Changing combi boiler thermostat (with pictures!)

[sm1thson]

History:
since moving into the house in summer it was clear the previous owners use the thermostat as a switch (heating was ‘on’ on the programmer and '0c' on the stat), when the cold came it was clear the thermostat didnt perform right (too much hysteresis) but i figured it was probably due to it been previously used as a switch, now i suspect its because it was wired wrong. today i was passing screwfix (its quite far from my house so i dont normally pass it) so i popped in and got a new thermostat (this one).

doing the change:
Now Ive had a look at the old wiring (yes I should have done that before buying a new one!) and it looks like this:

diagram inside:

measuring things i get: (contacts are labeled 1-4 going top to bottom)
4-3: (neutral to earth): 0v
4-2: (neutral to other end of heater): 250V (stat in any position!)
4-1: (neutral to Live): 250V or 0v when stat turned down!!!!

so im sure 1&2 are the wrong way round? anybody agree or disagree?

I suspect if i change these round it will work a lot better, but i have bought a new one and it looks better so i think i will put it on!

this is the diagram:

so i recon i wire it:
1: red (or yelow if the above assumption of it been wired wrong is wrong!)
2: yelow (or red if the above assumption of it been wired wrong is wrong!)
3: not connected (it does state no earth required in the instructions and I checked with my multimeter and, as the diagram indicates, 3 does connect to 1 when the stat it turned ‘low’)
4: Blue

any comments before I change it over?

(ps sorry pictures are from phone in low artificial light)

 

[philrosenberg]

By the sounds of it you were right in saying that the old one was wired incorrectly. The way it's done would mean it would switch the boiler but the heater resistor in the thermostat would always be on. This would increase hysteresis and the thermostat would always think the room was warmer than it actually was.

I think your assumpition is correct for the new one (I'm assuming the old terminals were numbered 1 at the top to 4 at the bottom on your photo)
1-red, live,
2-yellow, switched live,
4-blue, neutral.
3 is so that the unit can be used as a cooling thermostat (for an air con unit or something i guess)

Don't be tempted to cut the earth wire right back. Sleave it, put a chock block on it and hide it behind the thermostat. If this one ever needs replacing you may need the earth connection in the future

 

[sm1thson]

Done now seams to be fine.