Combo boiler heating when thermostat is off

[Tristy]

Combo boiler heating on when off issue

I have an Oil Firebird 70 Combi boiler. When the thermostat is off, the rads warm up (upstairs mostly) whenever I turn on hot water tap or whenever the boiler comes on periodically to keep its internal water reservoir hot (for instant hot water). I’ve disconnected the house thermostat just incase it was faulty. But same problem.

I was expecting to find a 3 way diverter valve. But can’t see it. Anyone know what I should check in my boiler from this pic? Also read that these Firebird boilers don’t have a diverter valve.. Thanks

 

[texasholden1980]

AFAIK the boiler doesn't use a diverter valve. Instead there are two paths for the primary flow to take when leaving the main heat exchanger. When there is a heating demand, the left hand pump operates driving hot primary water round the radiator circuit. When you have hot water demand the right hand pump operates to drive primary water to the plate heat exchanger (directly above the right hand pump but hidden in insulation) so you get hot mains fed water produced.

As far as I can tell both paths are connected at the main heat exchanger so even if there is no heating demand you could well have hot primary water rising from the boiler through the left hand pump (which shouldn't be operating at that point) through to the radiators above. I can't see that the boiler has any method of preventing this from occurring. You could potentially look to resolve by adding a motorised valve onto the heating circuit to prevent unwanted primary circulation by having the valve close when there is no heat demand - but that might create an overheat situation so best to have a heating engineer make any such modification.

If I'm right then this unwanted heating would have occurred since installation.

 

[Tristy]

Thank you for the reply

Makes sense what your saying.

How comes it’s seemed to have gotten worse recently? It used to be maybe the bathroom and back bedroom rad getting slightly warm. Now it seems all rads upstairs get really hot and some downstairs getting luke warm.