Non-motoring > Heating Thermostat Question Legal Questions
Thread Author: Bromptonaut Replies: 19

 Heating Thermostat Question - Bromptonaut
Would welcome the panel's thoughts/endorsement on this.

My Baxi CH/HW boiler is, for heating, controlled by an thermostat on the hall wall. It was a like/like replacement for the original which failed ten or so years ago and is simply a rotary dial and a stat that clicks on/off. Neither accurate nor reliable, was set today at 15 but was firing up the boiler even though the adjacent thermometer read over 20. Replacement with a modern and more sophisticated device is called for. Honeywell DT90E looks like the right choice - same fitting etc.

The old stat has 4 wires. Usual live, neutral and earth (sleeved green/yellow) and an additional yellow wire.

My researches suggest that the yellow is a switched live analogous to that used in switching ceiling lights.

The new Honeywell is all plastic and double insulated so needs no earth. As it's battery powered it requires no neutral either so both those wires can be abandoned and taped off etc for safety.

All I need are the live connection (red) to terminal A and B is the switched live (yellow) so only connected when the stat is closed.

Does that sound as though it's right?
Last edited by: Bromptonaut on Tue 4 Oct 22 at 17:58
 Heating Thermostat Question - Terry
With increasing costs of energy, it may make sense to do something more sophisticated than simply replacing a failing single thermostat with a new improved working version.

A single thermostat (even a very accurate one) will only respond to temperature changes close by.

Effectively the assumption is that the rest of the house has exactly the same energy profile as the place in which the thermostat is located.

This is, of course, a nonsense as sun and wind exposure, different sized radiators, variable window openings etc will all affect the temperature in that room. The temperature required in each room may also be variable.

We have a single thermostat in the hall, usually turned up high, with thermostatic valves on each radiator. Those rooms rarely used - the valve is off until heating required. Living room may have a higher setting than (say) the bedrooms.

It would be possible to fit motorised valves controlled over wifi is desired - although for me this may be a solution a little too complex.
 Heating Thermostat Question - tyrednemotional
...having had a look at the DT90 instructions, it seems logical.

To be sure, though, I'd want to check the wiring instructions for the existing thermostat though to ensure you've identified the live and switched live wires correctly.

(My, simple click room thermostat is double-insulated - no earth - but has 4 potential connections - neutral and live, and make when rise and make when fall - only the make when fall being used).

In general use, I've never worried at all about accuracy, simply set it at an offset that represents the desired temperature, and forget the numbers on the dial.
 Heating Thermostat Question - smokie
... and as someone who loves his tech I am still with the single stat in the hall (replaced about 2 yrs ago) and mostly individual rad stats in each room.

Some of the WiFi stuff is very expensive and really I don't see much point in it, unless you are going the whole hog like some I know, who adjust room temps if someone opens the window etc.
 Heating Thermostat Question - Fullchat
I'm with you.

A lot of these tech gadgets are solutions to problems that don't exist.

However i suppose being able to turn oon the heating before you get home has an appeal. But I'll manage. :)

 Heating Thermostat Question - Kevin
I'm still on a simple (electronic) thermostat and TRVs.

A while ago I had some work done that involved moving and fitting new rads and moving the thermostat. The CH guy fitted a new Timeguard thermostat that allows you to select the hysteresis (temp swing) before it switches on or off. The hysteresis on the old mechanical stat was ridiculously wide.

I see there's a new version available with the ability to select 6 different temperature periods per day.


I think TnE's answered your question but there should also be markings next to each connector on your existing stat that tells you what each wire is doing ie. L - Live, L1 - switched live or similar. Don't rely on conductor colour unless you're sure it's a direct point to point cable.
 Heating Thermostat Question - tyrednemotional
>> Don't rely on conductor colour unless you're>> sure it's a direct point to point cable.

...absolutely do not! I've recently replaced an outside security light with an LED unit. When off, the LEDs were showing some fluorescence.

When I checked the wiring with a multimeter, the live and neutral (identified by their colour) were in fact reversed. No problem with an incandescent fitting, but

Reversing the connections resolved the issue.
 Heating Thermostat Question - R.P.
We had Nest in the last place - very good and 2xHives here which are good. Our lives are pretty random and these wi-fi things suit our lifestyles. Fire up the heating remotly works well for us !
 Heating Thermostat Question - bathtub tom
I've had a new boiler, rads and ESI wireless programmable thermostat fitted. Almost need a degree to program it. It has optimum start (switches on early so the house is at the desired temperature at the start time). Delay start (doesn't switch on if the house is at the desired temperature). Optimum stop (switches off if the house is at the desired temperature).
So the last two do nothing more than a regular stat!

Something I did learn was you need one rad without thermostatic valve, or a by-pass to prevent the pump driving at a fully closed system if all the valves are shut.
 Heating Thermostat Question - Timeonmyhands
Re the “non thermostatic valve radiator” it used to be practice to use the bathroom radiator as the bypass so it could act as or in fact be a towel rail, useful in summer months when just hot water was selected.
 Heating Thermostat Question - bathtub tom
>> Re the “non thermostatic valve radiator” it used to be practice to use the bathroom
>> radiator as the bypass so it could act as or in fact be a towel
>> rail, useful in summer months when just hot water was selected.

The back boiler in my old house used thermo-syphon for the hot water cylinder in the airing cupboard. That meant all the upstairs rads and to a smaller degree the downstairs rads would come on without the pump going. During the very hot weather I'd close down all but the bathroom rad, which would be left open at a minimum.
Can't do this at the new place. SWMBO's not happy, because she no longer has an effective airing cupboard. The airing cupboard in the new place has a small rad in it, but that only gets warm when the t/stat turns the heating on.

Those designers of the older systems knew what they were doing!
 Heating Thermostat Question - Bromptonaut
>> ...having had a look at the DT90 instructions, it seems logical.

These are a bit confusing:

Item 3 includes several scenarios including controlling a cooling device. In practical terms I think mine is item b where T1 and T2 are live feed and switched live respectively; like a light. See also:

As long as the yellow wire is the switched live I think I'm OK.

Worst case is I fuse something.

Is there anything I should be doing with the no longer required earth and neutral other than carefully taping them up?
 Heating Thermostat Question - Crankcase
Our house is cold enough that I’ve never seen the point of trvs. If there’s a rad, it needs to be on full blast anyway to keep ice from the windows.

So all ours were set on max the day they went in and haven’t been touched since.
 Heating Thermostat Question - Dog
>> all ours were set on max the day they went in and haven't been touched since

Same 'ere = 10 rads, 1930s 3 bed detached owse. I only have the 'eating on 3 hours morning, 5 hours eve.
 Heating Thermostat Question - Bromptonaut
Did the replacement yesterday.

The red live and yellow switched live are definitely correctly identified. Neutral and earth both taped out of harm's way in case they're needed again. I'd forgotten how much of a PITA mains cable with a single solid wire conductor was to connect in confined spaces and considerable sweariness was involved.

Other problem was getting the b***** thing mounted to the wall; plasterboard dot glued to breeze block. Although the backplate was same size/shape as the old one it's mountings were a mirror image. One fixing was OK with a plasterboard Rawlplug. The other was too close where the cable emerged from the wall and the plasterboard crumbled. In the end it was solvedwith an extra long screw and a Rawlplug in the blockwork.

Had to use that technique before for heavy stuff but not a plastic switch fitting!!

All working now but as it's been warmer the last few days the heating's off again.

Meanwhile SO energy seem to have remotely kicked my smart gas meter into action and it's now registering gas on the in home display.
 Heating and Water - Economy Drive. - Bromptonaut
The new thermostat kicked me to think again about how we used the heating (and hot water).

When we both worked it came on (weekdays) an hour or so before first of us up, usually me for a London train, then off around 08:00 and back on 15:30 for when Mrs B/kids were home. Off around 22:00.

When I left the London job and was home all day we just left it on all day. By that point Mrs B had stopped supply teaching so even once I was working for CAB one of us was at home all day.

Experimenting with the new thermostat I tried heating on at 07:00 (if homeworking I start at 08:00) off at 10:00 then a boost midday to 14:00 and an evening burn from 17:00 to 20:30.

Thermostat is set at 18.5.

What I'm finding is that the temperature displayed by the thermostat (confirmed by a mercury thermometer) is pretty stable at 18.5 all day. The stat audibly clicks on and off at irregular intervals but seems to respond to draughts, the door out being opened or maybe heat from the kitchen. May be different if it's really cold outdoors but we've not had less than 5 degrees so far.

I've got a 600watt oil filled rad under my desk for the really chilly days.

If I left the boiler on for heating it would, no doubt, fire up at regular intervals even if only for five minutes.

I've also mucked about with the Hot Water so it's only on twice a day and we're making a concerted effort to only do hand washing up once a day.

At the moment SO energy have failed to send me a statement since mid August but have had my (smart) meter readings. It will be interesting to compare my Kw/h consumption for October/November and November/December with last year.

 Heating and Water - Economy Drive. - sherlock47
I have recently installed a Tado smart system - thermostat controlled only. First comment is that it is not as smart as you think it should be. It gets smarter if you pay a monthly fee - buried in the the sales bs small print, but I will persist with it.

The smartest free bits (free) are the weather and internal temperature compensation, allowing to start earlier than programmed time to achieve target temperature at desired time. The geo-fencing is useful, but requires manual intervention unless you pay.

The Android app is pretty good, but lacks some sensible features, eg HW timed boost. But this has been identified by many users so maybe it will get an update. The Html browser interface is somewhat limited with reduced features.

I am playing, with limited success, wit6h implementing the unofficial API control via Nodered - I can monitor but not yet control.

The reason i went for this is that having monitored the whole house for the last 2 years using Nodered on a RPi - I could not put together an interface that was wife- compatible. If it went wrong whilst I was away or sick, no heating or HW would be unacceptable!

From an economy pv I would probably been better off spending my time and money on just adding insulation to the loft!
Last edited by: sherlock47 on Fri 25 Nov 22 at 09:21
 Heating and Water - Economy Drive. - smokie
" could not put together an interface that was wife- compatible"

That's a problem here too but she is getting the hang of stuff when it's not too complex.

However a friend of hers house-sat for someone recently and couldn't turn on the telly or half the lights. And when I think about it, nor could visitors here.
 Heating Thermostat Question - Dave_
Bit late to the party here, but does the old thermostat function as intended if you turn it down until it just clicks when your thermometer shows (e.g.) 20degC? Regardless of what number that corresponds to on the 'stat itself.
 Heating Thermostat Question - Bromptonaut
>> Bit late to the party here, but does the old thermostat function as intended if
>> you turn it down until it just clicks when your thermometer shows (e.g.) 20degC? Regardless
>> of what number that corresponds to on the 'stat itself.

No. Part of the problem was that any touch on the dial caused it to click either on or off meaning it was nigh on impossible to set accurately.
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