My room thermostat does not appear to be working , i.e it is not cutting off the heating when it reaches its temperature, therefore the heating is just constantly belting out heat, I have took the casing off and cleaned it out in case it was the connection but no difference. Can anyone advise what else I should do or do you think it needs replacing? Cheers

Daz G wrote:


My room thermostat does not appear to be working , i.e it is not cutting
off the heating when it reaches its temperature, therefore the heating
is just constantly belting out heat, I have took the casing off and
cleaned it out in case it was the connection but no difference. Can
anyone advise what else I should do or do you think it needs replacing?
Cheers



Hi

First I would check the thermostat electrically. With the usual provisos of
"be careful, it's probably got 240V across it, don't do this if you are not
confident of your competence etc":

Stick a multimeter set for an AC range 250V across the contacts with
the system powered. Twiddle the knob until you hear the click.

Either side of the click, you may read 0V and 240V - if so, thermostat is
OK.

If 240V, twiddle again until it goes click and you should get a reading of
0V.

If it always reads 240V irrespective of contact position, then the contacts
are probably knackered or some other open circuit fault.

Used this method last week to prove my Dad's HW thermostat - transpired the
2 port valve was seized. I proceeded with extreme care as the
"professionally" installed wiring was a fecking rat's nest of crap stuffed
into a junction box, in fact to call it so is disrespectful to rats. Had to
remake one connection for being half out of the (one of many) bits of choc
block. Why can't the cheap gits use a proper terminal box? Labelling would
be nice too. As would strain relief. And they wanted to ban DIY electrics.
I ask you. Wasn't dangerous when I'd fixed it, to be honest it wasn't
particularly dangerous before, but it was still a disgrace.

Sorry - back on point:

If it always reads 0V, then either the contacts are stuck (or some other
short circuit) or your have a DC control signal to the boiler (how likely I
don't know - all the boilers I have experience with present mains at the
room thermostat). Or you have a very low AC voltage and your meter isn;t
sensitive enough. A digital meter should be fine though.

You should probably at this point remove the thermostat from the circuit
after proving the boiler supply is isolated and the thermostat has no stray
live voltages in it (no accounting for previous wiring and possible
multiple live feeds to different parts of the heating controls).

Then test with the meter on a low resistance range following the same
procedure.

If your brave and you know your meter will stand it, you *could* test in
situ on a DC range of about 400V (to allow for mains peaks if you were
wrong). In this case, I'd have a good probe around on the 250V AC setting
to known earths/neutrals to convince myself that there really wasn't mains
present.

This is USENET - if you don't feel comfortable with the above, don't do
it

HTH

Tim

Daz G wrote:

My room thermostat does not appear to be working , i.e it is not cutting
off the heating when it reaches its temperature, therefore the heating
is just constantly belting out heat, I have took the casing off and
cleaned it out in case it was the connection but no difference. Can
anyone advise what else I should do or do you think it needs replacing?
Cheers

Ah, diybanter. If your stat is managing to switch the 240v, its
something else, most likely the 2 or 3 port valve. If your stat doesnt
switch the 240v, the stat aint right.

Stats have 2 3 or 4 wires.

2: live, switched live
3: as above plus neutral to power the compensation resistor
4: as above plus earth

If you dont know how to do it safely, dont, as it will be live while
testing.

If its a bimetal stat, as it probably is, and it isnt switching, stuck
contacts can be wiggled apart and the surfaces smoothed with a rats
tail file or very fine sandpaper. Obviously dont do this with power
connected to it.

Doing this could knock the alignment off, so afterwards slowly turn it
till it clicks, to see if air temp matches dial temp. If not, adjust.


NT