Heating System Noise

[TB204]

I have a standard open vented system.
I recently replaced a radiator and took the opportunity to drain the system and refill with fresh Fernox (something I have done before). The water was pretty clear and there was no indication of any problems when I drained the system.
I had a few problems with air when refilling which resulted in the boiler locking out, but after a couple of attempts I managed to clear it. The system then ran ok for a while but then I would get the occasional lock out. This has now stopped. However, I have noticed that when the system first comes on I get a knocking sound which lasts for a few minutes or so which then clears. If the hot water only is switched on I get the noise and then if the heating comes on I again get the noise for a few minutes then the noise goes and all is ok. I haven't managed to track down where exactly its coming from, but I suspect its air trapped somewhere as I can also hear some movement of water in the pipes when the system starts. But I don't seem to be getting air in any of the radiators as they are fully hot and if I bleed them there is no air coming out of them.
Am I right in suspecting its air trapped in a pipe somewhere?
I'd really appreciate any advice on how to shift it.
Many thanks,

Terry

 

[centralheatking]

I have a standard open vented system.
I recently replaced a radiator and took the opportunity to drain the system and refill with fresh Fernox (something I have done before). The water was pretty clear and there was no indication of any problems when I drained the system.
I had a few problems with air when refilling which resulted in the boiler locking out, but after a couple of attempts I managed to clear it. The system then ran ok for a while but then I would get the occasional lock out. This has now stopped. However, I have noticed that when the system first comes on I get a knocking sound which lasts for a few minutes or so which then clears. If the hot water only is switched on I get the noise and then if the heating comes on I again get the noise for a few minutes then the noise goes and all is ok. I haven't managed to track down where exactly its coming from, but I suspect its air trapped somewhere as I can also hear some movement of water in the pipes when the system starts. But I don't seem to be getting air in any of the radiators as they are fully hot and if I bleed them there is no air coming out of them.
Am I right in suspecting its air trapped in a pipe somewhere?
I'd really appreciate any advice on how to shift it.
Many thanks,

Terry

It might well be trapped air ...I would blow it up from the bottom with mains pressure ...if you don’t know how to do this ..let me know...or others might. Centralheatking

 

[TB204]

Many thanks for your response.
In terms of using mains pressure to clear the trapped air. I've never had to do it, but remember reading that you connect a hose to a rad and apply mains water pressure. Presumably, through a drain cock would do. I assume that you apply mains pressure and gradually open the drain cock.
I guess I should make sure the heating and hot water 2 port valves are open and presumably the water pressure will push water through the vent pipe into the F&E tank and hopefully clear the air. I'd appreciate any tips on how to do this e.g is it best to close all the other rads and just blow through the main pipe runs?

Is there anything I need to be careful of?
Thanks in advance,

Terry

 

[centralheatking]

Many thanks for your response.
In terms of using mains pressure to clear the trapped air. I've never had to do it, but remember reading that you connect a hose to a rad and apply mains water pressure. Presumably, through a drain cock would do. I assume that you apply mains pressure and gradually open the drain cock.
I guess I should make sure the heating and hot water 2 port valves are open and presumably the water pressure will push water through the vent pipe into the F&E tank and hopefully clear the air. I'd appreciate any tips on how to do this e.g is it best to close all the other rads and just blow through the main pipe runs?

Is there anything I need to be careful of?
Thanks in advance,

Terry

Connect a hose from an outside tap to the lowest drain off using a jubilee clip . Gently turn outside tap on ..stop and check water tight. Then off you go gently ...start at the lowest , nearest rad and work your way up and away bleeding each in turn. Always watch the level of the f and e tank if it over fills it will overflow hopefully out of the overflow pipe.It is often good to blip the pump off and on. Leave all rad valves open and trvs at max
You have it basically anyway but it’s a great way to blow air out. Good luck and let us know how you get on .....Centralheatking

 

[TB204]

Connect a hose from an outside tap to the lowest drain off using a jubilee clip . Gently turn outside tap on ..stop and check water tight. Then off you go gently ...start at the lowest , nearest rad and work your way up and away bleeding each in turn. Always watch the level of the f and e tank if it over fills it will overflow hopefully out of the overflow pipe.It is often good to blip the pump off and on. Leave all rad valves open and trvs at max
You have it basically anyway but it’s a great way to blow air out. Good luck and let us know how you get on ...Centralheatking

Great, That's all clear.
Thanks again for your help. Things are a bit fraught at the moment what with the lock down and my wife is also high risk so pretty much shut in. So given its not causing a problem at the moment. I'm going to leave for a couple of weeks or so rather than risk causing any other problems at the moment.... just being a bit cautious. Terry