RWB9 Fault?

[Chris Baker]

Hi all.

Please can I ask for some assistance if possible?
We have a Gloworm Micron boiler controlled by a Siemens RWB9 programmer.
The timer function works correct at the programmer but the boiler runs continuously if allowed to! Regardless of thermostat, frost stat or programmer setting.
The only way to turn off the heating is to switch off the boiler or switch off the programmer.
Gas engineer came out and replaced one of the motorised valves and I think the 3 port valve. Then left.

Does anyone have any solutions I could try please?

Many thanks

 

[stem]

If you have a 3-port motorised valve, that is responsible for operating the boiler when the central heating only is running. Assuming that the valve has been replaced and so working correctly, the room thermostat could be faulty in that it is permanently calling for heat, keeping the valve in the heating only position and hence the boiler running.

When the hot water only, or hot water and heating are running, the boiler is controlled by the hot water cylinder thermostat, that too could be faulty and permanently calling for heat. Is the cylinder thermostat held tightly in place against the metal of the cylinder so that it can sense the temperature of the hot water. If not that could cause it to be permanently calling for heat.

The following tests might give you a clue where to start looking.

1) Have you tried the programmer with just the heating on? does the boiler run continually then? (if so, possible room thermostat fault)

2) Have you tried the programmer with just the hot water on? does the boiler run continually then? (if so, possible cylinder thermostat)

I'm not sure from you post if the programmer is working or not?

The timer function works correct at the programmer

but the boiler runs continuously if allowed to! Regardless of....programmer setting.

The programmer's sole function is to switch the heating and water on/off at the set times. If it switches the boiler off when the 'off' time is reached, and back on again when the 'on' time is reached, it's working.

 

[Chris Baker]

Hi Stem.
Firstly, many thanks for your reply.

I have made the assumption that the programmer is working as it switches on (respective CW or HW lamp illuminates) when the timer function tells it to.
I have tried your recommendations and can advise that:

Mains available at boiler and programmer.

Programmer CH switched to off. Programmer HW switched to timed function but not being called = Boiler is on.
Programmer HW switched to off. Programmer CW switched to timed function but not being called = Boiler is on.
Programmer CH and HW switched to off = Boiler is on.

The caveat to the above is that the boiler is on as stated. then after about 5 minutes, it switches off, then after a couple of minutes it switches on again. As th

As though it is being controlled by temperature.

The room thermostat shows open circuit above room temperature and short circuit below room temperature.

There is a bimetallic contact thermostat on the HW feed from the boiler. I have taken this off and at low temperature it is open circuit. It goes to short circuit as the temperature rises. Picture attached


Should this bimetallic contact thermostat be the same as the room thermostat in operation? For example. If I set it to say 45 deg C. The contacts would be short circuit until it reaches this temp and then go open circuit?

Once again many thanks for your input.

Chris

 

[Chris Baker]

Sorry I should have said that on this bimetallic contact thermostat there are 3 connections.
The installation leaflet says:
Terminal 1 = common
Terminal 2 = opens the circuit when temperature rises.
Terminal 3 = closes the circuit when the temperature rises.

The wires are connected to terminals 1 & 2 but as per my previous post it “appears” to work the opposite way round.
Should I swap the wire from terminal 2 to 3?

Thanks once again

Chris

 

[stem]

Unfortunately it looks like it will need someone with test gear to follow the live back from the boiler and find out where it is coming from. Over a forum we can but guess, and there could be many different causes of the problem.

Unless you have changed anything, there is no point is swapping wires around. If the system used to work OK, then the wires will be OK, they won't have moved themselves.

Also, your photo is not of the cylinder thermostat. The cylinder thermostat will be clamped onto the hot water cylinder about one third up. The one in the photo is clamped onto a pipe and is the sort of arrangement that would normally be associated with a frost protection system, it being there to stop the system overheating when the frost thermostat (which will be mounted elsewhere) switches the heating on. The air sensing frost stat should be set at 5 degrees, and the pipe stat at 20-25 degrees.

 

[Chris Baker]

Thanks Stem.
I will take a look at the PCB and start chasing the wire that gives the +vet feed.

The only reason I was going down the bimetallic contact thermostat was I remember that the engineer who serviced the boiler said that same pipe was leaking. During the summer so heating was always off at that time.
We have gone through the winter controlling it in manual.
Thank again
Chris