Why is my PRV only passing when cold?

[Fultonius]

Hi knowledgeable folks,

Background info. We have a very normal system consisting of a Worcester 24i Junior (non condensing) and DHW/CH. It has a filling loop.

The Issue: Over time the system pressure drops and the boiler cuts out. This typically takes a month to 3 months depending on time of year; however, the other week when it was very cold it happened twice in short succession. When filling via filling loop, the pressure climbs when the taps is open, but above about 1.3 to 1.5 bar it always drops down immediately when I close the filling loop isolation valve. It usually settles around 1.2 Bar. We notice some leakage out the overflow/prv outlet when this happens. However, we NEVER observe any discharge when the boiler is running and pressure can go up to 2.5 Bar.

Recently I noticed that when hot, the system was getting up to 2.7/8 Bar which is too high according tor the manual, so I thought maybe the expansion vessel was under pressure.

What we've checked / tried: Followed the instructions for checking the expansion pressure, and increased from 0.7 Bar to 0.9 (I think, forgot to write it down). This was with the boiler off, and pressure vented via radiator.

Since then, we've also had a boiler inspection / maintenance carried out and they've recommended checking (again) the expansion vessel pre-charge pressure, and replacing the PRV and this is where I'm confused....

Conundrum:

If it were the PRV, why does it only open when the cold water filling loop is open? When it's hot and running at 2.3-2.5bar, the PRV is closed and no water comes out the overflow. Is that system separate? Is there a non-return valve between the hot side and filling side? If so, is that not the issue?

I'm loath to spend £100+ to replace a PRV without it being fairly certain it's going to fix the issue.

 

[Fultonius]

Any thoughts on this?

 

[terryplumb]

If an engineer has visited , inspected,and recommended the PRV to be replaced ,and EV checked out ,it would seem to be wise to follow that recommendation.
Unless you have a rather large heating system ,lots of rads ,large rads, I wouldn't expect the pressure to rise to the levels you indicated.
The PRV is held closed under spring pressure ,maybe the inrush of cold mains water through the filling loop lifts the prv . If the pressure is dropping with such regularity then water is very likely escaping / being lost somewhere.

 

[Fultonius]

If an engineer has visited , inspected,and recommended the PRV to be replaced ,and EV checked out ,it would seem to be wise to follow that recommendation.
Unless you have a rather large heating system ,lots of rads ,large rads, I wouldn't expect the pressure to rise to the levels you indicated.
The PRV is held closed under spring pressure ,maybe the inrush of cold mains water through the filling loop lifts the prv . If the pressure is dropping with such regularity then water is very likely escaping / being lost somewhere.

Thanks. While I get the obvious thing to do is get the work done as instructed, it wouldn't be the first time I'd had work done on a boiler as recommended by the engineer for it then to be something else. In this case, it doesn't add up to being a PRV fault (or I'm yet to understand how).

I'm really just trying to understand the situation. It's an old (ish) boiler and would rather not spend unnecessary cash on things that may or may not fix it.

You might be on to something with the high pressure when hot thing... I've just very slowly topped up to 1.25 Bar cold (no prv passing) and now running the CH it's at 3.25 bar. To me, too big a difference from cold to hot points more at the expansion vessel not working/being set correctly? If this is happening and it's actually hitting the PRV set point, dropping a bit of pressure?

 

[terryplumb]

Your EV isn't functioning . The PRV Will lift around 3 bar ,and it's quite common for them not to seal fully once they have opened a few times. Is it possible that you haven't noticed water coming out the PRV pipework,which I assume terminates externally. Given the recent weather it wouldn't be very noticeable.

 

[oilhead]

When filling, the pressure can take a little time to equalise through the system, so falling slightly after turning off the loop is not unusual. If the PRV passes a little when cold, it is possible that when running, it is hot enough to produce a little steam which you would not notice. If the PRV passes when cold then obviously it is not functioning properly.