ReedRichards

I know nothing about well-insulated houses but it looks to me as if the "shortfall" occurs because your installers think your UFH will not be sufficient to heat this one area of your house. In my house, which is entirely heated by radiators, this issue was fixed by adding an extra radiator. I suppose it could have been fixed by running the existing radiators at a higher temperature, which might then have required a more powerful heat pump but that is entirely the wrong way to go. So if the calculations are correct you would need an extra heat source. Is there any scope to plumb-in a wet radiator if there really is a problem?

So your system will gradually shut off radiator after radiator as each room reaches its set temperature until the last room reaches temperature, all the TRVs are off, so the heat pump is shut down?

@JohnMoSo can we agree that whilst installers may or may not be obliged to fit TRVs (to all radiators in rooms without another form of heating control) they would be incautious not to fit those TRVs? Even if they intend not not use them.

@JohnMo The acid test would be to omit all TRVs and thermostatic controls, rely on WC then call in a Registered Building Inspector to see if it passes muster. I believe HeatGeek installers favour pure WC control (although how they find the time and opportunity to balance every radiator I don't understand). So do they not fit TRVs? Are there heat pump installers with the courage of your convictions?

That's not my reading of the regulations. This section is "in addition to meeting the general requirements for heating and hot water systems in Section 5." Weather Compensation and TRVs or other room thermostats are entirely compatible. Most gas boilers do WC, not just heat pumps. Yes, I know there is a "hard core" school of thought that if you get your radiators perfectly balanced then you can keep every room at the right temperature by relying entirely on WC without any internal temperature controls. But Building Regulations tend to be conservative and I don't think they so much as dreamt of that.

@JohnMo from https://assets.publishing.service.gov.uk/media/662a2e3e55e1582b6ca7e592/Approved_Document_L__Conservation_of_fuel_and_power__Volume_1_Dwellings__2021_edition_incorporating_2023_amendments.pdf

Aren't installers obliged to fit TRVs to all radiators to conform to current regulations?

Extra radiators aren't more efficient in terms of running costs, they are equally efficient to a mixture of radiators and UFH (in your circumstances). But I imagine that extra radiators would be much cheaper to install than UFH.

Can I just mention that there are no economic benefits of having a mixture of underfloor heating and radiators, particularly in a small property when there is little point in having multiple zones. If you like underfloor heating and don't mind digging up your floors to install it then all well and good. But the heat pump will need to make the water hot enough to meet the requirements of the radiators and it is how hot that heating water needs to be that determines how economical your heat pump will be to run. Unless you have really large area radiators they will probably want hotter water then your UFH could get away with but if so then you need that water hotter.

I have an LG Therma V heat pump with a Drayton Wiser controller. When a cycle stops, the controller will not allow a new cycle to start until 20 minutes after the start of the previous cycle (it would do the same for an oil boiler). I suppose there is still potential for each individual cycle to be short but there will be at most 3 cycles per hour. And my house maintains the set room temperatures (two zones) so it cannot be short cycling and failing to provide enough heat. All radiators, 4-port buffer built-in below DHW cylinder. Heating control by LWT only (there are alternatives that I haven't tried). I'm not aware that I can control the overshoot temperature; I have observed a degree or two overshoot in winter. On is via the controller which immediately starts the internal circulation pump. Heat pump spends several minutes in "start-up mode" before it reports "internal unit" in operation. Heat pump starts at low input power and ramps up (stepwise) as it sees fit. WC settings : -14,55 20,22 (determined by experiment).

I think a heat trap works because hot water rises. And I'm sure that they help overall, but I'm not at all sure how they influence heat conduction in the copper pipes.

Wandering off-topic but I have a horrible feeling that these quoted losses are for an unconnected tank and that the loss goes up considerably when you connect copper pipes to a tank. Copper is a superb conductor of heat and the pipes must wick the heat away.

You really ought to get a quote from a regular installer just to check that you are correct that the scheme on offer would work out cheaper. A company acting on the basis of a limited stock of radiators (which presumably they buy in bulk at a discount) rather than what you actually need does not bode well. That said, even an inefficient heat pump ought to be a lot cheaper to run than your Fischer radiators.

But the video suggests there is an option of incorporating an immersion heater in the tank. That must increase the chance of catastrophic overheating, no matter how small that chance is.

It wasn't the nomenclature I was worried about, it was the control. But I have now watched the video and it is clear that as far as the heat pump is concerned it is heating a normal DHW cylinder but with the control parameters tweaked to match the needs of this particular abnormal cylinder.

So it's a thermal store but you tell the heat pump it is the DHW cylinder?

Just to remind everyone, if you had your ASHP fitted without planning permission as a permitted development then it should not be used for cooling. To be clear, this does not apply to new builds where the planning permission covers the entire build including the heat pump.

Any scope to use a water source heat pump?

All heat pumps will cycle when it gets too warm outside. The heat pump will have a minimum electrical power it can draw and if the heat that it generates at this minimum power is more than enough to keep your house warm then the temperature of the water retuning to the heat pump will get too high and the heat pump will shut off for a while.

I don't think there has been enough studying of set backs to know if they save you money and if so how much. I know of one individual who worked out his set back saved him about 15% but it must differ in every case. But I do it because I don't like my bedroom to be so hot at night.

In my system the kitchen already had a chunky T3 radiator which my OH liked to put things on to dry. The heat loss calculation said it needed to be bigger but I thought with cooking and a bit of heat from the fridge and the freezer it would be okay. I was wrong. After a cold winter's night the kitchen is pretty chilly first thing. It warms up during the day but it's a few hours slower than the rest of the house. Even if your whole heat loss calculation is over-egged you need about the same ratio between calculated heat loss and actual radiator heat output for each room. Your OH may not like noise from fans but if they use the study in the morning I bet they will also dislike it being chillier than the rest of the house.

But only when you need it. Most of the time it will be warmer out, you won't need as much storage capacity so you can reduce the thermal store temperature. This would be a sort of DIY Weather Compensation, unless you can figure out a way to automate it. Could you get away with a plinth radiator in the study in addition to replacing the existing one; not the fan-assisted type, just one that is long and low? Having to modify your entire heating system to conform to the needs of one room seems like the tail wagging the dog. My retrofit system was designed around 50 C target flow but with hindsight I regret not pushing to make this a bit less, say 45C.

Any chance of adding another radiator if you can't resize an existing one to make it big enough? We added another radiator in a bathroom which my OH loves because it helps to dry/air clothes in winter.

Yes, but this is the actuality of cooling, not what somebody in an office devising regulations thought it might be like. But I agree that it means you should get away with it without your neighbours noticing (as necessary).

Says the regulations for installing a heat pump as a permitted development. So of course this doesn't apply to new build projects where the whole development needs planning permission and the grant of planning permission encompasses the heat pump.