Likely Suspect for Cause of Heating Problem?

[Michael D]

My central heating is misbehaving & I want to check my thinking before spending =A370.

The problem is that if the 3 port mid-position valve is on =91Water=92, 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 =93W=94 & doesn=92t 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=92s 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 =91On=92 permanently and use the room thermostat to control it. - My impression is that the wiring is generally =91antique=92. - I=92m 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=92ve missed out before I shell out =A370?

Many thanks,

Michael

[geoff]

In message , Michael D writes

Sounds like the synchron motor in the actuator head has died

not difficult to replace and should cost more in the range of £10-15

[Tabby]

Why dont you test it first and see, a multimeters only =A32.50 now

NT

[Roger Mills]

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 should.

[cynic]

Is that particular thermostat battery powered and the "duracell" in it has finally died after 4 years?

[Michael D]

Thanks for the replies. To respond in order:

1. New motor: I=92d thought about replacing just the motor, but as a couple of bits have fallen off the valve anyway I=92d thought I=92d probably have to replace it. It=92s definitely a cheaper option - is it a job for a (very average) DIY=92er?

1. Multimeter: I would like to test it to make sure before I spend money on a new actuator. I=92m going to buy a multimeter tonight: how do I test it to prove it=92s the valve that=92s faulty?

2. The label says it=92s a =91Mid-Position Actuator=92. I'm not sure why i= t does work when I move it to heating only!

1. Thermostat battery: I=92ll check them, but I think they were replaced last year. The display=92s fine & there=92s no low battery indicator.

As mentioned I=92d be very interested to know more about testing to make sure it=92s the mid-position actuator. I called Drayton to confirm the replacement model number & they suggested checking the =91white wire=92 with a multimeter. How should I set the controls up to test it? How do I test and what would I expect to see?

Thanks,

Michael

[Tabby]

not sure why it

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 =A32.50 multimeter will do all you need on this one. A nicer =A310 job is also useful for tracking down RCD trips. Add 50% if you buy from a retail shop.

NT

[Michael D]

Thanks, very much appreciated. I=92m 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=92m planning to go to Maplin as I need to get this sorted (grumblings from my wife!) I was going to go for this one

but if anyone thinks one of these is a much better buy for standard DIY tasks =96 please shout: /gxkiFL

Apologies for the dumb questions, as you can tell I=92m a novice at this, so your help is very much appreciated.

Michael

[Dave Liquorice]

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.

[Tabby]

link goes to a selection of 2 items

[Michael D]

Ooops, not sure what I did there ... the 1st link was meant to be to this multimeter:

2nd was meant to go to a list of multimeters:

[Hugo Nebula]

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, so YMMV.

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 thermostat problem.

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 & VAT.

[John MacLeod]

is perfectly good enough for diy use around the house. I bought a couple of them when they were =A32.50 or =A32.99 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 =A34.99 Maplin ones. Even the test leads on them are reasonably good quality and store neatly on the back of the meter.

[Michael D]

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 =A315, 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.

Michael