Hi, Our central heating has just started misbehaving. Even though the temperature near the downstairs thermostat is above that which should start the central heating, whenever we run the hot water and the bolier fires up to heat the hot water tank, the radiators also start heating. Now I assume this must be the two way valve not closing off the central heating. The light on the downstairs thermostat stays on while the hot water is heating even if I turn the dial right down to zero. Does anyone know if this could be just a faulty valve or could it be something electrical in the controller. Has anyone any ideas, please, before I have to get someone in to look at it. Lawrie
Can you take the head off the two way valve, if so you will be able to see if the actator is turning when you turn the room stat up/down and you can also try turning the valve itself with a pair of pliers (don't force it though as they can start to leak), been there and done it :-( . It's probable that the valve itself has seized and that probably means draining the system to change it.
However it would be useful if you can tell us exactly what make/model the various 'bits' are.
If the thermostat is telling it to turn on then that is the first thing to suspect. Sounds like the contacts are stuck together.
As for the valve, in my experience the mid position valve is not what gets stuck but the microswitches in the actuator which get burnt out. However, in your case it sounds like the thermostat.
Anyhow, if you have the normal type of timer selector it will be easy for you to check if the valve or actuator are stuck in the mid position. Simply turn off the CH side. This will override the wall thermostat, and you should also hear the actuator driving the valve fully across to the HW only position. Your rads will then cool down unless the valve is either stuck in the mid position or being held there by a faulty actuator.
If the rads do cool down then you have a faulty thermostat. (You might just be able to take it down and clean the contacts - remembering to turn off the power first...) If the thermostat (disconnected!) itself looks good and a resistance meter shows you that it turns on and off when you wind the dial back and forth: then it should work. Mind you, the dial may have slipped round, so when you think it's set to 15 say, it might really be set to 25. You do well to keep a thermometer nearby it as an extra means of calibration.
If the rads don't cool down even when you have sorted out the problem with the thermostat and its light is going on and off properly (ours doesn't have a light but we can still hear it click and then the boiler start up if it's working): then you may also have an actuator fault, but it would be quite bad luck to have both at the same time.
A simple way to detect if your actuator is giving trouble, is to stick an fm radio next to it, and then alternately run ch on its own, hw on its own, and both together. Listen for the motor driving the valve back and forth with each one you press. When the actuator is having trouble settling, due to burnt or burning or wearing out contacts, the radio goes pop pop pop pop pop. If it does not settle down after a short time and continues to interfere with the radio, you need a new one (the switches inside can be changed but in my experience it is very fiddly dealing with all the springs and then fine tuning.).
Also, if you can get your hands to the outlet pipes from the valve, you can feel if the heat is going down the rad pipe when it shouldn't, without having to wait for the rads themselves.
Luckily it looks like yours is just the thermostat playing up.
What's it got to do with the thermostat? all it does is override the CH switch: and if the light says it's on then it obviously isn't overriding it! For this to be a sticking valve or actuator the rads would have to be hot when all the switches were off: not when one of them is clearly indicating that it is on!
A stuck mid position valve is a very common problem. One of the main failure modes is that the CH comes on when only HW is called for. This occurs when the valve is stuck just after the mid position and both internal microswitches have operated. If the valve is stuck in such a position then calling for hot water will heat up the radiators as well as the HW.
The big clue is that there is a light showing on the room stat when the CH is not being called for but HW is being called. This suggests that the internal switch 2 of the mid position valve has operated (or has broken) and the HW signal is backfeeding through the mid position valve via the white wire to the room stat.
Well I certainly concur with the crap nature of the internal switches in the actuators, as I've replaced three sets myself - and in units that can cost £60 a time this is pretty disgraceful performance. Anyhow, our room thermostat doesn't work like that, and it seems completely unnecessary for current to flow through it when it's contacts are open - unless the light is to tell you that the heating *isn't* on, which seems daft to me.
I'll bet your room stat does operate like that if you back feed a voltage.
You have 3 terminals at your roomstat. Live in, Call for heat and neutral. If the call for heat terminal is made live due to a faulty 3 port valve back feeding a voltage to it then the neon in your thermostat (which is across the call for heat and neutral) will light up when HW is called for and thius will not turn off whatever the setting on the room stat.
No: as I would expect a thermostat to be, my Servowarm has an in an out and an earth and no neon. (Just as is shown in the uk-diy faq in fact:
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) And if it did need a neon I would put it on the controller side of the contacts: not on the side of a subsidiary circuit that could feed back false information. It cannot 'call for heat' unless it is both powered up by the main selector switch, and the temperature has closed its contacts. It is just a switch, and when it is off there is nowhere for any 'back feeding voltage' to go. Which, as your design seems to make it difficult to distinguish between a thermostat fault and an actuator fault, I'm very glad to have my version!
S
No: all that is needed to determine a stuck valve or actuator, is to turn off the CH and see if the rads come on when the hot water is on. Your neon or heater resistor is just one more thing to go wrong.
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