Replacement stat - advice please!

[matthew.larkin]

Hi

I'm replacing the stock honeywell stat that controls my combi with a Drayton Digistat3. Should be straightforward, but I just want to check the wiring.

The existing stat is wired as per this photo:-

Drayton doesn't need most of the wiring, but needs the following connecting:-

- Common

- Heating satisfied

- Call for heat

I can't quite figure out how these correspond to the pics in the wiring diagram on the inside cover of the Honeywell stat. I know that there will be fewer wires in the Drayton though! Any advice gratefully received!

Matt

[matthew.larkin]

photo:-

> The Drayton doesn't need most of the wiring, but needs the following > connecting:-

OK, I've done some reading around now. My understanding is that the Drayton only needs two wires connecting - common and call for heat.

Honeywell T40 as per photo linked above) I'm reading this as Common = Terminal 1 (marked L) and Call for Heat = Terminal 3 (marked N via a resistor?).

Any help gratefully received!

Matt

[Roger Mills]

You're basically right - though not necessarily for the right reason.

Your old Honeywell stat had a built-in accelerator heater - in the form of the resistor connected between 2 and 3. This has a small current passing through it whenever the heating was on, and warmed the sensor slightly in order to reduce the hysteresis of the stat. This needed a return connection to Neutral and is the *only* reason for having a neutral wire anywhere near the stat. The neutral wire must *not* be connected to any terminal on the new stat - otherwise you'll get a loud expensive bang when the stat switches one way or the other.

In my view, the diagram in the lid of the old stat is a bit misleading. The little rectangle just above 3 represents the heating load (boiler etc.) the

*other* side of which is connected to neutral - but not anywhere near the stat.

Both stats are of the changeover variety - and can be used for either heating or cooling applications, or for applications which require both a heating demand and a heating satisfied output. The unused terminal 4 on the old one is the 'heating satisfied' connection.

On your new stat, as you rightly say, you need just a live feed (the brown wire which is connected to 1 on the old stat) which needs to be connected to Common, and a heating demand - currently connected to 3 on the old stat - which needs to be connected to 'Call for heat'. Don't connect *anything* to 'Heating satisfied' and make sure that the bare bits of the old neutral and earth wires are insulated and taped off safely.

I hope that clarifies things a bit.

[matthew.larkin]

Thanks! All wired up and working perfectly (no nasty bangs!). Glad I was on the right track, even if it was for the wrong reasons.

Matt