Thoughts on commissioning a Vaillant Arotherm Plus 12kW

[sharpener]

18 minutes ago, JamesPa said:

I'm pretty sure plumbers connect these up if they choose to, and I'm pretty sure there is no regulation that says it has to be an electrician, its just connecting to an existing circuit via the 'wiring box'.  But then again that's why I still think an electrician may turn up and disagree with the current installation!

 

Both my potential installers had their own electrician and I gathered this to be the usual practice. (The ppl who in the end did the job are actually a firm of electricians who specialised in heating systems and then took on their own plumbers.) He turned up for a recce the previous week which was useful as we could sort out the final details without the other trades on site or any time pressure.

 

22 minutes ago, JamesPa said:

I'm pretty sure plumbers connect these up if they choose to, and I'm pretty sure there is no regulation that says it has to be an electrician, its just connecting to an existing circuit via the 'wiring box'.  But then again that's why I still think an electrician may turn up and disagree with the current installation!

 

Not sure what you mean by "wirng box". The CU they fitted has a B6 mcb for the electronics indoors - the 13A socket for the VR940F internet interface has been labelled as you can only draw 6A from it - and a C32 mcb for the 4sq mm feed to the mandatory AC isolator on the outside of the house, did your man install that already?

 

30 minutes ago, JamesPa said:

There is a switched fused spur and typical heating wiring where all the electronics will be

 

... but reckoned to be good practice to have them both on the Type B RCD that is specified in the MIs. It does not matter if either is energised when the other is not, so an indepent supply is I suppose OK if yr spur box is on a dedicated boiler cct, I would not want it on a general ring main though. Maybe I am not fully understanding your setup.

[JamesPa]

10 hours ago, sharpener said:

Not sure what you mean by "wirng box". The CU they fitted has a B6 mcb for the electronics indoors - the 13A socket for the VR940F internet interface has been labelled as you can only draw 6A from it - and a C32 mcb for the 4sq mm feed to the mandatory AC isolator on the outside of the house, did your man install that already?

By 'wiring box' I mean one of those typical central heating 'wiring centres' that many use to do all the interconnections between thermostat, valves, call for heat for the boiler etc.  Not necessary so far as I can see for the Vaillant, or indeed most ashps, as almost all wires go back to the ashp control, unless you are doing something silly with external controls (which I'm not).

 

Yes the ac isolator is fitted and wired already.  Actually a 32 A one with a 6mm feed (but a 16A MCB) just in case it needs to be upgraded in future.  I put the cable in, sparky fitted the isolator and connected to the cu.

 

10 hours ago, sharpener said:

but reckoned to be good practice to have them both on the Type B RCD that is specified in the MIs

They are.  The current boiler is run off the ring main.  I'd ideally prefer to have it separate as we do get the occasional nuisance trip, but not worth the effort until I replace the cu for some other reason.

Edited November 1, 2024 by JamesPa

[sharpener]

5 minutes ago, JamesPa said:

Not necessary so far as I can see for the Vaillant, or indeed most ashps, as almost all wires go back to the ashp control, unless you are doing something silly with external controls (which I'm not).

 

No but may provide a handy route to previous boiler or tank location if that is where the new cyl is going to be, saved me some awkward cable runs.

 

9 minutes ago, JamesPa said:

Yes the ac isolator is fitted and wired already.  Actually a 32 A one with a 6mm feed (but a 16A MCB) just in case it needs to be upgraded in future.  I put the cable in, sparky fitted the isolator and connected to the cu.

 

Sounds like you are good to go then apart from wiring the e-bus and SensoComfort itself. Best of luck!

[sharpener]

Back on the original topic some further wrinkles that have arisen.

 

1. I fitted an indicator panel with separate lights for the HW valve and each of the 3 heating circuits so I can see what the machine is doing at a glance. (Very helpful for checking all the different time schedules are set up correctly.)

 

This showed that when in Away mode the heating circuits are energised for less than a minute at 1025, 1030 and 1035 respectively every day (but AFAICS not the DHW). The installers have not responded to my question but presumably it is some kind of anti-blockage routine.

 

2. System pressure has been varying wildly between 0.6 and 1.5 bar depending on what it was doing. The label on the 25 litre expansion vessel says it is pre-charged to 1.5 bar. I don't think this was adjusted by the installers so with a nominal water pressure of 1.0 bar it will not be doing anything at all.

 

I have changed these round so with the system depressurised the expansion vessel is at 1.0 bar, and then topped the system pressure up to 1.2 when in standby. Now it works properly over the comparatively small range of 1.2 - 1.5 bar.

 

 

Edited December 5, 2024 by sharpener

[JamesPa]

4 hours ago, sharpener said:

This showed that when in Away mode the heating circuits are energised for less than a minute at 1025, 1030 and 1035 respectively every day (but AFAICS not the DHW). The installers have not responded to my question but presumably it is some kind of anti-blockage routine.

My boiler, now siting on the driveway waiting to be removed, had a pump anti-seize function doing exactly this.  I imagine heat pumps do also.

 

4 hours ago, sharpener said:

I have changed these round so with the system depressurised the expansion vessel is at 1.0 bar, and then topped the system pressure up to 1.2 when in standby. Now it works properly over the comparatively small range of 1.2 - 1.5 bar.

 

Mine seems to be varying by about 0.2bar around 1.7bar

 

One thing I have noticed is very long cycle times in the mild weather - too long in fact - the heat pump can be off for more than an hour and the house cools by 0.5C in that time which is noticeable.  I thought this was governed by the 'energy integral', but it seems that the room temperature mode (inactive/active/expanded) may be dominating the cycling as the compressor doesn't switch on even when the energy integral is as low as -150 degree minutes (I have a suspicion that it comes back on at -180 degree minutes but cant prove it).

Edited December 5, 2024 by JamesPa

[sharpener]

2 hours ago, JamesPa said:

I thought this was governed by the 'energy integral', but it seems that the room temperature mode (inactive/active/expanded) may be dominating the cycling as the compressor doesn't switch on even when the energy integral is as low as -150 degree minutes (I have a suspicion that it comes back on at -180 degree minutes but cant prove it).

 

If you have it on expanded this is more likely to happen.

 

Energy integral is -90 by default, is in the Appliance Interface settings under "Compr. heat. start from", try changing it to -60. Much discussion on the Arotherm Plus FB page.

[sharpener]

44 minutes ago, sharpener said:

Energy integral is -90 by default, is in the Appliance Interface settings under "Compr. heat. start from", try changing it to -60.

 

My mistake, according to MIs default is  -60 and correct name is Compr.start heat. from.

 

There is a well documented s/w bug which affects yr ability to change this but if yr machine was mfd this year it should be OK.

 

Also you could try reducing Compressor hyster. Heat. from its default value of 7.

 

 

Edited December 5, 2024 by sharpener

[JamesPa]

46 minutes ago, sharpener said:

f you have it on expanded this is more likely to happen.

Reading the expanded mode description I think the controller may ignore energy integral if this mode is active, relying on oat to restart.  My installer has also enabled adaptive WC (which requires expanded mode).  I guess his thinking is that this will eventually sort out the WC curve, which is probably true!

[Dan F]

Expanded doesn't ignore energy integral.   Adapative WC is typically not used or recommended.  Energy integral is -60 by default, not -90.  Don't think pressure should be varying like that, mine doesn't.

[Dan F]

7 hours ago, JamesPa said:

One thing I have noticed is very long cycle times in the mild weather - too long in fact - the heat pump can be off for more than an hour and the house cools by 0.5C in that time which is noticeable.  I thought this was governed by the 'energy integral', but it seems that the room temperature mode (inactive/active/expanded) may be dominating the cycling as the compressor doesn't switch on even when the energy integral is as low as -150 degree minutes (I have a suspicion that it comes back on at -180 degree minutes but cant prove it).

 

Start with inactive and try to find your correct heat curve, you can then switch to active which is more forgiving and adjusts better to solar gain etc.   Expanded has its uses, but it's typically not a good idea.  Adaptive WC is not really recommened either.

 

Don't touch integral or hysterisis configuration until you've done the above, only then are you really in a position to understand if either of these need tweaking.

Edited December 6, 2024 by Dan F

[JamesPa]

12 hours ago, Dan F said:

 

Start with inactive and try to find your correct heat curve, you can then switch to active which is more forgiving and adjusts better to solar gain etc.   Expanded has its uses, but it's typically not a good idea.  Adaptive WC is not really recommened either.

 

Don't touch integral or hysterisis configuration until you've done the above, only then are you really in a position to understand if either of these need tweaking.

Thanks.  I was coming to that conclusion myself (and its the general approach I have been preaching for some time - get the WC curve right before you anything more sophisticated). 

 

My installer is back today to fit two rads that weren't available when he did the main job last week.  I don't want to touch too much while he is still around, but once he is gone I consider it to be open season!   That said, I can see why he has set it up the way he has.  Most wont bother with or understand the tweaking of WC curves and the way he has set it up will get somewhere close eventually I think, without human intervention.

[Jerry32]

Hi Sharpener,

 

I'm hoping you might still be looking at build hub occasionally, as I'm really interested in your initial information on the Vaillant Arotherm Plus 12kW ASHP.

 

I'm almost certainly going to be 'recommended' to have fitted at least a 12kW ASHP, the same model as yours, because that seems to be the way of the MCS heat calculations, even though with an LPG Combi boiler, I've never used more than 80kWh on heating, in any given winter day, during the last 2 years.

 

I'd contacted Vaillant Tech Support, and to my surprise, they have stated that the compressor within the Arotherm Plus range is not an Inverter driven one ?

 

So I'm now even more confused, because you've managed to measure compressor modulation depending on the heat output required, assuming that the reduced percentages down from 100% means reduced power output / input.

I'm concerned that I'm going to end up with an ASHP which spends a lot of time short cycling, and from just about every forum discussion I've read, this is definitely not going to help the COP.

 

I've pasted in the reply I received to both my tech support 'requests', and am wondering what you might think of them ?

The earliest is the first, the last is the latest, (opposite to email style presentation).

 

-----------------------------------

Hi, I've been in contact with Swift Renewables in Clevedon, Somerset. As I've seen a really competent Vailliant installation in the village, completed by their company.

They are going to do the MSC heat loss calcs, and I know it will come back with probably a 12kW ASHP as a minimum.

(Even though I've calculated from the last two years, my winter time daily energy heat usage is a maximum of 80kWh per 24 hours.)

If I'm 'coerced' into having a 12kW ASHP, (to meet MCS requirements / Govt grant etc.), is there any reason why this could not be a 5kW & a 7kW ASHP, running either one, or both, depending on the heat output required at the time?

I'm really concerned that a solitary 12kW ASHP is going to be short cycling for a large majority of it's use, as it would only operate 'properly' during the very worst cold spells (we live in lowland Somerset, we don't do snow as a rule).

This is going to kill the overall COP efficiency of the ASHP.

What is the minimum power output of your various Arotherm Plus ASHPs ?

Can they all run with a volumiser not a buffer cylinder?

This is another concern for me, why do I need to run a separate circulating pump?

When the ASHP has it's own circulating pump already.

What is your recommended volumiser cylinder size for each of your ASHP's mentioned above?

I believe it relates to the ability to successfully defrost the ASHP, so presumably it will be different for 5, 7, 12kW ASHP's?

I think I've finally found a competent installation company, who will fit an up to date efficient Vailliant Arotherm Plus based system for me.

(I'm an ex Offshore Oil & Gas Dept Head, you might be really surprised at how many Installers do not know the simple basics of heat retention / Transfer, they just want to fit whatever 'kit' they always have fitted previously !

-------------------------------------

Hello,

 

Apologies you did not hear back last month, I can only assume the email got lost somehow, as we aim to respond to all emails within 24 hours.

 

 

In terms of your question, as a general rule we do not advise cascading different size heat pumps. The concern is that having different build confused the EBUS system enough to cause an issue with performance. Therefor, you are free to cascade several heat pumps, but only same heat pumps. In your case, 2 5kws or 2 7 kws, for example. 

 

There is a chance of cycling a little more than you would like if the unit is oversized dramatically, however any design should account for a certain avarage temperature, not for worse case scenario. In england the avarage is  0 degrees outside with 45 degree flow temperature.  

 

In terms of your question about minimum output, our compressor is not inverter driven, meaning a 12 kw heat pump will aim to output 12 kw of heat energy at all times, with mild outdoor conditions and a good install it can technically output as much as 16 =- example from manual being 2 degrees outside with 35 degree flow. Obviously the running costs lessen the milder it is, but yeah, the output increases, if anything. 

 

You are indeed correct that the minimum system volume is enforced for de icing reasons,  minimum values for all our Arotherm Plus - 40L for 3.5/5kw 45L for a 7 kw and 150 litres for a 10 and 12  kw. Volumisers are more practical with smaller units whereas buffers are usually used in larger systems, we effectively do not care how you meet your minimum system volume, as long as it is met. In certain houses with large radiators ( and a few of them) minimum volumes can be met without any added volume at all.

 

Kind regards, 

----------------------------------

 

I've just read through your reply, and I'm slightly concerned about the comment you made, that the ASHP is not inverter driven, and that it will try to output 12kW at all times ?

 

Surely the whole reason for having an inverter motor drive, is because for the vast majority of the time the heating is on, the required output will be less than the full 12kW capability ?

Therefore because it will be overproducing heat for the majority of the time, presumably the house thermostat is going to have to stop the heat pump whenever the set house temperature is reached ?

I'm sure I've understand the inverter driven theory correctly, in that the reason for an inverter driven compressor, is so that the actual heat pump output can be modulated down to eventually match the level of heat output required, so that the delta T (inside to outside temps), keeps the house at/near the thermostat controlled temperature ?

So, how does the Arotherm Plus manage to modulate the heat output to maintain a set temperature ?

If it's by switching itself off when the thermostatic temperature is reached, and then back on when the temperature falls, then that is short cycling, i.e. exactly what our present LPG combi gas boiler does ?

It would appear from all the internet based sites, that repetitive short cycling is an inefficient way to operate an ASHP ?

 

Best Regards

----------------------------------

 

Hello,

 

Yes in our unit the inverter does not module the control output, but controls the electronics in general. The output of the compressor is not modulated down based on outdoor temperature does a supposed heat output requirement at the time, which is why the advice is to size the heat pump very accurately. There are some safe guards built into the software of the unit to avoid excessive short cycling but they are not really something I have in depth knowledge on as that would be sensitive internal information, however it was fairly public a few years ago that a software upgrade on our HMU board lessened cycling by 2 fold by more accurate calculating energy integral temperatures to decide when to activate/deactivate in heating demand.

 

Kind regards, 

[Dan F]

13 minutes ago, Jerry32 said:

In terms of your question about minimum output, our compressor is not inverter driven, meaning a 12 kw heat pump will aim to output 12 kw of heat energy at all times,

This is nonsense.  All the aroTherm+ models modulate down.  10/12kW models don't modulate down as much as 7kW though, and 7kW doesn't modulate down as much as 5kW can.  There isn't a single value for miniumum kW output as this depends on flow/external temperature, but you can find this info in the data tables.

 

You don't need an additional circulation pump(s) unless you have hydraulic seperation (buffer, HEX etc.).  There is a preference to avoid hydraulic seperation in most install currently as an "open loop" approach is simpler, cheaper and generally gives best efficiency also.  That said, there are cases where a buffer may make sense, this depends on how many circuits you have, what they are used for etc.

[JamesPa]

4 hours ago, Jerry32 said:

I'm hoping you might still be looking at build hub occasionally, as I'm really interested in your initial information on the Vaillant Arotherm Plus 12kW ASHP.

 

I'm almost certainly going to be 'recommended' to have fitted at least a 12kW ASHP, the same model as yours, because that seems to be the way of the MCS heat calculations, even though with an LPG Combi boiler, I've never used more than 80kWh on heating, in any given winter day, during the last 2 years.

If the 80kWh figure is even faintly correct and assuming you heat your house, a 12kW heat pump is quite likely to be a disaster.  It will be cycling all the time and running permanently at the least efficient part of its modulation curve, you will pay 30% more for your heating than you need to (possibly worse) and will quite possibly be less comfortable.

 

Installers vary in their interpretation of the 'mcs calculations', the better ones recognise that they tend to overestimate and compensate accordingly. If you can afford to hang on, collect some more evidence and find some better installers. 

 

As a reference point I had two full 3 hour surveys done, one of which I paid £300 for.  Both came to 16kW.  I eventually had the 7kW Vaillant fitted, and it's an almost perfect match, perhaps 0.5kW over.  This was fitted by one of two of the installers I found who used an ach of 0.5-1 instead of the standard 2-3 for my type of house and, unlike the survey monkeys, accounted for fabric upgrades that they couldn't see (but which I made a point of telling them about) and didn't double count room to room losses.  I also produced smart meter readings to support their calculations

 

If you can get your calculation down to 8kW then the 7kW Vaillant will do.  Also worth considering is the 8kW R290 Mitsubishi which has 2 compressors, 6kW variable and 2kW fixed.  It therefore has a wider modulation range than most.  However your 80kWh figure suggests you need even less!

 

Possibly the only exception to the above is that if your house is super low loss with a large concrete slab, then batch heating with an oversized heat pump is a possibility, effectively treating the slab as a storage heater.  With the right tarrif this can be highly cost effective. 

 

Also, I would strongly advise against allowing any installer that wants to fit a secondary pump and buffer tank anywhere near your plumbing, unless there is something weird about your house (perhaps you could post some details of construction, floor area and rough location).  A 2 port volumiser plumbed in the flow (or even in the return) is ok.

 

Feel free to post any questions this gives rise to.

Edited Wednesday at 20:46 by JamesPa

[Jerry32]

Thanks for the replies given, as they essentially back up my thoughts entirely.

 

I'm pretty sure that as the Installation company that fitted the really neat / tidy installation in the village, used a volumiser, not a buffer tank, on that installation, I'm hoping I can reinforce this approach on his next visit.
( It's a small firm, owner owned, and he seems to have a handle on all the questions I asked of him, unlike several other firms, which seem to think they can tell you what they're going to fit in your house, even before they've site visited or done any calcs ! )

 

My 80kW per day is the highest total daily LPG usage during the last two winters, but, it's not because we have a near passive style modern house, it's because I know how to put on a jumper when required !

I've been running our LPG Combi boiler 24/7 during the 4-5 months of winter, but with the output temperature ramped right down, on/off firing at 28/38 degs C, which for all but the worst winter days, keeps the house above 18 degs C.

That's warm enough for us.

 

But the standard MCS calcs do not allow for personal preference on house temperatures, and if the OAT is 8 deg C, IAT is 18 deg C, then if you try and increase the IAT to 20 deg C, that's a 20% increase in LPG / Energy input requirement.

i.e. if we ran the house to try for 20 deg C IAT, then that 80kW pd becomes 96kW pd.
Even so, as I've tried to explain to so many other installation companies, 96kWh / 24 hours, does not equate to anything like a 12kWh ASHP requirement. One installer even wanted to fit a 16kWh ASHP.

 

This was why I also requested information as to whether 2x smaller ASHP's could be installed, so that for most of the heating season, I'd really only be running one of the two fitted ASHP's.

In Offshore Installation terms, fitting two dissimilar output 'units' gives maximum flexibility, i.e. a 4kW and an 8kW, gives 4 or 8 or 12kW max outputs, and all the modulated down capabilities in between, and probably down to 2kW as well.

 

I'd really like to know if 'Sharpener' managed to get the information he posted, from the Vaillant control panel, as I could take the people a bottle of wine, just to get to find out if their system does the same as his ?

[Dan F]

3 hours ago, Jerry32 said:

I'd really like to know if 'Sharpener' managed to get the information he posted, from the Vaillant control panel, as I could take the people a bottle of wine, just to get to find out if their system does the same as his ?

What information specifically?  I have a 7kW which is quite a bit oversized.  Yes it cycles, but having adjusted the integral settings I have minimized this.  Also, 7kW heats DHW faster than 5kW would.  Cycling is to be expected and not something you should try to avoid at all costs, that said you clearly don't want to significantly oversize and you want to ensure you have enough system volume to minimize frequency of cycles.

 

[JohnMo]

3 hours ago, Jerry32 said:

MCS

You could always skip the MCS nonsense and do a self funded install. Something that does 5 to 6kW at -3. Always something on eBay in £1200 to 1500 price range.

 

Bet they want a couple of arms and legs plus the £7500?

[JamesPa]

5 hours ago, Jerry32 said:

But the standard MCS calcs do not allow for personal preference on house temperatures, and if the OAT is 8 deg C, IAT is 18 deg C, then if you try and increase the IAT to 20 deg C, that's a 20% increase in LPG / Energy input requirement.

i.e. if we ran the house to try for 20 deg C IAT, then that 80kW pd becomes 96kW pd.

That is true but still there is a massive gap to be explained before fitting a 12kW becomes even faintly sensible.  As I say above, MCS installers can be flexible,  not on room temperature, but certainly on the assumptions made with loss calculations.   That can make a massive difference, in my case a factor of 2!

 

Obviously it's your choice but personally I wouldn't proceed until this is resolved.  It's not cycling as such that kills performance, it's that compressors generally are at peak efficiency at about 40-50 pc of their max modulation frequency and the efficiency declines quite dramatically at lower frequencies.  If this operating point corresponds to when you need relatively little heating energy, then it doesn't matter too much.  However if you are permanently operating in the low efficiency part of the curve, then it can matter a lot.

 

If you wanted to provide some more details perhaps I or someone else could suggest more specific actions.  Interesting info would be

 

House construction and floor area

Rough location (or design oat)

How do you/can you measure LPG consumption 

What is the heating pattern for the house that corresponds to the figures you have provided

 

Also, if you have survey results and calculations they would help, you can of course redact anything sensitive.

 

You don't have to do any of this of course, but I do fear that, if you proceed with 12kW, you will be posting again about a disappointing experience.

 

 

Edited Thursday at 23:05 by JamesPa

[JamesPa]

1 hour ago, JohnMo said:

You could always skip the MCS nonsense and do a self funded install. Something that does 5 to 6kW at -3. Always something on eBay in £1200 to 1500 price range.

 

Bet they want a couple of arms and legs plus the £7500?

The new pd rules (in England) remove the (admittedly arguable) need for installations to be carried out by an MCS installer.  So I agree you could DIY or have your local friendly plumber/electrician do it to a sensible spec if you forgo the bus grant.  The job probably amounts to (depending on your house)

 

fit an electrical feed

fit a uvc

connect the heat pump to where the boiler was formerly connected

swap out any necessary radiators

 

None of this is anything that a reasonably competent plumber (plus electrician for the feed) can't do.

 

Mine (which was MCS) took 6 long man days for the plumbing, and half a day for the dedicated feed.

 

That said, unless you are being ripped off to the max, it's probably still just about cheaper to get the grant and pay the mcs overhead.   However cheaper isn't necessarily better if it's the wrong system!

 

 

[MikeSharp01]

11 hours ago, Jerry32 said:

One installer even wanted to fit a 16kWh ASHP.

You could interpret this as just trying it on, the apparently poor MCS size matching and perhaps a lack of technical knowledge / understanding by many putative installers has been discussed here, and elsewhere, frequently and the experience we see above tends to reinforce this.

 

[sharpener]

Sorry, I have come late to this discussion but most of the points have already been covered by others.

 

Yes @Jerry32, the information on compressor modulation was taken from my VWZ AI Appliance Interface module's Live Monitor screens, IIRC the field in question is helpfully called "compressor modulation". Minimum observed value is 27.2%. My system has been consistently averaging 2 starts per hour which I think is acceptable, anything less than 3 is generally reckoned to be OK, so cycling is not a problem.

 

Now that heating demand is virtually over I am again grappling with the issue of whether it is worthwhile to use the HP for DHW, recent tests seem to have come out better than earlier trials though I have made no changes. What I really want to do is bypass the return path via the bottom part of the thermal store but unfortunately it does not have the necessary isolating valve so I will have to drain it and then refill with expensive inhibitor. Or muck about with pipe freezer. You will not have this problem!

 

The 12kW graphs referred to above (the "Czech tables") start here on p51

 

https://www.facebook.com/groups/488794632317506/permalink/1093926388470991/

 

I don't understand at all the exchange you have had with Vaillant technical. I have dealt mostly with their Lyndon Stevens <Lyndon.Stevens@vaillant-group.com> who is pretty sound.

 

If you know your annual gas usage Michael Podesta has a method of predicting HP size from this, you might have to adapt it slightly for LPG, he is easy to find on the web.

 

15 hours ago, JamesPa said:

The new pd rules (in England) remove the (admittedly arguable) need for installations to be carried out by an MCS installer. 

 

Is this actually in force now? Although much-trailed, the last time I looked it was still in the works.

 

 

[JamesPa]

7 hours ago, sharpener said:

Sorry, I have come late to this discussion but most of the points have already been covered by others.

 

Yes @Jerry32, the information on compressor modulation was taken from my VWZ AI Appliance Interface module's Live Monitor screens, IIRC the field in question is helpfully called "compressor modulation". Minimum observed value is 27.2%. My system has been consistently averaging 2 starts per hour which I think is acceptable, anything less than 3 is generally reckoned to be OK, so cycling is not a problem.

 

Now that heating demand is virtually over I am again grappling with the issue of whether it is worthwhile to use the HP for DHW, recent tests seem to have come out better than earlier trials though I have made no changes. What I really want to do is bypass the return path via the bottom part of the thermal store but unfortunately it does not have the necessary isolating valve so I will have to drain it and then refill with expensive inhibitor. Or muck about with pipe freezer. You will not have this problem!

 

The 12kW graphs referred to above (the "Czech tables") start here on p51

 

https://www.facebook.com/groups/488794632317506/permalink/1093926388470991/

 

I don't understand at all the exchange you have had with Vaillant technical. I have dealt mostly with their Lyndon Stevens <Lyndon.Stevens@vaillant-group.com> who is pretty sound.

 

If you know your annual gas usage Michael Podesta has a method of predicting HP size from this, you might have to adapt it slightly for LPG, he is easy to find on the web.

 

 

Is this actually in force now? Although much-trailed, the last time I looked it was still in the works.

 

 

Not quite, it comes into force on 29th May

 

https://www.legislation.gov.uk/uksi/2025/560/contents/made

[sharpener]

@JamesPa you will no doubt be dismayed by the following in the Explanatory Memorandum which apparently reinforces the MCS stranglehold on the business. And their potential competitors can legitimately complain that previous encouragement by HMG to develop alternative schemes has been a big waste of their time and effort.

 

<5.16 Article 5 also removes “or equivalent standards” in order to provide clarity to
installers that the MCS 020 a) – Air Source Heat Pump Sound Calculation (For
Permitted Development Installations) is the relevant or applicable standard for heat
pump installations.
5.17 Article 6 of this Order amends paragraph P of Part 14 (interpretation of Part 14) to
make references to the updated MCS Planning Standards.>

[JamesPa]

14 hours ago, sharpener said:

@JamesPa you will no doubt be dismayed by the following in the Explanatory Memorandum which apparently reinforces the MCS stranglehold on the business. And their potential competitors can legitimately complain that previous encouragement by HMG to develop alternative schemes has been a big waste of their time and effort.

 

<5.16 Article 5 also removes “or equivalent standards” in order to provide clarity to
installers that the MCS 020 a) – Air Source Heat Pump Sound Calculation (For
Permitted Development Installations) is the relevant or applicable standard for heat
pump installations.
5.17 Article 6 of this Order amends paragraph P of Part 14 (interpretation of Part 14) to
make references to the updated MCS Planning Standards.>

I was dismayed until I read article 6 and the document it references.  Then I was delighted.  What it apparently does is not what it actually does, at least as I read it.  Please tell me if you think I'm wrong!

Edited 19 hours ago by JamesPa

[sharpener]

Have tried to follow the convoluted interaction of the SI, the explanatory memorandum and the original legislation but gave up.

 

Can you say what you think it all means?