We are on mains gas. We have two boilers, one for the UFH and one for the upstairs rads and DHW.
We basically have the UFH on the lowest settings, we get a reasonable amount of solar gain so don't need more than this. We have two wood stoves which we run all the time in winter.
It's b-build with insulated boards on external walls. It wouldn't be "sealed" as such, although I have taken the time to run extra sealant around window opes, sockets etc. Other people will have much better air tightness than us. I haven't had it tested.
The HRV we find great as we never have to have windows open. Due to the dust and the flies (oh God the flies) this has become a major boon, also the filters seem to take out much of the pollen.
SSE