Vaillant EcoTec Plus 837 – F.20 Error / Temperature Spike on Partial Load

[Lucyspirit]

Hi everyone, looking for some advice on a stubborn Vaillant EcoTec 837 issue.

The History:A few weeks ago, I was getting constant low pressure and F.75 errors. My regular, reliable engineer found a leaking diverter valve. While fixing it, he replaced the diverter valve, pressure sensor, and expansion vessel.

The F.75 stopped, but then I intermittently got F.83, F.22, and finally a persistent F.20 (overheating) error.

Recent Repairs:To fix the F.20, the engineer has since replaced the complete pump (with auto air vent) and the pressure relief valve. He also removed and cleaned the plate heat exchanger (confirmed it was clear) and tested both NTC thermistors with a multimeter (both tested fine).

The Current Symptoms:

• In "Auto" mode: The boiler runs perfectly with no errors.
• In "Chimney Sweep" mode: The boiler runs perfectly, and the temperature climbs steadily.
• On "Partial Load" (manually locked to 14kW): With all radiator TRVs fully open, the temperature climbs steadily to around 60°C, then suddenly rockets up to 97°C and trips the F.20 error.

My engineer is honest and has looked after my system for years, but he’s currently stumped. Given that the pump, diverter, expansion vessel, sensor, and AAV are all brand new, what could cause the temperature to spike erratically only under manual partial load?

 

[CountryFan]

How many radiators do you have and what size/type are they? Auto i believe tries to work out your KW rating

 

[Lucyspirit]

3 large column radiators. 3 doubles and 1 smaller one. The engineer had put it down to 14kw 10 years ago. I can’t remember how he worked it out but said the size etc of your radiators you only need 12-14kw.

 

[CountryFan]

Obviously depending on the actual size, that would seem ok, buy perhaps it's struggling. Maybe change it to 16Kw? Maybe get him to speak to Vaillant technical? If anything it should overheat in chimney sweep.

 

[Lucyspirit]

I should add. Boiler is about 10 years old. It has only needed a replacement leaking diverter valve about 4 years ago. Has worked perfect since then and before then.

 

[Lucyspirit]

Change it from auto to 16kw?

The engineer did try the 14kw again but it was giving the temperature spike issues.

 

[CountryFan]

Hmm, does seem odd. That said, one of our engineers had a new diverter with a faulty motor, maybe get him to look at that?

 

[CountryFan]

Change it from auto to 16kw?

Worth a punt

The engineer did try the 14kw again but it was giving the temperature spike issues.

It is odd, see reply above regarding motor. It could be a flaky pcb.

 

[Lucyspirit]

He did loads of stuff in three hours. The motor from the leaking diverter valve. He tried both motors and I could see the pin moving up and down and he tried both but the issue still persisted. Only fixed in auto mode but the underlying issue is still present.

 

[Lucyspirit]

I will message the engineer and see when he can pop round.