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Discuss Gravity feed system sucking air open vent in the Plumbing Jobs | The Job-board area at PlumbersForums.net

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Evening all, I have been researching what could be the reasons behind my system sucking air through the open vent on the gravity feed central heating system. What I know....
The system was installed more 30 years ago and I believe looking at the pipework/soldering of the fittings that Jack was holidays and he left the pipefitters mate to install. The company I worked for as a mate delivered that quality of pipework I and fitter would have had to rip out and do it properly. No motorised three port valve fitted, only hand wheels to isolate the central heating and bypass circuits. The DHW is fitted with a danfoss cylinder valve/thermocouple and electrical connection to controller
1 - Around three years ago had an issue with a loss of domestic hot water. Believed the pump was at fault and was replaced a like for like danfoss unit. The pump was fitted in correct direction.
2 - After replacement the primary feed to the DHW was hot but the output was cold. Now the central system has always been a bit of pig for air locking but now amount of bleeding identified air locks.
3 - Started to chase and replace the primary feed line through to the return to the boiler. No blockages found, only a small amount of sludge in the pipework.
4 - By altering the open/close position of the manual bypass we were able to 'force' more flow through the DHW feed. But when the the system is on shutdown the primary is hot, output will go cold almost immediately.
5 - Always suffered varying levels of air in one particular radiator on the first floor closet to the boiler (physically) but joint second with another from the pump.
6 - Past nine to twelve months noted that air was being sucked intermittently through the open vent, reduced the pump speed from 3 to 1 and there was significant reduction in air being sucked and I thought had been cured.
7 - An intermittent thud can be heard up and around the pump, say once every twenty seconds.
8 - since following this site I placed a small tub of water (round 400ml) under the open vent which was emptied over a period of 18 hours. Whilst this small tub of water was under the open vent the thud/air in the rad was not noted and the system was as quiet as it was 25 years ago.
9 - a small neodymium magnet used and shows a 'small' amount of attraction either side of the pump and for about 300mm beyond the output side of the pump.
10 - the open vent is located with 50 to 75mm of the feed to the central heating pump

No central heating engineer has reported any problems with the installation except it is not the best in respect to spacing of pipes, soldering etc. is pig to fill with trapping air.

Any suggestions for going forward please?

Shaun Jones
 
It appears that you need a decent engineers pls post your location
 
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