Sticking motorised valve

I'm assuming that you have a separate valve for the CH, and that that is ok.

The motor in your valve is switched on by the programmer and cylinder stat, so that the valve opens when there is a HW demand, and closes when the demand is satisfied. Once the valve is open, some contacts in the actuator close and switch on the boiler and pump. This is obviously not always happening in your case. The valve is either not opening at all, or it's opening but not far enough for these contacts to close. Operating the manual lever opens the valve and enables the contacts to close.

Leaving it permanently in the manual position will mean that the boiler and pump will run all the time, and the water will get too hot because the valve won't close when the cylinder stat is satisfied - so *don't* do that!

You could try removing the actuator from the valve and turning the valve spindle backwards and forwards a number of times, and then replacing the actuator. That might just free it sufficiently to make it work more reliably.

If , when you remove the actuator, you find that the valve does turn freely over its full range, there may be a problem with the actuator itself, but that sounds less likely.

I only have experience of Honeywell. The sympoms you describe are consistent with the synchron motor having lost the oomph to fully open the valve and then drive the contacts closed, though with a Honewell switching to manual doesn't normally make the contacts. Is it possible that the CH was also calling for heat?

If its a Honeywell the head can be replaced which doesn't require draining down but does require the wiring centre to be accessed. It's possible you could takle the task safely as a novice diyer - but only if you are confident you can isolate the mains to the wiring centre.

Once you get more confidence the synchron motors are replaceable.

Jim A

On mine it won't keep the boiler and pump running but yes it usually means that the HW will be heated(or cooled) when demand for CH is also called for but if you set your boiler output to a lower temp there' little risk of the water getting "too hot". I suspect the motor in the valve is at fault. One from screwfix should do it.

I changed the motor on one of these recently.

Very simple (if it's Honeywell):

1. Buy the part.
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   Surprisingly inexpensive
2. Install it. In my case I used crimp connectors within the valve head. No need to do anything at the wiring centre (except isolate the power of course).

David

Problem is that if it's the 'wet' part of the valve which is partially seized, changing the motor won't help - and that hasn't yet been bottomed out.

In that case, remove the valve cover (1 screw, just loosen it a turn and the lid will slide off), and then IIRC you will see 2 screws that fasten the head to the valve.

Remove those then lift off the head and you should be able to easily rotate the valve itself and feel for any "stiction".

....I bet it's the servo taht's the problem!

The message from "Vortex3" contains these words:

YMMV.

I have have 3 failures over the years. Two motors and one valve body.

I have had a number of motor failures, never a body. The guy who invented the MV must have had a little probably explaining to his fellow engineers the validity of a design that relies on stalling an electric motor and keeping in this state for ages at a time.

but not to the accountants, eh ?

Too true! :)

I last bought one in 2005 - cost me around £35 with the "trade counter discount".

Every time British Gas quote "£12 a month for peace of mind" I giggle to myself.

/jk