Where is the heat pump controller going that will be the thermostat it just needs to be in a sensible location.

If you are running weather comp the whole point is for the heat input to match the heat loss to keep the house temp constant  You won’t be able to change room temp by 2C easily that will take hours and you would have to change the weather comp curve if that’s really a requirement you will need a secondary heater you can turn on as and when.

Hi Gary.

If there were to be an internal thermostat then it would probably be on the landing in the middle of the house away from the front door.

I agree that the point of weather comp is for the heat input to match the output. But I'm talking about times when a single thermostat located in the shade on the North side of my house does not know how much heat is being input, i.e sun shining in, ovens on, lots of bodies in the house, fire on. There seems to be no way to adjust how much heat the ASHP is providing in these instances, when it could be modulating down and saving energy.

Yes, it would probably take hours to change a room temp but that's fine. If it's the weekend and the kids are in their bedrooms more, I can increase the temp on Friday afternoon. If we want it a bit warmer in our bedroom in the morning we have several hours overnight to get the rooms to a higher temperature then let it drop by one or two degrees during the day.

I'm only talking ranges of 1-2 degrees in some areas at some times. The average temp of the entire 400m2 house is probably only changing by a few %.

I cannot see why this is going to destroy my COP or make the heat pump go into a short cycling death loop! 

Unless someone can tell me precisely why this will be such a problem on what will be quite a large system volume.

There are a couple of general points to make here.

Contrary to what the manufacturers of heating control systems would like us to believe:

• Lots of controls does not equal lots of control!  In most houses where the external insulation is better than the internal insulation its very difficult to (a) maintain significant temperature differences between adjacent rooms or (b) change the temperature of any given room (or the house) by more than about a degree every few hours.  All the 'smart' technology in the world does not change the fact that houses have a very significant heat capacity and so react rather slowly to any change you make to the controls, not does it change the fact that rooms share heat quite significantly.
• Micro-zoning, whether in time or space, is unlikely to save any significant amount of money and may well cost money (see various heat geek videos on this including worked examples)

The above is true in many cases for a boiler as well as a heat pump, albeit that the characteristics of a heat pump emphasise both.

Put another way, whatever you want to do for comfort may not be possible at all, or may be possible without active/add-on control, and anything you are doing to save money will quite likely not save money.

So starting from there my strong advice to you is to heed what you are being told and work out what is the minimum you must do (in terms of control) for comfort alone.  You may surprise yourself and conclude that keeping the whole house at a constant temperature during the day, and dropping it a degree or two at night (which is the most you are going to achieve whatever your heating system), is actually all that is needed, or indeed that keeping the living areas at one temperature 24*7 and the bedrooms a couple of degrees cooler also 24*7 is equally comfortable.

Once you have worked that out you will very likely find that much of the complexity can be swept away and (for example) the whole house run on a single zone, with the lockshield/loop valves turned down a bit in the bedrooms, and with a couple of 'limiter' stats/TRVs/valves for the rooms that suffer/benefit from solar gain/excess heat from cooking, everything else being done the native controller.

As I have learned about the myths of the heating world (as part of my own prep for an eventual move to an ASHP) I have tried various control strategies for my house.  I have moved progressively over a period of about 5 years

• following conventional wisdom is - gas boiler set as the installer left it, Thermostat/ TRVs/smart timed TRVs in every room to, operated most of the season during evening and morning only, to
• gas boiler with the flow temperature turned down to the minimum, several TRVs removed (ie run open loop, LSVs adjusted to balance if necessary), smart TRVs deleted, run 24*7 except at the ends of the season, to
• ashp run 24*7 on pure weather compensation without any room influence (except at the very ends of the season only when I have it shut down if the temperature goes more than half a degree above the design temperature), only two rooms (a room that gets lots of solar gain and a guest bedroom) on any kind of room stat.  The bedrooms are kept slightly cooler than the living rooms by (permanently) turning down the LSVs

Each time comfort has materially increased and cost has reduced, overall by 25%.  I am now in a position where my whole house is comfortable 24*7 with no need to fiddle with controls and for materially less than I was paying (duly adjusted for changes in prices) when I had a gas fired system with lots of bells and whistles.  I would put this down to chance if it weren't for the fact that there are very good thermodynamic arguments (in most houses other than those with the thermal characteristics of a tent) for the simple approach 

Obviously houses and needs differ, but keeping it simple is good advice IMHO.  Control system manufacturers have to convince you that they can do wonders to keep themselves in business, that doesn't mean that its true.

Finally, avoid like the plague any 'smart' thermostats, these don't play well with ASHPs.  Only controls specifically designed for ASHPs (or which there are only a handful currently) do the right thing, and in the vast majority of cases the only controller you need is the one that comes natively.  The Vaillant controller certainly has plenty of functionality (including the ability to shift the WC curve on a timed schedule - ie do setback in an efficient way); I don't know about Viessman but I imagine it does similarly.

Ps. 12kW is a lot, how big and what is the construction of the house?

Thanks for the reply! So continuing some of these threads...

Posted by: @jamespa

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the ability to shift the WC curve on a timed schedule

This is the answer I was looking for! Right, so the WC curve is adjusted by time of day. In addition to the time settings, is there usually any sort of plus/minus boost setting for a set period of time? So move the curve down/up for the next X hours/days, which is effectively the same as the traditional boost or holiday mode.

Posted by: @jamespa

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dropping it a degree or two at night (which is the most you are going to achieve whatever your heating system), is actually all that is needed, or indeed that keeping the living areas at one temperature 24*7 and the bedrooms a couple of degrees cooler also 24*7 is equally comfortable

It seems most people on these forums and YouTube videos agree that TRVs on rads in bedrooms is perfectly acceptable so that temps can be kept a little lower than other rooms. Lets say 19 degrees in bedrooms and 21 degrees elsewhere. Yet when I suggest that I might want to sometimes increase the bedroom temps from 19 degrees back up to the same temperature as the rest of the house, 21 degrees, everyone loses their mind! Why is this so controversial? 

If, indeed, TRVs in bedrooms are acceptable, does anyone know of a type that can be set by time of day rather than just manually going around and turning them as and when you want to change the temp? AND something that gradually opens and closes rather than simple ON/OFF which I want to avoid for all the reasons everyone seems to agree on.

Again, to be clear, I only want to adjust things by a couple of degrees, not turn things off completely, not have it 10 degrees cooler or warmer! Those two degrees are very important in terms of comfort levels for sleeping at night and living in the same room during the day.

Posted by: @jamespa

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keeping it simple is good advice

I am trying to keep things as simple as possible and I want to keep any zoning to a minimum. However, I do want to have some sort of control as and when needed, as described in OP. If this costs a few % more to run at times I'm not particularly concerned. At the moment we have a fully electric immersion heater system fitted in the 1980s which is very expensive to run even when heating up on overnight cheap electricity (it was designed for Economy 7). I'm pretty sure that with the system volume I will have on open loop/UFH the house will be heated very efficiently even if I do tinker at the edges by a couple of degrees on 2-3 rooms.  

The house is approx 375m2 living area and as I said in my original post, it is going to be fully renovated and will be almost entirely insulated to current standards. Two independent heat loss calculations rate the house at around 26 w/m2 heat loss, so about 11kw at -3.2C with standard room temps and all emitters appropriately sized. Three separate quotes from "renewable experts" that didn't even do proper heat loss calcs have all suggested two heat pumps. I thought this sounded a bit over the top which is why I started looking into this subject in more detail and have done Heat Punk calcs myself and have got a Heat Geek qualified engineer on the case.

Thanks for your input so far!

Posted by: @ccap

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This is the answer I was looking for! Right, so the WC curve is adjusted by time of day.

Yes exactly that.  The UI presents it as adjusting the target room temperature.  What it actually does is shift the WC curve (whether or not room influence is enabled).  If room influence is enabled it both shifts the WC curve and shifts the target for the room influence function.  That sounds complex and, under the hood it is, but as a UI its incredibly simple - if you want it warmer/colder shift the set temperature and the machine sorts out how to do it in an efficient way.  

Vaillant (and some other European manufacturers) have had to deal with WC for decades because in some EU countries more enlightened than the UK its been compulsory for boilers for quite a while.  Thus they have had time and incentive to work on a simple UI to WC

Posted by: @ccap

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In addition to the time settings, is there usually any sort of plus/minus boost setting for a set period of time? So move the curve down/up for the next X hours/days, which is effectively the same as the traditional boost or holiday mode.

There is a holiday mode yes

Posted by: @ccap

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It seems most people on these forums and YouTube videos agree that TRVs on rads in bedrooms is perfectly acceptable so that temps can be kept a little lower than other rooms. Lets say 19 degrees in bedrooms and 21 degrees elsewhere.

TRVs on a minority of radiators are generally acceptable but: TRVs are not the optimum way to keep the temperatures in bedrooms lower.  The optimum way is to turn down the lockshields.  You only need TRVs if you want to change the relationship between the bedroom temperature and the house temperature with time.  Actually you can do this with the lockshield also but a TRV may be easier because many lockshields have adjustment only over the last quarter turn.  The optimum is probably a TRV with a decorators cap (ie a flow restrictor not a controller).

When I first got my ASHP I had TRVs in all bedrooms.  Within a month all but one was set to fully on and unscrewed (ie effectively disabled but left in place for cosmetic reasons) because adjusting on the lockshields gave a comfortable result and increases the active system volume which helps with defrost and general stability. 

The remaining bedroom TRV is in the 'guest' bedroom because Im conscious that different people like different temperatures.

Posted by: @ccap

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If, indeed, TRVs in bedrooms are acceptable, does anyone know of a type that can be set by time of day rather than just manually going around and turning them as and when you want to change the temp?

Lots of 'smart TRV heads can do this, you can get them quite cheaply if you are prepared to program them individually. 

I had several for a  while with my gas boiler including one programmed to make a bedroom warmer during the day than at night.  TBH it became too much of a faff and anyway the time that the room took to cool meant that, whilst I had a control mechanism, I didn't actually have much actual control.  So I reverted to regular TRVs (with the gas boiler) and now to lockshields and constant temperature operation albeit in the case of the bedrooms slightly lower than the living rooms.

Posted by: @ccap

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AND something that gradually opens and closes rather than simple ON/OFF which I want to avoid for all the reasons everyone seems to agree on.

SOFAIK many (including the ones I had for a while) gradually open and close but do nevertheless completely close so end up doing on/off modulation. which is not what you ideally want.  Basically they have too short a time constant to do what is ideally needed (essentially an automated LSV).  Adia thermal are developing a system that does 'the right thing' for ASHPs and includes what they call 'flow regulating valves'.  However they will still be subject to the fact that rooms take a long while to cool, so what you want to achieve may not actually be possible!

Posted by: @ccap

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I'm pretty sure that with the system volume I will have on open loop/UFH the house will be heated very efficiently even if I do tinker at the edges by a couple of degrees on 2-3 rooms.  

I think that's probably true, but see above comments about whether you will actually achieve room temps that vary on a daily basis as a function of time, particularly as the house has been insulated to modern standards!

Posted by: @ccap

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The house is approx 375m2 living area and as I said in my original post, it is going to be fully renovated and will be almost entirely insulated to current standards. Two independent heat loss calculations rate the house at around 26 w/m2 heat loss, so about 11kw at -3.2C with standard room temps and all emitters appropriately sized. Three separate quotes from "renewable experts" that didn't even do proper heat loss calcs have all suggested two heat pumps. I thought this sounded a bit over the top which is why I started looking into this subject in more detail and have done Heat Punk calcs myself and have got a Heat Geek qualified engineer on the case.

OK that is quite big so 11kW is at least plausible.  For comparison my house is 200sq, 1930s two modest 1980s extensions.  Most of the  (originally solid) walls have about 50mm IWI, the loft has 300mm, the 1980s double glazing has had new glass but not new frames.  There are three Crittal metal framed windows which have perspex sheets over them as 'secondary glazing'.  None of the flooring is insulated.  Two 3 hour surveys said 16kW - they ignored fabric upgrades which they couldn't see, double counted room losses and overestimated ventilation loss.  The measured loss is 7kW, and a 7kW Vaillant works very well.

Hope that helps.

@ccap, running the system mainly on weather compensation is definitely the most efficient baseline, and I agree with your instinct that it makes sense for most of the house, most of the time. However, you’re also right to be thinking about practical flexibility, because real-world living (kids, guests, burners, and variable solar gain) rarely behaves perfectly in line with theoretical design conditions.

To answer your main concerns, you’re absolutely right that weather compensation alone can leave you exposed to internal heat gains that it can’t see, but it’s worth adding these are usually isolated moments during the heating season. It’s great at responding to outdoor conditions, but it’s completely blind to what’s happening inside the house. That’s where load compensation can be incredibly useful.

Load compensation involves using an internal sensor or thermostat to measure the actual indoor temperature and subtly modulate the system accordingly, either by shifting the flow temperature up or down without completely turning the system on or off. We recorded an episode of our podcast on this subject on Friday, and it’ll be out in about two weeks.

Both Vaillant and Viessmann offer systems where you can run primarily on weather compensation but allow internal feedback to trim the flow temperature or initiate a setback if indoor temperatures start to drift significantly. For example, Vaillant’s SensoComfort controller can combine outdoor weather compensation with an internal room sensor for finer adjustment, and Viessmann’s Vitotronic controls offer similar flexibility. It won’t destroy your COP if configured carefully… it just adds a layer of intelligence rather than pure indoor thermostatic switching.

Mars,

Thanks for your response. Load compensation sounds like the solution I'm looking for! I don't want to adjust temperatures by large amounts but it does seem to be that in 2025 it shouldn't be beyond the collective intelligence of the heating industry to come up with some form of adjustment to internal conditions.

I look forward to the next video!

@ccap you will find that Vaillant have a mode called’expanded’ which is weather compensation plus when the house gets too hot (cooking, sun, lots of people, wood burner too) it switches the heating off. It doesn’t seem to create discomfort through hysteresis. The bedrooms would go off too but all of that excess heat would rise upstairs to keep the kids warm.

The mode (which is still WC) is called active just reduces the flow temperature gradually when the sensocomfort (the thermostat) reports hot enough where it is placed. This might be the best mode for your family. You will need over large radiators in the kids bedrooms to keep them up to a higher temperature, but gaming machines add heat too!

@Judith, @ccap

The control problem is not actually as simple as it might seem, even in 2025!  There is a whole branch of engineering ('control theory') that deals with stabilising systems subject to external influences, the maths is horrid and the whole area quite complex. 

The problem with pure load compensation (ie a thermostat measuring internal temperature) is essentially delay.  The house reacts very slowly to any changes made by the control loop and thus instability/oscillation can result.  Worse still the external driving conditions may well change during the time it takes for the house to respond to any changes made by the controller.  That's why weather compensation has an advantage - the controller knows what it needs to do in time for the house to respond to the stimulus it gives. 

I found with my Vaillant that running on pure weather compensation for most of the season, and then switching on expanded mode in March when solar gain became significant, gave an excellent result.  During the height of the heating season, expanded mode resulted in too much oscillation in my house, whereas pure WC proved very stable indeed.   This will of course be house, and even person, dependent.  

As @mars says a heat pump controller that allows you to run essentially on WC (ideally providing the ability to schedule WC shifts, as Vaillant and doubtless others do) plus an element of load compensation to tweak provides a level of flexibility that should sort out most challenges, together with perhaps the odd TRV in rooms particularly sensitive to extraneous heat sources.  How precisely you use this is going to be house, and person, dependent.  

@judith, @jamespa

It looks like this sort of issue has been thought about by the manufacturers, so this is all good news. Most of what I have read/watched with regards weather compensation tends to suggest it is only the outdoor sensor controlling everything, and until now I hadn't seen anything about these extra modes/functions.

I'm starting to get more confident that I can achieve what I want in terms of control and efficiency. With approx 285m2 of UFH and towel rads running as open loop, I'm reasonably confident that adding some form of extra control on rads in the remaining 85m2 of bedrooms won't completely destroy my COP. If it reduces it by a few % then so be it - the house is for living in, not just a giant science experiment (as much as I love a good science experiment!)

Colin

Posted by: @ccap

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It looks like this sort of issue has been thought about by the manufacturers, so this is all good news. Most of what I have read/watched with regards weather compensation tends to suggest it is only the outdoor sensor controlling everything, and until now I hadn't seen anything about these extra modes/functions.

This aspect of control is a bit variable from manufacturer to manufacturer so far as I can tell from reading manuals and discussions here.  For example I get the impression that Vaillant is one of the more flexible controllers and Samusng one of the less flexible, which may be why Samsung is often paired with Homely.  

Posted by: @ccap

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I'm starting to get more confident that I can achieve what I want in terms of control and efficiency. With approx 285m2 of UFH and towel rads running as open loop, I'm reasonably confident that adding some form of extra control on rads in the remaining 85m2 of bedrooms won't completely destroy my COP. If it reduces it by a few % then so be it - the house is for living in, not just a giant science experiment (as much as I love a good science experiment!

Agreed.  The only thing I would say is that, for the rooms where you just want a slightly lower temperature as opposed to a temperature that varied during the day (eg bedrooms that arent also used as living rooms), turning down the LSV is the best approach.  Equally if you expect a bedroom to cool from a 21C daytime temperature to an 18C night time temperature in a couple of hours, you are probably out of luck