On 25/10/2019 10:21, harry wrote:

There are two sorts.
One has a float.
The other has a material which shrinks when it dries out (usually a washer) and swells when wet. You have to soak them in water (or let them be soaked) before they close off.
Haven't seen the latter for years though.


The Aladdin ones fitted to radiators are effectively the latter type. I
wouldn't expect them to work too well if the system were drained down
and then refilled soon afterwards as they wouldn't have had time to dry
out. In my case the radiators had been removed and drained down for a
few weeks allowing time for the valves to dry.

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