Boiler only fires up when tap fully open Help and information on all topics relating to your central heating, air conditioning and ventilation issues.

[srawstron]

Our Combi boiler will only 'fire up' when the hot water taps are turned up to virtually full. It's almost that the boiler doesn't detect that it it needs to heat the water unless its a strong flow. The central heating is working perfectly, 'firing up' instantly when the thermostat kicks in. Pressure of the system is in the 'Green Zone' so assume it's not related to that. Any help would be much appreciated.

[screech]

would need more details on your boiler.

most likely (although not all boilers use this system) would be a worn diaphragm in the diverter valve. it's just rubber and moves a spindle out to make a microwsitch every time you turn a hot tap on as the mains water forces it open. the diaphragm wears with time and use - can stretch or get holes in it so the water struggles to move it enough

[srawstron]

Thanks Screech, the boiler is an Alpha CB24X, not sure of age as it was in the house when we moved in. If this is the problem is it something I could replace myself or an engineer job?

[htg engineer]

It'll be the diaphragm, maybe stretched or split.

You know your capabilities, read the instructions if you think you're up to it - give it a go. But it can sometimes cost more for an engineer if you mess it up.

Get a price first, you may find it wont cost that much for an engineer.

htg

[Steve the gas]

Its not a DIY job really but many do it.

Only an RGI should open a boiler up!!