Is there a no-frills R290 heat pump?

[JamesPa]

7 hours ago, sharpener said:

Snag in my case is that the source of the DHW is harvested rainwater, according to this web page the pH needs to be 7.5 or more to avoid corrosion of the copper brazing. Raw pH is about 5.3 from dissolved atmospheric CO2 and despite treatment with NaHCO3 I can't get it above 7.0. I was going to try a mix of calcite and dolomite based media next but they sent it with the wrong size of filter bag so it is in the all-too-difficult pile ATM.

Swimming pools often use sodium hypochlorite as a disinfectant, pH balanced with hydrochloric acid.  I don't know if sodium hypochlorite is sufficiently alkaline to neutralise your rainwater, but swimming pool chemicals are relatively cheap and relatively easily available if that helps.

[sharpener]

Yes, swimming pool suppliers sell special "pH builder" but this is just sodium bicarbonate (as above), however I use food grade as after the UV sterilizer it is potable water, though we do in fact have a separate mains supply for drinking water taps in kitchen and bathrooms.

 

NaHCO3 will not increase pH above 7.5, however much you put in (sodium carbonate is much stronger but then there is a chance of overdosing). I make up a stock solution and then run it through a small dosing pipe into the 5000l underground tank and rely on the incoming rainwater to mix the contents which is a bit hit and miss. "Experts" have suggested various alternatives but they are mostly expensive (automatic dosing pumps) or bulky (5' high reaction vessels) or both. SWW just add NaOH at their treatment works but even that has been known to go catastrophically wrong!

 

Favoured approach now is the combination of calcite (slow reacting) with 10% magnesite (fast but too strong to use on its own) in a filter bag round the bottom of the incoming 4 in pipe. Getting it there will be the problem as it is riveted in place as part of the tank construction, they were supposed to send me a bag which could be lowered down the inside of the pipe but it is too big and has a rigid plastic frame.

 

If I can achieve consistent results higher than pH 7.0 I might think about a PHE. However we have had pinhole leaks and a totally failed s/steel spring in a pressure reducing valve in the past. OSO wanted >£150 for a replacement valve group but I managed to find a bare cartridge on the web, only £40 and a much easier job to fit.

 

So now you know. Another project in itself. After consumables and replacement parts not much saving. We inherited the system, it was very badly engineered by the original contractor and I have installed a new tank and re-done much of the treatment plant. Also the (Chinese made) Stuart Turner jet pump has been very unreliable and ST will not help under warranty as they cannot repair them and regard them as disposable. I have just built a new one out of bits of two others, the plastic impeller shroud had a big crack in it. if it fails again I will fit a DAB as at least you can get spare shaft seals etc for those.

[HughF]

Another vote for CoolEnergy…. Super helpful, good product. 
 

Ask them for a written quotation, I think I paid £3k with the vat for my iVT-9

 

[sharpener]

On 28/06/2023 at 12:53, sharpener said:

Went to the Installer Show at the NEC yesterday, lots of interesting stuff. You can still catch it tomorrow.

 

Many more HP companies have got their newly-developed R290 models now on test and will be bringing them to market late '23 to early '24.

 

Among these I talked to were Grant (will be made by Chofu), Toshiba (branded as Carrier), Panasonic, Trianco. Nothing surprising technically but I imagine as the technology penetrates there might be a bit more competition on price.

 

Stiebel Eltron are pursuing a different route and have chosen to go with R454 as a replacement for R410a. Their 15kW machine is incredibly chunky to look at!

 

Ebac have their new British designed and made 5 and 9kW R32 machines (based on Mitsi compressors) with a test house currently, and are working on a bigger inverter to expand range to ?14kW. Will sell direct from factory. Was highly impressed talking to their Head of Development.

 

Another thing which caught my eye were several manufacturers of pipe fittings designed to be pressed/swaged/crimped rather than using grab rings a la Tectite. Apparently public contracts are now calling for these. Rothenberger man was very proud that his battery-powered on-site press was under £1000(!). So I think I will stick to Tectite for my (longer) replacement radiators, don't fancy the large number of Yorkshire fittings involved.

 

Have now had three quotes for MCS HP installation, none very satisfactory, will post details separately in a short while.

 

 

Interesting post on an earlier exhibition in Frankfurt by Graham Hendra, some further comments much in line with the thoughts of @JamesPa here. All of which inclines me to wait until some more of these R290 machines hit the market.

 

Will still require a change of attitude on the part of MCS installer hierarchy though to design for flow temps higher than 45C. Part L allows 55C for retrofit but even that does not really make best advantage of the higher temps achievable IMO.

[JamesPa]

10 hours ago, sharpener said:

 

Interesting post on an earlier exhibition in Frankfurt by Graham Hendra, some further comments much in line with the thoughts of @JamesPa here. All of which inclines me to wait until some more of these R290 machines hit the market.

 

Will still require a change of attitude on the part of MCS installer hierarchy though to design for flow temps higher than 45C. Part L allows 55C for retrofit but even that does not really make best advantage of the higher temps achievable IMO.

Yup  Vaillant R290 has a scop of 3.1 (3kW)- 3.6 (10kW) at 55, which is good enough. 

 

I reckon, at a helicopter level, the way it might eventually go for many retrofits is:

• Right-size system based on actual measurement, not GIGO spreadsheetery plus 20% for bad luck
• Design for 55 and offer 45 as a business case led upgrade (which will likely stack up in many cases) not a 'must do or else I wont play'
• If necessary use one or two fancoils for any difficult radiator replacements (where the replacement is too physically large)
• Put a PHE in to deal with the DHW issue, rather than a cylinder with a heat pump coil.  If retrofitting a combi, use a smallish cylinder which can be tucked away somewhere (high level in a room, in the loft, etc).  
• Cut out all the ancillary stuff often done (upgrading primaries, upgrading mains feed to DHW, adding poorly plumbed buffers etc) because its no longer necessary as you haven't massively oversized the system

= sane retrofit system, at a sane price.   A day's worth (maybe 2 days worth) of design by a specialist designer who has some software, measurement tools and can actually think, then fitted by your local plumber.

 

Actually it has to go this way otherwise 1.4M retrofitted heat pumps per year is unachievable (or even 600,000, the Government target).

 

MCS say that they are revising their scheme and standards but dont say how.  Will they 'get it right' and build in flexibility so the market can innovate and the cowbows get forced out because new entrants actually think about the problem - hopefully but I'm not holding my breath.

[joth]

1 hour ago, JamesPa said:

• Cut out all the ancillary stuff often done (upgrading primaries, upgrading mains feed to DHW, adding poorly plumbed buffers etc) because its no longer necessary as you haven't massively oversized the system

 

Not included a buffer tank is tough call. So long as current building regs require per-room controls, if retrofitting ASHP to a house that already has per-room controls, you pretty much have to include a buffer unless you're confident the legacy room controls will reliably not call for heat one room at a time.

If the owner has already shelled out $lots on Hive or Nest or whatever, they'll be reluctant to rip it out nor replace with new controls, and the ASHP supplier arguing for whole-house rather than room by room control is flying in the face of current regs and putting themselves into a risky position if it's a larger draughty house that genuinely does benefit from leaving some areas on a lesser used rooms less heated.

 

[sharpener]

38 minutes ago, joth said:

Not included a buffer tank is tough call. So long as current building regs require per-room controls, if retrofitting ASHP to a house that already has per-room controls, you pretty much have to include a buffer unless you're confident the legacy room controls will reliably not call for heat one room at a time.

If the owner has already shelled out $lots on Hive or Nest or whatever, they'll be reluctant to rip it out nor replace with new controls, and the ASHP supplier arguing for whole-house rather than room by room control is flying in the face of current regs and putting themselves into a risky position if it's a larger draughty house that genuinely does benefit from leaving some areas on a lesser used rooms less heated.

 

 

This. Describes my situation exactly. Honeywell Evohome took a long time to get working (because there are no diagnostics to show which (defective) wireless TRV is calling for heat) but I am not going to scrap it now and install a new Vaillant ambiSense controller and new valves throughout at £81-28 a pop.

 

Just tried to ring Installer A at 1205 on a working day. Biggish firm but just rings out, can hear it divert (twice), no recorded message or answering m/c or anything.

 

BTW1 I see the Heat Pump Installer Network has been set up specifically to train everyday plumbers as HP installers but looks in the very early stages ATM. Investors include EDF

 

BTW2 I see also that Freedom Heat Pumps are now owned by Certas (the oil distributor who were forced to divest themselves of BoilerJuice after an anti-competitive outcry) - notice a pattern here?

 

BTW3 I think I have worked out why my cyl has 15mm F&R, the piping is so short it might have starved the rad circuit if done in 22. Is S-plan so both can be on at once. A turn-down valve might have been more useful (and cheaper than all the 22-15 reducers).

 

 

Edited July 3, 2023 by sharpener

[JohnMo]

58 minutes ago, joth said:

Not included a buffer tank is tough call.

 

59 minutes ago, joth said:

So long as current building regs require per-room controls,

If you are running weather compensation, which all ASHP installs should be, then the room by room temperature control will be for overheat protection, rather than direct room temperature control. So there is zero requirements for the room temperature control to call for heat.

 

Heating room more or less than others is simply a matter of balancing.

 

So by default why would anyone require a buffer?  

[joth]

44 minutes ago, JohnMo said:

 

If you are running weather compensation, which all ASHP installs should be, then the room by room temperature control will be for overheat protection, rather than direct room temperature control. So there is zero requirements for the room temperature control to call for heat.

 

Heating room more or less than others is simply a matter of balancing.

 

So by default why would anyone require a buffer?  

Because people with per-room controls use them to shut off rooms when not in use, not for overheat protection, and owners have been conditioned to expect this. see point 4 on https://www.hivehome.com/discover-hive/smart-heating/five-ways-to-save-money-with-a-smart-thermostat for example.

 

Also it's very possible a older, lesser insulated and non-cuboidal home would have a LOT of heat loss through in a single room, meaning that room will still require heat even when ~all other rooms are at temperature and are shut off to avoid overheating. Typically that leaky room would be the most used living area which the household also like to keep at the highest temperature for the longest periods. Obviously fixing the fabric would be the better answer, but that also doesn't meet the drop-in-replacement for combi boiler being wished for here.

 

I wouldn't go as far as say the buffer is required by default, but it certainly can't be excluded without some careful thought. 

 

Edited July 3, 2023 by joth

[JohnMo]

25 minutes ago, joth said:

~all other rooms are at temperature and are shut off to avoid overheating.

But that's down to poor system balancing. Existing room controls can easily be reconfigured to operate what ever regime you require. How they interface with the heat pump require change anyway as it's operation is different from a boiler.

 

What you are explaining is why heat pumps are deemed expensive to run, it all the wrong ways to operate a heat pump for good efficiency. Having one room hot and all others cold, means the heat pump has to work harder to compensate for the heat transfer to the colder areas. It would be more efficient with an electric panel heater.

 

You can't have a drop in replacement for a combi, as you will require a cylinder or a means of giving DHW on demand.  

 

Any new heating system has to be designed for a max heating temp of 55, so some work would be required to radiators in most cases anyway.most likely.

 

Lots of ways to get around pipe size issues, so that's not an issue.

[SteamyTea]

18 minutes ago, JohnMo said:

Lots of ways to get around pipe size issues, so that's not an issue.

Care to share them?

[sharpener]

23 minutes ago, JohnMo said:

Any new heating system has to be designed for a max heating temp of 55, so some work would be required to radiators in most cases anyway.most likely.

 

What counts as a "new" heating system for the purposes of Part L? If you are doing a like-for-like gas boiler replacement then surely you are not expected to recalculate the emitters for 55C and upgrade all the rads at the same time.

 

Why should changing the heat source to an HP affect this? Does that somehow make it more "new"?

[HughF]

Air-to-air multi-split and a direct UVC - simple and easy. Much better than an a2w setup in my opinion. This has the potential for cheap, low carbon heat, with very easy on/off controls, and allows full zoning as required by the regs.

[DanDee]

32 minutes ago, HughF said:

Air-to-air multi-split and a direct UVC - simple and easy. Much better than an a2w setup in my opinion. This has the potential for cheap, low carbon heat, with very easy on/off controls, and allows full zoning as required by the regs.

 

A multi split not run with all the units on does have a lower COP, but for those that find it ideal solution, then it's the best for their situation.

ras3.pdf

[JohnMo]

57 minutes ago, SteamyTea said:

Care to share them?

One is an additional pump in the system return, will require anything from 12 to 33W depending on pressure drop experienced, this will compensate for increased pressure loss through the system.  I use one on my DHW heating, due to very long length of 22mm plastic pipe. Works a treat, mine is on only during DHW heating.

 

51 minutes ago, sharpener said:

Why should changing the heat source to an HP affect this? Does that somehow make it more "new"?

Sorry I don't make the rules, but I believe, in the interest of boiler efficiency etc (aim should be over 100% efficiency - mid 80s to low 90s efficiency should be no longer acceptable. Therefore all new boilers sold should not be capable of more than 55 degs for central heating and should, if not a combi, be configured on to work only on priory hot water or X plan. S an Y plan banished to history books, the same as 70 deg radiators. As both should have been in the 1980s when condensing boiler first became available.

 

Also while I'm on a roll all new cylinders should by law only be able to be sold if they are heat pump ready i.e. 3m2 coil installed, even in a direct cylinder.

 

As @HughF if want an easy drama free install go for A2A and direct cylinder (but with a 3m2 coil pre installed).

[HughF]

1 minute ago, JohnMo said:

 

Sorry I don't make the rules, but I believe, in the interest of boiler efficiency etc (aim should be over 100% efficiency - mid 80s to low 90s efficiency should be no longer acceptable. Therefore all new boilers sold should not be capable of more than 55 degs for central heating and should, if not a combi, be configured on to work only on priory hot water or X plan. S an Y plan banished to history books, the same as 70 deg radiators. As both should have been in the 1980s when condensing boiler first became available.

 

Very wise words there....

[joth]

3 hours ago, JohnMo said:

Also while I'm on a roll all new cylinders should by law only be able to be sold if they are heat pump ready i.e. 3m2 coil installed, even in a direct cylinder.

 

Right, and while you're on that roll, can you go ahead and re-write Approved Document L section 6.51? That's the part that requires each room to have it's own thermostat with per-room schedule, adjustable by the home owner, and that will ruin the system designer's attempt to run a carefully balanced whole-house heating strategy.

For a self-builder this is moot (you can skip the controls and chance it, or install the controls and not use them - so just some wasted capital cost) but for the target of this conservation, it's installing an "energy saving feature" that the more the home owner uses it, the worse the system performs.

[DanDee]

26 minutes ago, joth said:

Right, and while you're on that roll, can you go ahead and re-write Approved Document L section 6.51? That's the part that requires each room to have it's own thermostat with per-room schedule, adjustable by the home owner, and that will ruin the system designer's attempt to run a carefully balanced whole-house heating strategy.

For a self-builder this is moot (you can skip the controls and chance it, or install the controls and not use them - so just some wasted capital cost) but for the target of this conservation, it's installing an "energy saving feature" that the more the home owner uses it, the worse the system performs.

Are you referring to this?

Edited July 3, 2023 by DanDee

[JohnMo]

48 minutes ago, joth said:

Right, and while you're on that roll, can you go ahead and re-write Approved Document L section 6.51? 

If only I could. Although living in Scotland it's not applicable to me.

 

24 minutes ago, DanDee said:

and that will ruin the system designer's attempt to run a carefully balanced whole-house heating strategy.

For a self-builder this is moot (you can

Not sure it does. If system is running weather compensation and system balanced, they can turn the heat up as high as they like it will have little or no affect, without changing WC curve also. Turning it down will act as temperature limit stop. Admittedly if they screw most of the thermostats down and leave one one high, they could so only one zone is engaged. If it a radiator system the type of TRV installed so the flow is never completely closed off would fix this.

 

Owner education on how to operate the installed heat pump, how to use controls including thermostats should be part of a well engineered and balanced system.

 

[JamesPa]

7 hours ago, joth said:

 

I wouldn't go as far as say the buffer is required by default, but it certainly can't be excluded without some careful thought. 

 

Probably right in principle, but not realistic in the real world.  Buffers are not a long term solution in retrofits, insufficient space (particularly if you have also got to fit in a new water cylinder) and too much opportunity to mess it up.  Lets get over this fetish and move on, retrofits will and should wherever possible be without buffers and this will become the norm.  Lets find ways to make that work!

 

 

8 hours ago, joth said:

Not included a buffer tank is tough call. So long as current building regs require per-room controls, if retrofitting ASHP to a house that already has per-room controls, you pretty much have to include a buffer unless you're confident the legacy room controls will reliably not call for heat one room at a time.

 

I agree the regs are mad.  As others have said the room by room controls should be used as temperature limiters, but of course the public are not accustomed to seeing it this way.  However, why not argue that the lockshield is a 'room by room control'.  Put a knob on it and it is exactly that.  Then provide one knob for the whole house.  We aren't obliged to fund Drayton (however good they may be).

 

4 hours ago, JohnMo said:

Also while I'm on a roll all new cylinders should by law only be able to be sold if they are heat pump ready i.e. 3m2 coil installed, even in a direct cylinder.

Hmm, not sure I agree with that.  A very compact heat exchanger and a small pump can do a lot for this problem, and a 3 sq m coil takes up a lot of space which people don't have.

 

55 minutes ago, DanDee said:

Right, and while you're on that roll, can you go ahead and re-write Approved Document L section 6.51? That's the part that requires each room to have it's own thermostat with per-room schedule, adjustable by the home owner, and that will ruin the system designer's attempt to run a carefully balanced whole-house heating strategy.

Currently only for comfort cooling based on the snip you provided.  Is it the same for heating?

 

4 hours ago, HughF said:

Sorry I don't make the rules, but I believe, in the interest of boiler efficiency etc (aim should be over 100% efficiency - mid 80s to low 90s efficiency should be no longer acceptable. Therefore all new boilers sold should not be capable of more than 55 degs for central heating

Couldn't agree more with this one.  How on earth did boiler manufacturers get away with advertising the benefits of condensing boilers of 20 years, and at the same time installers get away with setting them to 75/80 flow temp.  I am ashamed that I only recently spotted in my own house (as a direct result of my interest in heat pumps) that I could be more comfortable and use less gas by simply turning down the boiler temperature!  My excuse is that, until recently, I had little reason to know much about the detail of heating systems!

 

6 hours ago, sharpener said:

 

What counts as a "new" heating system for the purposes of Part L? If you are doing a like-for-like gas boiler replacement then surely you are not expected to recalculate the emitters for 55C and upgrade all the rads at the same time.

 

Why should changing the heat source to an HP affect this? Does that somehow make it more "new"?

I guess this will be down to the building control officer (if anyone bothers to involve them when they replace a boiler, which I doubt). 

 

Replacing a boiler like for like would not be a 'new' heating system.  So what's the argument that replacing the source of hot water with a different technology is any more a 'new heating system'.  I guess the grant harvesters will prefer the 'its a new heating system' interpretation and your local plumber will probably prefer the 'its just replacing the boiler with a different heating source' interpretation.  Once the grants evaporate the two will hopefully converge.

 

8 hours ago, sharpener said:

This. Describes my situation exactly. Honeywell Evohome took a long time to get working (because there are no diagnostics to show which (defective) wireless TRV is calling for heat) but I am not going to scrap it now and install a new Vaillant ambiSense controller and new valves throughout at £81-28 a pop.

 

 

Of course not, that would be stupid.  Keep Evohome and valves, adjust the target temps to 1 C above what you actually want, balance the system and you are good to go.   Then play if you wish with broad brush zoning (If I remember correctly your house is basically divided into two halves, quite likely zoning of this into the two halves makes sense even with an ashp).

 

 

20 minutes ago, JohnMo said:

Owner education on how to operate the installed heat pump, how to use controls including thermostats should be part of a well engineered and balanced system.

 

Id go for that one!

 

This is clearly an interesting debate and shows that there is unlikely to be a one size fits all solution, which means some intelligence needed at the design stage (thus ruling out many of the current players).  But at the same time we cant make every solution totally bespoke.  My bet remains on something like  (added text in italics based on this discussion):

 

• Right-size system based on actual measurement, not GIGO spreadsheetery plus 20% for bad luck
• Design for 55 and offer 45 as a business case led upgrade (which will likely stack up in many cases) not a 'must do or else I wont play'
• If necessary use one or two fancoils for any difficult radiator replacements (where the replacement is too physically large)
• In many cases put a PHE in to deal with the DHW issue, rather than a cylinder with a heat pump coil.  If retrofitting a combi, use a smallish cylinder which can be tucked away somewhere (high level in a room, in the loft, etc).  (the volume occupied by a typical combi is about 80l, not very different from the 90l (rectangular) tank that was fitted in the flat in Nice which I recently stayed in.  A water efficient shower uses about 9l/min.  Just saying...)
• in most cases cut out all the ancillary stuff often done (upgrading primaries, upgrading mains feed to DHW, adding poorly plumbed buffers etc) because its no longer necessary as you haven't massively oversized the system
• Give some very basic education to the customer on how to run the system efficiently..

 

 

Edited July 3, 2023 by JamesPa

[JohnMo]

19 minutes ago, JamesPa said:

I am ashamed that I only recently spotted in my own house (as a direct result of my interest in heat pumps) that I could be more comfortable and use less gas by simply turning down the boiler temperature!  My excuse is that, until recently, I had little reason to know much about the detail of heating systems!

Must admit being similar - but building the house I calculated everything I could and heating and DHW I had a good idea of what my bills should look like. Then I got my first bill it was twice what I was expecting. I really started monitoring gas usage, daily to understand what was happening. By the end I had the boiler operating at around 110% efficiency, so getting better than the calculations.

 

Got that sorted then installed an ASHP using all the learning from the gas boiler on very low flow temps.

 

Previous houses just paid the bill - didn't know or care to much bills weren't that expensive anyway. Gas was so cheap.

 

 

[joth]

2 hours ago, DanDee said:

Are you referring to this?

 

LOL yes that section is clearly for cooling only. I'm sure there was a requirement for heating controls to have room by room schedule but I can't find it now... nearest I can find now is this table.

 

 

Anyway yeah I agree the point has to be a set of room-wise controls designed and installed for the combi aren't going to be a automatically re-applicable for an ASHP without some redesign / reconfig, which is what the OP was asking for, but this will need adding to the BOM on any retrofit (in place of the buffer and other stuffs)

[sharpener]

3 hours ago, JamesPa said:

Keep Evohome and valves, adjust the target temps to 1 C above what you actually want, balance the system and you are good to go.   Then play if you wish with broad brush zoning (If I remember correctly your house is basically divided into two halves, quite likely zoning of this into the two halves makes sense even with an ashp).

 

Yes, it is long and thin, originally two barns. The upstairs layout is Master Bedroom, Guest Bedroom/Study1, Boxroom/Study2, Guest Bedroom 2 all in a row, with 6 wireless TRVs. By default if it is nice outside and no-one has come to stay then only the MB gets turned on at all.

 

We have seriously considered in very cold weather living in the kitchen/dining room where the AGA is, and not heating the other end of the house at all. The AGA flue runs up inside the MB and also the boxing round it feeds into the MVHR system which helps further.

 

Downstairs there are four zones of UFH each with their own hardwired thermostat, plus the two rads I have added in the living room with their own wireless TRVs. If sunny there is quite a lot of solar gain so it does not need much heat until after dark.

 

Some simplification of controls is perhaps feasible. Installer C expressly quoted for separate stats upstairs and downstairs but we defo need something finer grained than that. As explained above zoning into the E barn and W barn would make at least as much sense. The other proposals were altogether very vague about controls.

 

I get all the stuff about long and low - the AGA, UFH and stone walls provide a natural setback level everywhere and then with the controllability we now have we can spot heat the rooms only as and when we come to use them. OK this is a boiler replacement philosophy, which is where I began.

[DougMLancs]

It’s this part for heating. 

[JamesPa]

9 hours ago, joth said:

 

LOL yes that section is clearly for cooling only. I'm sure there was a requirement for heating controls to have room by room schedule but I can't find it now... nearest I can find now is this table.

 

 

Anyway yeah I agree the point has to be a set of room-wise controls designed and installed for the combi aren't going to be a automatically re-applicable for an ASHP without some redesign / reconfig, which is what the OP was asking for, but this will need adding to the BOM on any retrofit (in place of the buffer and other stuffs)

 

 

1 hour ago, DougMLancs said:

It’s this part for heating. 

So taking these together I suppose that 'time and temperature zone control' is one time and temperature controller per zone (or house) and then the second bit mandates TRVs in each (or most) rooms. 

 

At least TRVs are cheap, but it does mean that the customer has to be educated to use them sensibly and begs the question, how do you ensure that a minimum number are left open to ensure sufficient system volume if they don't use them sensibly?  If you conclude you can't then the only option, because of the regulations, appears to be to add a volumizer/2 port buffer the volume of which is just a shade less than the minimum system volume required by the unit (because, at least in in theory, every TRV bar one could be closed and the system still running, or wore still the master temp setting could be set high, the rads completely unbalanced and installers rely on the TRVs to sort it out).   Two control systems working independently against each other, never good engineering, yet mandated by regulation!

 

Hmm, there is a system design/customer education problem to solve here.   Minimum system volume, as I understand it, solves two problems namely a) it reduces short cycling and b) it preserves a reserve of heat for defrost.  Interestingly a) is more likely at elevated OATs and b) occurs at low, but not lowest, OATs.   I wonder what the ASHP manufacturers themselves are thinking is the direction of travel with this one.