Help with Honeywell DT2 Thermostat install

[0y1eqmco]

I'm replacing my thermostat with a digital one as the old one always overheats the house.

The old one has three wires:

The new Honeywell has a few options for wiring in the manual:

I connected live (brown) to A and 3 (grey) to B, leaving the other two wires unconnected. When doing this the heating runs constantly no matter what temperature is on the thermostat. Not sure if it's a bad unit or I've got the wiring wrong, any advice appreciated, thanks!

 

[CBW]

It would help to know what the other thermostat was and which wires went where. Do you have a photo?

 

[ericmark]

I connected live (brown) to A and 3 (grey) to B, leaving the other two wires unconnected. When doing this the heating runs constantly no matter what temperature is on the thermostat.

That seems correct.

The black wire needs to be safely terminated in a block connector or similar and is not used with the battery powered thermostat.

I would temporary put grey in a terminal block to test the heating goes off.

as the old one always overheats the house.

Question is was the old thermostat faulty, or was there some other fault?

I had mothers house getting too hot, and first attempt was change wall thermostat, I used a cheap programmable wireless unit it was a failure, and I was not inclined to make the same mistake twice, for fitted better heats on the TRV's, they did work well, with a modulating boiler, but new owners did not want them so changed back to standard, and now the lock shield valve had be set, which was easier when I had a computer report of target and current temperatures .

So even if the wall thermostat is faulty, the home should not over heat. The boiler will cycle a lot and waste energy, but home should not over heat.

Unless that is if you live in an open plan house or have a hot air boiler so no TRV heads fitted.

Analogue and Digital can refer to human machine interface (HMI) or the electrics/electronics, in the main analogue is better, but often the boiler will not allow use of analogue controls. The whole idea of the digital thermostat it to turn off the boiler in warm weather, it is not to control room temperature, the TRV does that.

The problem is a radiator has two controls, the TRV and the lock shield valve, and mechanical TRV heads are marked *123456 not °C which is a bit useless, so without a differential thermometer to set the lock shield valve it is guess work which unit needs altering. With an electronic TRV head even a cheap one as shown here, I paid £15 each for these blue tooth heads, you can actually set the temperature in °C so if too hot you know the lock shield valve open too much.

The TRV takes time to open and close, around 3.5 minutes to do the exercise once a week Saturday at 12 noon to stop the valve sticking, and unless the lock shield valve is set in 3.5 minutes the radiator can get stinking hot, and this can initiate a hysteresis where the TRV closes and opens to try an control the temperature but keeps over shooting.

 

[0y1eqmco]

Thanks @CBW. Here's a few more pictures.

Old one :

has a programmer downstairs:

Strange thing is, now I've connected the old one back up as it was before, the programmer no longer has any effect on the boiler. If the thermostat is on, the heating comes, regardless of if the programmer is set to on/off. I did trip the circuit breaker at one point during the installation so I'm wondering if maybe something in the programmer has broken. Know of any way to test that?



Thanks @ericmark

I would temporary put grey in a terminal block to test the heating goes off.
​

I'll give this a try this evening. Guess if the heating remains on with the grey terminated the problem has to be with the boiler or programmer.

 

[stem]

To give you definitive advice, we would need to know what exactly is connected to the other end of each of the wires. How do you know that the black wire is N for example, and not the grey wire? There is no standard colour code, and the colours of the wires used for what function would have been down to the original installers personal choice, and the cable he had to hand. Sometimes, even green and yellow wires are used for live connections. It's not right, but it happens.

If you can't give us information about the other end of the wires, then the details of the old thermostat and what terminals that each of the wires were connected to will help us work it out for you, as per the request @CBW

Just one extra question, at any time during the installation has a fuse blown and had to be replaced?

EDIT
Just seen your previous post. Our posts were sent at the same time, so it wasn't there when I started mine. And now you will see why I asked the question abut the fuse. When you created the 'short circuit' and the breaker tripped, you have probably welded the switch together inside the programmer, so that the heating is permanently 'on' so, that will probably need replacing now.

 

[TimboTwo]

Connect grey to B. A is off.

 

[0y1eqmco]

The backplate on the Honeywell is slightly different.

Ah I see, the programmer must be broken then. I'll get a new one and then have another go. Thanks @stem

Then try,

Brown (live) to A
Grey (switched) to B

and terminate the rest.

 

[ericmark]

Personally I would simply use a programmable thermostat. So it does not turn off, it just turns down, saves having a frost stat as well, last house used Flowmasta 6259G not as easy to use to Horstmann DT2 it replaced, but did a good job.

This house use Nest Gen 3, which is easy to use with a big dial I simply turn, and large display I have set to temperature, but can show time if you want, however in real terms not really any better than the Flowmasta 6259G to control central heating, I use it as also controls hot water, and the old programmer had a minimum time of 2 hours once a day, I now have it set to 30 minutes 4 times a week.

Google Flomasta 22199X have seen it offered at £16 but normally around £36, you will have a problem getting a programmer for that price, so may as well get a programmable thermostat. Battery powered just two wire connection.

For wireless don't get cheap, wired cheap OK, but wireless cheap types don't have a fail safe, so can stick on or off if some thing stops RF signal, so wireless looking at Hive as about the cheapest with fail safe.

 

[0y1eqmco]

...

 

[0y1eqmco]

Hi @ericmark @stem @CBW

You were right the programmer was the issue. Fitted the replacement unit this morning and everything is working again.

Just wanted to run this past you before I fitted the new thermostat. The backplate on the programmer has the Live and 1 (which I think goes off to the thermostat) bridged. Is that expected? Looking at the diagram I think it's optional and as the new stat uses batteries I'm not sure if it's required or not.

Thanks

 

[stem]

Terminals 1 and 3 are connected to a switch inside the programmer. So, when the programmer switches on the two terminals are electrically connected.

Therefore, the live is connected to terminal 1 and when the programmer switches on, terminal 3 becomes live as well.

If there wasn't a connection on terminal 1 then terminal 3 would never become live and switch the heating on.

Did you notice this?

I find that if a 3A fuse is fitted, it usually prevents the switch welding together in the event of a short circuit, but not so if a 13A fuse is fitted.

Only a live and switched live are connected to the battery operated thermostat, terminals A & B

Insulate / isolate the neutral safely out of the way.

 

[0y1eqmco]

Wired it up and all working a treat - thanks all for the help