The central heating system in the house that I bought as new 5 years
ago is fitted with Pegler Terrier II thermostatic radiator valves.
About a year ago I noticed that on a couple of radiators even when the
valve was turned off (* setting) the radiator remained hot so I
assumed the valve had failed and replaced both valves. Having just
drained the system and refilled it with Fernox cleaner I have noticed
that another 3 radiators are staying hot when the heating is on
regardless of whether the valve is turned off or not! It seems that
the small pin is not being pushed down by the head. I have tried
swapping the heads with heads from other radiators which are working
okay but this doesn't seem to make any difference. This confuses me
as I thought the problem was in the head not in the valve (the part
connected to the radiator).

Can anyone explain how these valves work and what is going wrong? Is
there any way of repairing them or do they need to be changed? Have I
got a bad make and need to switch all the valves before they continue
to fail (I would rather resolve the problem in one attempt rather than
spend the next 5 years having to change individual valves as they
fail).

Thanks in advance for your advice.

Regards.
Paul.