Boiler / Pump switching on dispite programmer being off

[leftnothing]

Hi All,

I have a Y-Plan system with a Potterton PrimaF boiler, a Horstmann CentaurPlus C21 programmer, 2x 2-port valves, a tank stat and a Danfoss TP7000 room thermostat. I have just noticed that my boiler is switching itself on despite the fact that the programmer for HW & CH is set to off. I have done a little "basic" investigation as follows:

1, When I turn on the CH and/or HW via the programmer or room thermostat the boiler, valves and pump appear to work correctly.

2, If I turn off the CH&HW via the programmer (and thermostat), the boiler and pump continue to turn on/off on their own. When this happens, both the 2-port valves remain closed and the radiators (upstairs only) start to get hot (I'm guessing this is happening via some kind of bypass pipe/value due to both valves remaining closed and in "normal" operation the system is not zoned in any way to allow me to heat upstairs radiators only).

Is there an easy way for me to determine where the fault lies without replacing parts? My hunch is that it's the programmer at fault and I'm going to try and borrow one to test my theory)

The system has been working okay for some time now, the only things changed in the last 3-4 years have been adding the TP7000 2 years ago and a new pump 8 months ago.

Any suggestions for me to try would be most welcome! :-)

Regards,

LN.

[Owain]

My guess is the microswitches in the valve actuators are sticking and still calling for heat.

Owain

[Christian McArdle]

Yes, if the programmer called for heating (and the room stat agreed), you would see the zone valve operate. S-Plan systems are fundamentally easier to debug, as the microswitches provide complete isolation between the programmer/stat side and the boiler interlock side. Basically, if the zone valve doesn't move, then the problem is somewhere between (and including) the zone valve microswitches and the boiler. The programmer and room stat are beyond suspicion, assuming it is a standard S-Plan system.

Christian.

[leftnothing]

Is there an easy way for me to test the zone valve(s)? - They are "ACL lifestyle 2-port motorised valve" according to the labels.

I have a multi-meter but would like to know what I am checking before poking around :-)

L.N.

[Ed Sirett]

The key place to start is at the demand terminal of the boiler or if you would prefer the other end of the cable in the wiring centre. My strong suspicion will be a problem with one of the zone valves (it probably is also not fully shutting off on the water side too?)

[leftnothing]

There are Blue, Brown, Orange, Grey & Yellow/black (earth) wires attached to the valves.

The Orange/Grey wires appear to be for the microswitch. The question is when the valve is "closed", the programmer switched to "off" should this switch be open or closed if it was working correctly?

I am guessing from the other comments and googling that if this switch is sticking "closed", it would cause the symptoms I have? (e.g. Boiler/Pump come on despite the programmer being set to off and the valves being closed)

L.N.

[Phil]

I know nothing of this but have suffered the same problem in the last 24 hours. Unfortunately can't see OP. My programmer is set to off for both ch and dhw. Boiler is igniting from a yet unknown signal. Have turned boiler t'sat off. I had presumed that the programmer was duff but, as previously said, I know nothing. Given this, will the programmer pass a signal from microswitches in valves in either/both circuits when the programmer is off for both circuits? Phil.

[Owain]

What usually happens is the programmer/thermostat turn the valve on when heat is required. The valve mechanically operates the microswitch. The microswitch then turns on the boiler.

If the microswitch sticks, then there is a call for heat regardless of the programmer/timeswitch.

Owain

[leftnothing]

Hi,

I have just done some more investigation as follows:

- With the CH & HW switched off on the programmer, the boiler / pump was still running. I then switched off the power and tested the microswitches with a multimeter. Both switches were "open", manually operaing the valves causes the switches to "close" so appear to be working correctly.

- I then tested the tank stat with the multimeter. This has two wires and the circuit was "closed", I turned the temp setting up/down expecting it to "open" at some point but it made no difference. Do I need to wait for a period of time for something to "open" the circuit inside?

Could a faulty tank stat cause these symptoms (boiler/pump operating on their own)?

If not, can anyone suggest my next best step to diagnose the problem?

Thanks for all your help!

L.N.

[Ed Sirett]

They should be open circuit when the valve is unpowered and has returned under spring pressure to the closed-non flowing position.

[Ed Sirett]

Provided the tank (HW Cylinder) was not cold then at some point turning its stat down should have caused a click. However the usual way for these to be wired is from the output of the programmer so even if the stat had failed (plausible) then HW zone valve should still open and off should go the boiler (+ pump ).

[Ed Sirett]

There is a section in the main FAQ on CH control systems.