syncchron motor on 3 way valve

1 Mar 2013
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United Kingdom
I'm trying to diagnose why I've got only intermittent central-heating and no hot-water.

I took the cover off the Honeywell 3 way motorised valve and noticed that the cylindrical head of the synchron motor was very hot - almost too hot to touch. There is also a black carbon deposit on the inside of the cover implying overheating.

Does anyone know if this is normal? How hot do synchron motors normally get?

P.S. the heat in the synchron is still there even when all the connecting pipes are stone cold.
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The electrical configuration of your system has likely resulted in the motor being fully wound open most of the time.

What timings do you have your heating and hot water set to?
I ony have it on 2 or 3 hours a day, and I'm often away for a week at a time when it just comes on for a short burst in the moning and evening.
This may be difficult to understand, but the valve goes to CH over two stages. First stage uses power on the white wire from room stat to get valve to mid position. Then triggering micro switch drops the power from white wire and picks up new power source from the grey wire. This drives valve to CH.
where it stays until the power on the grey is removed. there is a second micro switch triggered during the second stage that allows power also supplied by the white wire to leave the housing via the orange wire to light boiler.
When CH becomes satisfied, the room stat cuts off the power to the orange and boiler stops. But the valve stays where it is because the motor is still energised.
You will find the motor is always energised through out the night and while you're away as long as the grey wire is live and its live when HW is OFF.
If there is a demand for HW when CH is turned off, then yes the grey will be cut off and the valve will drop back under the pressure of the return spring to HW position and when HW is turned off the grey will become live again, but now it can't have any affect because the valve is not at mid position.
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Hi Mandate and thanks for the informative answer.

Do you know how hot the syncron motor usually gets?

Is the black carbon residue on the inside of the valve cover (described in my initial post) normal, or does it imply overheating?
Can't help a lot! there seems to be quite a few burning out due to the continual use I suppose. I believe there may be methods of cutting off the supply when its not wanted by the use of 'relays'.
Might be worth searching on this forum for 'relays'.
Can't think where black carbon could come from, unless its graphite grease on the quadrant gears that has dried due to the heat
Hi Mandate

Seems you know what you are talking about, but do you know how hot the synchron motor cover should be - it's almost too hot to touch - is that OK or not?
The valve is most likely in a airing cupboard so can't dissipate the heat and it sits on top of a valve with hot water straight from boiler, so it's expected to get warm, but I'm fairly sure it was never intended to run that hot
So to me it would be a problem that needed attention and replacing the motor with a new one would be easy and should resolve it.
It would in my opinion better to deal with it now while its still operational rather than wait till it gives up, or at least have a spare motor ready.
A lot of heating problems are due to the motor giving up. fortunately I don't have a motor in my system.
When your timer switches off, if the heating is still in demand, then the motor will hold the valve open till next switch on. If you often leave it for a week, if you had a heating demand when switching off, the motor will be energised for the week. Some newer programmers have a 'latching' system that de-energises the valve.
Yoiur motor will often get too hot to touch.
thanks oilhead and Mandate

I replaced the Honeywell 4073 motor valve (apart from the brassware - cos valve in brassware seemed fine). But still no heating or hot-water.

Controller and room-thermo are only just over 2 years old.

I'm thinking maybe it's the HW tank thermostat. I will have to work out how to test this with a multimeter.

Anyone know how to test the tank thermostat with a multimeter?
Disconnect the wiring, check continuity between C and n/c with a cold cylinder. If cylinder is warm, then by turning thermostat up and down will enable you to check changeover of contacts.
hi oilman

Sorry to sound thick but what is C and n/c and how do I check the continuity between them?

The cylinder is cold as I haven't had any water heating for about 3 weeks.

Any idea how much a 'strap-on the side of the cylinder' type thermostat costs?
The cylinder stat is basically a two way switch. There are terminals for two outputs but there is only one input terminal. It's known as 'c' for common because it's common to both outputs.
The switch when cold is linked across from terminal 'c' to terminal 'n/c' .
This means 'normally closed'. If electric can't get form 'c' to 'n/c' then it can't feed the boiler.
When the cylinder gets hot to whatever its been set to, the link from 'c' to 'n/c' is broken, but a new link is formed to the 3rd terminal 'n/o' normally open. This is required to to make the grey wire live when HW has been satisfied
You check continuity using a resistance meter. eg a multi meter set to the resistance mode. If there is a continuous path the meter will give a clear indication

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