Vaillant Ecotec boiler losing pressure intermittently

BillG100 · 10 December 2017 at 6:32PM

Hi,

My Vaillant Ecotec boiler is losing pressure intermittently but quickly and only when it is switched off! What happens is...

1) The boiler runs fine for a period of time which could be a few days or weeks and the pressure is reasonably constant (varies about 10% to 15% with the temperature)

2) Boiler loses pressure quickly during the overnight period when heating / water is off

3) Boiler won't restart and an f.75 error code is displayed

4) I add some water to re-pressurise and it starts again without a problem

5) The cycle (steps 1 to 4) then repeat

The expansion vessel has been re-pressurised. There's no sign of a leak and it would have to be an odd one to behave in this way. The pressure gauge reading appears to be correct.

I'm at a loss. Any ideas?

Alex1983 · 10 December 2017 at 6:37PM

Repressurise the boiler and shut the central heating flow and return valves off and leave it for 2 days, if the pressure goes then the boiler is the source of the leak if it doesn’t then you can slowly open the valves and see if the pressure goes straight away, if this happens the the leak is on the heating system.

BillG100 · 10 December 2017 at 7:33PM

Alex1983 wrote: »

Repressurise the boiler and shut the central heating flow and return valves off and leave it for 2 days, if the pressure goes then the boiler is the source of the leak if it doesn’t then you can slowly open the valves and see if the pressure goes straight away, if this happens the the leak is on the heating system.

Thanks but sometimes the boiler runs and maintains pressure for several weeks. How would shutting off the valves for 2 days prove anything? It's also odd that the boiler only appears to de-pressurises when it is off. For example it shows 2bar at 11pm when I go to bed and the next morning it is "flat" and an F.75 error code is displayed.

Ruski · 10 December 2017 at 7:44PM

Is the filling loop connected? It could be letting by and then the PRV would activate at 3bar.... I doubt it would 'empty' the system, but it a scenario worth considering and testing for.

HTH

Russ

Alex1983 · 10 December 2017 at 7:56PM

It doesn’t matter if it’s on or off, it’s a sealed system if your loosing pressure it has a leak. Weather that’s the PRV dripping, a leak under the floor or on the system or the heat exchanger leaking internally. If you don’t want to isolate it temporary then the only other thing you can do is tape a sandwich bag over the PRV pipe outside and wait for it to loose pressure then see if there is any water in the bag.

BillG100 · 11 December 2017 at 11:39PM

If the heat exchanger is leaking internally how can it be diagnosed?

Alex1983 · 11 December 2017 at 11:45PM

Not easily, it will go down the condense pipework. Leave it off for a while and see how much water comes out of the pipework, if a constant drip comes out then that’s the likely location of the leak.

BillG100 · 19 December 2017 at 8:42AM

Still having the same problem. I've stuck a webcam on the boiler pressure gauges (analogue and digital) and record 24x7. Basically what happens is the boiler starts losing pressure as soon as it switches off at night. It falls below the threshold level and can't restart. It displays an F.75 error code. I put some water in and get it going again without problems.

Diagnosis:
1) Three British Gas engineers. They ended-up pointing the finger towards the underfloor heating system

2) Underfloor heating installers came back and took a look. Said all was good BUT to prove it they suggested taking it off the system (with the out and return valves closed off and programmer off so pump wouldn't be going). I have done this three times and the boiler still loses pressure

3) The rest of the pipework to the few rads we have is visible (nothing under concrete ets) and no damp patches or evidence of them anywhere

I'm not a heating engineer or plumber but to me the evidence points to a fault with the boiler. The last BG engineer said the condensate pipe was dry so it can't be a heat exchanger issue. However the Vaillant ecoTEC manual says....

"The ecoTEC plus appliances are fitted with a siphonic trap (The filling height is 145mm). The siphon collects the accumulated condensate in a container of approx 200ml capacity and directs the entire content into the drain pipe. The risk of the condensate drain pipe freezing is thus minimised"

...to me that suggests it is perfectly possible that the condensate pipe was dry when the engineer checked it because the water is stored before being purged from the system.

I've got another BG engineer coming round to investigate further. Any suggestions on how I should take things forward?

daveyjp · 19 December 2017 at 8:57AM

You may be better calling Vaillant and getting one of their engineers to visit.

Alex1983 · 19 December 2017 at 9:27AM

Do what I said in post 2 and you will prove where the problem is without all the hassle your having now.

BillG100 · 28 December 2017 at 3:45PM

Alex1983 wrote: »

Do what I said in post 2 and you will prove where the problem is without all the hassle your having now.

1) British Gas put a new heat exchange in and it made no difference
2) The underfloor heating was isolated and it made no difference
3) There's no sign of a leak on the old rads / pipework. I've tried wrapping tissue around the valves, probing the walls with a decent quality moisture detector and using an endoscope on stretches of the pipes not visible. Nothing!

I will now do your post 2 suggestion (haven't yet because there are others in the house!) although since the heat exchanger has been replaced and the PRV is ok I doubt it can be the boiler.

Sorry if this is a dumb question but are the central heating flow and return valves on the boiler? If not how do I find / identify them?

Vaillant Ecotec boiler losing pressure intermittently

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