Draining down central heating

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Hi all,


Could you advise on the correct procedure for draining down a sealed central heating system? I’ve done it once before out of necessity due to a leaking rad feed, but would prefer to know if there’s anymore to it?


Should TRVs be open or closed? If closing TRVs, is it also worth closing the lock shields to save draining the rads entirely? i.e. just drain the pipe work?


Last time I drained and refilled the system, some of the rads made a lot of noise when the heating was turned back on, like water gushing around inside them, is this normal?


Finally, how should inhibitor be added to the system? Does it matter which brand is used?


Thanks.
 
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Why are you draining it down?

If you're not removing the rads then close both ends and yes just drain the pipework, find the lowest drain point on the system and drain from there, never via the boiler PRV. Careful, the initial opening of the drain valve will be under pressure.

Once the system is filled back up and back under pressure the system will take time to work all the air out, that'll be the sounds in the rads you'll be hearing. It will take a few turns of bleeding and filling to remove all the air.

Inhibitor can be added through one of the rads so when the system is empty, leave the smallest rad to drain down too, use that one to dose the system. I'd recommend to use a well know brand of inhibitor, fernox/sentinel etc.
 
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Why are you draining it down?

If you're not removing the rads then close both ends and yes just drain the pipework, find the lowest drain point on the system and drain from there, never via the boiler PRV. Careful, the initial opening of the drain valve will be under pressure.

Once the system is filled back up and back under pressure the system will take time to work all the air out, that'll be the sounds in the rads you'll be hearing. It will take a few turns of bleeding and filling to remove all the air.

Inhibitor can be added through one of the rads so when the system is empty, leave the smallest rad to drain down too, use that one to dose the system. I'd recommend to use a well know brand of inhibitor, fernox/sentinel etc.

Thanks Madrab. Need to fit a new radiator, no rad present at the moment got the pipes capped off temporarily.
 
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You can use that rad to dose the system then and just leave the others sealed and filled.

You don't even need to drain the system down, just use one rad to release the system pressure via a bleed valve then shut them all down. Once the pressure's released then open one of the capped pipes and catch any water it releases, and do the same with the other and then fit the rad, the amount of water it will release should be minimal, especially if it's higher up.
 
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