I have a Horstmann Centaurstat 7 programmable room stat. It gives the
appearance of working OK. It clicks at the right times etc but the heating
circuit in the zone that it controls doesn't close down when it teaches the
set temperature, despite the appropriate click.
Normally I have the programmer set to constant for this zone. If I change
this to off or timed it switches everything off correctly. Even if I remove
the Centaurstat body from the backplate the circuit continues to work (if on
at the programmer)
I tried a re-set (set and enter buttons) to no avail. Would a replacement
unit solve the problem or is it something else?
Any suggestions most welcome. Many thanks.
Basically any room stat is just an on/off switch regardless of how clever it
appears to be.
Whilst not familiar with your model most stats when removed from their
backplates disconnect from the system.( when you take it off ,no wires are
If this is the case then the programmer is wired wrongly.
From your post it seems the system works fine if on 'Timed' but if set to
constant the stat does not work.
Again suggesting a programmer fault or wiring fault at the programmer.
To verify run the system on constant ,remove the stat (turn off power first
it may be mains switching) make sure no wires are connected to the stat
,insulate any bare wires ,return power .
If system still works the fault is either in the programmer or a secondary
We really need more information for further help.
What type of boiler do you have ?
What type of system (fully pumped 'y plan' etc)
What programmer controls the system?
The system has been running fine for 6yrs so, with respect, I doubt if
this is the case
Whether on timed "on" or constant at the programmer the circuit is "on"
and is not turned off when the thermostat is apparently satisfied. When
turned to off or timed "off" at the programmer the circuit is off.
Tried this and with only the base plate remaining, the circuit
remains on (dependent of course on the settings at the programmer)
Glow Worm. Ultimate 80 FF
Fully pumped two zones plus water to tank all with 2 port
Horstmann H37 3 channel
(not sure that it's series 2)
">> Many thanks again. The system has worked absolutely fine for
That is with no stat in place, just the baseplate with exposed terminals. No
voltage when turned to off at the programmer
Tested for continuity between the prongs of the stat when removed. (Lower
fits to common on the base plate, middle to call and upper to neutral)
Continuity between lower and upper when set to low temp; lower and centre
when set to high temp.
sounds like the stat is controlling a zone valve somewhere hence the
switching on high & low settings it could be that this valve is stuck open
so even when the stat (satisfied for heat) calls it to close it remains open
,could also explain readings at stat backplate.
Of course sods law will probably point to the new mirror you just fitted
being nailed through the stat wires shorting them out.
In any event the stat does not appear the problem unless the backplate is
broken causing a short,you need to investigate the wiring further I'm
A tip for motorised valves feel around the front end for a thin protruding
piece of steel this is a manual opening device.
Move it and if no resistance is felt the valve is open if it feels hard to
move and you hear the valve whirr as you move it the valve was shut and you
are forcing it open.
Maybe other on group may have other ideas but for now
You really are very helpful and I'm really grateful. It's a real puzzle and
I'm determined to solve it.
I guess if the valve is stuck open then that zone will heat up even when
switched off at the programmer, if the water or zone 2 circuit is on! Will
check that tomorrow. Also will ask whether the good lady wife has been
hanging decorations! I feel sure as you say that the stat is OK. Glad I
didn't buy a new one.
A wiring diagram is here for you
I would guess at the S plan plus.
The valve may only be partially open/stuck and so not making the microswitch
contacts it usually makes when fully open. You idea or trying the other
zones off is a good one.
The valve seemed to be working fine, controlled by the progammer as
suggested by CJ
So disconnected brown wire between valve and stat feed at junction box.
Still both common and feed terminals at stat backplate live when progammer
on. Tried disconnecting common to stat at the junction box. Both common and
feed now dead! Seems like there is short between the common and feed between
stat and junction box. This fits as it was the only piece of cable not
replaced when the whole system was refurbished 6 yrs ago. Propose to fit rf
stat, mounting receiver next to junction box and use my Centaurstat on Zone
2, which at present has non-programmable stat. Does this sound reasonable?
Thanks again both.
Does this sound reasonable?
Try this first. Rewire your prog stat with some flex to the existing system
(close to the wiring centre and just for testing purposes) and disconnect
the stats old wiring whilst you do this. If things work ok then it is the
prog stats wiring that is at fault.
HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.