Central heating control

[vexed]

My cental heating system is over 20 years old. The controls are all Switchmaster - tank stat, electronic (digital) programmer and motorised three way valve.

The tank stat seems to work (clicks on and off when you turn it). With the system on HW only, the tank stat determines whether the boiler runs or not.

But when the system is running on CH and HW, the motorised valve stays put, even though the water reaches temperture (60 degrees) so the water just gets hotter and hotter. I've tried all sorts of eliminations but can't find a pattern to show which unit is faulty. Does anyone know which component is the likely culprit, and how can I confirm this?

 

[gas4you]

Sounds to me like the valve is stuck mid position. Replace complete unit.

 

[vexed]

Sounds to me like the valve is stuck mid position. Replace complete unit.

The valve turns freely. I've had the actuator off.

 

[gas4you]

All I can suggest then is the actuator is faulty and not closing the valve when asked to by the stat.

 

[vexed]

All I can suggest then is the actuator is faulty and not closing the valve when asked to by the stat.

Possibly so. I've been doing tests this afternoon with the actuator off the valve. The actautar works ok moving from HW to HW plus CH. But it won't move to CH only once the water is hot and the tank stat is satisfied. All wires in the actuator are connected Ok. So I'm baffled.

Any idea where I can get a Switchmaster replacement actuator/valve Dave? I imagine any other type will have different wiring.

 

[MANDATE]

This is what it's supposed to do!
In the mid position it feeds both HW and CH until one side gets satisfied.
So HW gets satisfied, the power coming from cylinder stat to boiler is diverted to the grey wire of valve and valve should move to CH position.
In doing so power is fed from valve via the orange wire to relight the boiler.
The motors of valve often cause problem, there energised almost 24 hours a day.( yes even when HW and CH are off). Also the valve may move to the correct positon but it relies on a micro switch to power the boiler and somtimes that fails due to dirty contacts.

 

[MANDATE]

For what I can gather switchmaster no longer exists.
If you search for author 'andyswaindiy' on this forum you'll find he had the same problem.
You may be able to replace the motor within the actuator for about £15.

 

[vexed]

This is what it's supposed to do!
In the mid position it feeds both HW and CH until one side gets satisfied.
So HW gets satisfied, the power coming from cylinder stat to boiler is diverted to the grey wire of valve and valve should move to CH position.
In doing so power is fed from valve via the orange wire to relight the boiler.
The motors of valve often cause problem, there energised almost 24 hours a day.( yes even when HW and CH are off). Also the valve may move to the correct positon but it relies on a micro switch to power the boiler and somtimes that fails due to dirty contacts.

I've noted that there has been some seepage from the valve itself, though this has occured below any wiring in the body of the actuator. But could this have a bearing on the problem? Your comments about a microswitch are also interesting. There is one of these in the actuator. It fails to 'click' when the important change from CH plus HW, to CH alone, should occur. No valve movement ensues.

 

[MANDATE]

With a spring return valve there are two micro switches, each playing an important role.
The first MS (micro switch) takes power from two sources, but just one output to drive the synchron motor. One source is from the room stat and other is a dual source which is from (a) the statisfied terminal of the cylinder stat and (b) the programmers HW OFF terminal.
On modern valves the room stat feeds the white wire and the cylinder stat and HW OFF wires feeds the grey wire.
Initially the white is powered to drive the actuator to mid point whre it triggers the first MS. and changes over supply from white to grey wire.
So it all depends now if the grey is 'live' or not.
If the grey is 'live' then motor and actuator continue and almost immediate triggers the second MS. while continuing to the CH position.

The second MS also has a double role. One role is to supply power to a small PCB to generate a 'holding voltage'. This is required if the grey wire is not 'live' and it holds actuator at mid point. ( power from white wire is dropped when MS triggered)
The other role is to supply power out from the valves orange wire to light boiler when it is triggered. ( 'holding voltage no longer when second MS is triggered).
My own problem was related to the second MS, but I can see if the First MS was faulty the valve would not go beyond mid point.
It may be wise to verify the grey wire is live when it should be before buying a new valve.
So its looking like your problem may be the first MS or the motor in my opinion.