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Any cost comparison simulators are useless, as every house is different. This is why to get the grant, the installer must do a full heat loss survey on your actual house which will give a vague estimate of running costs.
This heat loss survey often over-estimates energy requirements; ours suggested a parity in running costs (in kWh delivered) with gas, but in reality so far we’ve been using half the estimated energy and the house is at 21deg, not the 20deg design temperature.
With regards to radiators: they cost so little (relative to the total cost of an ASHP install) that it’s a no-brainer to upgrade any that need to be. But even if you don’t, the beauty of a heat pump system is that they all modulate the flow temp according to the weather, so your ‘design flow temperature’ will only ever be reached on a couple of really cold days/weeks of the year. The rest of the time it’ll be coasting along at 30deg or whatever, just slowly and constantly topping up just the right amount of heat in your house, keeping every room at a constant temperature. The radiators will never, ever feel warm to the touch.
This last bit is what the UK seems to have a hard time understanding: it’s not a gas boiler, and doesn’t work well if you try to run it like one. Program it to come on for a few hours in the morning and evening at full blast, and you’ll have an uncomfortable house and high running costs.
ectoplasmosis-
Radiators don't cost much but redecorating where you've had to move pipework, etc costs a lot.
The theory on the temperature flow thing is interesting, maybe I'll give that a try by turning the boiler down and see what happens over the course of a day. Bringing the house up to temperature when cold takes a lot of boiler on time but I'm not sure what cooler flow would be enough to offset the temperature in the cold months.
Although I'm still not sure where I'd actually put a heatpump, they look massive in all the pictures.
aggi
I think I've mentioned on here before how information on how this works in reality is incredibly sparse though.
When I was hunting around before I couldn't find one decent website on fitting a heat pump into a poorly insulated property with small radiators so it's easy to assume it doesn't work.
When you look at the stuff to compare running costs they always have the heat pump running at maximum efficiency too, I couldn't find anything on costs when they're running at 60 degrees.