Central Heating Pressure Dropping

tony glenn

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

Have you checked the outlet from the pressure relief valve?

tony glenn

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

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

tony glenn

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

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

PRV pic.

tony glenn

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

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

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

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

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

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

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

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

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

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

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

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

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

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

tony glenn

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

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

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

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

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

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

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

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

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

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

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.

K.Flyer

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?

Two central heating systems I worked on during the year had the same symptoms, both were due to intermittent leaking push-fit connections.
Unfortunately both times in hard to reach places and it was only when staining started could they be traced.

tony glenn wrote: »

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

New ones have been known to fail. More importantly if it has been adequately sized and checked for correct pre-charge air pressure.

John.G

The expansion vessel will have to be removed/renewed to rule it out but the next time the pressure drops to zero, just press that Schrader valve again and see if there is still some pressure there. If connected to the system via a braided hose or whatever then there could be a blockage or partial blockage here which also should be checked out.
Also remember that those Grant boilers now have the PRV set at 2.5 bar.

What size expansion vessel is fitted and where is it located?.

tony glenn

John.G wrote: »

The expansion vessel will have to be removed/renewed to rule it out but the next time the pressure drops to zero, just press that Schrader valve again and see if there is still some pressure there. If connected to the system via a braided hose or whatever then there could be a blockage or partial blockage here which also should be checked out.
Also remember that those Grant boilers now have the PRV set at 2.5 bar.

What size expansion vessel is fitted and where is it located?.

Thanks both for continued assistance. Pic attached of vessel, opened loop to set at 1.0 bar half an hour ago, heating running now, pressure risen to over 2.0bar

tony glenn

tony glenn wrote: »

Thanks both for continued assistance. Pic attached of vessel, opened loop to set at 1.0 bar half an hour ago, heating running now, pressure risen to over 2.0bar

Could the apparent kink in the braided pipe be relevant?(amateur question, don't laugh)

John.G

It certainly could affect the pressure reading if this is the filling loop? When the pressure falls to zero does this affect the upstairs rads (assuming you have a upstairs) first or what??.

tony glenn

John.G wrote: »

It certainly could affect the pressure reading if this is the filling loop? When the pressure falls to zero does this affect the upstairs rads (assuming you have a upstairs) first or what??.

1)That is the filling loop.
2) The pressure never drops when the heating is running, both upstairs and downstairs rads work fine... although if it's a clue, the upstairs thermostat makes no difference, the rads heat upstairs even if the upstairs thermostat is set at zero.
3) Having said all that, we only first noticed this problem when we discovered the upstairs rads were cold after first winter switch-on, although the downstairs were ok. The system was ok after boiler change last Autumn btw.
How about if I switch on the heating with the pressure at zero, and see if downstairs heats and upstairs stays cold? Or is that risky?

K.Flyer

tony glenn wrote: »

Could the apparent kink in the braided pipe be relevant?(amateur question, don't laugh)

Is that not the filling valve hose??
If it is, then its not related to any of your issues.
Connections between vessel and boiler internal pipework can be quite small, so I wouldn't be overly concerned unless it was kinked almost flat.
Edit:
As said, expansion vessel Must be correctly sized and pre charge checked, otherwise you could be running around in circles forever with this.

tony glenn

K.Flyer wrote: »

Is that not the filling valve hose??
If it is, then its not related to any of your issues.
Connections between vessel and boiler internal pipework can be quite small, so I wouldn't be overly concerned unless it was kinked almost flat.

It is the filling valve hose, so no worries there then, thanks. How about my other suggestion.... running at zero for half an hour to see if upstairs and downstairs react differently?

John.G

I wouldn't be happy to advise that, when you first discovered your upstairs problem I presume you fixed it by opening the filling loop to give you 1 bar or whatever. The E.vessel seems to be located in your hot press, if so then if that pressure is rising to 2.0/2.2 bar that will mean that the pressure at the boiler is approaching 2.5 bar due to static head and it will lift the PRV.

So (1) Is the hot press/E.vessel located upstairs and (2) can you take a pic of the boiler PRV and look for any signs of water/staining.

Also if you have access to the E.vessel details (on a label) you will be able see its capacity in litres.

tony glenn

John.G wrote: »

I wouldn't be happy to advise that, when you first discovered your upstairs problem I presume you fixed it by opening the filling loop to give you 1 bar or whatever. The E.vessel seems to be located in your hot press, if so then if that pressure is rising to 2.0/2.2 bar that will mean that the pressure at the boiler is approaching 2.5 bar due to static head and it will lift the PRV.

So (1) Is the hot press/E.vessel located upstairs and (2) can you take a pic of the boiler PRV and look for any signs of water/staining.

Also if you have access to the E.vessel details (on a label) you will be able see its capacity in litres.

Yes, I have always opened the valve to replace pressure loss.
Pic of PRV just taken, all seems ok, no sign of leakage or water staining anywhere.

tony glenn

tony glenn wrote: »

Pic attached

Pic won't send. But all ok, no sign of any water anywhere in boiler housing.

John.G

Where is the hot press? and can you read the capacity 0f the E.vessel in litres.

tony glenn

John.G wrote: »

Where is the hot press? and can you read the capacity 0f the E.vessel in litres.

The hot press is on the upstairs landing. There are no visible markings on it, if it helps it's about 12" in diameter and a little less than 12" in height. That's a Lidl torch beside it for scale btw.

John.G

Thanks, probably a 12 litre which should be adequate. My last suggestion short of getting your plumber back is to get hold of a pressure gauge and check the pressure air side with water side pressure at say ~ 1.5/2 bar and then when water side pressure is 0 bar. If there is no change in the air side pressure it means that the diaphragm is stuck in one position. If working properly, the air side pressure will read the precharge pressure when water side is at 0 bar and when water side pressure exceeds this (precharge pressure) then both pressures should read exactly the same.

tony glenn

John.G wrote: »

Thanks, probably a 12 litre which should be adequate. My last suggestion short of getting your plumber back is to get hold of a pressure gauge and check the pressure air side with water side pressure at say ~ 1.5/2 bar and then when water side pressure is 0 bar. If there is no change in the air side pressure it means that the diaphragm is stuck in one position. If working properly, the air side pressure will read the precharge pressure when water side is at 0 bar and when water side pressure exceeds this (precharge pressure) then both pressures should read exactly the same.

Thanks again, I'll try and source a pressure gauge locally. But would that explain the continued need to 'top up' the water pressure after the pressure drop?

John.G

I don't really know, due to the location of the E.vessel, if the prepressure is too high of if the diaphragm is stuck then even if you loose a tiny amount of water (or any air is released) the pressure will fall to zero, and/or the PRV will release water every time the boiler heats up with pressure falling to zero, OR you either have a leak either in your system or very very unlikely in your brand new boiler.
Thats why its vital to rule E.vessel in/out. Remember you also stated in another post that it takes very little time to repressurise the system which would normally take 1.5 to 2.0 litres, all the more reason IMO to get a hold of that pressure gauge.

I see its a Firebird boiler so I don't know if the PRV is set at 2.5 bar or 3.0 bar.

K.Flyer

John.G wrote: »

Thanks, probably a 12 litre which should be adequate. My last suggestion short of getting your plumber back is to get hold of a pressure gauge and check the pressure air side with water side pressure at say ~ 1.5/2 bar and then when water side pressure is 0 bar. If there is no change in the air side pressure it means that the diaphragm is stuck in one position. If working properly, the air side pressure will read the precharge pressure when water side is at 0 bar and when water side pressure exceeds this (precharge pressure) then both pressures should read exactly the same.

Sorry, have to disagree on this method.
Vessel pre-charge should be checked with no pressure on wet side.
If pumping up is required, that any water still in the wet side of the vessel is able to discharge freely.

tony glenn

John.G wrote: »

I don't really know, due to the location of the E.vessel, if the prepressure is too high of if the diaphragm is stuck then even if you loose a tiny amount of water (or any air is released) the pressure will fall to zero, and/or the PRV will release water every time the boiler heats up with pressure falling to zero, OR you either have a leak either in your system or very very unlikely in your brand new boiler.
Thats why its vital to rule E.vessel in/out. Remember you also stated in another post that it takes very little time to repressurise the system which would normally take 1.5 to 2.0 litres, all the more reason IMO to get a hold of that pressure gauge.

I see its a Firebird boiler so I don't know if the PRV is set at 2.5 bar or 3.0 bar.

Thanks once again for your continued advice.
The plumber who fitted the new boiler was here for two hours last week, he was scratching his head at the end, just couldn't see any clue as to the reason. He suggested putting a stop cock either side of the immersion cylinder, so that we could rule out a leaking coil in there. I was hoping to avoid that, as we could do without any big plumbing bils before Christmas. So I think I'll push the expansion tank theory first. Watch this space!

John.G

K.Flyer wrote: »

Sorry, have to disagree on this method.
Vessel pre-charge should be checked with no pressure on wet side.
If pumping up is required, that any water still in the wet side of the vessel is able to discharge freely.

I thought thats what I said..."If working properly, the air side pressure will read the precharge pressure when water side is at 0 bar and when water side pressure exceeds this (precharge pressure) then both pressures should read exactly the same." I didn't say anything about pumping up the air side, just to check it with water side pressure at 0 bar.

John.G

tony glenn wrote: »

Thanks once again for your continued advice.
The plumber who fitted the new boiler was here for two hours last week, he was scratching his head at the end, just couldn't see any clue as to the reason. He suggested putting a stop cock either side of the immersion cylinder, so that we could rule out a leaking coil in there. I was hoping to avoid that, as we could do without any big plumbing bils before Christmas. So I think I'll push the expansion tank theory first. Watch this space!

If you have a unvented hot water cylinder and the pressure is greater than the coil side pressure then it should raise the boiler system pressure, if a vented system then the CH system water will leak the other way, theoretically the level should then rise in the CWST but very hard to monitor this as you are only talking a litre or two, if you had a old/dirty system then you would quickly see the hot water getting discoloured.