DIY Doctor UK DIY Forums

I have a problem with my central heating...the house was becoming too hot. There is no room thermostat and the heating is controlled directly from the boiler. I noticed that if I moved the hot water thermostat, the central heating was firing/shutting off as I altered the setting - the CH thermostat had no effect on the boliler firing at all.

With the CH timer on the run phase, the CH thermostat set to its lowest setting, I changed the DHW thermostat to max and the boiler fired.....once running, I turned the DHW thermostat to its lowest setting and the boiler shut-off. I'm assuming this is a PCB problem ? Anyone seen this behaviour before and any suggestions ?

Martyn_H wrote:I noticed that if I moved the hot water thermostat, the central heating was firing/shutting off as I altered the setting?

You would expect the boiler to fire but the radiators should not get hot.

Martyn_H wrote: the CH thermostat had no effect on the boliler firing at all.

It won't if the timer is off.

Martyn_H wrote:With the CH timer on the run phase, the CH thermostat set to its lowest setting, I changed the DHW thermostat to max and the boiler fired.....once running, I turned the DHW thermostat to its lowest setting and the boiler shut-off.

That would be expected.

To prevent the house becoming too warm a room thermostat must be fitted otherwise the heating will run continuously when the timer is on and the heating water temperature is set above minimum.

Sorry...should have been more explicit...The timer is on ( constant )for the CH, so I'd expect the CH thermostat for the radiators to govern the boiler firing and running if altered ( assuming no one opens a hot water tap !)

The Radiator Thermostat on the boiler is at its lowest setting. When the DHW thermostat on the boiler is increased, the boiler fires and runs and the radiators get hot.

If I change the bolier to DHW setting only, on the slector switch, the radiators do not get hot ( which is what we expect - so I'd eliminated the flow control valve ).

I only noticed this behaviour as I'd increased the hot water temperature on the boiler and then the house started getting hotter. Even with the radiator themostat turned all the way down, the radiators get very hot indeed during the CH 'on' period.

I understand what you are saying about a room 'stat and it'll be something I'll be installing when I can. On a slightly different topic...what do you think about wireless 'stats ?

Providing the timer is set to CH "on" mode even if the radiator thermostat is set to min the heating is still operative and possibly the water will be circulated through the radiators. That is something to bare in mind.

If increasing or lowering the DHW thermostat alters the temperature of the radiators then something is wrong.

Altering the DHW thermostat should not trigger the boiler to fire unless DHW is actually being called for at that time.

That's what I figured....I've set both 'stats to minimum and then let the boiler finish circulating and shut off ( after about 15 mins ).

I then turn the DHW 'stat to half way and the boiler sparks up and runs, warming the radiators.....I've checked the pipework and everything feels like it's piped correctly...e.g, the CH feed pipe gets hot, the DHW pipes feel cool/cold and the CH return warms gradually as it should....so I'm thinking it is a PCB fault ?

According to the instructions the boiler should not fire unless there is a demand for DHW. It can;t be a faulty flow sensor because the heating wouldn't work at all then.

I am not familiar with Baxi combi's so perhaps the best option is to phone their technical helpline and explain what you have said in your last post. It would probably be better to do that than just replace the PCB using a process of elimination. They are not cheap.

Thanks for taking time to review/help ....I'll get in touch with them