Help re Honeywell T40 wall thermostat.

[ianmcd]

Thanks for reply. My comment was just regarding old wiring colours but I did notice today, whilst wallpapering around it, that there is a tiny piece of red insulating tape on the black wire so I think that probably means switched live.

yes it does, and for future reference wiring colours mean diddly squat in a heating system

 

[Harry Bloomfield]

Thanks for reply. My comment was just regarding old wiring colours but I did notice today, whilst wallpapering around it, that there is a tiny piece of red insulating tape on the black wire so I think that probably means switched live.

Yes, that was usual practice so people following knew it wasn't a neutral. Today, with a brown and blue, they would/should mark the blue with brown sleeving.

 

[fixitflav]

yes it does, and for future reference wiring colours mean diddly squat in a heating system

Same with lighting circuits, where twin + earth, brown, blue, green/yellow the blue is often a switched live.

 

[ericmark]

You are correct in a way, if every room has a TRV then there is no need for a wall thermostat. On can manually turn heating off/on summer/winter.

However a wall thermostat can stop the boiler cycling when your in that transision time between summer and winter, often without neutral connected as a high hysteresis is good when using thermostat to stop cycling.

However a new thermostat likely looks better than a blanking plate, and they are cheap.

There is really no way around testing, for me I am happy to remove cover and test, if your happy testing then you need to test voltage with thermostat turned off, to find if 24 volt or 230 volt or even connected.

 

[JohnD]

You are correct in a way, if every room has a TRV then there is no need for a wall thermostat.

Except that the boiler will keep making itself, and its water, and the bypass hot, all the time. The unwanted heat will make the house warmer than needed, and the cost of energy is wasteful.

 

[Harry Bloomfield]

You are correct in a way, if every room has a TRV then there is no need for a wall thermostat. On can manually turn heating off/on summer/winter.

A stat is essential for economy, without it the boiler, not knowing the temperature was satisfied, would just continually cycle on its stat, never switching off. A very inefficient way to run a heating system.

 

[ericmark]

A stat is essential for economy, without it the boiler, not knowing the temperature was satisfied, would just continually cycle on its stat, never switching off. A very inefficient way to run a heating system.

I would agree, you ideally want an automated way to tell the boiler it's not required. The ideal way is for the TRV to tell a hub/thermostat which in turn tell the boiler what output is required Tado, EvoHome do this, failing that when it wants the boiler to run Hive do this, or linking the TRV with the thermostat so they both automatically change together Nest does this. However each of these systems is expensive.

So we try to get away with cheaper options, the traditional method was a thermostat in a ground floor room which is normally kept cool has no outside door, and no alternative heating, however often such a room does not exist, so some other compromise is required, for example setting the thermostat with same heating schedule as the wall thermostat.

However when I had gas, (now using oil so not a modulating boiler) the problem was to get the boiler to modulate without the wall thermostat messing it all up. Theory is as each TRV closes the water is either pushed through the remaining radiators at a greater pace, or it opens the by-pass valve, net result is the return water temperature starts to rise, the boiler then starts to modulate until it has reached it's minimum output, at which point it starts to cycle, the better boilers will have some anti-cycle software, so if when the boiler fires up, within a set time the water returns hot, the time before next cycle is increased, other wise it is decreased, so cycling is reduced to a minimum.

Adding a on/off thermostat can result in the boiler not modulating fully before being turned off, this wastes fuel as it is turned off hot and is cooled through the flue, and also increases the hysteresis, as to which wastes more fuel, being turned off premature, or turned off late by a on/off thermostat is open for debate, but it's not cut and dried, and would vary installation to installation and user to user depending on how much manual control is used. Where the user turns off the system slightly early and turns it back on slightly late, it may be the manual control wastes less fuel than the automatic.

Personally I was forced into fitting a thermostat which would talk to the TRV's mainly because of lack of wiring, I needed to switch on the boiler, pumps, and motorised valves as required using just two wires, so fitted Nest, not because it's a Smart thermostat, but because I only had two wires. Had this not been the case, I would have used a programmable thermostat both on the wall and as a TRV head running the same program. But unless the wall thermostat and TRV head can both be set to sing from same song sheet it is likely to cause problems, and most people want dormitory areas and living areas to follow different schedules, so the old mechanical TRV heads just don't cut the mustard.

OK electronic TRV heads start at £10 each, so not expensive, but then they need setting up which can involve some tricks, last house wall thermostat turned down at 4 am and back up at 6 am and TRV heads also changed temperature as 6 am so that turning down 4 to 6 ensured boiler was running when TRV changed the temperature set. It also had a second wall thermostat in parallel to allow for wind change as depending on wind direction, the front or back of house would become warmer. Yet the house before that one, worked fine with a single wall thermostat controlling all of the open plan down stairs, and TRV's stopped up stairs over heating.

My 4 houses that I have in my time owned, have all been different, there is no one size fits all unless you use expensive EvoHome or Tado. Doing it on the cheap, it takes some thought on what is best for that house.

However starting point is normally the TRV head, not the wall thermostat, hardly worth fitting a programmable wall thermostat if the TRV heads are not also programmable, and fitting a new TRV head is easy, compared with fitting a new thermostat on the wall.

However wall thermostat is likely a good idea in fullness of time, but the user still has a programmer, so it would seem we need to establish what he has and what he is willing to pay to improve the system?

The instructions I have found seem to show the boiler has either a 24 volt or Ebus control, as to what can be done DIY I don't know, for some daft reason some boilers have the electrics within the gas tight area, and although it says Ebus it does not say OpenTherm and some Ebus systems are not as good as using return water temperature.

However since the boiler uses a 24 volt thermostat there will clearly not be a line, neutral and earth at the thermostat, it will likely only have two wires connected if it is connected, and the replacement will have to be a two wire type. As to if a battery powered type, or simple mechanical if only to stop cycling mechanical is good enough, but if you want to use it to control temperature then battery powered likely the answer. There are some really good quality thermostats like the Nest e which are battery powered, and will work with opentherm, but I would think the flomaster programmable at £35 is ample.

 

[Harry Bloomfield]

The instructions I have found seem to show the boiler has either a 24 volt or Ebus control, as to what can be done DIY I don't know, for some daft reason some boilers have the electrics within the gas tight area, and although it says Ebus it does not say OpenTherm and some Ebus systems are not as good as using return water temperature.

However since the boiler uses a 24 volt thermostat there will clearly not be a line, neutral and earth at the thermostat, it will likely only have two wires connected if it is connected, and the replacement will have to be a two wire type. As to if a battery powered type, or simple mechanical if only to stop cycling mechanical is good enough, but if you want to use it to control temperature then battery powered likely the answer. There are some really good quality thermostats like the Nest e which are battery powered, and will work with opentherm, but I would think the flomaster programmable at £35 is ample.

My Vaillant has Ebus or the alternative mains thermostat control. It was installed last year with all mains controls from the earlier boiler system, since when I have removed all of that, to replace it with a full Ebus system for much better/efficient control. Timeclock + room stat + cylinder stat were ripped out and replaced with a wireless combined timer and room stat. The airing cupboard wiring centre, replaced with an Ebus controller and a sensor on the cylinder, rather than a switching stat., then an outdoor wireless temperature sensor.

Prior to fitting this kit, the boiler would simply run near flat out, until the stat switched off, with little modulation and was quite audible when running in the kitchen wall cupboard and pipes would creak. Now it fires up already modulating its output and I never hear it running, I have to check its panel to know its operation state. The pipes creak much less now and the temperature in the place is much more stable - none of the being cold when its off, too warm when it runs.

 

[ericmark]

Good to hear ebus works, since now using oil, I no longer have the option, and the problem with old house was one had no idea what the boiler was doing, a small blue light came on when running, that was all. And it was too early a model of the Bosch range to fit ebus control, I am sure the Honeywell thermostat's anti-hysteresis did not help, switching the boiler off/on so often, but it seems with wireless thermostats either they don't fail safe, or they have anti-hysteresis software. So for a modulating boiler thermostat needs to be wired unless using ebus.

The problem is the huge leap of faith buying the ebus system as it's not cheap, and few seem to report how well it does or does not work, so thank you very much for the report.

 

[Harry Bloomfield]

So for a modulating boiler thermostat needs to be wired unless using ebus.

The problem is the huge leap of faith buying the ebus system as it's not cheap, and few seem to report how well it does or does not work, so thank you very much for the report.

From a single self install, plus as a one winter user - yes it works absolutely perfectly. But is a complete lack of any hard information about how the Vaillent Ebus system works - the software. It seems it uses some sort of learning process, to determine how much heat it needs to produce, but that is only my guess from watching it at work. My one criticism is of the display on the wireless stat control. It displays the wrong date when you first wake it up and takes a few seconds to display the current data.