Intermittent Programmer Problem

[eddiespaghetti]

Hi there, I'm new and in need of help!

I've got a Baxi Solo 40 HE boiler (non-combi) and I've been having trouble with my CH/HW. I had an old Landis & Gyr mid-position valve that had jammed, which I've now replaced with an equivalent Danfoss HSA3. I also replaced the ancient Siemens programmer with a new Siemens (Wickes) RWB9 and re-wired the junction box at the same time as the wiring was a mess.

Most of the time, the system works perfectly. However, occasionally the CH will get 'stuck' on, even when the programmer is not calling for CH and/or the room stat is not calling for heat and the system gets stupidly hot. I turn the whole system off at the fuse and it goes back to normal.

I've triple checked the wiring and the terminations, and they're all fine.

I've had my multimeter on the various terminals and the only thing I can find out of the ordinary is when the HW OFF terminal is live (showing 240V), the HW ON and CH ON terminals seem to have 50V on them. I am aware of induced current in adjacent wiring, but this seems excessive?

I've been fiddling with it all day and I'm probably blind to the problem from looking too hard, so any input would be greatly appreciated.

Thanks for your time,
Ed

P.S. I'm a Mechanical Building Services Consultant, so you can be as technical as you like!

P.P.S. If anyone needs any advice on building regs, rad-sizing, Part-L Compliance problems etc. feel free to PM me

 

[OilLecky]

It's a bit difficult diagnosing these control problems from afar but I'll try to help.
The supply from the programmer hot water "OFF" will hold the mid-position valve actuator in the heating position with everything else off.
It is possible to pick up stray induced voltages but also the resistor / diode network inside the valve actuator produces a reduced voltage when it needs to hold the valve in the mid-position. (Maybe that's your 50 volts.) You could be getting feed back through this network. However in the circumstances that I describe the boiler should NOT be getting a supply. Find out where that supply is coming from. (The wire from the valve actuator that normally provides the boiler supply is the orange one. Is anything else connected to this that should not be? It should be connected to the Cylinder Stat call for heat, that is all.

 

[D_Hailsham]

The Honeywell Wiring Guide has very detailed diagnostic procedures which should help you identify the problem. Mid position valves all have the same wiring.

You only get 50V on the orange wire after heating is satisfied from the heat only position (port A only open). The voltage is reduced by a resistor to 50-150V. This is supposed to be low enough to prevent the gas valve opening. See How a Mid position Valve Works.

 

[jackecole]

if all wires are tight and the wiring is correct, i'd say the programmer is faulty (sending wrong signals) - replace it (get one so that you only need to change the face and not the baseplate, makes it a lot easier and quicker).

 

[eddiespaghetti]

Thanks everyone for your help, I'll work through the mid-position valve fault finding section of that Honeywell guide at the weekend and let you know how I get on.

 

[jackecole]

The guide is good (the fault finding section especially) - the fault finding procedures make sure that you definetely find the fault and that there is no chance of a backread signal, etc giving you a slight chance of the wrong diagnosis.

Look for the fault finding for a programmer too (same page) - i think your prgrammer is buggered!