tl;dr : For the millions of dwellings that have significant physical constraints imposed by the physical fabric of the building, its configuration and location, that rule out heat pumps, biomass and solar thermal, how can we decarbonise our space and water heating effectively?
The specifics of my home, for context
At the moment there's natural-gas-fuelled central heating, with water-filled pump-driven loop going into wall-mounted radiators that have been sized for an operating temperature of 60-70°C. That's pretty typical for most British homes.
The property is solid-wall construction, and there isn't much external wall - it's mostly glazed. Most rooms are generally tall - about 3m. (a fairly standard late-Victorian British terrace, with some subsequent improvements, including secondary glazing).
It's still quite leaky for air, but we're working on that, and planning to build tight, ventilate right, over time.
We don't have much unshaded south-facing surface. And we're nowhere near any district heating schemes.
There is a garden of about 180m2, with access only through the house. We live cheek-by-jowl with our neighbours, so a heating system of 40dB or more would be too loud. So no heat pumps - the defrost cycle alone is unacceptably noisy, let alone the constant throbbing during normal operation.
Our first emphasis is on reducing space-heating needs through good insulation and air-tightness. But there will remain a big chunk of space-heating requirement, as well as water-heating.
Heating season is roughly October to March, and outside temperatures in deepest winter can get as low as -15°Celsius at times. There may be 30 days a year when the temperature goes below 0°Celsius.
And although our home is unique, it is in many ways representative of millions of homes. So I'm not looking for a unique solution for my own home, but a generic solution or set of solutions for the millions of homes that share the significant features with mine: there are severe fabric constraints that rule out air-source heat pumps, ground-source heat-pumps and solar thermal. Biomass seems an unlikely route, given the local pollution impact of millions of homes with individual biomass burners.
So how can I decarbonise my domestic water and space heating, given the severe fabric constraints?