Central heating - logic check

[IsleofWightPete]

I have a large central heating system, split into two zones, controlled by two thermostats, one for each zone.
Yesterday, when Zone A was calling for heat, the rads in Zone A weren't getting warm, even though the motorised valve for that zone appeared to be open. I then forced Zone B to call for heat, and which point the boilers* fired up, and rads in both zones warmed up.
* there are 2 boilers, but they are not dedicated to zones; they work in tandem and their output goes through a mixing column, where it interfaces with the circulating water. However, given both boilers behaved the same, it allows me to rule out the boilers as a cause of the fault.

I am therefore tentatively concluding that it must be a signalling failure from the Zone A valve - it appears to be getting the signal from the stat (hence it was open, and allowed the water to circulate once the boilers fired up on command from Zone B) but isn't passing on that call for heat signal to the boilers. However, on thinking about it when I turned off the call for heat from Zone A, I don't think the valve closed again. So maybe it is stuck in the open position (lever was floppy)??

Any help with finalising the diagnosis, please?

And, is that likely to be a replacement motorised head required, or full valve replacement?

[jfgw]

If the valve is stuck but not fully open, the switch that sends a signal to the boiler will not operate. The problem is probably the valve body, not the actuator. If you take the actuator off, you can check whether the spindle turns freely.

If it is a very old Honeywell valve, the body might come apart and cause a flood when you take out the screws to remove the head.

Julian F. G. W.

[stewamax]

There is an easy ways to check: with the ubiquitous Honeywell 4043s and similer (the newer ones with a dimple on the case ), the head itself comes off with two screws and no efflux of water (don't do this with the old non-dimpled ones or you will have a lot of mopping up!), and the spindle sticking out from the valve should be turnable (not much - about a third of a turn) with your fingers. If it isn't, then you may be able to free it with gentle manipulation but you probably need to replace the valve.

If the spindle turns easily:

a.try a replacement Synchron motor first (cheap and quick to fix). Get a genuine one: the real cheapos are a nuisance.

b. If this fails, buy the whole valve (not just the head): it sounds as if the spring-loaded mechanism driven by the motor is worn and stuck open. The valve itself could then be more or less open, but not open enough for the microswitch in the head (which starts the boiler and pump) to be triggered.

c. But - even though the spindle turns by finger pressure - the valve itself could be corroded and/or full of sludge and intermittently sticking - and you will curse if you have only bought the head!

Normally when the motor fails, the valve just stays shut (not your case), so my guess your problem is b. , and you can always keep the unwanted motor as a spare.

[pendas]

I'm not suggesting this as the cause of your problem, but if the micro switches do prove to be suspect, I've got years more life out of them by squirting WD40 into the switch body and pushing the actuator many times.

If you have an ohmmeter, any slight variation in the measured contact resistance with variations of pressure on the actuator indicates a dirty or worn switch.