I have a combi boiler which is becoming erratic - hot water downstairs is okay but the upstairs shower doesn't always trigger the hot water demand. When that happens I've been turning on the downstairs hot tap first, then the shower and once it's heated up, slowly turning off the downstairs one. Sometimes the boiler stays on, sometimes it doesn't. I wondered if it was the shower thermostat but it does happen from time to time with the bath (which is separate), although not as often. The mains pressure and the boiler pressure are okay and so is the central heating. Is it more likely a boiler problem or could it be a pipe problem? Thanks.