Combi boiler loses pressure when idle but regains it when working

shiraz99

Bendy_House said:

The only issue regarding DIY repressurising, is that the system side should be at zero or else you won't get a true reading on the air side - it'll be being pressurised against an unmoveable object, the water. So, ideally, a rad bleed screw should be opened until no more water comes out = zero pressure - and kept open with someone catching subsequent ejaculations as the air side is given a blow.

Having said that, if the EV has just lost pressure and isn't leaking, then fitting a car or bike pump and giving it 6-12 pumps will very likely get it good enough; just monitor how the pressure fluctuates betwixt hot and cold. As you pump, and the pressure gauge rises, keep 'bleeding' it when it hits, say, 1.5+ bar, and bring it back down to 1.

Before pumping, press the centre pin of the Shrader and see if it hisses. And if water comes out, don't bother pumping...

No, as mentioned, flow and returns on the boiler need shutting off which completely isolates the boiler enough for you to check the current pressure of the EV. Then you drain down the boiler only, to get zero pressure to pump it back up.

Bendy_House

I don't know about oil boilers (even tho' I have - naughtily - installed one many moons ago), but I would imagine the sealed system side would be the same as for a gas boiler. So 'all' you'd have to do is to open a bleed screw on a top-floor radiator, zero the system pressure, and leave it open whilst the EV is pumped. Once the EV 'air' side has been pumped to spec, then close bleed screw and repressurise the water side.

Not that I'm suggesting the OP do this...

shiraz99

Bendy_House said:

I don't know about oil boilers (even tho' I have - naughtily - installed one many moons ago), but I would imagine the sealed system side would be the same as for a gas boiler. So 'all' you'd have to do is to open a bleed screw on a top-floor radiator, zero the system pressure, and leave it open whilst the EV is pumped. Once the EV 'air' side has been pumped to spec, then close bleed screw and repressurise the water side.

Not that I'm suggesting the OP do this...

That really isn't the way to do it for any type of combi boiler. You can't accurately check the pressure of the EV on a "live" boiler, ie, not isolated from the CH system.

Fubar1953

Thanks for all the input guys. The consensus of opinion seems to point to the expansion tank so I'll call my local guy in. 

Bendy_House

shiraz99 said:

Bendy_House said:

I don't know about oil boilers (even tho' I have - naughtily - installed one many moons ago), but I would imagine the sealed system side would be the same as for a gas boiler. So 'all' you'd have to do is to open a bleed screw on a top-floor radiator, zero the system pressure, and leave it open whilst the EV is pumped. Once the EV 'air' side has been pumped to spec, then close bleed screw and repressurise the water side.

Not that I'm suggesting the OP do this...

That really isn't the way to do it for any type of combi boiler. You can't accurately check the pressure of the EV on a "live" boiler, ie, not isolated from the CH system.

I do stand ready to be corrected, Shiraz.

My understanding is that a new EV comes pre-pressurised, and that is obviously with the 'system' (wet) side unconnected. Once fitted, the wet side is then pressurised to roughly the same setting - around 1bar, at which point the diaphragm will presumably be roughly central.

Once air is lost from the EV during use, water will replace the void. If you isolate the boiler using its F&R valves, the boiler water side will be sealed, so when you try and repressurise the air side, the diaphragm cannot move, hence the pressure on the air side will shoot up with little pumping as the diaph has nowhere to go.

The only way to allow the air side to be pressurised like a new unit, is for the water side pressure to be removed completely.

I don't know that this is the correct way, but is how I've always done it for the above reason. When I did sil's boiler a good 2 years ago, I depressurised the system via a bleed screw, but then nipped it up again as it was only me there, and I didn't want a squirt spaffed across the wall (sorry - too visual? ) What I found was, the air pressure side rose dramatically with a single pump as the diaph couldn't go anywhere. Had I left it at that - supposedly 'correct' air pressure reading - there would hardly have been an air bubble inside the EV. So I had to keep running to the nearest rad and bleeding it back down to zero after every pump - and the air pressure reading fell accordingly. Took a bludy age... To fully air-charge the EV took, I dunno, around 20-30 strokes, and 20-30 runs.

At this point - correct air pressure achieved, coupled with zero on the water side - the diaphragm would have been squished right tight against the far wall of the EV leaving no room for the water, just as if delivered new.

Once the air side is at the recommended pressure (0.75b, I think it was), coupled with the system side being at 'zero', then the water side can be repressurised. At the recommended ~1bar, the diaph should then be somewhere in the middle again.

shiraz99

Bendy_House said:

shiraz99 said:

Bendy_House said:

I don't know about oil boilers (even tho' I have - naughtily - installed one many moons ago), but I would imagine the sealed system side would be the same as for a gas boiler. So 'all' you'd have to do is to open a bleed screw on a top-floor radiator, zero the system pressure, and leave it open whilst the EV is pumped. Once the EV 'air' side has been pumped to spec, then close bleed screw and repressurise the water side.

Not that I'm suggesting the OP do this...

That really isn't the way to do it for any type of combi boiler. You can't accurately check the pressure of the EV on a "live" boiler, ie, not isolated from the CH system.

I do stand ready to be corrected, Shiraz.

My understanding is that a new EV comes pre-pressurised, and that is obviously with the 'system' (wet) side unconnected. Once fitted, the wet side is then pressurised to roughly the same setting - around 1bar, at which point the diaphragm will presumably be roughly central.

Once air is lost from the EV during use, water will replace the void. If you isolate the boiler using its F&R valves, the boiler water side will be sealed, so when you try and repressurise the air side, the diaphragm cannot move, hence the pressure on the air side will shoot up with little pumping as the diaph has nowhere to go.

The only way to allow the air side to be pressurised like a new unit, is for the water side pressure to be removed completely.

I don't know that this is the correct way, but is how I've always done it for the above reason. When I did sil's boiler a good 2 years ago, I depressurised the system via a bleed screw, but then nipped it up again as it was only me there, and I didn't want a squirt spaffed across the wall (sorry - too visual? ) What I found was, the air pressure side rose dramatically with a single pump as the diaph couldn't go anywhere. Had I left it at that - supposedly 'correct' air pressure reading - there would hardly have been an air bubble inside the EV. So I had to keep running to the nearest rad and bleeding it back down to zero after every pump - and the air pressure reading fell accordingly. Took a bludy age... To fully air-charge the EV took, I dunno, around 20-30 strokes, and 20-30 runs.

At this point - correct air pressure achieved, coupled with zero on the water side - the diaphragm would have been squished right tight against the far wall of the EV leaving no room for the water, just as if delivered new.

Once the air side is at the recommended pressure (0.75b, I think it was), coupled with the system side being at 'zero', then the water side can be repressurised. At the recommended ~1bar, the diaph should then be somewhere in the middle again.

Yes, which is why you isolate the boiler from the CH system by closing down the flow and return. Taking a reading from the EV will then be no longer influenced by the CH system pressure. Then you "simply" drain the boiler if you need to re-inflate the EV, wait 10 minutes or so to see if it holds pressure, then you can make the boiler "live" again by opening the flow and return and and filling if necessary.

None of which would be a "simple job", as was previously mentioned by another member, for that Heatslave boiler.

shiraz99

Different boiler but this is the correct way to check and re-pressurise your combi.

https://www.youtube.com/watch?v=ZrAo1YKQvEo

FreeBear

Bendy_House said:

I don't know that this is the correct way, but is how I've always done it for the above reason. When I did sil's boiler a good 2 years ago, I depressurised the system via a bleed screw, but then nipped it up again as it was only me there, and I didn't want a squirt spaffed across the wall (sorry - too visual? ) What I found was, the air pressure side rose dramatically with a single pump as the diaph couldn't go anywhere. Had I left it at that - supposedly 'correct' air pressure reading - there would hardly have been an air bubble inside the EV. So I had to keep running to the nearest rad and bleeding it back down to zero after every pump - and the air pressure reading fell accordingly. Took a bludy age... To fully air-charge the EV took, I dunno, around 20-30 strokes, and 20-30 runs.

At this point - correct air pressure achieved, coupled with zero on the water side - the diaphragm would have been squished right tight against the far wall of the EV leaving no room for the water, just as if delivered new.

Once the air side is at the recommended pressure (0.75b, I think it was), coupled with the system side being at 'zero', then the water side can be repressurised. At the recommended ~1bar, the diaph should then be somewhere in the middle again.

Just reducing the CH pressure to zero still leaves water in the system - You need to take in to account the "head" of water still behind the diaphragm in the EV. There will always be some pressure there unless you drain the system down or disconnect the EV completely.

Bendy_House

FreeBear said:

Bendy_House said:

I don't know that this is the correct way, but is how I've always done it for the above reason. When I did sil's boiler a good 2 years ago, I depressurised the system via a bleed screw, but then nipped it up again as it was only me there, and I didn't want a squirt spaffed across the wall (sorry - too visual? ) What I found was, the air pressure side rose dramatically with a single pump as the diaph couldn't go anywhere. Had I left it at that - supposedly 'correct' air pressure reading - there would hardly have been an air bubble inside the EV. So I had to keep running to the nearest rad and bleeding it back down to zero after every pump - and the air pressure reading fell accordingly. Took a bludy age... To fully air-charge the EV took, I dunno, around 20-30 strokes, and 20-30 runs.

At this point - correct air pressure achieved, coupled with zero on the water side - the diaphragm would have been squished right tight against the far wall of the EV leaving no room for the water, just as if delivered new.

Once the air side is at the recommended pressure (0.75b, I think it was), coupled with the system side being at 'zero', then the water side can be repressurised. At the recommended ~1bar, the diaph should then be somewhere in the middle again.

Just reducing the CH pressure to zero still leaves water in the system - You need to take in to account the "head" of water still behind the diaphragm in the EV. There will always be some pressure there unless you drain the system down or disconnect the EV completely.

Ah, cool, than you. That makes sense, provided the boiler is drained down.

A lot more hassle than a spaffing bleed screw, tho' :-)

Bendy_House

FreeBear said:

Bendy_House said:

I don't know that this is the correct way, but is how I've always done it for the above reason. When I did sil's boiler a good 2 years ago, I depressurised the system via a bleed screw, but then nipped it up again as it was only me there, and I didn't want a squirt spaffed across the wall (sorry - too visual? ) What I found was, the air pressure side rose dramatically with a single pump as the diaph couldn't go anywhere. Had I left it at that - supposedly 'correct' air pressure reading - there would hardly have been an air bubble inside the EV. So I had to keep running to the nearest rad and bleeding it back down to zero after every pump - and the air pressure reading fell accordingly. Took a bludy age... To fully air-charge the EV took, I dunno, around 20-30 strokes, and 20-30 runs.

At this point - correct air pressure achieved, coupled with zero on the water side - the diaphragm would have been squished right tight against the far wall of the EV leaving no room for the water, just as if delivered new.

Once the air side is at the recommended pressure (0.75b, I think it was), coupled with the system side being at 'zero', then the water side can be repressurised. At the recommended ~1bar, the diaph should then be somewhere in the middle again.

Just reducing the CH pressure to zero still leaves water in the system - You need to take in to account the "head" of water still behind the diaphragm in the EV. There will always be some pressure there unless you drain the system down or disconnect the EV completely.

I don't think a boiler is going to quibble over a ~0.1bar discrepancy.

Tbh, I personally won't be bothering with anything as involved as draining a system, or even just a boiler. Bleed screw for me.