Baxi Duotec HE 33 CH cycling on off

[Rickytickytavi]

Hi

Wonder if anyone can help? I’m having a problem with my combi boiler, the CH keeps cycling on/off on a three minute cycle instead of modulating at the set temperature. It will fire up and ramp up to max fire before dropping back again but then overshoots the set temperature (65degC) and cuts out at 70degC. It’ll then wait for about two minutes before starting the cycle again. Thinking it is either a flow problem or a control fault (faulty PCB), anyone have any ideas what may be causing this?

Thanks

Richard

 

[Johntheo5]

Most boilers will fire up at ~ 65% of max output (if this is what you mean by max firing) and then modulate down and will undershoot the target temp so will then again ramp up again but normally will only overshoot the target temp by ~2C or so and not reach targettemp+5C and burner trip + recycle before refiring again which is what is your problem but repeatedly??, possibly a circulation problem if rad TRVs have throttled down the flow or only one CH zone on?. On lots of boilers you can set the (CH) part (max) heating output to your heat demand, you may have 33kh HW output but only rquire say 20kw heating demand which helps to avoid this overshoot.

 

[Rickytickytavi]

Hi John, thanks for the reply, don’t think I can adjust the heating output on this boiler, just the water temp set point. I had all the trv’s open in a cold house and still does the same so suspecting now to be a circulation issue caused by the pump or blocked/gassed up heat exchanger. Had the boiler 10 years and been fine up till now so something must’ve gone amiss. Was wondering if the heat exchanger may be gassed up with air as have had a few rads out recently, notice on some of the boilers of the same model as mine have automatic de-aerators on the top of the primary heat exchanger, mine doesn’t for some reason. Also found I can get the boiler to stay on by running hot water when the boiler fires up for CH, I wait till the temp stabilises at 65deg then shut the tap. Works for now.

Cheers

Richard

 

[Johntheo5]

Could be air.

You have a combi boiler so as soon as you run HW then the boiler will only supply HW as a diverter valve changes over, you can't have both HW & CH, you can run the HW only but the boiler flow temperature isn't fixed then, it increases/decreases depending on the HW demand and its (the HW) set temperature, if its working the way I'm interpreting it then looks like a diverter valve problem.

 

[Rickytickytavi]

Good point, didn’t think of that. Maybe the diverter valve is slightly open causing reduced flow in the CH system when on CH mode, would explain my problem, I will look into that in the morning.
Cheers Mate