Vaillant F75 - pump or sensor?

[Daljit]

Hi

Yesterday morning awoke to a cold house with Vaillant Ecotec Plus (fitted 2012) showing f75 code. After adding a little water to the system, switching on/off, it started working again. However, same happed this morning although took a few hours before it started working again.

Having read online, it seems issue is most likely pump on last legs or faulty pressure sensor. My question is how can I find out which?

If it helps, this morning when it wouldn’t start after switching on/off, the pump didn’t run at all when the boiler was resetting.

Cheers.

 

[Johntheo5]

Is that your old EcoTec Plus 831?.

When (if) the circ pump starts then you should see the pressure increasing by 0.2/0.3bar, also ensure the pressure is ~ 1.3/1.5bar, cold.

 

[Daljit]

Is that your old EcoTec Plus 831?.

When (if) the circ pump starts then you should see the pressure increasing by 0.2/0.3bar, also ensure the pressure is ~ 1.3/1.5bar, cold.

Yes! It’s still the 831.

I noticed pressure did jump up when it started working, as had to drain the system a little (an hour after adding pressure) to get it back into green zone.

Altho, that doesn’t tell me if it’s the pump on last legs or sensor does it?

 

[Johntheo5]

No, but if it jumps up when you press the start button or very shortly afterwards then that would point to the pump actually starting, don't know what increase the boiler needs to restart but would have thought ~ 0.2bar or so.

 

[muggles]

Could be the pump, could be the sensor, could be blocked rubber hoses, could be a blocked canoe filter, could be an incorrectly set expansion vessel. All of these are known causes of F75.

 

[CountryFan]

Could be the pump, could be the sensor, could be blocked rubber hoses, could be a blocked canoe filter, could be an incorrectly set expansion vessel. All of these are known causes of F75.

Agreed. Best getting an engineer in rather than guesswork

 

[Daljit]

I replaced the seals on a leaking magnaclean about 3 mths ago, so possibly some debris in the system. As heating working fine for last 6 hrs, during which there’s been regular demand for HW, I think it’s less likely to be pump. Had to drain system a tiny bit this pm, removing what I added this morning, same as y’day.

Replacing sensor is relatively cheap and simple job, and local p merchant has original part in stock so will do tmrw morning.

Will call engineer if that doesn’t fix it.