Persistent air lock in hot water circulator pump

SteveP02 · 1 Dec 2019

I've changed the pump circulating water to the hot water cylinder in an attempt to cure this but the air keeps getting back and blocking the water flow, within a few hours. It's driving me slightly crazy now. Any ideas how to track down where the air is getting in?
Many thanks

CBW · 1 Dec 2019

Is the pump in correct location and orientation? Has this problem always existed?

SteveP02 · 1 Dec 2019

We moved to the house in January. All fine for the first couple of months then we had 3 or 4 random mornings without hot water. I was just thinking I needed to get it sorted when the weather warmed up and then it was fine all summer. The problem came back as soon as the central heating came on again, although it's the hot water circulation pump, rather than the central heating pump, that is playing up. Pump looks like it's positioned normally, vertically in the inlet pipe to the cylinder.

MeldrewsMate · 1 Dec 2019

It's that word 'normally' that appears anything but normal.

Do you have two separate circulating pumps?

My first impression is that you have an un-normal system, and that the pumps have been installed on the return pipework, thus the extra circulation caused by having both pumps running causes air to be sucked-in via the open vent over the loft mounted feed and expansion tank.
Try turning the speed of the CH pump to minimum. Does that alleviate the problem?

Ultimately you should consider having the system re-designed to eliminate the problems.

SteveP02 · 1 Dec 2019

Thanks for that. Yes two pumps and the heating pump feeds an underfloor heating manifold, sealed system I think. Hot water pump is between the furnace and cylinder. My terminology might be out but I'll take photos tomorrow. Come to think about it, the bleed screw on the old pump looked well used so the problem may be longstanding.

SteveP02 · 2 Dec 2019

Here's the system, problem pump circled in red

SteveP02 · 4 Dec 2019

Update: After stripping off the insulation I can see a small leak from the isolation valve marked in green. Not good but I'm thinking unlikely to be the cause of air getting into the pump on the adjoining pipe. Currently the pump fills with enough air to stop the water every few hours. I can't see any othe r leaks but feel like I'm missing something obvious.

polesapart · 4 Dec 2019

I take it you're not in the UK?

I'd try opening the two right hand of the four cocks located on ?1/2" pipework dropping down, a few inches above the guages, starting with the one second from right. Use a normal radiator bleed key. Use a hose or hold a container as some water as well as air may come out.

It looks to me that a conscientious installer has placed vent points from the high points of the pipework tee'd down to to a handy height to facilitate deaeration.

Edit; check system pressure afterwards and repressurise to 1 bar if required.

SteveP02 · 4 Dec 2019

Many thanks for that. Yes, Brit abroad, in CH but with only enough German to order a beer. The motor and a solenoid in the big blue box (oil fired) have both packed up in the last couple of months and I've attempted to get across this other problem to the engineer but he just wants to replace the pump, which I've already done without any improvement. I'm beginning to wonder if there's water leaking somewhere inside the boiler itself, perhaps causing the other two failures. I've bled the 1/2" pipes as suggested, have done so previously but the air keeps coming back. I can clear the pump by rocking the speed control (big glug then can hear the water running in the cylinder) which sorts it for a few hours.

polesapart · 4 Dec 2019

What system pressure do you have and what are the top-up arrangements?

SteveP02 · 5 Dec 2019

I'm guessing this is the system pressure gauge located at the back of the boiler, but sorry, I don't know about top up. I'll google, but looks like a red valve on the back of the gauge for that purpose? Should I be keeping the pressure higher? Many thanks

Mottie · 5 Dec 2019

Surely it should be in the green? Is that another leak?

SteveP02 · 5 Dec 2019

Good point, I'll figure out how to top it up and will keep an eye on it for w few days.

oilhead · 5 Dec 2019

The red valve top looks more like a pressure relief valve. Your filling loop will be a connection between mains supply and the primary pipework, normally with aan isolation tap at each end.

SteveP02 · 5 Dec 2019

You're right, thank you, after folllowing it properly the pipe off the back of the gauge and red valve just goes to a drain. It's all very neatly built but I can't see the filling loop, just a connector for a hose at the bottom of the boiler that could be it or may be a drain.
I think I'm going to throw in the towel and try to get someone in as I'm at the fringes of my competence and anyway have run out of ideas.
Many thanks for the pointers guys and I'll let you know if there's an "Of course!" moment at the end of it.

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Persistent air lock in hot water circulator pump

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