Boiler losing pressure every day, replaced PRV, Gauge not showing high pressure

[a546345]

Boiler is a British Gas (Bosch) 537i which is about 10 years old. We have 14 radiators. There is an auto bypass valve fitted.

The pressure gauge reduces from 1 bar to 0 over about a day. During this time water is released from the PRV discharge pipe, but only 100 to 200ml.

It then takes only about 10 or 15 seconds to repressurise using the filling screw on the boiler turned about 1/4 turn.

During all this time, the pressure gauge reading does not increase abnormally other than a 1/4 bar or so as the temperature rises. It does not get anywhere near 3 bar. I have a CCTV camera pointed at it at the moment so I can check what the gauge has been doing.

If I deliberately fill the system up to and indicated 3 bar on the gauge then the PRV releases and a lot of water comes out until it closes again.

I think the expansion vessel is ok. It had plenty of air in it. I emptied it and plenty of air came out. Some water also came out but only about 50ml or so. I then repressurised it up to 0.7 bar with the system drained and the drain tap open. If this was faulty I'd expect the pressure gauge to rise?

We had someone in to replace the PRV (a pain as the whole hydraulic block needed to come out), but that has made no difference.

Any ideas what could be causing the PRV to let by, but not register high pressure on the gauge, and only let out a small amount of water?

Cheers.

 

[terryplumb]

If the prv is allowing water out of the system ,when system pressure is much less than 2.5 bar ,then its faulty. Possibly just not closing fully and allowing a small amount of water to trickle out.

 

[a546345]

The PRV is letting water out when the system gauge is showing less than 2.5 bar. The PRV has been replaced and the new one is doing the same.

I'm wondering whether there is fault condition that could allow the pressure behind the PRV to build up, yet the gauge still read normally.

Or whether the gauge could be partially faulty, yet working enough to show pressure when I fill the system.

 

[Daveydub]

If you want to confirm that the pressure gauge is not lying, you could screw a gauge in to any convenient place, remove plug or vent from radiator, then fill to 1 bar & compare gauges
Alternatively using the gear you already have, vessel is filled to 0.7 bar with system unpressured, so if you fill system to 1 bar, air in vessel should also now be 1 bar

 

[a546345]

Thanks both for the replies. I'll check the vessel pressure.

 

[andytw]

While checking pressure vessel it would be worthwhile checking any hose that connects it to the boiler is not blocked.
I had a similar problem with neighbour's boiler. Changed expansion vessel and PRV to no avail, but found the flexi hose was clogged solid.
New hose was required (and system clean and Magnaclean).

 

[a546345]

I've tested this now.

Gauge pressure was at 0 this morning. The pressure in the expansion vessel was 1 bar using a tyre pressure gauge. I must have over filled it slightly a few days ago as it should be 0.7 bar... Unless the system pressure was actually 1 bar.

I opened the boiler filling screw a little and pressured to 1.5 bar on the boiler gauge. During filling, I continued to measure the pressure in the vessel. As soon as the boiler gauge pressure increased past 1 bar, the expansion vessel pressure tracked it.

The filling took around 20 seconds. At the end, both boiler gauge and tyre gauge on the expansion vessel read 1.5 bar.

Also the expansion vessel is connected with a copper pipe in the boiler, not flexi hose.

Other information - the discharge water is clear. We had a Maganclean installed about 3 years ago.

Is the next step is to plug in another gauge to confirm that the boiler gauge is correct over time, and that 0 bar was really 0 bar?

As a crude measure perhaps I could just undo the bleed valve a turn in an upstairs radiator now with the gauge reading 1.5 bar, and see the flow rate. And then repeat again tomorrow with it reading 0 bar. If it is really at 0 bar, I should only get a dribble out tomorrow.

What are the odds of the replacement PRV being faulty?

 

[Daveydub]

You have checked the gauge so it isn't lying
It could be a bit of muck on the seat of the valve so if you fully open it it may wash off
Let it snap shut as well rather than slowly shut.
Only a vague chance of this working but is always worth trying

 

[a546345]

Yes I think the gauge is accurate. The flow from a radiator bleed valve is much higher when the gauge is higher.

If the boiler is on, but all radiators are closed, and the auto bypass is faulty, would that cause this problem?

I've taken the TRV off the hall radiator for now so that there is always a flow through.

I tried snapping the PRV shut but made no difference.

The pressure gauge never gets near 3 bar though.

 

[a546345]

There is only about 20ml difference between 0 bar and 1 bar pressure. About 20ml comes out of the PRV, and I add 20ml back in.

Does this still sound like the new PRV is faulty?

 

[terryplumb]

See my post of Sunday 12.18. If water is coming out the prv pipework its letting by when it shouldn't . I think you MAY also have a slight leak somewhere adding to the pressure drop.

 

[a546345]

Back with an update. The water is not leaking from the PRV - the leak was actually coming from a corner join in the pipe above it which is the condensate drain. The icicle from it this morning was the giveaway!

In the mean time, I have done tests with the heating circuit to the house shut off completely so that all flow was through the bypass valve. The pressure on the gauge still reduced slowly over a period of a few hours.

I can't see any water leaking in the boiler or boiler area. Where is it going? Up the flue?

If the condensate pipe was blocked would that cause this problem?

Any ideas much appreciated.

 

[terryplumb]

You either have a leak on your pipework / rads ,or within the boiler Internally. To determine which...turn central heating off and allow two hours to cool. Isolate boiler electrically. Pressure to ,1.5 bar. Then close FLOW and RETURN valves at boiler. Leave overnight then check pressure gauge on boiler. If pressure has dropped the problems in the boiler. If pressure has not dropped ,open the flow and return valves. If it then drops its a system leak. Tie a bag over prv pipe outside to rule out any loss via prv.
If pressure is lost within boiler ,assuming no water visible within it ,it could well be the HEX.

 

[a546345]

Thanks Terry, I've tried some of the tests you describe. I'll wait until the weather warms slightly and try the full test and report back.

If it is the heat exchanger - I assume you're referring to the primary heat exchanger? Is that something worth repairing, or is it new boiler time?

 

[terryplumb]

With all due respect , you misled yourself with the prv and wasted money having it changed ,and the info you posted here led to anyone reading it drawing the wrong conclusions .you don't want to risk wasting More money ,the main Hex is an expensive item to change ,( and I personally wouldnt on a ten year old boiler). My advice to you is to follow my instructions above when boiler / heating is COLD ,to at least determine if it is within the boiler. Engage a good gsr engineer to diagnose on site if it turns out to be so.
Regards terry