Standard vs. Changeover wiring in thermostat

[silve]

Hi, I'm looking to replace my Honeywell dial thermostat with a Heatmiser touchscreen thermostat PRT-TS.

It can be bought with "standard" or "changeover" wiring. I would like to know which wiring type to buy?

Old:
http://www-2.plumbworld.co.uk/honeywell-t6360b-spdt-room-thermostat-001361M.jpg

New:
http://www.heatmisershop.co.uk/prod...mable-Thermostat-%2d--Heatmiser-PRT%2dTS.html

My existing wiring has 3 cores (red yellow and blue) plus earth. They are connected to terminals marked 1, 2, 3 in the Honeywell.

I notice someone asked a very similar question a while back but got no clear answers

//www.diynot.com/forums/viewtopic.php?p=703798#703798

Does anyone know about this? Heatmiser's site says standard wiring should be ok unless I have a power control valve (??)

Thanks!

 

[ChrisR]

not as simple as I first thought... hang on I'll be back...

 

[ChrisR]

It depends

YOU won't need the changover contacts unless you've got a very unusual motorised valve.
If it's a Honeywell tin box one or other spring return type you'll be fine.

You'll almost certainly be OK with the wiring on the left, where eg the wire I've makrked "2" on the honeywell goes to the terminal shown on the new stat.

BUT the new stat really needs a permanent live, which the honeywell stat may not have had - it may just have come from a boiler output which would compllicate things. It probably would have come from the old programmer CH ON output, which you'll be leavin set to "ON 24 hrs".

CHeck your current wiring, you should be able to work out where things come from.

 

[silve]

Awesome reply ChrisR thanks for all the detail!

That thermostat photo you posted is what I have almost exactly. The tin box in the airing cupboard does indeed have Honeywell on it - I assume this is what you mean (i.e. the tin box is the wired box on pipes that isn't the pump?)

I'll poke the thermostat with a multimeter later and try to figure out if the live is switched or not. It's not the end of the world if I have to leave CH permanently on "24 hrs" as the Heatmiser stat will handle the timing etc. I assume I could still leave the HW switching via the "old" clock? It's one a Potterton EP2000 clock - black rectangular job with yellow writing, with separate sliders for CH and HW.