I have recently replaced my very expensive to run, electrically heated water cylinder with a gas combi boiler recommended by my gas engineer which just supplies hot water. My central heating is supplied by a gas fired warm air system which I love and don't want to change. The combi-boiler has a rating of 35kw, the warm air unit 24kw and they are impossible to run together. Every time I switch on a hot tap it cuts out the pilot light on the warm air unit so no central heating.... I am now considering changing the combi-boiler for a multi-point water heater but am struggling to find a suitable model with a kw rating which is significantly less than the boiler. They mostly seem to be 24 - 30kw? I have been told by my Gas Network providers that I only have a 3 cubic metre supply (my gas installer assumed it was 6 cubic metres) despite having a 5 bed house. They want to install a bigger supply but it involves ripping up my driveway to do it and I can't afford to have it resurfaced. Any suggestions please?