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Andrew Smith
Andrew Smith, Plumber
Category: Plumbing
Satisfied Customers: 3767
Experience:  City and Guilds qualified plumber
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Just replaced mechanical Potterton PRT100 thermostat with

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Hi, just replaced mechanical Potterton PRT100 thermostat with new Honeywell CM907. Connections made RED (live) to A, YELLOW (heat) to B, BLUE made safe/redundant. Have original Potterton EP3002 dual channel programmer still in situ with CH channel set to ON so that CM907 programmer can control heating. CM907 calls for heat (flame symbol shown on display) but boiler does not fire. Please can you help with suggestions as to why this might be ? Thanks in anticipation. Peter B.
Can you upload a photo of the wiring?
Use the paperclip
Customer: replied 2 years ago.
Relist: Other.
No answer received yet. Do I need to post this in a different category ?
Customer: replied 2 years ago.
This really should be with HVAC and I'm not paying again to relist, that's a con ! The wiring has already been carefully described in the original question. Someone familiar with Honeywell thermostats would know what A x B y C connections were for in any case.
You should not have to pay.I need to know you have wired it correctly - it is mains powered or battery?
Ok, got it.Lets prove it is not the stat, can you wire the red and yellow into the same connection and see if the boiler fires?
Customer: replied 2 years ago.
If it helps, this is a good example of the wiring involved. It shows the old Potterton PRT100 wiring and the requirment for the new CM907 thermostat. This is how I have wired. Attempting to relist the same question into HVAC did request repayment and I can so no way around it. I originally selected HVAC but for some reason it ended up as Appliances. Also our comms are crossing, I will attempt the RED to YELLOW link. Standby please.
I will wait for your reply.The problem with HVAC category is you will end up on the american end.
Customer: replied 2 years ago.
OK thanks for the info. RED and YELLOW joined, ie. NO thermostat. CH channel on Potterton EP3002 programmer set to ON position. Indicator light on 3 position Myson EP322 motorised valve shows CH channel ON but boiler does NOT fire. Looks like the thermostat is probably OK. Also DHW channel is working fine ! Any further ideas greatly appreciated, thanks.
Is the motorised valve opening to fully open to make the microswitch (do you get a click)?
Customer: replied 2 years ago.
I can cycle freely between only DHW and only CH on the programmer. The motorised valve shows the correct indications in each case. If both are selected, then both indicators are on, on the motorised valve, as expected. I can use the override lever on the m/valve to switch too and the appropriate indications show BUT the bolier only fires for DHW. To be honest, I don't hear any clicks from the motorised valve when it's cycling, it is very quiet and I have no problem hearing eg. the thermostat click in and out.
Customer: replied 2 years ago.
BTW. Just have to give the dog a short waolk before dark !!! Back soon, Thanks
No problem.If you dont get the click at the end then the boiler will not fire.Try taking the head off and seeing if it will fire the boiler without the restriction of the valve underneath.
Customer: replied 2 years ago.
OK back again. You were definitely on the right track with this. I removed the actuator for the m/vlv. Indicator lights were always correct whatever mode. However the motor had stopped working ! It had clearly been intermittent in operation for the last few days and has just stopped altogether. I've had probably 4 m/vlvs in the last 20 years, typically they actually broke their baseplates with the motor trying to grind round a stiff valve. I've not seen this before though, valve is easily moveable and instead, actuator motor broken. Fortunately due to past experience I keep a spare actuator head :-) I still don't hear clicks from the new one, it's very quiet but it does of course actually turn the valve and work ! Problem is now solved. The Honeywell CM907 is a very flexible thermostat in operation so it will still be a good addition to the system for CH and I now have the added advantage of being able to time the CH and the DHW separately. You were 98% there, so as far as I'm concerned that's money deserved and we can call it fixed. Your time and effort has been genuinely appreciated, regards, ***** *****
No problem, when working with someone like yourself it makes things easy and logical.
Many thanks
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