I have a 3 bed detached 70s house with a larger 90s extension. The original house is double skin breeze block and the extension is block with timber frame. Both are insulated with underfloor spray foam insulation and loft insulation. It is a big 10 room house run on gas central heating with a combo boiler. Would this mean an air source heat pump is a no no?
Whils’t the majority of older homes would have upgraded their loft insulation to 270mm or the equivulent R value in other materals,. The majority of said homes either have a solid concrete floor or suspended wooden floor insulated with only underlay and carpet.
You have insulated floors which is good but, the skin (walls) of your homes remains the sames as they were built in the 70’s and 90s . The cavity wall will be at the standards back then which will likely be 50mm (air) gap between the inner and outer skin of any external walls, likely filled with insulation by your energy company, which drilled a series of holes and injected it into the main building. This way of dong it does not guarantee the whole cavity is filled, meaning you will have a few cold spots here and there. Homes of this era were sesigned to breath.
New building falling under the lates’t building regulations will have a minmum of 150mm PIR insulation in the floor construction. The cavity wall will have 150m of taped PIR insulation or similar bat material providing the same R value and the loft insulation will have 270mm insulation. Thes homes can be considered airtight and therfore suitable for a heatpump. These homes will have an EPC rating of A (I had to build porch to these regs as per building regs attached to a leaky 1960s home!!)
To be entitled to the heatpump grant your home must have an EPC rating of C or higher , however thats still someway off an A rating or possibly B rating.
I gas boiler has a flow rate of 60c, where as a heatpump is much lower and would require radiators of at least two and a half the size of your current ones as @wigwam said. This increases cost of both materials and installaton charges. On the note of radiators, the radiator fitted in my porch is a single 200x40o In a similar sized room in the house its a 500×600 double panel single convector rad.
In my opinion your best keeping your boiler and fit an hydrogen ready one when the time comes!
Do check your buding regs though as those Ive quoted are not for Scotland 🙂