hot water no heating - Honeywell V4073A1039 zone valve

...here's yet another central heating question...
The radiators have stopped getting hot but the water is still doing so.
Boiler fires but when set to heating only, quickly stops again.
Zone valve is the Honeywell V4073A1039 which is in the airing cupboard.
I've read through <http://www.diyfaq.org.uk/plumbing/controls/midpositionvalve.htm#Heating which I think describes this type of valve.
It seemed to me that the valve control might have failed but having taken the cover off I can see movement when the CH timer is set to water. However, when the CH control is set to provide heating nothing moves.
I found a replacement head for the V4073 but at GBP80 <http://www.onlineplumbingsupplies.co.uk/acatalog/Motorised_Valves.html it's GBP15 more than the complete valve assembly from my local Screwfix. However, I can't see exactly how to remove the head is it supposed to be held in place by 4 screws, one at each corner? (in which case only 3 were used when it was fitted). Is it really just a case of setting the lever to manual, undoing the screws and lifting the powerhead off and replacing it? Have I diagnosed correctly? Is there anything else I should be thinking about?
(...and, errrm, are these wires running 240V AC? and where are they likely to be getting fed from (I don't really want to have to disconnect mains power to the whole house.)
Many thanks people, from a shivery house this morning. :(
--
Si

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In an earlier contribution to this discussion,

OK, so your valve isn't moving from the HW position to either the mid or CH position. There are two possible causes for this (more if you consider things outside the valve, but let's assume for the time being that it's a valve problem). These are motor failure on the one hand, or seizure of the 'wet' part of the valve on the other. You need to determine which.
Remove the actuator head from the valve [1] (via the 4 screws you mentioned) but leave it connected electrically. This will uncover the operating spindle of the valve, which has a D-shaped end. You should be able to turn this to and fro with finger and thumb or, if necessary, using *light* pressure with a pair of pliers. If it is very stiff to turn, you may be able to free it by turning it back and forth with pliers. If that doesn't work, you'll need to replace the wet part of the valve which will require a partial drain-down of the system.
You can test the actuator by putting the heating through its paces with the actuator removed from the valve. Make sure that both cylinder stat and room stat are calling for heat and select HW-only, HW+CH and CH-only on the programmer. [Try to put the valve spindle in the mid-position before you do that]
You should see the internals rotate to the mid position on the HW+CH setting, and to the CH position on CH-only. If you then switch to HW-only, it should move back to the HW position using its spring return. The boiler and pump should run in all cases.
If you *do* need to replace the actuator, make a careful note of the wiring and wire the new one exactly the same as the old one. You should have a dedicated FCU (fused connection unit) somewhere for the heating system, enabling that to be turned off without needing to throw the main switch.
[1] Very old Honeywell valves apparently can't be separated without causing a flood, but I'm assuming that yours is a relatively recent one.
--
Cheers,
Roger
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<bigsnip>
>[1] Very old Honeywell valves apparently can't be separated without >causing a flood, but I'm assuming that yours is a relatively recent >one.
The info I found on it said it's designed to be separable.
Thanks for all that helpful detail Roger. I've only just got back from work so this'll have to wait til tomorrow. The good news about having only just got home is that I'll have all day tomorrow to fix this. :)
Meanwhile, our cat continues to complain at the change in ambient temperature
--
Si

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Whatever ...
It depends, as said above, on the age Some of the older ones do not allow you to separate the head from the valve
Those that do have, IIRC, two screws and two studs
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geoff

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writes

even within the same valve model?
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Si

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Yes
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geoff

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As of this morning the hot water isn't working either. :(
Having taken the head off the valve the movement seems to be ~15 degrees total, which doesn't seem large enough.
However, more problematically the pump also seems to have stopped working now - it's buzzing continuously but makes no sounds that I'd associate with water moving in pipework. :(
Looking at the lack of space around all the pipework in the airing cupboard and the probable lack of possible movement, I decided that I'd rather let a professional deal with this, (especially if there are complications :).
...and having contacted our home insurance domestic helpline it turns out that we are covered for work like this :) (I always forget that) so I'm definitely now going to spend my surprise day off doing something else.
Thanks again for the info tho' Roger.
--
Si

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