesPa said:

   

 

Edited July 15, 2023 by Beelbeebub
Sorry can't get rid of the quotes!

[Beelbeebub]

Sorry about ☝️ couldn't get rid of the boxes  

 

Anyway....

 

Yeah something like that.

 

I did wonder if we stored the DHW at about 35/40C and then used the HP to instantaneously heat up the DHW.flow by another 5-10C?  The power required to lift a shower flow from 10C to 45C is too much for a HP, without it being oversized for heating.

 

But the power required to lift a shower flow from 35C to 45C is much closer...

 

 

[Beelbeebub]

As I mentioned earlier, if the HPncan achieve pasteurisation temps (65C or so) on it's.own then we no longer need any immersion booster. 

 

That means it is physically impossible for the heat source to overheat the cylinder and cause a catastrophic event.

 

Which means the safety features aren't needed, just a PRV, venting to the drain in case the expansion vessel fails.

 

Which simplifies the installation back to that of a vented cylinder (DIY) probably easier (thus cheaper)

[markocosic]

1 hour ago, JamesPa said:

Begs the question, why not just run both dhw and Ch at 55 and use the dhw tank, as you imply, intelligently as a buffer.  

 

The volume of a washing machine, subtracting 50mm all round for insulation, is about 170l which should be plenty of dhw/buffer if people could get used to showering for a sensible amount of time.

 

Yep.

 

Winner of the show at ISH this year.

 

https://www.qvantum.com/ISH23/GSHPM

 

It's a thermal store with DHW produced on demand by plate heat exchanger AFAIK.

 

Allows you to run the store even lower than 55 whilst still producing useful DHW.