Combi boiler pressure issue

[mark6789]

Hi, hoping for some advice on problems I’ve had with a combi boiler.

I have a Vaillant ecotec plus 831 serving 4 radiators and 1 towel rad plus water to a bathroom and small kitchen.

Only recently bought the house and there were signs of leaking at all rad thermostatic valves and the stains were brownish. There were also historic brown stains inside the boiler and on the shelf below it (they looked like treacle). I had a initial problem with it not working at all which a plumber solved by changing the programmer, albeit only two rads plus hot water then worked.

Had a different plumber round yesterday to change all the valves, add a magnetic filter and flush the system. I’m not sure how much flushing was actually done but he obviously drained and refilled and everything was fine for a couple of hours i.e. both hot water and all rads were working.

Then it stopped. Boiler showed F75 fault and boiler pressure gauge was at ‘0’ (its one of the small gauges that doesn’t have numbers, just a small disk with colour coded sections). After speaking to the plumber on the phone I tried filling up the system by opening the taps underneath and could hear water flowing into the system. After at least 4 minutes of this the needle eventually started to move very slowly up but still only 1/2 way up the red section. At this point black/brown water started flowing through the tundish.

Really hoping someone may have some ideas of what to try next! Thanks in advance.

 

[Stigster]

The pressure relief valves are famously finicky and when systems are dirty and sludged up they often fail to seat properly.

My suspicion is that the expansion vessel has lost its charge or the diaphragm has failed and when the heating was on there was nowhere for the heated water to expand, setting off the pressure relief valve and dropping the pressure to zero. Gunk has got into the pressure relief valve seat and prevented it from closing properly.

When you are filling the system via the filling loop, the water is passing through the pressure relief.

Do you know if the system was powerflushed or was it just drained and refilled?

 

[mark6789]

The pressure relief valves are famously finicky and when systems are dirty and sludged up they often fail to seat properly.

My suspicion is that the expansion vessel has lost its charge or the diaphragm has failed and when the heating was on there was nowhere for the heated water to expand, setting off the pressure relief valve and dropping the pressure to zero. Gunk has got into the pressure relief valve seat and prevented it from closing properly.

When you are filling the system via the filling loop, the water is passing through the pressure relief.

Do you know if the system was powerflushed or was it just drained and refilled?

Pretty sure it was just drained and refilled and not powerflushed.

I've now managed to get the boiler to fire up with the analog gauge only halfway up the red zone. The boiler digital display is showing 2.8 bar though so there is obviously an issue with a faulty pressure gauge. I presume it is the digital one that is faulty and that must be the one triggering the pressure relief valve.

 

[oz-plumber]

From what you just posted, I would assume the digital gauge is correct and the relief valve is passing due to it being set at 3 Bar.
The expansion tank has probably lost its charge and the relief valve is probably knackered.
Could be worth the effort to have someone check the expansion tank pressure and replace it if need be. At the same time, replace the relief valve

 

[Last Plumber]

It would be unusual for both those gauges to be out but not impossible.

You need to be Gas Safe registered to work on Gas appliances. The issues you are discussing are a simple fix for an experienced Heating Engineer. You can use the Gas Safe website to find someone close by but recommendations are always better if you can get one.

 

[Scott_D]

It sounds like the boiler is in a sorry state
Have the plumbers been gas safe registered and checked the boiler is safe to use?

 

[mark6789]

It sounds like the boiler is in a sorry state
Have the plumbers been gas safe registered and checked the boiler is safe to use?

Yes, of course they are gas safe registered.

The boiler is not that old and looks very well maintained other than the pressure problem. It’s now been running all day with no problem. I now have the plumber booked in to powerflush the system, replace the pressure release valve and the pressure gauge. Hopefully that will be the end of the problems.

Thanks for all the helpful replies!

 

[Scott_D]

Yes, of course they are gas safe registered.

The boiler is not that old and looks very well maintained other than the pressure problem. It’s now been running all day with no problem. I now have the plumber booked in to powerflush the system, replace the pressure release valve and the pressure gauge. Hopefully that will be the end of the problems.

Thanks for all the helpful replies!

The treacle in the boiler, blocked pressure gauge, brown water and leaking valves suggest it was poorly fitted to a dirty system.
If it’s 4 years old is it not under warranty?