Advice on replacing old manual thermostat with digital progr

[elsmandino]

Hi there,

My old manual turn-dial thermostat has given up the ghost and I was thinking of buying a digital programmable version as a replacement - I have read that this not only gives you much better flexibility but also can help reduce energy bills.

Anyway, I would be really grateful for any recommendations/advice as to what to look out for.

Having done a bit of research, I think I need the following:

* 5/2 or 5/1/1 programmability
* Frost protection (though at the moment I cannot see that this would actually be necessary if you keep the hearing at a minimum temperature at all times).
* The ability to "learn" so it eventually learns how long to turn the heating on in advance to have it at a particular temperature at a particular time

Any help would be much appreciated - especially with the particularly cold winter that we are supposedly expecting this year.

Thanks

 

[kayedee]

If you currently have a time programmer, there will be little scope for energy savings with a programmable room thermostat, but if you replace with an new electronic thermostat the operating differential will be smaller and give you more acurate control at the room setpoint setting, over a bimetallic strip type mechanical thermostat.

 

[elsmandino]

Thanks for that. Before I look further into what products are available and the various options, I would really grateful if you could help me just work out why there are more wires in my current thermostat than are required for any of the digital types.

My current dial thermostat is a pretty old Honeywell and has three wires attached. The contacts are numbered (but not in order for some reason and are in order 2, 1, 4, 3 going downwards).

I have a red wire attached to contact 1, a blue to contact 2 and a yellow to contact 4.

All the digital thermostats I have been looking at seem to require only two wires and I cannot work out why. Which of the two would I need and what do I do with the third?

Also, why do they all need batteries? Surely, if there is a power supply, they would be powered off this wouldn't they?

 

[denso13]

You don't need the blue (neutral) and it should be terminated safely at the thermostat. You need the batteries for power. The digital stats are volt free and act as a switch therefore the "live" does not actually power the stat.

 

[elsmandino]

Right, I think I get it now - so the red (live) remains disconnected unless the thermostat senses it to be too cold - at this point, the red and yellow wires are connected and the current is sent back down the yellow wire to tell the boiler to kick in?

This being the case, why is there a neutral wire at all in my current thermostat?

 

[denso13]

The neutral is required on mechanical stats to make the anticipator work, described here http://danfoss-randall.co.uk/PCMPDF/Introducing Room Thermostats.pdf under room thermostats

 

[Charnwood]

Also, why do they all need batteries? Surely, if there is a power supply, they would be powered off this wouldn't they?

Because 240v would probably fry most of the components, unless you fitted a transformer which would be 4 times the size of the stat.

I just replaced my antique stat with a Honeywell CM927.
Features:
Inteligent; learns how long it takes to achieve temperatures therefore switches the heat on so the rooms is at the required temp when you want it. No more guesswork or wasted energy setting heat to come on too early
Programmable for 7 individual days (days can be copied from previous day's settings)
6 time zones per day
Temperature can be altered at will, but will reset at start of next time zone

 

[kayedee]

Also, why do they all need batteries? Surely, if there is a power supply, they would be powered off this wouldn't they?

Because 240v would probably fry most of the components, unless you fitted a transformer which would be 4 times the size of the stat.

I just replaced my antique stat with a Honeywell CM927.
Features:
Inteligent; learns how long it takes to achieve temperatures therefore switches the heat on so the rooms is at the required temp when you want it. No more guesswork or wasted energy setting heat to come on too early
Programmable for 7 individual days (days can be copied from previous day's settings)
6 time zones per day
Temperature can be altered at will, but will reset at start of next time zone

But if you read the post he only needs to replace a thermostat, a time programmer is not required, there is no benefit in the poster having two time programmers.
An electronic thermostat is around £10 this programmer/thermostat unit is nearly £100.

 

[elsmandino]

Firstly, thanks very much for the explanation in respect of the wiring for stats. That guide you referred to is great and has clarified a number of issues that I did have generally.

Moving on to the electrical vs programmable thermostat debate. I did wonder whether I would open up a bit of a can of worms with this as there are loads of differing views on whether they are indeed worth it.

At the moment, my heating is controlled by a 5/2 programmer that has three on/off timers.

I have it to come on about an hour before I get up, have it turn off about half an hour before I leave for work.

I then have it come on about an hour before I come home from work and have it switch off about 11 o'clock.

Thoughout this cycle, we have the thermostat constantly on at 18 degrees.

My house gets pretty cold during the winter, so I would like the ability to set the temperature at a low minimum at all times to make sure that it never gets too cold - unfortunately, my current set up does not do this.

Secondly, my heating coming on an hour before I get up is purely based on guess work and I suspect that it could come on later than this and still be warm enough - this being the case, wouldn't the ability to "learn" the heating patterns allow me to save money by accurately allowing the heating to only come on when actually needed.

Furthermore, especially when it gets to the heart of winter, we note that once the timer goes off in the evening, it can get too chilly in bed, especially around dawn. Wouldn't a programmable timer allow the temperature to stay on at the time I am in bed but keep it at a cooler, say, 15 degrees?

Do you think that I am indeed a prime candidate for a programmable thermostat or Charnwood, do you think I could still get away with just an electronic thermostat?

One other quick questions, if I may - can you get non-programmable digital thermostats that have the "learning" ability and a minimum temperature than you can set?

I suppose that if I did get just a thermostat, I could have the heating switched on at all times that I am at home but just turn it down to 15 before I go to bed. This being the case, though, how would I get round the problem of it being too low in the morning?

 

[Dan Robinson]

But if you read the post he only needs to replace a thermostat, a time programmer is not required, there is no benefit in the poster having two time programmers.
An electronic thermostat is around £10 this programmer/thermostat unit is nearly £100.

Twaddle.

Op, if your current Stat is in the right(ish) location a cm907 would be ideal.

Kayedee obviously had no idea as the 907 is about £60 maybe less.

The predictive nature of the unit means much finer control based on what you want when you want it. Rather than guessing what is needed.

 

[kayedee]

But if you read the post he only needs to replace a thermostat, a time programmer is not required, there is no benefit in the poster having two time programmers.
An electronic thermostat is around £10 this programmer/thermostat unit is nearly £100.

Twaddle.

Op, if your current Stat is in the right(ish) location a cm907 would be ideal.

Kayedee obviously had no idea as the 907 is about £60 maybe less.

The predictive nature of the unit means much finer control based on what you want when you want it. Rather than guessing what is needed.

Dan read the posts before you reply, we are talking about the cm927 which is around £100.
I do not deal in twaddle!

 

[Dan Robinson]

Check again.

Charnwood was discussing HIS 927. The op was discussing the wiring for a fixed stat.

I amalgamated the two into the 907 suggestion.

Now kindly jog on.

 

[D_Hailsham]

Nobody has bothered to ask the OP the vital question: what type of boiler does he have?

If it is a combi, the existing timer is redundant, so it can be set to permanently on and control transferred to the programmable stat.

If he has a hot water cylinder, the existing timer will still be required to control HW times; but the CH can be set to permanently on, as control will be done by the programmable stat.

I don't know if it's true of other brands, but I found out, by accident, that Optimization on a Honeywell CM9XX is not restricted to the first ON period. (Optimization is the feature which varies the ON time according to the weather). So you could set the following times/ temperatures, knowing that the house will always be up to 20C by the set time:

7am - 20C (ON)
8am - 10C (OFF)
6pm - 20C (ON)
11pm - 10C (OFF)

 

[mikely]

A programmable stat is exactly what you want. As you say, keep a night set-back temperature of 14 or 15C, then set the optimum start function that will decide when it needs to bring the heating on in order to reach your set temperature. This is how I operate my heating system using a Honeywell CM907 which will do everything you want and more. In my view £60 well spent.

 

[Charnwood]

Whoops. My new stat is the 907 wired version.

As I said, it removes the guesswork of setting it to come on "about at hour before.....". It will have the room at the temperature you want, at the time you want it.

Regarding nighttime, we have ours set at 15 degrees between 22:00 and 07:00.

Non-programmable 'learning' stats are not available, because the learning element is to do with the programmed times. They learn how long it takes to raise the temperature from A to B.

You are IMHO possibly a prime candidate for a programmable stat.
What type of boiler/system do you have?

Our time/temp settings are:
1) 22:00 - 07:00 15c - stop the dog getting too cold
2) 07:00 - 09:00 21.5c - OH likes it warm to shower in
3) 09:00 - 10:00 19c - OH works from home
4) 10:00 - 13:00 19c - ditto
5) 13:00 - 18:00 19c - ditto
6) 18:00 - 22:00 21.5c - cosy for watching the idiots lantern and keeping an eye on this forum.
In effect we only use 4 of the 6 available time zones.

60 nicker well spent? Most definitely!