Is draining & refilling bad for the system

[ZACCCC]

I've got two or three jobs to do on my Combi Boiler CH system:-

1. take out some redundant copper pipework
2. fit 2 new radiators
3. fit TRVs

I'm a bit of a novice so I'd like to do these a bit at a time and this will involve draining and refilling the system three or more times. My question is: Is this bad for the system, to drain and refill it so many times?

When I had the old CH system, before I got the combi boiler, I seem to remember that it was supposed to be bad for your system to keep draining & refilling it.

 

[ALCPlumbing]

It shouldn't be a problem. Just make sure you put inhibitor in when filling back up. It could be beneficial to add a cleaner the first time you fill up. You can then let this circulate with the heating on for a couple of weeks before draining again. At least this will help remove any dirt from the system.
The problem with old vented style systems when draining and refilling is that you can easily get airlocks which can hinder circulation and cause the boiler to overheat. This isn't a problem with combi boilers and sealed systems.

 

[ZACCCC]

It shouldn't be a problem. Just make sure you put inhibitor in when filling back up. It could be beneficial to add a cleaner the first time you fill up. You can then let this circulate with the heating on for a couple of weeks before draining again. At least this will help remove any dirt from the system.
The problem with old vented style systems when draining and refilling is that you can easily get airlocks which can hinder circulation and cause the boiler to overheat. This isn't a problem with combi boilers and sealed systems.

Thanks for that info. So do you mean I should put inhibitor in each time I refill after draining or just the last time ie. when I've done all my jobs?

 

[jackthom]

Ideally it should have fresh inhibitor (quantity calculated to match up with the amount of water drained) at each refill, but it also depends on how long the system will be running between each spell of DIY.

If it's only a day or two between lots of smaller jobs, that could start getting unnecessarily expensive so I'd just add some inhibitor if it's going to be left much longer than this.

 

[ZACCCC]

Thanks for that Jack.

 

[Agile]

It is not good to keep draining.

But if you close rad valves and only drain the pipework associated with the work then you retain the inhibitor in the closed rads.

Every time you refill you are adding new oxygen to rust your rads!

Tony

 

[ZACCCC]

Thanks Tony. Bearing in mind that it's not good to keep draining, is it possible to drain the CH system but still have the domestic hot water on. That way I would be able to get all the work done over a period with one draining and refilling. I can manage without CH but it would be difficult without HW on tap!

 

[St0rmer66]

Even a week or two with no inhibitor isn't going to cause any trouble, it can take years for damage to be done. Just put inhibitor after the last time you're going to refill it.

 

[ZACCCC]

Even a week or two with no inhibitor isn't going to cause any trouble, it can take years for damage to be done. Just put inhibitor after the last time you're going to refill it.

Ok mate, thanks for that. However, what I was thinking of doing was turning off the combi system and the water supply to it, draining down the CH system and then doing my work as quickly as I can. I'll have to use a kettle for a few days to heat any water for washing but I'll be able to manage.

Btw, I'm assuming it is possible to turn off the water supply to the combi i.e. without having to turn off the water to the whole house. Am I right in thinking that? Please comment.

 

[jackthom]

Not sure this is what you mean but you don't need to worry about turning off the cold supply to the combi if all your work is on the heating circuits only.

 

[Agile]

If the heating circuit is drained you must turn the power OFF to the boiler otherwise it will try to fire up when you take hot water and will be likely to damage the boiler.

Most boilers can be isolated with pressure in them and continue providing hot water.

Most manufacturers will not recommend that though.

Practically it is very unlikely to be any problem on most models. But you have not told us what your boiler is.

Tony

 

[RigidRaider]

I'm reading this with interest because yesterday I drained our system to sort out a radiator problem. After refilling I switched on the boiler (condensing type) and at first it refused to fire despite the pump running. After turning the pump on and off two or three times to the sound of a lot of air in the pipes the boiler fired briefly then ramped down as the water temperature shot up towards 90 - I'm sure it does this to protect itself against overheating. On the third or fourth attempt the temperature went up to 105 briefly then dropped just as fast. Following that the boiler fired and ran normally and this morning all the air seems to have worked its way out and the system is running perfectly.

My concern is that overheating is obviously something to be avoided with a modern condensing boiler (I can hear the pros sucking their teeth as I write) so how can I be sure the system is fully purged of air before I fire it up? Once the boiler was running I went around again with the bleed key and found that the rad immediately above it was completely full of air; bleeding it produced some curry and beer style gurgles and bubbles from the pipework so I guess that's the rad that collects most of the air and ought to be bled before switching on the system.

 

[St0rmer66]

It's inevitable that it'll have some air in the pump and boiler pipework even if you bleed every radiator first. Best thing to do is put the boiler thermostat for heating temperature onto its lowest setting (usually around 40 degrees) so that it cuts out sooner but still allows some circulation. That's assuming the boiler doesn't have an air purge cycle that can be enabled in the service settings!

 

[RigidRaider]

That's a good suggestion, thanks.

 

[WabbitPoo]

If the heating circuit is drained you must turn the power OFF to the boiler otherwise it will try to fire up when you take hot water and will be likely to damage the boiler.

Most boilers can be isolated with pressure in them and continue providing hot water.

Most manufacturers will not recommend that though.

Tony

Can you clarify? If CH is drained, a combi shouldnt be used for DHW?

Thanks