Central Heating Fault Diagnosis

[thescullster]

Hi all

Hoping someone will be able to confirm my suspicions regarding lack of heat to ground floor.

The system is an open vented S Plan Plus (I think). Essentially it has

3 motorised valves, one for hot water, one for upstairs heating and one for downstairs heating. These are controlled by a tank stat and timer for the hot water and two Honeywell programmable wireless stats for the separate upstairs and downstairs heating.

The issue seems to be when the downstairs stat only is calling for heat. It looks like the radiators are heated for a short while, but then not. The stat is calling for heat and this is indicated as received by a green light on the base station in the airing cupboard.

If I then bring in demand for the upstairs heating circuit, both upstairs and downstairs get hot.

There are a couple of photos of the valves and piping for download here, the most relevant being the first. The valves are labelled. But as the system has generally been working fine for ten years, the actual layout is probably not too relevant.

The two valves in plain sight are for heating, the hot water one being towards the back of the cupboard horixontally mounted.

Can anyone can suggest the cause, diagnosis and fix method please?

Thanks

Phil

[Bob Minchin]

When the downstairs only problem arises, is the demand for heat signal arriving at the boiler (maybe dodgy demand switch inside downstairs zone valve) if so is the pump running or has the boiler shut down due to internal overheat? I'm thinking that maybe downstairs flow is constricted somewhere or has air in so that heat is not flowing to the rads, boiler gets over hot and shuts down. In this circumstance the boiler should slowly cool, fire again and quickly shut down as it heat itself up to over heat once more.

Also what happens if you manually open the lower zone valve.

We need more info as to the state of all the system components when the "fault" is active and if they change over time eg any type of slow cycling

[Tim+]

If I understand these things correctly, they way they should work is that the thermostat controls the motorised valve and then the micro switch in the motorised valve powers the boiler/pump.

If the motorised valve is opening, which it seems to be, then I would suspect a faulty micro switch that is perhaps failing is some way as it heats up causing the boiler to shut down again.

Should be easy enough to test.

Tim

[Roger Hayter]

Sounds as though the CH circulating pump is not coming on when only the downstairs is calling for heat. This might be due to failure of a switch internal to the downstairs valve, if it ihas one. Otherwise, an obscure type of fault most likely a failed connection somewhere.

[thescullster]

Hi Tim

This was my initial thought also. That the valve is opening, but not far enough to operate the micro-switch jobby. If the valve is partially open, this would explain how come the downstairs rads get hot when both upstairs and down stats are calling for heat.

Is it possible to remove the top "cover" on the valve motor to see what's happening with the valve in situ?

If not, how is this failure "easy enough to test"? I didn't install the valves or wire them, so am not clear what would present if I open the wiring centre/junction boxes linking the valves and programmers.

Thanks

Phil

[thescullster]

I don't believe the demand for heat signal can be reaching the boiler. It has behaved fine serving the hot water circuit over summer and reacts correctly when upstairs stat calls for heat.

As mentioned, the downstairs heats up OK if the upstairs stat also calls for heat, so don't suspect a blockage. I also think a boiler overheat is unlikely as this tends to show as a flashing indicator light on the boiler which didn't happen.

I'll look at manually operating the zone valve a few times and see if it's just a lack of use/partial seizure type situation.

I have not seen any cycling occurring, but it's a bit of a complex system so takes a while to test each circuit.

Many thanks

Phil

[GB]

I would swap the two valve heads, without changing any of the wiring. Then crank up the "wrong" thermostat and see what happens. It'll tell you whether it's the electrical or mechanical part of the system that's at fault.

Also, wasn't there a thread about gate valve problems. You haven't fiddled with any of them?

[thescullster]

Thanks GB

Good call swapping the heads over, that would give me a start.

No I haven't been twiddling gate valves yet. Although I got the installer to fit them, I suspect with the pipe layout I might end up having to release both valves to get enough pipe movement to free one of them IYSWIM.

Phil

[GB]

I was able to get a new insert for my 3 way valve. That replaced all the valve gubbins inside the body. That's easier than changing the whole valve.

[Davey]

As mentioned in a different thread, remember that manually operating the valve does not always trip the microswitch. But the switch can be operated directly, with a pencil or ballpoint pen, to see if this does then operate the pump and boiler. You can try this with the valve powered open, too, to see if anything happens. It may be that the switch is not being activated.

[thescullster]

I'll bear that in mind.

Phil

[thescullster]

Hi Davey

Is it necessary/possible to take the cover off the head to do this?

Thanks

Phil

[Davey]

Yes. It should be one screw, that only needs loosening, and is probably on the bottom of the cover. If you can find the most awkward place for a screw to be located, that's where it will be! Once it's loose, the cover should slide directly away from the valve, revealing the motor, wires, and switch.

[thescullster]

Thanks Davey

I'll do my limbering up exercises ready for attempting to contort myself (probably upside down) into the smallest space in the airing cupboard, then moan cos the exertion has caused my glasses to steam up so I can't see the hot water cylinder let alone a screw in the most inaccessible recess possible.

Phil

[GB]

Unclip the head off the valve first?

[Bob Minchin]

yebbut the demand signal must be getting through as you said the downstairs rads did heat up to start with? So the demand signal is getting interrupted after a while.

Bob

[Roger Mills]

I think from reading the discussion so far, that you're on your way to fixing it.

Just to recap on the way in which an S/S+ Plan system works.

Each zone has a 2-port zone valve driven by a programmer plus stat (or programmable stat). When there is a demand in a particular zone, its zone valve opens.

Each zone valve has some auxiliary contacts which close when the valve is fully open. These are wired in parallel, and are used to switch on the boiler and pump. The boiler and pump will run when any one or more zones are calling for heat, but only those zones whose valves are open will get hot.

In your case, the troublesome zone's valve is opening but its contacts are not switching the boiler and pump on - so that zone is only heated when another zone is calling for heat at the same time.

There are 3 possible causes for this problem - starting with the most likely:

1. The zone valve's microswitch has failed
2. The mechanical (wet) part of the valve has become partially jammed, allowing the valve to open enough to allow water to flow but not enough to operate the microswitch
3. A wire has become detached in the parallel microswitch circuit

Swapping two of the actuators without disturbing the wiring should enable you to home in on the problem. [Remember you will need to generate a demand for upstairs to test the downstairs circuit, and vice versa while swapped]. Whilst you have the actuators off, check the valve spindles to make sure that they turn freely. I suspect that they will, and that the problem will transfer to the other zone - indicating that the mircoswitch isn't switching.

[Davey]

That may be the valve opening, and the heat already in the boiler getting used up, but because the boiler is not getting a 'Run' signal, the heat runs out. A failed valve microswitch would fit this scenario.

Maybe.

[thescullster]

Hi Roger

Thanks for the clear and concise synopsis. Looks like the issue was No 2 above, but see separate post.

Phil

[thescullster]

Thanks to all responders on this.

Took the cover off the downstairs heating motorised valve and called for heat on that circuit. The motor rotated but stopped before the actuator tab reached the micro switch - presumably this is down to sticky valve internals. So I "helped it" through the rest of its travel with a broggle from a screwdriver. The pump and boiler fired up fine, so microswitch OK. Tried raising and lowering stat temperature to see how it behaved and the motor/actuator tab now travels far enough to make microswitch contact unaided. Heating came on fine this morning.

One additional comment - moveable wireless stats are useful here cos I could be increasing temperature for downstairs (calling for heat) while sat in front of the valve in the airing cupboard upstairs.

Cheers guys

Phil