Having Googled, FAQ'd, and looked in here, I suspect that I might have 2 faults :-(
Installation: Potterton EP2000 (the old brown one), anonymous beige wall stat, Grundfos Super Selctric pump, Potterton Prima.
Due to unusual occupancy patterns, we've always run the timer on '24 hours' for both CH and HW and just set the temperataure on the wall stat and boiler stat (Yes, I know we don't need a timer, but it was here when we moved in!). This always worked fine. Boiler cycles, pump cycles.
As of today (maybe since yesterday) the boiler is still cycling as usual, but the CH pump is running continuously, and it's getting hot in here!
Thinking that the wall stat had gone short (despite it 'clicking' in a sensible place on the dial), I turned the EP2000 to 'off' on both HW and CH. But the pump still runs. I'd assumed that the wall stat was in series with the ouput from the EP2000, so if it had gone short that turning the output off would kill the pump. Apparently not :-(
The EP2000 has always had a flickering display (not a problem, we don't use it as a timer) and I've just checked the battery and it's dead, but I assume that's just to back up the clock in the event of power failure.
So to me it looks like the EP2000 is faulty (and may always have been) as 'off' doesn't seem to do anything, and that now that wall stat has gone short.
Does this sound sensible? I don't really want to replace the EP2000 if the stat is faulty, and vice-versa. Also, I'm not sure if the newer EP2000 is compatible with the backplate of my older one (any ideas?).
I don't know if there's a frost stat anywhere - I've never been able to figure out the wiring in this much-extended/much-bodged house.