Replacing a 2-wire Programmable Thermostat

[greenmekon]

I have encountered a problem whilst attempting to replace a British Gas PT5 progammable thermostat with a Homexpert (Honeywell) THR870CUK. A 15-minute job which has so far taken all day! (The combi-boiler is an Ideal Isar I'm afraid!)

The original PT5 has terminals 1, 2 & 3. The incoming red wire (with a brown sleeve) was connected to 1 and the yellow wire to 3 (2 is unconnected). According to the user guide, 1 is Common, 2 is Heating Satisfied and 3 is Call for Heat.

The Homexpert unit has terminals A, B and C. According to the scant instructions, A is switched and B appears to be a live feed. There are no more helpful decriptions given. C is unconnected and has a plastic break-off tab over the terminal to prevent a connection except under certain circumstances.

When I first connected the new unit, I connected the Red (brown) to A and the yellow to B. The boiler seemed to "kind of" work, but did not drop into standby when the target temperature had been reached, it did seem to work when the thermostat was set to Manual, but this may have just been a fluke.

When I looked again at the connections I decided I had them the wrong way around, so I swapped them After this, the boiler was supplying heat permanently, regardless of the thermostat setting.

I am now totally confused! My second configuration would seem to be the correct one (i.e. yellow to A and red (brown) to B), but neither configuration seem to work properly.

I am hoping you guys may have some suggestions (preferably clean ones!)

 

[ianmcd]

A is the feed B is the switched live but if you got them the wrong way round it would still work so if it is not working with the two existing wires in A & B then you have done something else or it is the settings on the new programmable stat

 

[greenmekon]

Firstly, thanks for the super fast response, I really appreciate it.

I'lol take your comments on board and have another go tomorrow when my brain is less addled!

 

[ianmcd]

your programmer is simply a volt free switch so even if you get the wires the wrong way round it will still work so if it isnt doing what you expect the problem is elsewhere good luck

 

[greenmekon]

Hmm! It now seems to be half-working. When the boiler is in standby and the stat switches on, the boiler start heating. However, when the stat switches off again, either because the target temperature is reached or because of a new time setting, the boiler does not automatically switch back to standby; if I switch off the power supply to the boiler then switch back on, standby is achieved until the next time the stat kicks-in.

Any ideas?

 

[greenmekon]

Update: The system seems to have sorted itself out and now appears to be behaving itself! Thanks for your input.