Worcester Danesmoor 20/25 + Nest on gravity hot water system – burner lockout only in winter, could Nest wiring be wrong?

[Sam94]

I'm hoping someone familiar with older Worcester oil boilers and gravity hot water systems can point me in the right direction.

System:

• Worcester Danesmoor 20/25 oil boiler with Bentone burner.
• Bentone TF832.3 control box (the burner lockout reset button is on this unit).
• Old gravity hot water system.
• Nest fitted soon after I moved in, replacing/bypassing the original Worcester programmer.
• Hot water and heating are both controlled by Nest.

Symptoms:

• Boiler runs perfectly all summer when only hot water is needed.
• Problem only occurs during the heating season.
• Eventually the burner goes into lockout and the red lockout light comes on.
• To get it going again I have to press the Bentone reset button.
• If I reset it immediately it often locks out again.
• If I leave it to cool down first, the reset usually works straight away.
• At the time of failure the radiators and hot water are often extremely hot.

Work already done:

• Boiler serviced multiple times.
• System powerflushed.
• New circulation pump fitted.
• Photocell replaced.
• Nozzle replaced.
• Solenoid coil replaced (more than once).
• Oil filter, oil line and fire valve replaced.

None of the above made any difference.

Important detail I recently found in the Worcester manual:

The manual says that on a gravity hot water system the programmer must be set to "G" because the system will only function correctly on heating if the hot water programme is also on at the same time. It says fully independent CH and HW programming is only for fully pumped systems.

This made me wonder whether the Nest installation may be treating the system as if it were a modern fully pumped setup.

Questions:

1. Could a Nest installation on a gravity hot water Danesmoor cause this type of burner lockout behaviour?
2. Has anyone seen problems caused by replacing the original gravity-system programmer logic with Nest?
3. Does the fact that the burner only resets reliably after cooling point more towards a burner fault (control box, ignition transformer, electrodes, etc.) rather than a controls issue?
4. Would it be worth testing with hot water permanently enabled for a few days to see if the problem disappears?

Any thoughts appreciated because several parts have already been replaced and I'm not convinced the root cause has been found.

Thanks.

 

[CountryFan]

It seems more a heating problem rather than a control issue, especially it being ok after it cooling down. @Exedon might be familiar with that boiler.