So, I’m in the throes of retrofit right now: EWI is going on next month, we’re getting quotes for windows and doors, the loft has been insulated to within an inch of its life and I’ll air tightening like a madman when our MVHR goes in. The project is… pricey, but going ahead.
What I’m wondering is if anyone has used an EAHP like the Tesy Aquathermica 200 to provide space heating as well as hot water?
This is possible because there’s a heat exchange coil in the unit, and the pump claims to be able to hit 65C. Obviously the heat demand is a massive factor here as the EAHP only supplies 1.5kw of heat, but I’ve been toying with the idea that once all the work is complete, we’d be able to avoid ripping the old wet heating system out and instead drive it at the cylinder temperature.
Working out how this is done without compromising the hot water temperature is hard, though. Aiming for 40C to the tap gives 25C of wiggle room, so 4.18kj per kg of water (200L) times 25 makes about 20Mj to play with.
The output of the radiators in the old CH is supposedly around 2.6kw, and assuming the heat exchanger can supply it, that might do the trick. I need to re-run the heat loss calcs, but with EWI alone we got to 6kw for heating requirements. New windows and doors, plus best recovery, will, I think, cut two thirds off this, but I need to do the maths.
What I do know, though, is that removing a net 1.1kj/s from the cylinder via the hx (that’s radiator output minus heat pump supply) the cylinder when it is at 65C will run for just over 5 hours before the temperature hits 40C.
Skipping to the end, it is still fairly borderline as to whether or not this is a goer, with the main questions relating to recovery time and how long the CH and DHW are in use each time.
So, I’m wondering: has anybody done this? And… does it sound insane? I feel like it might be.