My central heating is misbehaving & I want to check my thinking before
The problem is that if the 3 port mid-position valve is on ‘Water’,
when the room thermostat then calls for heat the valve does not move
across to the mid-point or 'Heat'. If I move it to Heat using the
manual override on the mid-position valve, the heating then clicks on
immediately and works normally (... until water is next needed and the
valve moves to “W” & doesn’t move back).
A couple of facts:
- The room thermostat is a 4 year old Honeywell CM907 (ie a fairly
sophisticated digital one).
- The mid-position valve is a 10 year old British Gas model. It’s
generally in poor condition, with a couple of bits of plastic that
have come off.
- The time clock is maybe 20 years old. I leave the CH switched to
‘On’ permanently and use the room thermostat to control it.
- My impression is that the wiring is generally ‘antique’.
- I’m want to replace the whole set-up & boiler in the spring.
My thinking is that the mid-position valve is the most likely
suspect. Anything I’ve missed out before I shell out £70?
Sounds like a problem with the mid-position valve but, as others have
suggested, you may be able to replace just the actuator - or even just
the synchron motor - rather than the whole thing.
One thing I don't quite understand,though. With most valves, using the
lever to select the manual position only moves it to the mid-position -
which is not far enough to tell it to fire the boiler once the HW demand
is satisfied. UNLESS it isn't a mid-position valve but is simply a
diverter valve - so that you have a W-plan system rather than Y-Plan.
When it's working normally, does it heat the HW and the radiators at the
same time, or only one at a time?
Either way, is the actuator easily removeable from the wet part of the
valve? If so, remove it and check that the shaft of the wet part rotates
freely - because that may be your problem if it is very stiff. Also, put
the actuator through its paces whilst removed (but still connected
electrically) by selecting just water and just heating and (if
appropriate) both together - and see whether the innards rotate as they
Thanks for the replies. To respond in order:
1. New motor: I’d thought about replacing just the motor, but as a
couple of bits have fallen off the valve anyway I’d thought I’d
probably have to replace it. It’s definitely a cheaper option - is it
a job for a (very average) DIY’er?
2. Multimeter: I would like to test it to make sure before I spend
money on a new actuator. I’m going to buy a multimeter tonight: how
do I test it to prove it’s the valve that’s faulty?
3. The label says it’s a ‘Mid-Position Actuator’. I'm not sure why it
does work when I move it to heating only!
4. Thermostat battery: I’ll check them, but I think they were replaced
last year. The display’s fine & there’s no low battery indicator.
As mentioned I’d be very interested to know more about testing to make
sure it’s the mid-position actuator. I called Drayton to confirm the
replacement model number & they suggested checking the ‘white wire’
with a multimeter. How should I set the controls up to test it? How
do I test and what would I expect to see?
To change position the unit needs power. One multimeter prod to wire
under test, other to neutral or earth. Now, get your CH system to
change the valve's position, and the meter just confirms (or not) that
the valve is getting the necessary power. You'll need to have it
change position in both directions to see exactly where the system's
failing. More likely its your valve, but the problem could also be a
controller or wiring fault, with the valve not getting power to move
in one direction.
A basic £2.50 multimeter will do all you need on this one. A nicer £10
job is also useful for tracking down RCD trips. Add 50% if you buy
from a retail shop.
Thanks, very much appreciated. I’m assuming that the wires will have
their normal colour codes. How long should I expect to see the power
lift for, just a second or two to move the valve or until the room
stat stops calling for CH?
In terms of buying a multimeter, I’m planning to go to Maplin as I
need to get this sorted (grumblings from my wife!) I was going to go
for this one http://bit.ly/eSe0wp but if anyone thinks one of these
is a much better buy for standard DIY tasks – please shout: http://bit.ly/gxkiFL
Apologies for the dumb questions, as you can tell I’m a novice at
this, so your help is very much appreciated.
On Thu, 9 Dec 2010 05:19:57 -0800 (PST), Michael D wrote:
The only dumb question is the one that isn't asked.
Just be very aware that the electrics are mains. It will hurt if you
touch a live bit or make a loud bang/flash if you accidentally short
something out with the meter probes, say one slips off a terminal.
Cheap meters don't come with decent probes, might be worth getting a
set to go with the meter when you buy it.
http://www.maplin.co.uk/Module.aspx?ModuleNo=37279 is perfectly good
enough for diy use around the house. I bought a couple of them when
they were £2.50 or £2.99 and they're both cheap enough and robust
enough to keep in a toolbox. I'm all for good meters -- and have a
couple of analogue AV) Model 8s, but you'll have no problmes with the
£4.99 Maplin ones. Even the test leads on them are reasonably good
quality and store neatly on the back of the meter.
My local Maplin didn't have the one I wanted, so I waited a day &
ordered one from Screwfix. Based on the advice I didn't go mad, but
bought one for £15, which seems to have more than enough for my needs
for the forseeable future.
I tested the actuator & it seemed to confirm that the room thermostat
was behaving itself, so it definitely was the actuator.
I've now ordered & fitted a new valve and everything's working again.
Many thanks for the advice, much appreciated.
[Default] On Thu, 9 Dec 2010 03:45:13 -0800 (PST), a certain
keyboard and wrote:
I speak from experience here, having had exactly the same symptoms
last week. However, I still don't have a clue what I'm talking about,
To test w/a multimeter: firstly don't plug the leads into the wrong
sockets and blow a fuse in the boiler at 15:30 on a Sunday afternoon,
otherwise it's a frantic trip to Wickes (who don't sell 1 amp fuses)
and B&Q (who do). Secondly, don't ask me how I know this.
Next, turn the HW off or the cylinder 'stat to low, and the room stat
to high (calling for heat). Using the multimeter correctly, stick the
neutral prong to the 'common' wire (usually blue) and the live prong
to the white wire. You should get a reading of 240V. If not, it's a
Mine needed a replacement valve, including the body. I'd replaced the
synchron motor, but I'd knackered up the rest of the actuator in the
process. It was one that couldn't be replaced without replacing the
whole thing, and I wasn't prepared to attempt it myself in the middle
of a cold spell, so I found a plumber to do it for £170 inc valve &
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