best room thermostat

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Had a conventional Worcester Greenstar 18ri fitted about a year ago. In the hall they fitted a basic clocklike Honeywell thermostat to control room temperature - you simply turn clockwise to increase temperature. (the hot water is controlled by a separate thermostat on the hot water tank) .
I have had the honeywell room thermostat replaced but it is still very pretty inaccurate. For example, turn it to 19c and the boiler turns off when the room temperatures actually gets to about 24c. I have TRV's in the bedrooms but does anyone know of a more reliable hall thermostat which is compatible to my boiler. Would a digital one be more accurate ?
 
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I have had the honeywell room thermostat replaced but it is still very pretty inaccurate. For example, turn it to 19c and the boiler turns off when the room temperatures actually gets to about 24c. I have TRV's in the bedrooms but does anyone know of a more reliable hall thermostat which is compatible to my boiler. Would a digital one be more accurate ?
Electromechanical thermostats are not as accurate as digital ones. This type of stat keeps the boiler running flat out until the required temperature is required. The problem is that the rads continue giving off heat after the boiler stops, so the room temperature continues to rise. However a rise from 19C to 24C is exceptional, unless your rads are considerably oversized.

They also have a wide differential (difference between on and off temperatures). This means that users are continually turning the stat up and down to achieve a comfortable temperature.

If you have a very old timer, it's worth considering a programmable thermostat such as the Honeywell CM900 range. If not, the Honeywell DT90E or DT92E are very good. You will not get the room temperature rising above the required setting with these temperatures as they take into account the fact that the rads continue to give of heat. It's called TPI control and you can read how it works in the Honeywell FAQ.
 
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Electromechanical thermostats are not as accurate as digital ones. This type of stat keeps the boiler running flat out until the required temperature is required. The problem is that the rads continue giving off heat after the boiler stops, so the room temperature continues to rise.
Once again, our armchair expert has his facts wrong. You should spend a bit more time on Google before you post your nonsense here.

1. All modulating boilers will throttle down when they approach settisfacting point, you don't need a special thermostat for that.

2. Those boilers that are not specially designed to take variable feedback from special thermostats, will not react any differnet to them than they do to a £10 mechanical model.

3. Overshooting of target temprerature is not due to residual heat in the rads, but to the lag in response from the stat.

4. Even it were due to residual heat in the rad, the temperture would still go up after the boiler turns off, unless you could design a thermostat that could calculate how many minutes before reaching the target temperature is reached.
 
1. All modulating boilers will throttle down when they approach settisfacting point, you don't need a special thermostat for that.
The boiler modulates down to maintain the correct water temperature. It has nothing to do with the room temperature.

Those boilers that are not specially designed to take variable feedback from special thermostats, will not react any different to them than they do to a £10 mechanical model.
You don't need specially designed boilers.

Overshooting of target temperature is not due to residual heat in the rads, but to the lag in response from the stat.
Both are true.

Even it were due to residual heat in the rad, the temperature would still go up after the boiler turns off, unless you could design a thermostat that could calculate how many minutes before reaching the target temperature is reached.
It's been done by many thermostat manufacturers; it's called TPI.

Isn't it about time you caught up on modern control technology?
 
The boiler modulates down to maintain the correct water temperature. It has nothing to do with the room temperature.
That's what I said.


Those boilers that are not specially designed to take variable feedback from special thermostats, will not react any different to them than they do to a £10 mechanical model.
You don't need specially designed boilers.

You do if you want them to modulate.


Overshooting of target temperature is not due to residual heat in the rads, but to the lag in response from the stat.
Both are true.

But the residual heat part tends to be far smaller than the lag part

It's been done by many thermostat manufacturers; it's called TPI.
TPI is one of these things that sound great on paper but have several significant downsides one of wich is potential for excessive cycling.

The fact that the POTENTIAL saving is far less than the average savings in the real world is another one.
In fact, when installed on the average system today, there is virtually no saving at all, other than the part resulting from lowering the average temperature in the room by switching the boiler off sooner. This can be equally adequately achieved by simple lowering the setting to the same end temperature as that on which the tpi would turn the boiler off.
Nothing more than a gizmo.

There is virtually ZERO saving from tpi technology in itself.
 
The boiler modulates down to maintain the correct water temperature. It has nothing to do with the room temperature.
That's what I said.
No, it wasn't. You said:

1. All modulating boilers will throttle down when they approach settisfacting point, you don't need a special thermostat for that.
I don't' know what you mean by the "settisfacting point", but the discussion has been about controlling room temperatures, not boiler temperatures.

You don't need specially designed boilers.
You do if you want them to modulate.
We are not discussing modulation, but controlling room temperatures.

But the residual heat part tends to be far smaller than the lag part
What evidence do you have for that?

TPI is one of these things that sound great on paper but have several significant downsides one of which is potential for excessive cycling.
What is the problem with cycling. provided it is properly controlled?

Don't you think that Honeywell et al have conducted tests to see what effect this will have on components, particularly gas valves?

The fact that the POTENTIAL saving is far less than the average savings in the real world is another one.
Evidence?

In fact, when installed on the average system today, there is virtually no saving at all, other than the part resulting from lowering the average temperature in the room by switching the boiler off sooner. This can be equally adequately achieved by simple lowering the setting to the same end temperature as that on which the tpi would turn the boiler off.
So a mechanical stat set to, say. 19.5C will achieve the same result as a TPI stat set to 21C?

There is virtually ZERO saving from tpi technology in itself.
Evidence?
 
I don't' know what you mean by the "settisfacting point"
Typo; satisfaction point.

but the discussion has been about controlling room temperatures, not boiler temperatures.
Can you tell me another reason for investing in more advanced controls than hoping to use less gas and save a few bob? If you want to eliminate unnecessary gas use from too high a room temp, simply turn it down a bit.

As for the controlling and related savings: a SE boiler has a virtually constant efficiency between 50C and 80C, and thus any and all savings will come virtually exclusively from lowering the average room temperature; whether the boiler is on for 2x5 minutes or 5x2 minutes makes no measurable difference outside a lab.

With HE boilers, savings are identical with SE in lowering room temp but have an additional factor, which is lowering the return temperature. Switching the boiler on and off more frequently does not bring that average down in a significant way compared to either a constant low temp design or weather control.

Since you have never been a heating engineer, I don't expect you to understand what I explained, but even you can find it on Google if you read enough so you can absorb it in small steps.

It does however form the basis of all the evidence you asked for. On top of that, as you are a real regulation lover; I am qualified and certified to sign off for par L ( energy efficiency ), you are not, never have been and never will be.
 
Hello, thanks for the replies. all too tecnical for me.
The temperature is read from the halway and basically dictates the general downstairs areas.
Are we all agreed that the cm900 range of thermos are decent for the job ....and presumably digital to be more accurate ?
 
You still haven't justified this:

But the residual heat part tends to be far smaller than the lag part

but the discussion has been about controlling room temperatures, not boiler temperatures.
Can you tell me another reason for investing in more advanced controls than hoping to use less gas and save a few bob? If you want to eliminate unnecessary gas use from too high a room temp, simply turn it down a bit.
But that is a completely different subject; the OP is concerned with maintaining a constant comfortable temperature.

I've been around considerably longer than you and can remember the times when mum/dad would be forever turning the stat down because the room was too hot and not long after turning it down because it was too cold. The stat may have been set to 20C but the room still heated up to about 22C, due to the residual heat in the rads. The stat was turned down until it clicked (about 18 or 19C) but the room cooled down below that due to the hysteresis in a bimetallic sensor.

It's a well documented fact that if the temperature is maintained with close limits, a lower temperature can be tolerated.

You have introduced a lot of irrelevant information about the efficiency of SE and HE boilers, but no concrete evidence to justify your earlier assertions.

Since you have never been a heating engineer, I don't expect you to understand what I explained, but even you can find it on Google if you read enough so you can absorb it in small steps.
I will treat that with the contempt it deserves.

If you want evidence that a TPI controller works, I can provide it.

When I moved to my current house, about five and a half years ago it had the traditional electro-mechanical stat with anticipator. The first years gas consumptions was about 30,000kWh. I then installed a Honeywell CM67 followed by a CM927. The consumption has gone down to about 24,000kWh; a reduction of 20%.

And before you reply that the saving will be due to it being warmer in the winter, think again; winters have actually been getting colder. Look up the degree-day data for your area.
 
can remember the times when mum/dad would be forever turning the stat down because the room was too hot and not long after turning it down because it was too cold. The stat may have been set to 20C but the room still heated up to about 22C, due to the residual heat in the rads. The stat was turned down until it clicked (about 18 or 19C) but the room cooled down below that due to the hysteresis in a bimetallic sensor.

You can't cite that as evidence unless you at least know whether the stat had an accelerating heater in it. There have been versions WITH one since the 50's at least, and adjustable mechanical hysteresis as well, like on the old Satchwells.
You can of course still buy crappy mechanical room stats without acceleration.
I have also put in loads of Honeywell prog stats with cleverness built in - and now always disable it because of call-outs when the user fails to understand it.
 
You can't cite that as evidence unless you at least know whether the stat had an accelerating heater in it. There have been versions WITH one since the 50's at least, and adjustable mechanical hysteresis as well, like on the old Satchwells.
Knowing my dad, he would have insisted on the best and latest. So I would assume it had an anticipator.

I have also put in loads of Honeywell prog stats with cleverness built in - and now always disable it because of call-outs when the user fails to understand it.
That couldn't possibly be because you did not take the time to explain the features to the users??
 
I've obviously started a heated debate here.
Anyway, am I correct that a Honeywell digital is probably the best to get ?
 
1. All modulating boilers will throttle down when they approach settisfacting point, you don't need a special thermostat for that.
Garbage. Boilers "modulate down" to maintain exactly the same (hot) radiators even though they don't require full power to maintain that temperature. Without special controls (sometimes referred to as weather compensation controls, although that is not strictly accurate because they don't need to have any external sensors) they have no idea what the temperature in the room actually is, nor how close to the set temperature it is.
2. Those boilers that are not specially designed to take variable feedback from special thermostats, will not react any different to them than they do to a £10 mechanical model.
Correct. Only WB CDi boilers can modulate down the flow temperature and they require special optional WB controls to do it, not just any control meeting the open standard. The "TPI" controllers referred to are compatible with the WB Ri boiler range, but they do not modulate the flow temperature. They actuall switch the boiler on and off quite frequently even when the room temperature is below the desired temperature, in an attempt to reduce the flow temperature. This results in the boiler firing at full power for short periods as it tries to do the exact opposite, which is maintain the full flow temperature. Personally I think this type of control is a load of crap and wouldn't have one if you paid me. They are more successful with lower powered system and regular boilers, almost useless with high powered combis.
3. Overshooting of target temprerature is not due to residual heat in the rads, but to the lag in response from the stat.
Garbage. It is due to both. Digital electronic thermostats can switch almost arbitrarily close to the set temperature, but it is necessary to deliberately maintain a certain gap to avoid switching the boiler more frequently than the heating system can sensibly respond. The better controls give you control over this switching gap, and also a calibrating feature where you can adjust the measured temperature in either direction to compensate for any difference between the thermostat location and the perceived temperature elsewhere in the room.
4. Even it were due to residual heat in the rad, the temperture would still go up after the boiler turns off, unless you could design a thermostat that could calculate how many minutes before reaching the target temperature is reached.
Garbage. Modulating controls don't specifically calculate "time to temp", but they model the temperature differential vs flow temperature to achieve a "soft landing" at the desired temperature. Perfect operation of modulating controls requires some adjustment of the model to match the characteristics of the system and heated area.
 

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