I recently did some voluntary gardening work for a retired lady and whilst I was there she had a new gas cooker fitted with a new bayonet connector installed. After the gas was switched back on the gas boiler failed to relight.
This is a baxi type gas fire with a conventional gas boiler behind and a pilot light ignited by a piezo electric igniter. It looks like the igniter hadn't been used for years and the pilot light had been burning all that time.
A gas safe firm came round and complained the boiler was in a bad state (never looked at for 30 years, about the same time she abandoned the garden), parts for the igniter were not available so it was beyond economic repair. He cleaned the fire and said it was fit for continued use.
I barely know the lady so have to be wary of advising her but it seems to me that the boiler behind the fireplace could be capped off and abandoned. The gas fire still being a source of warmth in the only sitting room.
A 24kW combi will fit in the kitchen and as there is no running water upstairs, plumbed to the adjacent bathroom. There are only 5 radiators in the house so the valves can be replaced by TRVs on all except the one in the living room where the thermostat can be.
What she doesn't like the sound of is losing her supply of DHW from the vented tank with its secondary heating by immersion. I have suggested using a local firm offering a 7 year warranty on the combi which may well see her out, at which stage the property would be a candidate for a rebuild.
What I did wonder is rather than remove all the vented system could a separate zone from the combi run through the DHW water tank coil, the tank and cold water feed in the loft left in place? Then a separate DHW tap in the bath would provide hot water back up, the combi just being plumbed to the shower head?
Cost is a significant issue.