I remember watching a heat geek video about the effects of unheated rooms and how it would impinge on efficiency as the flow temperature in other rooms would have to be higher, unless your bedroom has very good insulation in its walls and floor (assuming its upstairs).

What are the designers or installers reasons for not allowing you to do what you want other than its makes the system slightly inefficient?

@ashp-bobba I think the starting point was soaking up the patsy who qualified for the £10K grant in 2020 and then added more....
I have learned to fight my corner but I am intimidated by Heat Geeks who really seem to know what they are talking about but even they don't address the wishes of the resident about how much of their house they want to heat.

@bontwoody the non standard part of my house is that it is an old stone building with 600mm thick walls some of which are external but due to two extensions the old core is wrapped round with modern fabric, making quite a lot of internal walls 600mm thick too. I notice that when I turn on my electric blanket, perhaps two hours before bedtime, the room is noticeably warmer, not warm enough to sit in but warmer.

Posted by: @jswhite

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The insistence on heating all rooms produces heat loss calculations which listed heat loss for three rooms (a store room, utility room and junk room) which do not even have emitters and never intend to have them! The calculations then add even more to the total for the rooms that I do not want to heat and an enormous heat pump is offer up to cover it. The enormous device never gets called on to heat all that space and so is hugely inefficient.

Its not quite as simple as that

Unless the insulation between rooms is very good, or the insulation between the house and the outside world very bad, you will still lose nearly the same amount of heat from the house (which is what determines the amount of heat you must supply) whether you heat a room or not. 

If you doubt this try this try this experiment on a cold day: Turn off the heating in the room(s) you dont want to heat.  Turn on the heating in the other rooms.  Leave for a few hours.  Then go into the rooms you aren't heating and compare the temperature in them with the temperature outside.  In most houses the 'unheated' rooms will be much closer in temperature to the temperature of the heated rooms than they are to the outside, which means that they are still losing heat to the outside at nearly the same rate as if they are heated. 

Of course there are exceptions, long thin houses can be warm at one end and cold at the other, and if the insulation between rooms is very good and there is little air movement, then a sizable temperature differential can exist, but most houses aren't like that.  The heat loss from the house will be 'correctly' calculated by setting the room temperature of the unheated rooms to the temperature that they will settle at.  For most houses, because this is closer to the temperature of the heated rooms than it is to the OAT, its not going to 'save' much on the calculated loss.

In this circumstance the argument for actually heating the rooms, given that the temperature differentials are relatively small so the overall heat loss doesn't change much, is that more radiators = greater total emitter area = lower flow temperature = better efficiency.  Heat geek do a worked example showing its (in the case they illustrate) actually cheaper to heat all rooms than to heat some, because the efficiency gains outweigh the saving on loss from leaving rooms unheated.  Of course this is house dependent, but it shouldn't be dismissed as installers trying to max the size of a heat pump etc.

There is nothing to stop you, as a householder, turning down radiators in some rooms (eg bedrooms).  The point is that, for most houses, its probably more efficient to heat them, albeit to a lower temperature, than to leave them unheated and rely on heat spilling from adjacent rooms.  A proper heat loss template allows for this.

All that said there does appear to be a tendency for heat loss calculations to overestimate the required capacity, but its for different reasons.  Many installers will make 'conservative' assumptions about the fabric construction 'for safety' and will also assume air change values which may be way in excess of the actual.  They have also been known to 'double count'.   My own house is a case in point.  Two full three hour surveys, one of which I paid £300 for, came out with 16kW.  I asked for the detail of the one I paid for and it turned out they had ignored fabric upgrades they couldn't see, even though Id made a point of telling the surveyor about them and confirming with him/her that they would be accounted for.  Worse still they had counted loss from warm rooms to cooler rooms in the house total (without counting the gains).  Correcting for these errors, but otherwise using MCS assumptions, gets to 10.5kW, 11kW tops.  Fortunately I have 2 years of smart meter readings from my gas boiler which pretty much unequivocally show that the house loss is ~7.5kW, maybe a bit less, and that's how the heat pump has been sized.  The difference can be accounted for by assuming ACH=0.5-0.8, instead of the default 2-3.  Some more candid installers will admit that using a lower ACH than the default is probably a good call.  All this is nothing to do with heating rooms or not, but it did lead to an oversizing by a factor of 2.  

If you have any independent evidence of your heat loss then my advice is to use it, if only as a sense check. There are installers out there that will take notice!

@jswhite If you design the system to suit the whole house and then have TRV's at each rad you could run all the rooms you don't want to heat on low point just above frost protection only which would be 16 Deg C and then any heat not used by the rooms would cycle back to the ASHP which would inverter down to a lower capacity. Providing you are running the system @ above 50 % of design  so number of rooms and their size greater than the number not heated, it would not or should not short cycle to much. This would make the system slightly less efficient but If the systems max efficiency say was SCOP of 4.2 on an 8kW system and you are only using it for 5kW then perhaps you would get 3.7 SCOP over all (this is a guess) Remember all systems need to be designed accurately to gain max efficiency but these are designed at peak -2.7Deg C Ambient, 35 / 55  flow rate. but heat loss in a property varies from 12pm daytime to midnight, month by month season by season, the ASHP copes with this, also if the heat loss is 8kW its only that at initial points of the day across the year, the rest of the year the system will do its magic and modulate. There is just a limit of efficiency to this modulation.

Its likely the Grant funding and UK standards dictates the system cannot be deliberately designed to be half a house, so design and install the whole house and turn the TRV's down to very low after commissioning as when you own it, its is your heating system.

 Also just to prove that its not the machines capability to run less but rather design rules that limits, most ASHP also heat stored water, most stored water coils run at 3 or 4kW even on an 11, 12, 14 and 16kW ASHP in this mode the ASHP just runs at 30% or what relevant to its capacity, its just less efficient than the heating circuit but way more efficient than electric heater elements and boilers (from a carbon producing point of view) Design is looking to lower your carbon footprint as well as give you comfortable heating but across the whole house.