CH gone on lower floor. Any suggestions??

Hi-
I'm having problems with my central heating and was wondering if someone could tell me what they think has gone wrong.
The boiler was only fitted 3yrs ago by British Gas, and I'm a bit annoyed that it is already causing problems (even more so, since I did not take out British Gas insurance after the 1st year warranty).
The boiler is a Potterton Supreme. Basically, the water heats up, upstairs is fine, but downstairs radiators do not work.
I've looked at the boiler, and underneath I can see two 'Valve Actuators' - little blue boxes which attach around 2 pipes (I'm assuming one is for lower floor, one for upper floor).
When both floors are set to heat, the top floor thermostat clicks when I turn it, but the bottom floor thermostat does not.
When I manually set the top floor to heat I can see a little lever on the side of the top valve actuator move, then moments later the boiler springs to life.
When I do the same for the lower floor, the other valve actuator lever does not move at all.
Can someone tell me whether they think the problem is with the lower valve actuator or with the lower floor thermostat. I'm assuming one is affecting the other, I'm assuming if the valve actuator doesn't work it will stop the equivalent thermostat from clicking.
Sorry to sound so vague - I am not experienced with any of this.
If it is the valve actuator, does anyone have any idea how much another one would cost? and how much British Gas would charge for labour? Would it be cheaper to get a non British Gas engineer?
Thanks for any replies.
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The usual system is that the programmer/time switch feeds the thermostat which, in turn, feeds the zone valve. If the zone valve isn't opening, it could be due to one of several reasons: * duff zone valve * duff thermostat * no feed from programmer to stat * duff wiring berween stat and zone valve
The fact that the stat doesn't click when you move it past the actual temperature of the room it's in (I think that's what you're saying?) means that the stat is the prime suspect. If you feel comfortable doing so, short out the two wires going to the stat ** and see whether the zone valve then opens. If it does, it's definitely the stat.
** turn the power off before joining the wires together - and make sure that nothing live is accidentally shorted to earth or neutral
Roger
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Thanks for the advice Roger. I've taken the thermostat cover off - I will try this tomorrow (during the daylight). I just want to confirm one thing :
there are 3 wires going into the thermostat base (unfortunately all brown).
However on the thermostat base moulding where the 3 wires go in, the moulding is labelled 'N', 'L' and '3' . Your last comment made me wary of going ahead before I get a reply. Can you just confirm that I should connect a shorting wire between the N and L wires. (I'm assuming the wire which is marked '3' on the thermostat is the earth....).
Thanks
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means
short
then
that
This 'stat sound like a mains fed one, so you shouldn't (Should Not) short over L and N. The 'stat is more likely to be a switched live which controls other mains fed appliances in the system.
Is it an electronic thermostat ? Does it show numbers on a little screen ? Does it have a light inside ?
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Seems very analogue to me. No lights, no display, just a plastic rotary dial on the front... going to keep on waiting a bit :)
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No comment. ;-)
--
*On the other hand, you have different fingers.

Dave Plowman snipped-for-privacy@argonet.co.uk London SW 12
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means
short
then
that
No, DON'T short L & N - you'll get a big bang! Try to work out which wires are connected to the contacts - almost certainly L & 3.
[Some stats have a neutral connection because they have a little heater resistance in them which gets warm when the system is on, and improves the response.]
Roger
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Many thermostats will click regardless if they have power to them or not - assuming you can 'rock' them either side of the actual room temperature.
Have you tried moving the arm on the valve by hand? That's what it's there for - to operate it if the electrics fail.
If this works, then either the thermostat, valve, or the power to them has failed. If you know a half decent electrician, I'd get him to look at it rather than a plumber - central heating systems often have the most appalling standards of wiring installation.
--
*When I'm not in my right mind, my left mind gets pretty crowded *

Dave Plowman snipped-for-privacy@argonet.co.uk London SW 12
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Dave- firstly, the working thermostat (upstairs) does not click when the heating is not on.
Secondly, I'll try moving the valve by hand when I get back tonight. Thing is, that still won't tell me whether it's a broken thermostat or a broken valve actuator (motor) or loss of power will it.. I think I need someone with a multimeter... or buy one.
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Right - older basic mechanical ones do as you're just hearing the contacts mating. Dunno about new basic ones - I've got a programmable one which is battery operated and gives a vast clunk when it operates. ;-)

If I had to guess, I'd go for the motorised valve as thermostats rarely fail.
--
*You sound reasonable......time to up my medication

Dave Plowman snipped-for-privacy@argonet.co.uk London SW 12
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