DIY Doctor UK DIY Forums

My girlfriend just had a combi boiler installed.

She used to have a conventional heating system with an expansion tank in the loft, but the new system is sealed with an expansion vessel in the boiler unit.

I know that the system is filled by a flexible hose between the mains cold water feed and the central heating circuit, but how on earth does inhibitor get added to a system like this?

When the system was installed we had to bleed a large amount of air out of several radiators. Without an expansion tank and the usual supply of cold water into the system when needed, how does the system stay topped up? Do I have to open the feed from cold water supply to ensure the system stays full?

The favourite way to get inhibitor into a closed system is through the top of one of those ladder towel rails. Failing that, remove a blanking plug in the end of a radiator.

Of course this must be done when the system is empty or partially drained. Generally, because of the concentrate level of inhibitor, one dose is sufficient to last several minor top ups.

A good sealed system may last many months without the need to top which is then done via the hose you describe until the pressure is between 1.0 & 1.5 bar. If the system needs topping up every few weeks or even more often, then the system is leaking and the frequent refills will dilute the inhibitor over a period of time.