Heating that only works when the hot water is turned on is a frustrating issue, but it’s one that often points to how the boiler is prioritising demand rather than a complete fault.
In combination boilers, internal components decide whether heat goes to the radiators or the hot water tap. If a valve inside the boiler isn’t moving correctly, the system may default to hot water mode and only allow heating through when both are calling for heat.
Controls can also play a part. Faulty timers, wiring issues, or thermostats sending mixed signals can confuse the boiler into behaving this way. The boiler isn’t broken, but it’s responding to incomplete or conflicting instructions.
This issue can also be linked to pressure or circulation problems, where the boiler struggles to deliver heat unless demand is high enough. That’s why it’s important to look at system behaviour as a whole.
Understanding how different parts of the system interact is covered in this wider guide, where boiler behaviour is considered alongside radiators and heat loss.
If the heating consistently depends on hot water being on, professional diagnosis is often needed, but recognising the pattern helps avoid unnecessary guesswork.