Analog to Digital thermostat

[Paul7376]

Hi I am wanting to replace my analog thermostat and have purchased the Honeywell DT90

anyone able to say on the old one that the wire in terminal 3(the bottom black wire) is some sort of earth and won’t be needed in my 2 wire digital thermostat

otherwords connect red in terminal 1 from old to new live and black in terminal 2 to new neutral?

 

[EFLImpudence]

You do not have a new neutral.

The rectangle on the diagram represents the appliance therefore the wire from thermostat to appliance is the switched live; only becoming neutral after the appliance.

 

[EFLImpudence]

What model is your old thermostat - or is there a diagram on it?

 

[Paul7376]

Honeywell it’s T6060

 

[EFLImpudence]

Ok.
Mistake corrected
1 is L
2 is N and
3 is SL

So,
L to A and
SL to B
You do not need the wire in 2 any longer.


However, it would appear someone has used the bare earth wire (with black sleeve) for the SL - not really allowed.

Therefore it would be better if you swapped Black and earth wire at the other end of the cable and use the black for the SL.

 

[Paul7376]

Ok so can put
2 to A
3 to B
Discard 1

thanks very much. Great help

 

[EFLImpudence]

Ok so can put
Discard 1

Should be discard 2 - the neutral.

Well, yes, terminate safely, although,

as I said, it should really be swapped with 3 AT BOTH ENDS.

 

[Johnmdc]

2 is NEUTRAL
1 is LIVE
3 is switched live
So 1 to A and 3 to B and make 2 safe
If you wire it as advised above you will blow the fuse and possibly damage your new thermostat and boiler.

 

[ericmark]

If I could fit my thermostat as analogue I would, analogue is far better than digital (off/on) I would check boiler first and see if analogue can be used.
Old boiler the digital thermostat controlled room temperature, new boiler all it does is stop cycling in warm weather.
Working and working efficiently is not the same, with my oil boiler is does not matter turning boiler off/on, but with a modulating gas it can cause boiler to run inefficiently, electronic thermostats in the main switch more often then mechanical and this can result it boiler running less efficient.

 

[Mottie]

If I could fit my thermostat as analogue I would, analogue is far better than digital

I found the opposite! With my old analogue there was such a range between it clicking off and clicking back on again it was uncomfortable. With my Honeywell DT90e, once it reaches the set temperature, it fires up the boiler up to 6 times an hour for 5 minutes or so to maintain the set temperature to within .5 of a degree.

 

[bernardgreen]

I think there is confusion.

A thermostat display can be analogue or digital

but an analogue thermostat can send digital ( on / off ) commands to the boiler.

 

[EFLImpudence]

2 is NEUTRAL
1 is LIVE
3 is switched live
So 1 to A and 3 to B and make 2 safe
If you wire it as advised above you will blow the fuse and possibly damage your new thermostat and boiler.

Thank you John.

Of course I got 1 and 2 the wrong way round. I will edit my posts.

 

[Paul7376]

Johnmdc. So what about the advice to change wire at bother ends?

 

[EFLImpudence]

Yes, sorry John about my mistake.

The red live wire should be to A.

The advice about the black wire still stands.

 

[ericmark]

I think there is confusion.

A thermostat display can be analogue or digital

View attachment 221276

but an analogue thermostat can send digital ( on / off ) commands to the boiler.

Not really

To my mind the important bit is how it talks to the boiler, the thermostat to left can only tell the boiler off/on, the thermostat to right has the option to be connected off/on or using OpenTherm when it tells boiler to turn up or down, connection to boiler ebus with for example OpenTherm is analogue control, it is also duel digital and analogue display round the edge showing both set point and current temperature is analogue, and in the centre digital and the centre display can be selected so you can have it show the date if required.

As one would expect the one to left around £35 and one to right around £190. When controlling a modern gas boiler with digital the main idea it to stop the boiler cycling, it does not control room temperature, it simply switches it off when weather warms up, with both thermostats they can be programmed, to the left it doubles as a frost stat, so instead of turning boiler completely off, it will allow it to fire up over night if it gets too cold, but on heating it can easy over shoot and it relies on the TRV to do their job, the one to right has built in algorithms so it works out the over shoot and compensates.

Since I use oil there is little advantage between the two, as my boiler is simply on or off, but with modulating gas boilers there is a huge advantage using the ebus control. The cooler the return water the more efficient the boiler runs, gaining the latent heat from flue gases, also as long as room is to temperature the cooler the radiator the quicker it can turn off when for example sun shines through bay windows and heats the room, and with ebus control hysteresis is eliminated.

However the Nest Gen 3 shown I would not rate as a good wall thermostat because it does not interact with the TRV heads, maybe the new Gen 4 does, I have not looked at the specs.