System Ignoring controls

[lloyda]

My parents have just moved into a retirement flat, and it seems that the heating has a mind of its own. Hopefully someone can give me ideas on where the problem might lie.

Boiler is Baxi, about 15 years old. It looks like the controls may have been updated recently, as there is a Honeywell programmer which should control water and heating. Separate wall mounted stat. Tonight I've turned both hot water and heating to off, and turned the stat down for good measure, but the boiler still fires and the pump runs.

I think the system is S-plan (two 2-way valves), and looking at the cables coming out of the wiring centre I don't think there is a frost stat (and I can't find one anywhere). There is an over run timer.

Any thoughts appreciated.

Thanks

 

[stem]

There could be several possible scenarios, here are the two I find most common.

1) If the controls have been recently changed they could be wired up incorrectly.

2) The boiler is not directly controlled via the programmer and thermostats, but rather via a microswitch inside each of the motorised valves. A motorised valve may have stuck open, or just the microswitch may have stuck in the 'on' position which would keep the boiler and pump going despite all of the other controls being 'off'.

You can make a basic test of the central heating motorised valve as follows:
1. Programmer set to heating on / hot water off
2. Set the room thermostat to it's lowest setting and wait a few seconds
3. Get someone to wait next to the motorised valves
4. Turn up the room thermostat and wait. After 10 / 15 seconds a click should be heard from the heating motorised valve? That's the microswitch operating
5. Turn down the room thermostat until it clicks off, you should hear a wining noise as the valve closes and a click as the microswitch is released.

Repeat the test for the hot water motorised valve:
1. Programmer set to heating off / hot water on
2. Set the cylinder thermostat to it's lowest setting and wait a few seconds
3. Get someone to wait next to the motorised valves
4. Turn up the cylinder thermostat and wait. After 10 / 15 seconds a click should be heard from the hot water motorised valve? That's the microswitch operating
5. Turn down the cylinder thermostat until it clicks off, you should hear a wining noise as the valve closes and a click as the microswitch is released.

If all of the above is OK, then the valves would appear to be operating as they should be, so possibly a wiring fault keeping the boiler on, but the only way to be sure is for someone competent with a multimeter and a working knowledge of S-Plan wiring to check each part of the system bit by bit.

If one of the valves doesn't respond, or you don't hear the click then it could be faulty, or wired up incorrectly, so you are still back to getting it tested.

 

[lloyda]

Thanks.

If the rads are still getting hot, does this indicate the issue is likely to be with the heating valve, rather than the water one?

The problem does seem to be intermittent, which leads me away from a wiring issue (but I'm not dismissing the idea).

 

[John D v2.0]

Excellent post from stem, the only thing I can add is that it could also be frost prevention or a frost stat. Is the boiler or any pipework located in a lean to or garagegar similar unheated space?

 

[lloyda]

Excellent post from stem, the only thing I can add is that it could also be frost prevention or a frost stat. Is the boiler or any pipework located in a lean to or garagegar similar unheated space?

As I said, can't locate a frost stat, and I can account for all the wiring coming out of the wiring centre. This is a retirement complex, so no garages, lean to or similar. The whole complex is so hot, there is very little chance of anything freezing up, even if left empty.

 

[stem]

If the rads are still getting hot, does this indicate the issue is likely to be with the heating valve, rather than the water one?

Yes.

 

[lloyda]

Ok. Thanks

Will try and get the maintenance man for the complex to take a look tomorrow.