Potterton EPP2 - Voltage on both H and C at same time?

22 Jul 2008
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United Kingdom
Hi everyone,

18 months ago I replaced a dead PTT1 with a PTT2, and then found that the PTT2 needs 5 wires, but the PTT1 only used 4. I eventually called Potterton, and they sent a wiring diagram which explained what to do, although that lead to another problem, and for a while I settled on a work-around, which was all described in a previous post //www.diynot.com/forums/viewtopic.php?t=142686
I decided to look at this again, and try and get a proper solution, and so spent a while mapping all the wires in the Potterton EP2000 programmer, and the wiring box in the Airing cupboard. I feel as though I finally understand how it's supposed to work, and am slowly testing each piece, trying to find what's actually wrong. And here is my question:

The PTT2 has 5 connections, Neutral, Permanent Live, Switched Live (TL), Heating and Cooling. As far as I understand, the Heating circuit will be raised when the Stat is calling for Heat, and the Cooling is raised when satisfied. The PTT2 has a green LED labelled 'Below' to show when temp is below, and heat is required, and a Red LED labelled 'Reached' when temp has been reached

However, when the red 'Reached' LED is lit, their is still voltage on BOTH H and C. Although the two Potterton diagrams disagree whether H or C is 1, and whether H or C is 2, I would expect to see voltage on one OR the other (no matter whether H is 1 or 2, or C is 1 or 2). Am I missing something simple (quite possible!) I can upload the wiring diagrams if required.
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