What might cause pressure to rise when boiler not running?

[Lemoncurd]

We've had a few repair people out to look at our 3 yr old Potterton Performa 28i but they haven't managed to fix it.

When we bought the house six months ago we switched the boiler off and there was a leak at the back left of the boiler coming from high up in the boiler unit. About 1/2 a litre came out over a couple of days and then stopped. There were no problems when the boiler was running so we were advised that it was probably just somethign contracting when cold and not to worry about it as it was fine when the boiler was switched on.

The pressure was quite low for a few months but when I found out how to top it up I topped it up to 1.5 bar. It was fine for about a month until one evening I noticed a few drips beneath the filler loop (although we thought the source was higher up above the filler loop). At the same time the pressure started to rise and was staying just above 2.5 bar.

Repair man 1 came along about 10 days ago, he said the filler loop wasn't closing properly and replaced it. He also said that the expansion vessel might need replacing as he had pumped it up but it had already gone down in the short time he had been there.
The pressure went back up over 3 and the PRV was dripping constantly, I noticed that it seemed to rise most in the daytime when there was no heating on and no hot water being heated (turned preheat off and stopped using the hot taps). It would drop down a bit for a few hours after each time the heating came on.

Today repair man 2 (from a different company) said that the expansion vessel had been OVERinflated and this was why the pressure was high because the expansion vessel was too inflated to do its job. He checked the filler loop and this wasn't letting anything through. He serviced the boiler and said that there was absolutely nothing wrong with it.
When he left the pressure was 2 bar and he said this was just because he had just been running the boiler and that it would go down when it cooled down.
2 hours later, the boiler hadn't run at all and the pressure was reading 3.25 and it has been staying around that level since. Why might this be happening?

 

[lor]

fill loop not shut properly or plate heat exchanger has a hole in it .
disconnect the loop to check if it is the later

 

[Agile]

Its a very rare fault for the plate heat exchanger to leak ( except on one of the Worcesters ).

I only see one about every year.

To check make sure you take the filling hose off to ensure that is not leaking and see if it still rises.

If it does then close the cold water inlet to the boiler and open a hot tap. If its the plate leaking then the pressure should fall !

Tony

 

[croydoncorgi]

Based on what you say has already been done / part changed, I'd say it was a racing certainty that the secondary heat exchanger (water / water, producing the hot water output from the boiler) has failed.

The reason for the high pressure is that mains water is forcing its way into the primary side of the HX.

This is a Very Bad Thing because there will be water running out of the pressure relief valve in the boiler and down the discharge pipe to the outside wall. (It may be the connection to the discharge pipe that's leaking.)

To fix this, the secondary HX must be replaced and also the pressure relief valve, that once having lifted will tend to leak forever.

To confirm that this is the cause of the pressure rise, turn off the heating and turn OFF the cold water supply to the boiler (valve on the 4th pipe from the left, underneath the boiler, then open the air vent on a nearby radiator and release enough water to take the pressure down to 1 Bar.
Do NOT open any hot taps. Turn the heating back on and leave it overnight. If in the morning the pressure's still 1 Bar, that's your problem. If it's increased again, there MUST be a problem with the filling loop - these are the only two places where mains pressure can get into the heating circuit.

It would be best NOT to turn the cold supply to the boiler on again until it's been repaired.

 

[Nige F]

I thought filling loops were supposed to be left disconnected ..

 

[billplumb]

too true nigel f and check the expansion vessel?