Combi boiler ... losing pressure ?

[Jethro]

Merry Christmas to all !!!!!

Just noticed the last few weeks that the dial with the pressure for the CH on our Vaillant Turbomax boiler is constantly dropping. I have to keep on opening the little valve the installer showed me to get the pressure back to between 1 and 2 bar. It seems low every day now.

Is this evidence of a leak somewhere in the radiator circuit (piping was all brand new 10 years ago) ? Looking above ground I can't see any signs of water - let alone enough to account for the amount of water I am putting *in* to get the pressure back up.

Do plumbers have a magic wand to locate underfloor leaks ?

Alternatively, could it be a fault in the boiler ? Maybe the valve I am using to top up the circuit is actually staying slightly open, and water is flowing back ?

As always, answers gratefully received. In the meantime, don't overdo it on the sherry !

[Tim Lamb]

In message , Jethro writes

IANAP and there will be an expert along shortly:-)

Perhaps your boiler system air reservoir has failed or needs pumping up? I think previous explanations in here suggest checking for system water being ejected through the pressure relief piping, perhaps by tying a plastic bag over the outlet.

regards

[Jethro]

Having been googling for a while now, it seems worth eliminating the relief valve, before pulling up floorboards. This excellent guide, by Ed Sirett

seems to say the relief valve should have a test button. However, I can't seem to find a valve matching that description - and the only pipe going to the outside of the property comes off a radiator the other side of the house from the boiler ...

Or do I need to look again ...

[Tim Lamb]

Oh dear! Pretty close to the end of my knowledge.

The pressure vessel could be part of the boiler or separate (airing cupboard etc.)

If the pipe you have found comes from the bottom of the radiator then this is likely to be for draining the system and nothing to do with over pressure.

Does the boiler manual give any clues. I thought pressure relief piping had to be obvious like overflow pipes that drip on your head.

I'll bet all the real experts are watching TV through half empty glasses.

regards

[Jethro]

Following up from this, I *have* found the relief valve ... it had been beautifully taken through the back of the boiler, and through the wall, and unobtrusively routed round our bin store, to terminate almost invisibly against the outside wall, below the threshold of the bin store door. You really wouldn't notice it (as indeed I didn't) unless you were looking for it.

And it's wet, and there's a small damp patch on the paving below it. Ordinarily that in itself should have alerted me ...

So it looks like the relief valve needs changing. I wonder if that is covered by our boiler insurance. We shall discover Tuesday ....

[Jethro]

Hi Tim,

thanks for your advice. I have found the PR valve outlet now (see above) and it *is* leaking. So at least my nightmare scenario of a rad pipe leaking under the floorboards is eliminated (or, strictly, less likely :) )

Anyway, Merry Xmas !

[gremlin_95]

Not a hard job to do :) There is a reason why it's leaking though and that's probably because the expansion vessel has no air left in it on the air side or the diaphragm in it could be split! If it is split, it's usually not worth the cost and effort required to replace the internal one within the boiler, so an external one is fitted somewhere on the system.

[Roger Mills]

Have you located your expansion vessel? It is likely to look something like:

unless you have a system boiler - in which case it might be *inside* the boiler casing, and might be a flatter (pancake-ish) shape.

The filling loop, pressure gauge and relief valve are likely to be very close to the pressure vessel. One side of the relief valve will be connected to the same bit of pipe as the pressure gauge - and the pipe on the other side should go the the outside world to allow water to be safely let out if the pressure rises too much. So you need to identify this pipe, and follow it to see where it goes.

The expansion vessel contains a diaphragm with air on one side and water on the other. The air pressure is supposed to be pre-charged to about

0.7bar (10psi). When the system gets hot, the water expands - and some of it enters the vessel, compressing the air a bit more. The air provides resilience, allowing the water volume to increase without excessive pressure rise. If the vessel loses its air - so that it effective starts off full of water - it can't provide any resilience with the result that the pressure rises until the relief valve opens and lets some water out. When the system cools, the pressure falls to zero - or a very low value. So you top it up, and the whole thing repeats!

I would put my money on that being your problem.

[John Rumm]

Watch the system pressure for a bit as the system warms up. You may find that its rising too much (i.e. 3 bar or more) and the relief vale is just doing what it is supposed to.

(typical cause of excess pressure rise is a failed or empty pressure vessel. A quick hack to get you through Christmas (assuming pumping it back up either does not fix it for whatever reason), is to half drain a rad in a room you can tolerate being a bit chilly. The pocket of air at the top will do the job for now. ;-)

Happy Christmas anyway!

[Jethro]

Thanks for all the replies - hope you all had a merry Xmas !!!!

Have now had a chance to have the cover off etc ... I've "splurged" the Pressure relief valve - hopefully if there is any crud there it's gone.

I want to check the expansion vessel. Careful reading of the wonderful instructions left with me by the fitter show it looks like a cymbal, tucked right at the back of the boiler. I have tried to reach it to touch it, but can only just get my fingers onto it. ISTM there is no no way to access the valve to test for water in the thing (or, to re-gas it, if necessary).

Is there a trick, or is it simply so unlikely for it to fail, there is no way to access it in situ ?

Having read the wonderful Ed Sirett guide, it seems we have a plan "B", by leaving a radiator with air in it. And the Mrs has already suggested our bedroom radiator, because for some reason, it's one room we never need to heat - even in the depths of last winter.

If it is that unlikely to be the expansion vessel, is it worth simply changing the pressure relief valve ? That seems a very straightforward job.

[Roger Mills]

It's far more likely to be the expansion vessel than the PRV. The fact that it's inaccessible is no guarantee that it's unlikely to fail! If it

*has* failed (as opposed to just needing to be re-charged) it's likely to be easier to fit an external one elsewhere in the system rather than trying to replace one inside the boiler.

[Jethro]

Well, a little wobbling about on a ladder, and dipping into the dark history I have as a mechanic, and I managed to locate the cap to the expansion vessel - which clearly has *never* been removed (so much for the service the boiler plan provides). I managed to take the cap off, and squeeze the valve ... just a little air came out - no water.

Thus encouraged, I drained down the heating circuit, opened a bleed valve, and used my digital tyre inflater to push the pressure to 0.85 bar (service manual recommends 0.7-0.9 bar). I then closed bleed valve. Topped up pressure to 1.2 bar, and fired the heating up. When things were nice and hot I gave the PRV a blast, just to blow any crud out ...

Heating has now run - got the house *toasty* warm (typically it's ludicrously mild for the time of year), and 4 hours later, pressure is still at 1.2 bar ...

Hopefully I can report tomorrow evening things are still 1.2 bar - problem solved.

Once again thanks to all - happy new year !

[Jethro]

UPDATE:

Managed to locate and repressurize the expansion vessel (on boxing day). Pressure has remained up since, so result - thanks to all who replied.

Can you use the specific gravity of the water to determine if there's enough inhibitor. I completely changed it in 2010, but can't tell how much was lost by the pressure blowing.

[MuddyMike]

Just run off a jar full, pop a clean steel nail in it, do the same with a second jar but use plain tap water. You should find the nail in tap water rusts and the one in heating water doesn't. If both rust you need more inhibitor.

Mike

[Roger Mills]

And if neither rusts, they were probably stainless steel nails.