Programmers and programmable room thermostats

[Alfie25]

My CH system is to be overhauled soon and I'm trying to understand these new controls. I understand a programmable room thermostat allows you to set the temperature to be different at different times of the day/on different days, but is it also a programmer, allowing you to simply set the heating to On/Off/Timed? And I mean a proper programmer, rather than setting the temperature to say 5 degrees when you just want it off.

From what I've read, some of these programmable thermostats let you adjust the temperature only, but don't offer the traditional 'programmer' functions of On/Off/Timed. Can anyone clarify please.

To be honest I would like to keep a roonstat and separate programmer. Do new combi boilers still allow you to connect these up?

 

[TCCHeating]

A programmable room stat it a programmer and thermostat combined. It gives you more control than a standard programmer and thermostat. I reccommend the honeywell cm907.

If you want a seperate programmer and thermostat then there would be no problem in wiring it to the boiler.

But as I say. Once you get used to the honeywell cm907 youll never want to go back to a standard programmer and thermostat!

 

[Alfie25]

Thanks, I'm sure I'd read on here about a Salus wireless programmable thermostat that didn't have simple on/off/timed features as per a traditional programmer.

Call me old fashioned but I'm not sold on wireless controls. Give me a wired programmer and roomstat any day.

 

[TCCHeating]

The cm907 that i recommended is not wireless. It is wired.

Ps. Avoid salus. Cheap crap

 

[Alfie25]

Sorry, I had a quick look at the brochure of the cm907 and by the look of it and it mentioning AA batteries I assumed it was wireless. I know some programmers have battery back up for if the mains power fails so it keeps the time and settings, but this one looked different.

Yes Salus doesn't strike me as a good brand, so when I had an RGI out to quote and he mentoned he normally fits Salus wireless programmable thermostats it didn't fill me with confidence. Why do some installers use them if they're so universally rubbish, same goes for cheap and nasty bottom of the range TRVs.

 

[Whitespirit66]

Why do some installers use them if they're so universally rubbish, same goes for cheap and nasty bottom of the range TRVs.

Because, many customers are only interested in the price of the job. Unfortunately, if the only criteria for chosing one installer over another is the cheapest quote, then the quality of controls and fittings, etc often suffers.

 

[Alfie25]

Just looking at the Honeywell CM0907 again. Why does it need AA batteries if it's wired rather than wireless?

The old style 2 channel programmers like mine (Drayton UP2) don't need batteries.

 

[JohnD]

it has an electronic clock and timer inside it. The batteries keep it going.

I still have an oldish Danfoss programmable stat, the factory-fitted batts are dead now, so if the power goes off, it loses its memory.

What is it that you want to do with on/off/timed that you can't do with a programmable stat (apart from forget to reset it the next day)?

 

[D_Hailsham]

Just looking at the Honeywell CM0907 again. Why does it need AA batteries if it's wired rather than wireless?

Because it is designed for use with boilers which have volt-free switching as well as those with mains voltage switching.

I wouldn't bother too much about batteries - my last lot (Duracell) in a CM927 lasted nearly three and a half years.

 

[Alfie25]

So are most combi boilers these days volt free switching or mains voltage switching? The RGIs Ive spoken seem to mainly fit wireless programmable stats, but I want a wired one for many reasons.

As for why a timer rather than a programmable stat, i prefer the ability to time the heating to come on and off, rather than setting temperatures. I know the ultimate aim is the same, but i prefer the simpler notion of on and off.

 

[Alfie25]

it has an electronic clock and timer inside it. The batteries keep it going.

I still have an oldish Danfoss programmable stat, the factory-fitted batts are dead now, so if the power goes off, it loses its memory.

You're getting mixed up here. In the case of the cm907, the batteries power it, where as in yours, the batteries are there as back up if the power goes off.

 

[Alfie25]

OK, decided to investigate these programmable stats a bit more, What I'm struggling to figure out with the cm0907 is the cycling when the set temp is reached. From what i understand, when the stat senses the temp is 1.5 degrees below the target temp, it cycles the boiler on and off 6 times an hour to maintain this temp.

How does this compare to how an old set up works. For example, my old style analogue stat would shut off the boiler once the target temp is reached, and then there's usually a biggish window (say half an hour to an hour) where even with the best insulation, the room temp will drop a few degrees and so the stat will call for heat again. I have to say I prefer this set up to the wear and tear caused by the boiler cycling.

The critical question is by how much does the temp need to drop on an old analogue dial before it calls again for heat. And... if I were to swap it for a digital stat, how do they compare?

Another programmable stat I've found is the Danfoss TP7000. This has the ability for the cycling function (or chrono-proportional control as it calls it) to be switched off. So... once the set temp is reached, stat tells the boiler to shut off. But when would it call for heat again? When it drops 0.5 degrees from the set temp?

 

[Stephen7]

I've used the CM907 for years - easy to use & as has been said the batteries are for the display only and they last years! It even gives you notice that they soon need changing.

 

[Armo74]

On older stats there would be a bi-metallic strip consisting of two materials that expand at different rates.
As the room is reaching the set temperature both metals are expanding until they break the electrical circuit.
As the room temperature decreases the metals retract back to there normal state to complete the electrical circuit which then would fire up the boiler again.
Modern ones a more accurate than the older type.

 

[D_Hailsham]

What I'm struggling to figure out with the cm0907 is the cycling when the set temp is reached. From what i understand, when the stat senses the temp is 1.5 degrees below the target temp, it cycles the boiler on and off 6 times an hour to maintain this temp.

Six times an hour is only a start point. If the room temperature has not dropped a lot the boiler can stay off for several 10 minute periods.

How does this compare to how an old set up works. For example, my old style analogue stat would shut off the boiler once the target temp is reached, and then there's usually a biggish window (say half an hour to an hour) where even with the best insulation, the room temp will drop a few degrees and so the stat will call for heat again.

The problem with mechanical stats is that they hot water circulating at the temperature set on the boiler stat until the room stat turns the boiler off. The rads then continue to give off heat, so the room temperature rises higher than needed. Cycling the boiler, with reducing on times as the required temperature is approached reduces the overshoot.

If the temperature has dropped a few degrees in an hour the house is not well insulated! In any case most people would notice the drop and turn the stat up, so the heating comes on. Then the room gets too hot, so they turn the stat down; and so the dance continues.

I have to say I prefer this set up to the wear and tear caused by the boiler cycling.

The wear and tear of boiler cycling is minimal.