Room thermostat advice please

[jonmac73]

Hi, wanted sanity check of thermostat replacement before proceeding...
Replaced original 70's Honeywell when last decorated, and also because the 'temperature swing' from on to off was too great. Could not find any original wiring for it so trial and error fitting and had DT90e working.
First one locked up - checked with HW and confirmed fault so replaced.
Present one has been fine for months, but now the indicated temp is clearly several degrees higher than actual. Net result is having to 'force' the desired temp up above whatever the thermostat indicates to force the heating back on. Normally leave at 17, and room reaches indicated 17.5 which is very comfortable.
Plan to exchange the DT90 for either yet another one, or Drayton Digistat which seems to have good reviews.

Basically is new thermostat wired correctly and just unlucky with faults, or is something wrong ?
Surely something this simple either works or doesn't work ?
Wiring is a 3-core cable colored red, yellow,blue.

DT90e wiring:
A - red
B - yellow
C - [empty]

For information, thermostat is on external wall in large open plan reception room. All rads have TRV. Timer is upstairs with simple twice daily on/off which is fine. Blue cable is chocblocked inside wall cavity. Can't find matching colored cabling at boiler end electric panel.
For aesthetics want a slimline surface mount thermostat, just for temp.

Sorry if long but tried to be thorough.
Thanks in advance - any constructive opinions welcomed.

70's Honeywell Thermostat (T403A1083 ??)

DT90e wiring installation note:

 

[D_Hailsham]

now the indicated temp is clearly several degrees higher than actual. Net result is having to 'force' the desired temp up above whatever the thermostat indicates to force the heating back on. Normally leave at 17, and room reaches indicated 17.5 which is very comfortable.

What are you using to check the temperature?

Have you checked the batteries in the thermostat?

Basically is new thermostat wired correctly

Yes

Surely something this simple either works or doesn't work?

It's not as simple as you think. There is some "intelligence" built into it.

All rads have TRV.

Do you mean ALL, ie 100%? If so, that's wrong. One rad should not have a TRV as it would conflict with the room stat. Set the TRV on the reception room rad furthest from the thermostat to Max.

 

[jonmac73]

What are you using to check the temperature?

No equipment but is obvious feeling to everyone in house. This started last week when weather was still really cold. Basically I'm expecting to set the desired temp and it to remain without CONSTANT adjusting.
Also this has just started recently been fine for 4 months since replacement.

Have you checked the batteries in the thermostat?

Will try fresh Duracells.

Do you mean ALL, ie 100%? If so, that's wrong. One rad should not have a TRV as it would conflict with the room stat. Set the TRV on the reception room rad furthest from the thermostat to Max.

Yes all apart from bathroom as advised by Plumber who put in a new radiator. I have read conflicting advice of this but that is what we have, and there are turned up pretty high all the time downstairs, obviously lower up in bedrooms.

 

[D_Hailsham]

What are you using to check the temperature?

No equipment but is obvious feeling to everyone in house. This started last week when weather was still really cold. Basically I'm expecting to set the desired temp and it to remain without CONSTANT adjusting.
Also this has just started recently been fine for 4 months since replacement.

Which boiler do you have and what is the boiler temperature set to?

Yes all apart from bathroom as advised by Plumber who put in a new radiator. I have read conflicting advice of this but that is what we have, and there are turned up pretty high all the time downstairs, obviously lower up in bedrooms.

The correct advice is as I said earlier: the rad in the room with the wall thermostat must not have a TRV; this is because it will cause a conflict.

If the TRV is set to a higher temperature than the wall stat, the wall stat will turn the system of first, so the rad will never reach the temperature set on the TRV. If the wall stat is set to a higher temperature than the TRV, the TRV will shut off the rad before the wall stat reaches temperature, so the wall stat will never reach temperature and the boiler will run and run and run.

The solution, in your case is to set the TRV on the rad furthest from the wall stat to max, so it is then controlled by the walls stat. The other rads in the room will be turned off by their TRVs.

If you have to have the TRVs turned up high, ie. not left on a fixed setting, then:

1. the rads are not large enough; or
2. the boiler is not giving enough output; or
3. the boiler temperature is not high enough.

 

[mysteryman]

Just a general point - all electro-mechanical room thermostats such as the original one here should have the neutral wire connected. This powers up the anticipator heater in the 'stat, so that it leads rather than follows room temperature changes. It thus gives closer control.

None of this is anything like as good as weather compensated variable temperature control, obviously.

 

[jonmac73]

Which boiler do you have and what is the boiler temperature set to?

Boiler is a not massively efficient (according to BG) admittedly Potterton Profile something, 20yrs+ probably but regularly serviced and no probs. Not sure exact temp - dial 0-6, normally on 3, upped to 4.5 in recent cold snap.

If the TRV is set to a higher temperature than the wall stat, the wall stat will turn the system of first, so the rad will never reach the temperature set on the TRV.

OK, have checked and TRV's are set to MAX on ground floor still from last week's cold spell.
Also maybe I didn't explain fully - the rads DO get sufficiently hot when the boiler is forced on by the thermostat. I have a problem with the indicated temperature on the thermostat believed to be higher than actual.

I may try another stat, either same make or different now I know wiring is correct (thought was as wouldn't work at all otherwise).

 

[jonmac73]

Just a general point - all electro-mechanical room thermostats such as the original one here should have the neutral wire connected. This powers up the anticipator heater in the 'stat, so that it leads rather than follows room temperature changes. It thus gives closer control.

I assume that the blue wire (connected on original) and not required here is Neutral then ?

 

[D_Hailsham]

Not sure exact temp - dial 0-6, normally on 3, upped to 4.5 in recent cold snap.

I would set it to 6 all the time and let the room thermostat control when it comes on and go off. If the weather is very cold the boiler will be on a lot, so the mean water temperature will rise. But in milder weather the boiler will be off more than it is on and the mean water temperature will automatically be lower.

I have a problem with the indicated temperature on the thermostat believed to be higher than actual.

Buy a thermometer and hang it next to the thermostat. Then you will be able to say with certainty if the thermostat is showing the wrong temperature.

 

[jonmac73]

Hi again - will get a room thermometer and check before replacing - thanks.

Re: the boiler I was always told to keep it lower when warmer weather and raise if it is really cold.