replacing very old Drayton room stat with Digistat +3

18 Sep 2013
Reaction score
United Kingdom
I have just replaced a very old room stat which had 2 red, 2 black and an earth. Drayton technical told me the wires to use from the old stat which involved taping off two of the wires as only two are needed for the new stat which are common and on.

When set temp calls for heat, boiler fires up, 3 port valve moves to heating position and all fine. When room gets to right temp, boiler switches off. All OK. When heating turned off at main timer unit and HW is called for, boiler fires, 3 port valve moves to HW position and all OK.

The problem arises when both HW and heating are on on the main timer box. When room temp is up to correct temp, Digistat 3+ turns boiler off, but 3 port valve does not "park" at the HW position which means that if HW is being called for and pump is running, hot water is still being pumped to the rads. It is only if the heating is switched off at the main timer box that the 3 port valve returns to the HW park position.

I've called Drayton who say that as the room stat is causing the boiler to fire the wiring is OK and it could be either a faulty tank stat (I've turned this through the temp range and the relay clicks when calling for heat so that seems to be OK) or faulty room stat. Any suggestions please.
Sponsored Links
in the old stat

did the two reds used to be in the same terminal

did the blacks used to be in the same terminal ? etc etc

how was it ?
Hi Andy

The two blacks and two reds went to separate terminals ie there were 4 terminals and therefore they were not connected.

so you have two wires which aren't connected to anything.

I guess one of them needs connecting to the 3rd terminal!

Don't guess though.

If you tell people the terminal numbers of the old one hopefully someone can help

I assume you have the model with 2 AA batteries.
Sponsored Links
The old thermostat was probably a changeover type, which would have 3 wires and a neutral.
If this is correct, the cylinder thermostat will only have 2 wires connected.

The solution is to connect the third wire (heating off), and also ensure that the two wires already connected are actually connected the right way round.

It is essential that all 3 wires are connected properly and the 4th one is not connected to anything. If not, it either won't work, or there will be a big bang, possibly causing damage.

If the new thermostat requires batteries, you have purchased the wrong one anyway - the mains powered version would have been preferable.
The battery one can still be used, you will just have to buy batteries for it every year.
Thanks Andy and flameport. I have a central heating electrician coming round tomorrow who is a mate of my brother in law who is a sparks.
The point you make about one of the 2 wires not currently being used makes a lot of sense. Hopefully the guys tomorrow will know which one. I will keep you posted,


DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local