Most people think that unless you have a modern, well insulated house, solar and batteries, heat pump systems are a gamble. I have to suggest that, while anecdotally there may be accounts that suggest this is the case, it is in my opinion a myth. We need to separate fact from fiction, and look at the real world and what happens in it. In practice, there is a bias towards old leaky buildings getting bad installations, but it doesn’t have to be that way, if you simply do the basics correctly.
Heat loss calculations
Heat loss calculations are not easy. They are tedious and boring and you need to concentrate and spend a lot of time to get things right. This is why installers cut corners and get things wrong.
Among the heat loss calculations done for me by installers, some were pure fantasy. One appeared to have been done by a blind surveyor, and this from a company with a full on ‘leave it to the professionals’ strap line. Others had careless mistakes (typos, entering 2.3 instead of 12.3) and some made wild assumptions such as reducing the house with rooms to a single giant shoe box and did the heat loss for that.
No wonder they all came out with vastly different results. But done correctly, they will give you a reasonable estimate of the heat loss. What you do is measure all the relevant areas (walls, windows, doors, floors, ceilings, etc.) which is very tedious and boring to do, and then apply (from standard tables) the heat loss in watts per degree centigrade per square metre to each area and add up all the losses to get a total loss, and then add in an allowance for air changes (a sealed room has less air changes than a kitchen with a back door that is frequently opened) and that’s pretty much it, apart from some minor refinements.
There are plenty of freely available spreadsheets available that are set up to do the calculations for you. You don’t have to set up the formulas – you just enter the measurements for each room, and the better ones even have drop down boxes with U values (the loss in watts per degree per area) for common building materials. You will nonetheless have lost several hours of your life that you will never get back, but you will know your heat loss. It’s worth doing.
A heat source is a heat source is a heat source
No heat source cares one iota about what it is heating. To any boiler or heat pump, there is nothing special about heat pumps in this. A huge, well insulated mansion with ten bedrooms on the south coast of England with a total heat loss of 10kW is no different to a battered, old leaky tiny crofter’s cottage in the highlands of Scotland with a total heat loss of 10kW.
In each case, under steady state conditions, which is what you are aiming for, each building loses 10kW. Match that with a 10kW input, and all will be well, however old and leaky the building.
Higher heat losses will always cost more (whatever the fuel)
The other unavoidable fact is that a building with a higher heat loss will always cost more to heat than a building with a lower heat loss. If you live in an old, attractive but leaky-as-hell cottage with a 12kW heat loss, as I do, it will always cost more to heat than a modern, super-insulated house with a heat loss of 6kW, whatever the fuel. I will always burn more oil or gas, or consume more electricity to heat my house than the owner of the aforementioned modern abode.
A pinch of the real world
Now we need to add the real world bit. By and large, most fossil fuel boilers are substantially oversized. In the past it didn’t matter because energy was relatively cheap, so just bang in a big one and be done with it. No need for all those tedious heat loss calculations. Just guess it, multiply by two and fit a boiler that size.
As a result, fossil fuel systems never get anywhere near being stretched beyond their capacity. They can almost always supply enough heat.
Heat pumps on the other hand tend to be matched much more to the predicted heat loss, and that’s where the problems start because there is far less margin for error.
Get the heat loss wrong, which as we know is very easy to do. My heat loss estimates from installers ranged from 9kW to 14kW. There is no way they are all correct. If the error is to underestimate, and then match the heat pump to that underestimate, then the system will fail.
A further pernicious problem is that heat pump suppliers deliberately mislead about their heat pumps output. The headline output (in my case 14kW) is for a sunny day in spring, when I don’t need 14kW. This very same heat pump, however, becomes progressively incapable as outside temperatures fall, and by the time it is zero degrees outside, it is only capable of putting out around 11.3kW. As my heat loss is 12.3kW, given a 14kW output branded heat pump, what could possibly go wrong? As you can see, quite a lot will go wrong once it gets cold outside. The heat pump will never supply enough heat.
Consider the entire heating system
The other real world bit is failing to consider the heating system as a whole. Because heat pumps run at lower temperatures, they need bigger emitters to deliver the same amount of heat.
A cooler-running heat pump delivers less heat than a hot fossil fuel boiler to radiators. Very roughly, the increase needed when moving from typical fossil fuel running temperatures to heat pump running temperatures is to double the size of the radiators.
Even if you match a heat pump’s output to the building’s heat loss correctly, taking into account the lower performance at cold temperatures, if you keep your old fossil fuel rated rads, they will be too small, and the system will not deliver enough heat in cooler weather, and your house will be cold.
Commissioning is critical
Lastly, heat pumps, because there is less margin for error, have to be commissioned correctly. Unlike a fossil fuel ‘fit-and-fire’ solution, heat pumps need to be tuned to match the building they are heating. This takes considerable time on the part of the installer.
Omit this step, as many installers do, and the system will either fail or your fuel bills will go through the roof because the heat pump has been left to run on its most inefficient settings. It will produce heat, but with appalling inefficiency.
My own personal experience is in line with the “underperforming heat pump in cold weather” problem. The rads are all upgraded to match heat pump running temperatures, but in cold weather my heat pump fails to deliver enough heat, and my house fails to reach, let alone stay, at design temperatures.
In warmer conditions, it is fine. The building is listed, so reducing heat loss options are limited, and there is no way solar panels are going to be installed. I am 100% reliant on the heat pump replacing my oil fired boiler.
The bottom line is there is no reason why within sensible limits a heat pump can’t heat any building (as long as you have somewhere to put the unit, and a supply of mains electricity). You just have to do the calculations carefully and thoroughly, because there is far less margin for error as I’ve stated several times.
The sad fact is that the heat pump industry in the UK has far too many jobbing workers who have done a half-day course on how to fit heat pumps, which they then install with their old fossil fuel fit and fire mentality. The forums on this site are littered with examples of this ‘workmanship’. Far too many have no idea how to do a valid heat loss calculation (how on earth did my prospective installers come up with heat losses ranging from 9kW to 14kW? They all had the same building to work with).
Result: failed systems. And this just happens more often on old leaky buildings because there is even less margin for error. It doesn’t have to be this way, so long as you design, install and commission the system correctly.
In closing, I had a number of installers tell me they couldn’t possibly fit a heat pump because my house didn’t fit the requirements as it was far too old and leaky. They tended to have a sanctimonious air about them, but they were just as incompetent as the installers who got other things horribly wrong.
To categorically say you can’t have a heat pump is just as wrong as fitting the wrong heat pump.