Refilling pressurized CH system

[Andy Bennet]

As I am getting on in years and no longer fancy climbing ladders to clean the upstairs windows, a while ago I bought one of those water fed pole brushes and a pressurized water deionizer to do the job. I must say that it does an absolutly brilliant job in next to no time and wished I had invested in such a kit years ago. I use it for the downstairs windows as well as it is so easy and fast. But I digress... I need to do a small amount of system pipework relocation on our (sealed) CH system next year and the thought crossed my mind, as we live in a very hard water area, that I could repressurize the system with deionised water and inhibitor. This would help to prevent scaling up and kettling noises in the boiler. Now I know that pure water can be quite agressive in terms of corrosion etc etc but is this a good idea or should I just forget it and top up as normal?

[harry]

Limescale is not a problem in CH systems as there is only a small quantity of water involved, ie the same water recirculates. The crap you may find in your boiler/radiators is the result of corrosion (sorted out by an inhibitor/additive to the water). There are different inhibitors for different metals in your system (eg you may have an aluminium boiler),

The other cause of "kettling" is poor water flow due to bad design of the CH system/pipework.

[John Rumm]

I would say that since you are talking about a recirculating system, where fresh water is introduced rarely, there is no particular advantage to using deionised water. Many boiler makers also caution against using softened water in the primary CH circuits.

You can get additives to suppress kettling if its already happening. Sentinel do a "boiler noise reducer" product that works - although needs replenishing each year IME.

[Reentrant]

Is that still the case? We had an unvented system and a whole-house water softener installed recently. The plumber said we should always bypass the softener when repressurising, but I've since found various pieces on the web saying otherwise, so long as there's inhibitor in the system

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