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Potterton Programmer not controlling stat?

I developed a fault on my Potterton system as a direct result of trying to change the in room stat (Potterton PRT2) with a new Honeywell DT90e. I wired the new stat incorrectly which blew the 3amp fuse on the fuse spur next to the Potterton EP2002 programmer. Before replacing the fuse with temp 5amp one as I didn’t have a 3amp one in the house – I tried reinstalled the previous stat as it was wired before.

I turned the fuse spur back on and the programmer seems to be working ok, all the lights come on, the program has been saved and the sliders work. However the stat in the room stays permanently on – triggering the Potterton boiler when the temp drops below the desired temp regardless of the setting on the programmer (programmer is switched off and the stat is still kicking the boiler on).

The problem only seems to impact the Central Heating as the programmer is controlling the hot water correctly. I tested this the morning with the stat set to lowest temp (reached light on so the boiler off) then HW set to Timed on the programmer – the boiler kicked in to power the hot water after the shower was running (after using existing hot water in the tank) – I then switched the HW off at the programmer and the boiler switched off.

I suspect I have either rewired something incorrectly or I have blown something on the circuit board – any ideas?


April 2013
Thanks for your reply Peccavi

Regarding your questions – The stat is somehow finding a constant live – it only turns off if I turn the fused spur off next to the programmer – I’m trying to turn the central heating off by switching the switch on the programmer to off – but the light stay on on the stat (green or red depending if the room temp is below or reached) – this lights on the stat used to go off before the wiring incident.

I have a photo of the internal wiring on the stat – where I have probably created the problem, but I’m unable to upload to here, so I have described it below:

Live (Brown) = Com (2)
Switch Live (Black wire with Brown Sleeving) = H (3)
Neutral (Blue) = N (5)
Link wire from Com (2) to TL (4)

April 2013
The hot water works correctly in all respects.
The Heating works correctly according to the room stat but it pays no attention to the timer / programmer.

The Control board and the boiler do not care and have no knowledge about why they're heating water - they are non judgmental - they heat water promiscuously.

The room stat is acting as an on/off switch for the boiler and takes no notice of the programmer.

Under normal conditions the room stat gets its power from the timer / programmer (are you sure that the timer / programmer is really off when you think it is?)

Assuming the timer programmer is behaving correctly -the Room thermostat MUST be finding power to turn the boiler on from somewhere - and the room thermostat is acting as an effective on/off control as it should.

There are two questions to answer -
Is the timer programmer doing the right thing?
If it is working correctly then where is the room stat finding power from so that the boiler fires up?

I am not a boiler person. My advice is worthless.

In some circumstances - I think the CH only condition - the call for gas comes into the boiler from the 3 way valve.

Good luck...

April 2013
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