So, heat pumps can, in a normal domestic situation, generate enough heat during all seasons to match the load requirements of older houses without significant modification to the building?
Yes. On the caveat if your using electricity it will cost more per KWh, and if the building has a high heat loss this will then amplify costs, but the physics of heating it will be no different to any other fuel source, you just match the heat source (heatpump & rads/UFH) to the heat loss of the building.
The issue with most of the "heat pump cost me thousands to run and house was cold" articles, is the people fitting them, do not have a clue about heating. Most gas fitters (and I include myself in years gone by) do not size up heating systems and boilers. They fit a boiler that is generally miles over the size required, and what ever radiators fit under the windows, and in general, that will heat most houses. The tolerances on a heat pump system are not the same as with gas.
You start by doing a full heatloss survey of the property (which gas fitters should do but typically dont) this will give you the heat loss of the house at a set outside temperature, for my location thats around -4°c outside temperature, and my heat loss is about 6.8kw at that temperature. This will include the heat loss for each individual room.
You then need to decide what temperature you want to run the system at, at -4°c outside in this case. I've opted for 45°c (as I cant fit UFH so will be using radiators, and much lower than 45°c will generally mean some hideously large radiators)
Knowing the heat loss for each room, and the flow temperature I will be using at design temperature, I need to know the mean temperature of the radiators to gauge their output at this temp. With boilers, we would use between 11 and 20°c temperature difference across the radiators depending on when the system was fitted (and assuming at that point any calculations were in fact even done (doubtful))
What this means is that giving 45°c flow to the radiator, on a system with a DT (temperature difference) of 11°c, we would have 45°c flow, and 34°c return to the heat source. (standard efficiency boilers were generally about 80°c flow/69°c return, condensing boilers 70°/50°c (again only if it were actually sized and balanced correctly)
With ASHP, we generally need a DT of 5°c. We do that by flowing much more water round the system. So in our case were sizing for 45°c flow temperature, and 40°c return temperature, which would require about 4 times the water flow rate through the radiator than the equivalent condensing boiler running a DT of 20°c. Thus why heat pumps often need larger water pipework fitted as they need much higher flow rates.
Knowing each rooms heat loss in Kw, the fact that were using 45/40°c flow and return temps, we can then workout the size of radiator needed for each room, which will also give us the size of pipework needed to each radiator and to the heat pump. The overall heat loss for the house is how we would determine the heat pump size also.
But you must be aware that unlike boilers, you don't select a 7kw heat pump where you would a 7kw boiler (if they had 7kw boilers) The heat pump output is dependant on the source temperature (air) and the emitter temperature (radiators) so we need a heat pump that will provide 7kw at 45°c flow temperature with a source temperature of -4°c outside. That same heat pump may not be able to provide 7kw if the flow temperature was 55°c or 65°c against -4°c as the heat output power is variable depending on conditions unlike a gas boiler which will be a fixed output regardless.
So we have the Heat pump selected, the radiator sizes, the pipe sizes, good to go? No. there's more to work out.
As mentioned in other posts, heat pumps have what they call a defrost cycle. You are extracting heat from the outside air and moving it to water pipes inside. This can only mean that the outside unit inevitably gets colder than the surrounding air. In winter, when temperatures are low, and humidity is high (as is the case for out wet little island) you will get frost formation on the evaporator (the bit that extracts the heat from the air) this looks like a finned car radiator, and as it frosts up the fins become blocked reducing its capacity to pull heat from the air.
This happens on all units. And is a perfectly normal and expected part of ASHP operation. Now begins the defrost cycle. The unit essentially goes into reverse. It draws warm water from the heating system, runs its internal refrigeration circuit backwards to heat the evaporator temporarily to de-ice the unit, before returning to normal heating operation.
To do this successfully in one short burst, there must be a minimum system volume (which is quite large) Most reports of units constantly defrosting and never getting the system up to temperature in the winter will be because this fact has escaped the installer, and they have not fitted adequate provision for de-icing, in the form of a buffer tank.
A buffer tank serves multiple purposes, but the primary one being to give the system enough heated water volume that it can run a defrost cycle when required, without adversely impacting the heat inside the property and get it defrosted in one hit, rather than cycling on defrost almost constantly.
Theres a number of other things that also need to be taken into account in the specifying of a system, but thats the basics. If system specification and installation is done properly, then any property can be heated with a heat pump. And any well insulated property can be heated very efficiently with very low CO2 production, which as said, is the goal of a heat pump.
