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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.

Hi Zacccc

Every time you introduce fresh water to the system you increase any build up of limescale. You also increase corrosion in the radiators due to oxygen in the water. Also, what about the inhibitor you'll be draining away, a bit expensive to replace every time.
So try to do the work in one go. I would think you'd probably need two days for those jobs. Just make sure you have everything ready before you start.

Hi proptech

Thanks for getting back to me. You say I'll need a couple of days to do the work. However, as well as being a novice I'm a real slowcoach and anyway I can only grab three or four hours here and there so I'm sure it'll take me considerably longer than two days. I understand that it's not possible to drain the CH system and work on it while still having the domestic hot water system operating. So I think what I'll have to do is turn off the combi system and the water supply to it and then drain down the CH system and do my work as quick 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. Thanks again.

The very fact that you are asking if you need to turn off water supplies, I find rather worrying. Can I suggest that you look up how a combi system works before carrying out this work, otherwise there's going to be problems when it comes to refilling the system.

proptech wrote:The very fact that you are asking if you need to turn off water supplies, I find rather worrying. Can I suggest that you look up how a combi system works before carrying out this work, otherwise there's going to be problems when it comes to refilling the system.

I have been consulting the manual that came with my combi and I think I have a very basic understanding of how things work but nevertheless I still like to check things out with a question on sites such as this. Hence my question about turning off the water supply to the combi. But I've even received different opinions on other sites on whether or not I can drain my CH system and work on it while still having my DHW system running. It seems that this can be done but as I didn't understand the explanation of how it's done I've decided to err on the side of caution and have the combi completely switched off while I do my work.

You cannot run a combi for hot water with the system drained. I hope all goes well.

Ok, thanks mate.