Gravity fed hot water airlock? real head scratcher


Postby Alexnp » Wed May 19, 2010 1:23 pm

Please help me! I have what I believe to be the mother of all airlocks within the coils of an indirect cylinder on a gravity fed hot water primary although other possible suggestions would be welcomed. Starting from the beginning, the boiler (an old potterton) is located in a basement it heats two separate circuits with their own flow and returns, a pumped central heating circuit and a gravity circulated hot water circuit (22mm not 28mm) to a indirect cylinder located in the attic literally right under the Feed and expansion cistern, there are no diverter valves / external controls. Heating options should be hot water only and CH + Hot water (not CH alone). I drained the system to add a new rad + fit TRV’s to existing rads, while there i added fernox heavy duty CH cleaner to the header tank and refilled – no problems. 48 hours later re drained the system to remove cleaner and refilled with inhibitor. CH system is working flawlessly but no hot water regardless of CH on or off, when set to hot water only flow pipe heats up near boiler (but not at cylinder) and after a few minutes the boiler starts to kettle and shuts down. Went through all known methods of moving an airlock (more on that in a min) but to no avail. At this point it did occur to me maybe some of the muck loosened by the cleaner had blocked the pipe so re drained the system cut both the flow and return connected the flow to the mains and return to a drain capped off open vent and closed cold feed. Managed to push water around the circuit no problem so re connected flow + return and refilled. Assuming now that it is an airlock here is what i have tried to move it. Loosened unions on cylinder – lots of air out followed by water, still no hot water. Attached mains to open vent – bubbles out of cold feed still no hot water. Added T to flow pipe in basement and connected this to mains (like a pressurised filling loop) opened this until overflow ran don’t know if it moved any air as i couldn’t leave basement in case i over filled F + E cistern, still no hot water. Have also had a gas safe friend rule out issues with the boiler. I am well and truly stumped on this one only other thing i have ever heard of to move an airlock is boiling it out but due to distance between boiler and cylinder and age of the boiler this is not something i want to try. Ideas please could it be something else? Are there any other tricks to moving an airlock? Just to clarify i am a professional plumber so answers including call out a plumber don’t help!
Many many thanks in advance
alex
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Postby htg engineer » Wed May 19, 2010 6:50 pm

Like you say, you could boil it out, but on a old boiler/cylinder i'd be abit dubious too.

Other than that, try to drain and refill again - sometimes you get lucky.

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Postby eggers » Mon Jul 05, 2010 9:54 pm

is it an old kingfisher mk I or mk II model boiler?
i have had faults where the return pipe near the bottom of the heat exchanger becomes blocked
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Postby eggers » Mon Jul 05, 2010 9:55 pm

is it an old kingfisher mk I or mk II model boiler?
i have had faults where the return pipe near the bottom of the heat exchanger becomes blocked
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