I’ve been planning to replace rads, I did the central areas of the house last year, now I want to do the living spaces. I’ve been working on making them slightly wider and double panel which will roughly double the area. This will make the current boiler much more effective and efficient. However the rule of thumb seems to be that for a heat pump you want 2.5x the area.
However, I think that the original rads were oversized when run at 80C, because the house was really warm. Even as they are I can run the boiler at 55C or even 50C when it’s mild and 60C when it’s cold. That’s a dT of around 40C. If I halved that, that would mean a flow temp of 40C in the coldest conditions we see would be enough to heat the house – therefore suitable for a heat pump. However there’s a couple of caveats to that calculation. The radiator isn’t the same temperature all the way across, so by doubling the area you’re not really doubling the heat output. However I’m more than doubling the area, but how much heat is output depends on the flow rate, so I’m not really sure how much extra heat I’ll be able to get.
The other thing to consider is that even at 55C in mild conditions the heating warms the house in say, 30 mins, and then stops for an hour or so. So in theory a heat pump could run at even lower temps for longer or even continuously, and still heat the house. And at the lower temps it could be very efficient and therefore cost less than gas (which is what I want).
I don’t want to replace these rads and then have to do it again for a heat pump install in the future. But the rads I am choosing now are the biggest I think I can fit in the spaces we have so.. dunno.