Combi boiler constant leaking from overflow pipe when heating on

nikkilou

My Vaillant EcoTec Plus combi boiler is streaming water from the overflow pipe when I turn my heating on, to the point where the pressure drops right down and I have to top up the system again.  It's fine when the heating is off
From reading other threads, I can see this is possibly a faulty pressure relief valve or expansion vessel.  My question is, is there a way of knowing/testing which of these it could be that's faulty?  Also are there any other common causes of this issue it could be that would be simple to detect/confirm?
The boiler is 13 years old so I would not be surprised if there were faulty parts

Le_Kirk

It is quite common for the pressure relief valve to stick open after it has relieved pressure, it only takes the smallest piece of grit to cause this.  Two solutions: -
1.  Replace the faulty valve (after having determined why it was operated)
2.  Remove the valve from the boiler and try to clean it by blowing through it, again after having determined why it was operated
Only try this is you are a competent person.

nikkilou

Thanks Le_Kirk.  We just replaced the water inlet valve because that was faulty (pressure was rising constantly and we had to keep bleeding the radiators to relieve it).  We replaced the faulty valve, flushed and refilled the system, added inhibitor and now this issue, so what you suggest makes sense as this would have been triggering the pressure relief valve.  Thanks

neilmcl

nikkilou said:

Thanks Le_Kirk.  We just replaced the water inlet valve because that was faulty (pressure was rising constantly and we had to keep bleeding the radiators to relieve it).  We replaced the faulty valve, flushed and refilled the system, added inhibitor and now this issue, so what you suggest makes sense as this would have been triggering the pressure relief valve.  Thanks

My dad went through all this earlier in the year with his Worcester Bosch boiler. Initially an inlet/filling loop fault went undiagnosed and so it was only the PRV that was replaced but that didn't fix the issue of the pressure constantly going up and although that's been fixed he's still getting an odd drip from the new PRV. Once they go they rarely reseat properly.

nikkilou

Thanks neilmcl - if it is the PRV they seem cheap enough to replace, not sure how tricky the job is?  I just wasn't sure if there was an easy way to tell which of the parts was faulty without going to the expense and effort of actually replacing them one by one. 

Jeepers_Creepers

Hi Nikki.

The fact that your pressure doesn't drop when the boiler is off might suggest that the pressure release valve is actually ok - it is seemingly seating properly when the system pressure is 'normal'. That also suggests that your system pressure increases significantly when your heating is on, and it's this wot's operating your PRV to release water. 

This boiler has a pressure gauge, analogue or digital; that's the thing to keep an eye on when you turn on your CH. When you top it up, I presume it's to around 1bar? Cool. Now keep an eye on that pressure when you turn on your heating - does it increase, and - if so - by how much? At what pressure does water start to 'stream' out yer pipe? (Normal safety release pressure is around 3bar).

If you could answer these Qs, we should be able to narrow down the cause - as you say, it could just be the PRV (~£30 plus a half-hour fitting) or the exp vessel (~£80? plus an hour fit). A cheap way of sorting the exp vessel (if you are presented with a large quote and don't wish to spend this on an 'old' boiler) is to fit a remote EV somewhere else on the system. But you need to find space for a big red ball.

neilmcl

nikkilou said:

Thanks neilmcl - if it is the PRV they seem cheap enough to replace, not sure how tricky the job is?  I just wasn't sure if there was an easy way to tell which of the parts was faulty without going to the expense and effort of actually replacing them one by one. 

Depends where it is. For some reason Worcester Bosch in their device wisdom chose to put their PRVs in one of the most inaccessible places on the rear of a lot their combi boilers which meant taking the whole thing off the wall in a lot of cases.

nikkilou

@Jeepers_Creepers.  Thanks.  Cold boiler pressure started at 1.1.  Turned on the heating and within 12 mins it had gotten to 3.2 and the PRV was dripping quite a lot.  Turned the heating off and the pressure returned to normal within the hour.  From what you are saying, it doesn't sound like the PRV is the fault? 

neilmcl

nikkilou said:

@Jeepers_Creepers.  Thanks.  Cold boiler pressure started at 1.1.  Turned on the heating and within 12 mins it had gotten to 3.2 and the PRV was dripping quite a lot.  Turned the heating off and the pressure returned to normal within the hour.  From what you are saying, it doesn't sound like the PRV is the fault? 

Possibly your expansion vessel then.

https://youtu.be/SNULc5fhY28

bris

It's the expansion vessel. Have it recharged or replaced.

Jeepers_Creepers

Yes, that's pretty convincing, Nikki! 

The pressure should ideally only raise a bit, if anything at all. I've only ever seen one 'normal' boiler where it regularly went above 2bar, and this was apparently ok for this particular Potterton. Most boilers would only experience a small increase if it's set up properly - 0.2 of a bar, something like that.

As said above, a plumber would check the existing EV and see if it's just slowly lost pressure over time - that's quite common. If it happened suddenly, that usually suggest a ruptured (the rubber diaphragm inside) EV. 

That vid above is pretty good. If you are happy taking the boiler casing off, and especially if you have a good boiler manual supplied, you can carry out that test; undo the schrader valve cap and briefly press the pin in t'middle. If any water comes out, then the diaphragm is gone and you need a new EV. If nothing comes out, then it might respond to a recharge - a pump-up to usually around 0.75bar. Bear in mind that technically, it should only be air-pressurised when the water side (your system) is at zero pressure - ie with say a bleed screw open - or else you'll be getting a somewhat false reading (the pressure of the water on the other side will make the air side seem higher.)

In practice, however, it's a fairly crude device and, provided it isn't letting out water, you can stick a hand or foot pump on there and give it a good half-to full dozen strokes. Keep testing the pressure, and stop before it reaches 1 bar. Then try your CH on and see how it behaves.

If no water at all comes out your discharge pipe when the boiler sits at the cold 1 bar, then you might be lucky and not have to replace the PRV.