@b0rn2sk8 Thanks for the detailed breakdown
. It's an interesting read as I'm looking at taking the plunge myself.
Mind if I ask what sort of house you have? Is it a modern build with good insulation? Did you replace your rads/pipework to work better with your heat pump?
No problem!
It's a 136m2 modern(ish) build from 2003, so not up to modern insulation standards but not terrible either. I've topped the loft insulation up from 100mm to 250 and 300mm (boarded and non-boarded sections) but otherwise its as it was when it was built, including 10mm plastic piping on all of the radiators.
Our hear loss is just under 6kw so we have a 6kw heat pump and a 180L cylinder. 10mm plastic piping is a problem as it limits how much water you can flow through a radiator and therefore caps how much heat I can get out of an individual radiator. Luckily that wasn't an issue for my specific install so no re-piping was needed of any of the radiators other than some small adjustments where a wider one was installed.
That £3500 install cost included swapping most of my radiators, the only ones which were not changed were in the tiny rooms (utility, downstairs loo, en-suit) and the main bathroom which I had previously changed for a larger towel rad. My airing cupboard had to be completely re-piped to install the cylinder and the pipework runs out to the heat pump were new of course.
Insulation levels doesn't really change anything between heat pump and boilers. The lower your insulation levels, the higher your heat loss, the bigger radiators you need and the bigger boiler or heat pump you need. The only requirement for the government grant is that you have no loft insulation recommendations on your EPC. Given how easy and cheap it is to top up your loft insulation to 250mm or 300mm, its a no brainer anyway. Your installer should do a full room by room heat loss survey and determine the required radiator sizes for each room based off the target flow temperature at -2C (for 21C room temp). The actual flow temperature will vary as it will work off weather compensation and will be significantly lower for 355+ days a year where I live.
One thing that people don't realise is that their boilers are probably gargantuan for their their heat loss (30kw is normal) and almost all run at a fixed flow temperature, this massively reduces the efficiency. Condensing boilers advertise >90% efficiency, hardly any run at this efficiency because they are not set up correctly. Like a heat pump, cycling (on, off, on, off, on, off etc) is not your friend and tanks efficiency.
To achieve those levels of efficiency you have to follow the same logic as a heat pump, you need correctly sized radiators for each room, a return temp of 55C or below which usually means a flow temp of no more than 60c at -2C, a weather compensation sensor to automatically reduce the flow temp when its warmer than -2c and minimal zoning (e.g. thermostatic valves). Hardly any installations have this and if the boiler isn't condensing, its in the low 80% range at best. It's also harder to correctly size a combi boiler for heating as you need a huge boiler to get decent hot water flow rates but you seldom need that power for heating. Some really good combi boilers can modulate down to 1-2kw but you still need to do all the steps mentioned above.
The boiler that was ripped out of mine was a Range Powermax 155x, they are not your usual boiler and they don't make them anymore. It was a 100L heat store with an integrated 14kw boiler (old, non-condensing so ~80% efficient). It heated mains pressure hot water via a plate heat exchanger. Other than the poor efficiency (further let down by terrible insulation of the heat store), its capability for a 1 box solution were not bad for its time.
Replacement options in order of price were:
1) a combi but we would sacrifice mains pressure hot water,
2) a heat pump with unvented cylinder
3) a system boiler with an unvented cylinder
We didn't want to sacrifice mains pressure hot water so the combi was struck out immediately and I doubt it would have come in any cheaper anyway as we would have needed to re-pipe the gas supply because it would have been too small. The heat pump was cheaper to install and cheaper to run so it was an easy decision in reality.