I am troubled by hysteresis. The thermostat seems to have a 2 degree 'delay'. Are the digital ones any better in this respect? If so, what's a good one to buy? I don't want anything fancy. Also, are the radio ones any good?
Bill
All thermostats will have hysteresis or they will "chatter" around the setpoint.
The electronic ones will have it programmed in, whereas the mechanical ones tend to have it as a natural result of the quick-snap switching action.
I suspect the digital ones will have it rather tighter than the mechanical ones, but it will be there and will be some amount that may or may not be documented.
Why not make them so they only switch once every five minutes, like the auto mono/colour switching in a CCTV camera??
Bill
That's another option of course, that is not readily available with a mechanical design.
Now I am feeling really thick. Never occurs to move ours into the dining room when we have the gas fire alight in the lounge where it normally lives.
Reliable though and keeps the temperature within 1 degree.
I was pondering on that as I screwed mine to the hall wall, but it had to go there because it covers the gap in the wallpaper where the old thermostat lived.
Apart from the small amount of thermal inertia in the bead thermistor, there is no reason why an electronic thermostat need have any hysteresis. This won't always be a good idea though, as short term cycling of boilers is an inefficient way to run them so I suspect the designers put a controlled amount of hysteresis back in.
So called "anti cycling" logic is often built into modern boilers...
For a basic programmable stat (i.e. the ability to specify different temps at different times of the day) I find the seven day Centaurstat quite good and reliable (under £30 at SF etc)
It does not try to do any of the optimum start type tweaks to the timing itself though.
Generally yes if you go for a decent one. Can save a bit of wiring, and /or let you move the sensor bit around with you.
But what happens if you have 2 combis and only one stat?
How have you measured the 'delay'? Have you simply twiddled the knob backwards and forwards, and noted a 2 degree difference between where it clicks on and clicks off? Or have you monitored the room temperature controlled by it, and noted a 2 degree range? If the former, there may not *actually* be a 2 degree hysteresis. Most manual stats have a built-in 'anti-hysteresis' heater resistor. This generates a bit of heat whenever the stat is 'on', fooling it into switching off a bit earlier than it otherwise would, and reducing overshoot.
Digital stats work on an entirely different principle. When near the desired setpoint, they operate like a proportional controller - and they proportion the on/off times in a way which usually results in maintaining the required temperature within no more than half a degree. They save energy because you don't have to set them over-hot in order to make sure that the room is still hot enough at the bottom end of the hysteresis.
As regards makes/models, my experience is limited to Honeywell CM67's (of which I have 3!). These work pretty well but, as someone else has said, may come into your 'fancy' class. The wireless ones are excellent
- and very reliable (unless you live in a Faraday Cage!). They have the advantage that you can move them around, to find the best position - and you can also programme them on your lap.
My quite old one goes into a 'proportional mode' once it is up to near temperature. It switches in a 10 minute cycle with the mark/space depending on where it is in a small band close to set point. I think it's +- 1degC for proportional mode.
More modern ones (Honeywell?) have this programmable.
I wouldn't call the CM6x Honeywell wireless stats reliable. The UK ones operate on 433MHz or thereabouts are very prone to outside interference and clashes of frequency allocations. It's just as well they default to a set period on off pattern when signal is lost as a house would freeze if left unattended.
The more recent Honeywell CM9xx series operate on 868MHz and are much better.
They there were probably plumbed by dribble, and won't work anyway ;-)
Is there any point if you are in the living room with the fire?
Jonathan
It will work. Duh.
The point is that, presumably, the gas fire isn't normally lit but the lounge and that is the room that is normally occupied. So you need the stat in there to make that room the most comfortable. Light the gas fire though and the lounge warms up, stat becomes satisfied and the rest of the house cools. Moving stat to dining room means it still keeps the rest of the house warm.
My daughter has had a programmable thermostat made by Honeywell for a year. The sensor is portable. She is happy with it. We had a programmable Honeywell thermostat with a fixed location sensor, which we were happy with We bought a new combi boiler which included a new fangled Dutch thermostat, which we have major difficulties programming. It isn't a language problem, it is just a problem of it being supplied with useless manuals. We have both Dutch and English language versions of the manual.
The modern ones are also adaptive, they will work out how long it takes to reach the required temperature and start up early enough for you to have a warm room when you get up/come home from work etc.
Honeywell TH4210D Programmable Thermostat Manual
"Adaptive Intelligent Recovery? (Setup Function 13) Adaptive Intelligent Recovery eliminates guesswork when setting your schedule. It allows the thermostat to ?learn?how long your furnace and air conditioner take to reach the temperature you want. Just set your program schedule to the time you want the house to reach your desired temperature.The thermostat then turns on the heating or cooling at just the right time to reach your scheduled temperature at your scheduled time."
There's an example here of one switching off earlier than the end of a programmed period.
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