I am considering fitting a thermostat to my central heating system which
at present is a combi boiler with TRV,s fitted to all rads.
I am told it is not advisable to have TRV,s fitted to the rads in the
room where the thermostat is fitted.
Would it be ok however to either set the TRV,s to max or remove the
screw on head to save me having to change the radiator valves?
Any thoughts appreciated.
How old is your system?
It is possible that an alternative system to a room thermostat is installed.
This achieves boiler interlock by detecting flow on an automatic bypass
circuit and shutting the boiler down when it is detected. If so, your
upgrade will not be appropriate. If such a system is not installed, then the
room thermostat is an excellent idea.
Although it could work by turning the TRV on full, it is better to install
lockshield type valves at both ends to prevent someone operating the TRV and
thus causing inefficiency in the controls.
Yes, I've heard of this - but don't understand how it works.
In particular, what starts the boiler up again? Does the pump have to run
all the time so that flow through the rads restarts when a TRV opens -
stopping the bypass flow and starting the boiler?
Either the pump runs at a slow speed (but with enough pressure to open the
automatic bypass), or the pump could pulse every ten minutes to determine if
any TRVs have opened. If there is no bypass flow, the boiler can be allowed
This has the advantage of acting as an anti-cycling feature. I don't know if
anyone produces a unit with the correct logic to do this, but it wouldn't be
difficult to do so with a little electronics knowledge.
It's starting to get complicated! I think that if I were going for this
level of complexity, I would rather have a zone valve on each rad controlled
by a room stat in each room - and with the volt-free contacts on the zone
valves being connected in parallel to control the boiler and pump when any
combination of rooms required heat (described as S-Plan-Plus in
[If each room stat was programmable, you could have a *really* sophisticated
But then that can get very expensive. Also, it is difficult to site the zone
valves so that they are accessible. The easiest way is to have a manifold
system with all the zone valves in a cupboard. However, this takes space and
the pipe runs have to be installed with this topography in mind.
An alternative is to use motorised radiator valves instead of TRVs. These
can be wired up to local room programmable thermostats. This will be cheaper
and easy to retrofit to an existing installation. The irony is that without
the microswitches of the zone valves to run the boiler off, you'd still need
to implement the automatic bypass interlock anyway, to detect when the
valves were open. (Although this could be done electrically too, with a load
of wiring and some OR logic).
As long as each valve has a pair of contacts which close when the valve
opens, and which are completely separate from the motor supply, the only OR
logic required is to connect all these contacts in parallel - then, whenever
or or more valve is open, the boiler and pump will run. You will still need
a by-pass circuit - but only to cope with pump over-run while the boiler
cools after the last valve has shut. This doesn't need any fancy pressure
switches - but a pressure-operated auto by-pass is preferable to a
permanently open one.
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