Zone valve replacement. Question on draining down

30 Nov 2010
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United Kingdom

I'm replacing CH and HW zone valves on a S plan unvented system. They are located just off the boiler on the 2nd floor. There are 2 small and 2 medium rads on this floor, 6 mixed size rads on the 1st floor and 6 large and medium rads on the ground floor. Will I need to empty the whole system before changing the units, or can I get away with just draining the top (2nd floor) rads?

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As long as drain down to just below the level of the valves you are changing that will be fine. Don’t forget to add inhibitor when filling up.
Last edited:
Thanks Mottie,

I've got the inhibitor at the ready!

I have an expansion vessel with a filling loop attached (off be default of course, see picture below). I can't see any other supply to the vessel? In this case do I just need to empty the top floor rads? Cheers.
Expansion vessel. Dec 2020..jpg
Sorry, edited my reply. I thought you had a vented system but reading again, it’s a sealed system. Same as mine and when I need to partially drain mine, I drain from the lowest point and open a bleed valve high in the system until I’ve let enough water out. You’ll know when you’ve let enough out because when you loosen the zone valves, with the system draining you’ll hear air being sucked in. If not drained enough, water will **** out!
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