Central Heating Pressure Dropping

tony glenn · 24-11-2019 12:12am #1

Advice appreciated! We converted our CH from Calor gas to oil last Autumn, a new boiler, expansion tank and all associated works. All has been well until about a month ago, when the upstairs rads didn't heat. (Downstairs ok)
I checked the pressure gauge, and it was showing zero. I opened the lever by the gauge until the pressure reached 2.0, closed the lever and all seemed well, upstairs rads all hot, downstairs fine. I assumed it was because the new system had been idle all Summer and was 'settling down'.
Now it's happening again, and this time the
pressure drops within a few hours.
No sign of any leaks, plumber involved calling next week.... just curious as to what the problem might be?

Wearb · 24-11-2019 12:23am

Have you checked the outlet from the pressure relief valve?

tony glenn · 24-11-2019 12:30am

Wearb wrote: »

Have you checked the outlet from the pressure relief valve?

If it's this, yes. Discovered It was loose and leaking slightly this morning, tightened it thinking that was the problem, but it's still happening.

Wearb · 24-11-2019 12:32am

No that's not it. It most likely has a red knob. It might be fitted to top of boiler.

tony glenn · 24-11-2019 12:36am

Wearb wrote: »

No that's not it. It most likely has a red knob. It might be fitted to top of boiler.

The (Firebird) boiler is outside, so I'll check in the morning. What should I be looking for? Is it ok to 're-pressure' and use the heating for maybe an hour each day until the plumber calls?
Many thanks for your late night advice btw, much appreciated!

Wearb · 24-11-2019 12:42am

It's difficult to advise without having identified the cause. Perhaps ask your plumber to make that call.

PRV pic.

tony glenn · 24-11-2019 12:47am

Wearb wrote: »

It's difficult to advise without having identified the cause. Perhaps ask your plumber to make that call.

PRV pic.

Thanks again, plumber calling on Monday, I'm sure we'll survive until then. Cheers!

tony glenn · 24-11-2019 4:09pm

tony glenn wrote: »

Thanks again, plumber calling on Monday, I'm sure we'll survive until then. Cheers!

Hope it's ok to talk plumbing problems on a Sunday!
Everything seems ok in the boiler cabinet, no sign of leaks etc. But I think I might have discovered a possible clue....the expansion tank was mounted in the hot press, on a shelf above the pressure gauge and pipe-work. I fitted a small door below the shelf to conceal all the pipes etc.
Recently I noticed the door was starting to jam against the shelf above. I've realised this morning that the shelf has started sagging, and as it's been ok since fitting last Autumn, the expansion tank must be getting heavier....which in turn means it could be filling with water? When I tap it, it doesn't ring like an empty cylinder. Could I have a faulty tank? I've double -checked every visible part of the entire system, and there's no sign of any leaks or water damage. All the rads, upstairs and downstairs, work fine if I just let in water to bring the pressure up. I switch off after an hour, and the pressure gradually drops to zero. Any comments appreciated!

Wearb · 24-11-2019 4:14pm

If you have a faulty expansion tank then when your water heats and expands it has to go somewhere.

Did you find the PRV?
Do you have a small expansion tank in the attic?

tony glenn · 24-11-2019 4:37pm

Wearb wrote: »

If you have a faulty expansion tank then when your water heats and expands it has to go somewhere.

Did you find the PRV?
Do you have a small expansion tank in the attic?

PRV seems fine, no sign of dripping or leaks. Water squirted out when I opened it slightly.
No expansion tank in the attic.
Is the expansion tank the same set-up as under the car bonnet? Is it possible that water is expanding into it, but not getting back out....hence the sagging support shelf?

tony glenn · 24-11-2019 4:44pm

tony glenn wrote: »

PRV seems fine, no sign of dripping or leaks. Water squirted out when I opened it slightly.
No expansion tank in the attic.
Is the expansion tank the same set-up as under the car bonnet? Is it possible that water is expanding into it, but not getting back out....hence the sagging support shelf?

PS It only takes max 5 seconds to re-pressurise to 2 0 bar each time, so I'm thinking the water loss isn't that great?

Wearb · 24-11-2019 4:44pm

A working prv (and faulty expansion tank) will only pass water as the heating goes from cold to hot.

A working expansion vessel will partially fill with water as the water heats up. This water will then return to the heating system as it cools.

You also could have a perfectly good vessel set at the wrong pressure.

Wearb · 24-11-2019 4:47pm

tony glenn wrote: »

PS It only takes max 5 seconds to re-pressurise to 2 0 bar each time, so I'm thinking the water loss isn't that great?

That might indicate a problem with the epansnion vessel.

Is your prv outlet piped away from the top of the boiler?
Post a pic of that general area.

Wearb · 24-11-2019 4:51pm

If vessel and prv is fine, then you could have a leak in the heating pipework or even cylinder coil.

If you have to keep putting in it has to be going somewhere.

tony glenn · 24-11-2019 5:02pm

Wearb wrote: »

That might indicate a problem with the epansnion vessel.

Is your prv outlet piped away from the top of the boiler?
Post a pic of that general area.

Pics attached of the PRV and boiler, plus the location of the tank and jamming door.
I lifted the end of the tank very slightly, it certainly seemed heavy!

Wearb · 24-11-2019 5:05pm

tony glenn wrote: »

Pics attached of the PRV and boiler, plus the location of the tank and jamming door.
I lifted the end of the tank very slightly, it certainly seemed heavy!

If it's hot it will be heavy, same if it's damaged or not at correct pressure.

tony glenn · 24-11-2019 5:07pm

Wearb wrote: »

If it's hot it will be heavy, same if it's damaged or not at correct pressure.

Thanks for all your help, I'll be well ready for the plumber's visit tomorrow! If you don't mind, I'll update you when the problem is located and rectified. Cheers for now.

Wearb · 24-11-2019 5:14pm

tony glenn wrote: »

Pics attached of the PRV and boiler, plus the location of the tank and jamming door.
I lifted the end of the tank very slightly, it certainly seemed heavy!

The out let from the PRV should be piped out of harms way. If it passes water as it is, it's likely to damage the burner elecrics.

I would also question the positioning of the other fitting there. It's an automatic air release valve and in that position might draw in air. However I'm not sure of that. Perhaps some of the other experienced lads on here would give their thoughts on that.

tony glenn · 03-12-2019 7:07pm

Wearb wrote: »

The out let from the PRV should be piped out of harms way. If it passes water as it is, it's likely to damage the burner elecrics.

I would also question the positioning of the other fitting there. It's an automatic air release valve and in that position might draw in air. However I'm not sure of that. Perhaps some of the other experienced lads on here would give their thoughts on that.

Well, a week or so gone and still baffled. So is the plumber! I open the loop, it takes up to 20/30 seconds to reach 1 bar, the I run the heating no problem. Then every time, the pressure drops within an hour of the heating switching off.
Interestingly, the pressure stays up and stable whilst the heating is running, only dropping after switch-off. Definitely no apparent leaks in the rads or boiler, (modern house, no underfloor pipes btw, new boiler fitted last Autumn, switched from Calor gas to oil). The next possible step is to fit taps either side of the cylinder to enable us to isolate the coil and see if the pressure still drops, if it's ok, as a last resort, try leak sealant? Any further thoughts appreciated!!!

K.Flyer · 04-12-2019 12:20am

Check the coil before using leak sealant, otherwise it gets into the hot water system.

tony glenn · 06-12-2019 1:12am

More mysterious by the day.... yesterday morning the pressure had stayed at 2 bar overnight (where I had set it) so I turned off all the rads, ran the heating for 30 mins, turned the rads back on, but this time only opened each one halfway. Also bled them all again. Now the pressure is still dropping, but really slowly. Instead of to zero within an hour, it's taking all day to drop from 2 bar to 1 bar. How can this be? Surely a leak is a leak?
Am I filling the loop incorrectly.... should it be done hot or cold? Could the reduced rad flow be a factor? The plumber still thinks it's the coil, but surely that wouldn't explain the change from fast leak to very slow?

whizbang · 06-12-2019 2:36am

Did you find out if the expansion vessel is ok, and at sufficient pressure ?

It still sounds like pressure vessel has now filled completely, and so isnt dropping pressure anymore.
However the Prv will now leak a bit when water is getting up to temp.

If there's still air in system, it could be this air thats released out of Prv and/or Aav.
get close up and listen as system is heating up.

tony glenn · 06-12-2019 10:30am

whizbang wrote: »

Did you find out if the expansion vessel is ok, and at sufficient pressure ?

It still sounds like pressure vessel has now filled completely, and so isnt dropping pressure anymore.
However the Prv will now leak a bit when water is getting up to temp.
If there's still air in system, it could be this air thats released out of Prv and/or Aav.
get close up and listen as system is heating up.

Well, that's exactly what I'm thinking, but when I cautiously suggested it, the plumber insisted the expansion vessel was ok, and he still suspects the coil in the cylinder. I've attached a pic of the expansion vessel, it's mounted on it's side and actually feels 'heavy' when I lift it slightly. It also sounds 'dull' when I tap it, if that's a clue.
Having said all that, the pressure is still slowly dropping, down to zero this morning from 1.0 bar so it's still going somewhere. It's just weird that the loss seems to fluctuate for no apparent reason.

John.G · 06-12-2019 7:40pm

With the system pressurised to ~ 1.5 bar, remove the black plastic valve cover that you can see on the end of the E.vessel, you will then see a (Schrader) valve exactly like the one on you car tyre, press this in with your finger nail and tell us what comes out.

tony glenn · 07-12-2019 10:45am

John.G wrote: »

With the system pressurised to ~ 1.5 bar, remove the black plastic valve cover that you can see on the end of the E.vessel, you will then see a (Schrader) valve exactly like the one on you car tyre, press this in with your finger nail and tell us what comes out.

Set the pressure at 1.5 bar just now, pressed the valve for a couple of seconds, air escaped but no sign of any water. The pressure rises from 1 to 2 bars when the heating is running, then takes several hours (most of the day) to drop to zero again. What puzzles me is that when this problem was first noticed, the pressure was dropping from 2 to 1 in an hour or two. Now it takes almost half a day 'per bar'.....and all that's been done in the meantime is closing each rad and re-opening by 3 turns instead of 5, then bleeding again. Surely a leak would be relatively constant?

John.G · 07-12-2019 11:05am

The E.vessel isn't water logged anyhow, the only way to see how much water you are loosing (if any) is to drain down the system, then pre pressurise (air side) the E.vessel to 1.0 bar (you will need a pump/pressure gauge for this) then open the filling valve until you get 1.5 bar. depending on the E.vessel capacity then you will have a water reserve of 2 to 2.5 litres so when/if the pressure falls to 1.0 bar then you know you have lost this reserve somewhere.

If you can get hold of a pressure gauge only you can do a reasonable test....make sure no pressure on the water side then measure the pressure on the air side, add 0.5 bar to this when refilling water side and see what happens.
You said earlier that the pressure rises very fast on refilling, this could be a indication that the E.vessel pre pressure is too high, the pressure gauge reading when water side pressure is zero should give a reasonable idea of this.

tony glenn · 07-12-2019 7:30pm

John.G wrote: »

The E.vessel isn't water logged anyhow, the only way to see how much water you are loosing (if any) is to drain down the system, then pre pressurise (air side) the E.vessel to 1.0 bar (you will need a pump/pressure gauge for this) then open the filling valve until you get 1.5 bar. depending on the E.vessel capacity then you will have a water reserve of 2 to 2.5 litres so when/if the pressure falls to 1.0 bar then you know you have lost this reserve somewhere.

If you can get hold of a pressure gauge only you can do a reasonable test....make sure no pressure on the water side then measure the pressure on the air side, add 0.5 bar to this when refilling water side and see what happens.
You said earlier that the pressure rises very fast on refilling, this could be a indication that the E.vessel pre pressure is too high, the pressure gauge reading when water side pressure is zero should give a reasonable idea of this.

Thanks, we're getting back into plumber territory here, so I'll have to ask him to call back asap.... I had asked him to give me a few days to monitor the pressure drop, time to decide the next step now.

tony glenn · 10-12-2019 10:33am

tony glenn wrote: »

Thanks, we're getting back into plumber territory here, so I'll have to ask him to call back asap.... I had asked him to give me a few days to monitor the pressure drop, time to decide the next step now.

This is getting weird. I hadn't called the plumber back yet, so did the usual last night.... opened the loop for ten seconds, raised the pressure from zero to 1 bar, and ran the heating for an hour. The pressure Rose from 1 bar to 2.2 during that time, then after switch-off I expected it to drop to zero after a while. But no! It dropped to 1 bar again and stayed almost there all night, it's now just one graduation under 1.0 bar. There is no constant situation here, a week ago it stayed at 2.0 bar for 24 hours until I ran the heating, then it slowly dropped to zero after switch-off, then repeatedly dropped to zero after use.
The plumber suggested that the 'leak' sealed itself due to expansion, explaining the steady pressure during use, then re-appeared on cool down. Fair enough, but how come it's not happening every time? And this is a fairly new timber frame house, no under floor piping downstairs. Surely after at least three weeks, there would be some small visible sign of leakage?

K.Flyer · 10-12-2019 10:39am

A rise from 1.0 bar cold to over 2.0 bar when hot shows that you have expansion vessel issues. Either lack of pre charge air pressure or the vessel is too small for the system.
If the pressure is falling to below the original cold pressure setting, and there is zero evidence at the pressure relief valve discharge pipe, then its an indication of a leak..

tony glenn · 10-12-2019 10:52am

K.Flyer wrote: »

A rise from 1.0 bar cold to over 2.0 bar when hot shows that you have expansion vessel issues. Either lack of pre charge air pressure or the vessel is too small for the system.
If the pressure is falling to below the original cold pressure setting, and there is zero evidence at the pressure relief valve discharge pipe, then its an indication of a leak..

Thanks for your input, I'm hoping that the expansion vessel is somehow 'involved' in the problem. As for the possibility of a leak, this brings me back to my other point, which is that there seems to be no constant situation. Why would the system hold it's pressure as long as it's running, and then after switch-off, drop to zero sometimes over a few hours, sometimes it takes all day and this time it's hardly moved since last night?

tony glenn · 10-12-2019 10:53am

tony glenn wrote: »

Thanks for your input, I'm hoping that the expansion vessel is somehow 'involved' in the problem. As for the possibility of a leak, this brings me back to my other point, which is that there seems to be no constant situation. Why would the system hold it's pressure as long as it's running, and then after switch-off, drop to zero sometimes over a few hours, sometimes it takes all day and this time it's hardly moved since last night?

Btw, the expansion vessel was fitted last Autumn when the gas boiler was replaced by a new oil boiler.

Central Heating Pressure Dropping

Comments