Replacing a wired room thermostat

These are intended for combi boilers, but your suggestion will work giving the features you describe. I have not seen a mains operated version, but the battery will last for more than a year. Why not go all the way and get a radio frequency thermostat that you can carry into the room you are using, thus controlling the temperature in that, the currently most important room.

 

Not sure why you need a thermostat if the rads are already controlled by TRVs, but each to his or her own, I suppose..

 

It comes under Building Regulations Part L 2002.

 

Hi Simon.

You have it sussed - it's exactly as you say.

And, yes, very few mains powered ones these days - I guess it's 'cos battery-powered means simple 2-wire connection.

As said above, battery units last at least a year, often more like two, so not a hassle in practice. Having said that, I personally went out of my way to get a mains one - a Heatmiser - for my own house.

This iQ is cracking value - the savings will pay for lots of batteries: http://www.screwfix.com/p/iqe-7-day-programmable-room-thermostat-with-li-ion-battery-back-up/53575

 

That is also my feeling, in mothers house we have TRV's to control each room, their operation is such that the TRV opens and closes gradually so during the heart of winter the boiler flame height varies to match demand from the radiators, giving a constant control, which is far better than the thermostat can do.

However as summer approaches then the TRV has a problem, the heat required is less than the minimum supply the boiler can give, so boiler starts to cycle, and although there is anti-cycle software in the boiler at some point you need something to switch off the boiler.

In an ideal world the eTRV connects to a hub which in turn tells the boiler to switch off, however that is expensive, so cheap way around the problem is to have a thermostat set to switch the whole system off.

There is again a problem, where do you put the thermostat? normal answer is coldest room, but in mothers house that is not easy, the hall is coldest room, however it has the front door so the radiator is large, so it can re-heat the room fast, this would mean the hall heats up faster than other rooms, even though we would like it to be colder than other rooms, so we also fit a TRV to hall in spite of it having the thermostat in that room, what we do is set TRV to start closing at 18°C and wall thermostat is set to 19°C so hall heats up quickly to 18°C but takes a lot longer to reach 19°C which allows for other rooms to also get warm.

So if you change the temperature of the wall thermostat you also need to change the temperature of the TRV to match, so although you can set a defrost of say 12°C and normal of 19°C you can't really have the wall thermostat change temperatures throughout the day, as the TRV would also need to change to match.

However there is nothing to stop you fitting eTRV's and having bedroom only heated at night and living rooms only heated in the day, mothers house I have two eTRV's one in mothers bedroom and one in her living room, and with the latter evening temperature is highest, night lowest, with something in between for the daytime.

I used a wifi linked eTRV but in hind sight and hind sight is easy, it did not need to be the wifi type, the Pegler Terrier i-temp i30 would have done the job just as well as the MiHome heads I fitted without need for a central hub, clearly not perfect as when the valve calls for more heat, you will only get that heat if the boiler is running. But unless you go whole hog and get something like EvoHome then you have to accept it is not perfect.

Of course there are exceptions, with a hot air rather than wet system then programmable thermostats work well, and with open plan homes again they can work well, but in the main the thermostat is only there to stop boiler cycling in the summer, it does little or nothing in the winter the TRV does all the work, and if you want to change temperatures through the day you use eTRV heads in those rooms.

With Nest you can set the MiHome eTRV to follow Nest using the ITTT system, however since I want the thermostat to be set at 19°C and TRV at 18°C that would not work, I need the TRV to be rather poor controlling temperature for it to work with the wall thermostat, the eTRV is too good, so to get what I have now, which is when front door is opened the hall quickly reheats, but it does not get hot enough to switch off the wall thermostat until whole house is warm, would be hard to set with a eTRV they are too good.

 

Again, you've got it.

Convention would have it that the wire link you'd take from the 'L' to the A terminal would go to A1, and then the 'load' ("switched live") would connect to A2. But of course it would work either way around - it's chust a switch.

Every wire that becomes 'live' at some point - L, A1 and A2 - should have a red or brown sleeve (eg tape) placed around it to ID this.

My thoughts on the Heatmiser - not as sensitive as I thought it would be (tho' perhaps that's an issue with location as much as anything else) and not at all intuitive to set up and use. Eg - I need to consult the 'help' card in order to change BSTs twice a year.

It certainly works and does its intended job, but I would need to consult the instructions if I needed to change anything - other than a quick temp over-ride which I've remembered how to do!

It also doesn't keep perfect time - it's currently around 10 minutes fast (not a huge concern, of course).

Good points? It sits in a back box so is very neat, and - that's it. Oh yes, the display lights up when you press a button...

 

It's nice, but not that nice...