Baxi duo Tec 2 GA temperature cycling when hot.

[Jamm89]

Hi all. Can anyone shed some light on this fault please. Boiler is cycling when system is hot.

It's a 40kw combi ~ 8 years old. we have 10 rads on the system. 1 towel rail isn't currently working in the bathroom maybe an airlock? Have previously had an airlock on one of the rads downstairs.

Thanks in advance.

 

[John.g]

You may or may not have a problem.

If the boiler starts cycling as soon as the system contents are up to temperature say after 20/30 minutes after a cold start in the morning but with all roomstats/rads/Trvs calling for heat then you have a problem.
If it starts cycling after a few hours or so when roomstats or TRVs, if installed, are shutting down then not a problem as the heating demand may be less than the minimum output of the boiler which on a 40kw combi may be as high as 6 to 8kw, the boiler will then cut out the burner at the boiler (set point) temperature + 5C and then do a pump overrun and anticycling time before refiring.

 

[Jamm89]

It is a bit unpredictable when it happens. The boiler was fine last night with no cycling occuring. This morning started doing it after about an hour.
It's worth nothing that I got the previously not working towel rail working year by releasing the airlock. The flow rate coming out of the flow side was a fair bit less than compared to the return side.
All rads have trvs fitted but set to maximum so they shouldn't be shutting down? 2 off towel rails one is throttled for balance and one is fully online.
Don't know if any of this extra information helps. Checking the manual the boiler has a min CH output of 5.7kW non condensing and 6.2kW condensing.
Thanks

 

[John.g]

With all TRVs full open then the heating demand will be far higher than 5.7/6.2KW (am assuming that all rads are hot from top to bottom) so no problem regarding that and the boiler flow temperature should not exceed its setpoint by 5C. Some boilers also stop the burner if the dT between the flow/return temperatures are greater than 30c but would expect a flow fault alarm if this occurs, the boiler schematics don't seem to show a return temperature sensor.

Can only suggest that if the cycling persists to get the boiler serviced and all sensors tested.

 

[Jamm89]

Thanks for the responses. My system has been particularly troublesome to balance perfectly. I have a big double panel vertical rad in the kitchen that I struggle to get above ~45c.
Will get the thing serviced and see where we go.
Any recommendations of boiler engineers in the Hinckley region of Leicestershire?
Cheers.