Combi boiler loses pressure when idle but regains it when working

FreeBear

Bendy_House said:

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.

https://www.engineeringtoolbox.com/pressure-head-water-d_1354.html

Do not discount the pressure from even a modest head of water. Just 2.4m would equate to 3.5psi or 0.23Bar - That would be a significant error if you were trying to repressurise the EV to 1Bar (14.7psi).

Bendy_House

I won't discount it. But I won't do it either

Solution - bleed a rad on t'same level as the boiler. (I think that would work...?)

GDB2222

FreeBear said:

Bendy_House said:

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.

https://www.engineeringtoolbox.com/pressure-head-water-d_1354.html

Do not discount the pressure from even a modest head of water. Just 2.4m would equate to 3.5psi or 0.23Bar - That would be a significant error if you were trying to repressurise the EV to 1Bar (14.7psi).

There’s clearly a problem with low pressure in the EV at the moment, but they normally work over a range of pressures, and they seem to continue to operate even when not at an optimal pressure.  

Bendy_House

Yes, in most cases they seem to only cause issues that need resolving when they have nearly emptied!

Don't get me wrong - Shiraz and FB are correct, but I know what's easier for me (and will be for most folk), what will give a full resolution, and what I'll continue to do.

GDB2222

Bear in mind that most boilers are happy working with a pressure anywhere in the range 1-2 bars, anyway, and there’s bound to be a fair bit of leeway.