open therm compatible wireless modulating room thermostat?

[johnnynuffin]

Thanks namsag, apparently although in the brochures they are not yet available for another six months.

 

[martinogden]

You need to get a Honeywell Chronotherm stat, this is wireless and will modulate your boiler, see the following link

http://europe.hbc.honeywell.com/products/ecatdata/pg_cm920.html

This area is a mine field the manufacturers don't seem to understand that people want wireless stats that modulate boilers, I bet if you asked 10 plumbers they would say that they fit wireless stats

Let me know if you get no joy with locating one, I did see some on ebay for a blistering price.

Good luck

 

[D_Hailsham]

You need to get a Honeywell Chronotherm stat, this is wireless and will modulate your boiler, see the following link

http://europe.hbc.honeywell.com/products/ecatdata/pg_cm920.html[/QUOTE]
The CM900 series of programmable thermostats are not modulating controllers; they are PI (proportional-integral) controllers which mimic the effect of a true modulating controller.

All they do is adjust the proportion of time that the boiler is on. When the room temperature is low the boiler runs continuously until the temperature is within 1.5°C of the set temperature. It then goes into an on-off cycle based on a 10 minute period, e.g 9 minutes on, 1 minute off. The ratio changes as the temperature approaches the set temperature.

A true modulating controller acts directly on the burner so the boiler output is reduced as the set temperature is reached.

 

[martinogden]

Thats interesting, I will talk to Honeywell and see if they can shed some light, if you are right (I suspect you might be) then this adds even more confusion to the issue, one of the aims is to stop the boilers cycling on and off, this flys in the face of that philosophy.
Cheers

 

[glasgowgas]

the honeywell cm 927 is not a modulating stat

there is a honeywell cm 957 that is wireless and modulating, check their
website for details

EDIT after confirmation from mehran

you have already been told

 

[D_Hailsham]

... one of the aims is to stop the boilers cycling on and off, this flys in the face of that philosophy.
Cheers

As far as I understand it, the aim of the CM900 and the earlier CM67 series was to reduce the over- and under-shoot typical of most thermostats which keep the boiler running at full output until the thermostat reaches the set temperature. The boiler will cut out but, due to the heat in the radiators, the room wil continue warming up, maybe a degree or more. Users will feel too warm, so they turn the thermostat down. But then the temperature drops too low before the boiler comes on again; so they turn the stat up. A vicious cycle of turn up/turn down.

As for modulating boilers the question I can't find an answer to is: what controls the modulation? Presumably a temperature measurement. But which one - Flow, Return, Differential? It can't be room temperature - unless you have Opentherm or a similar proprietary system.

So the user sets the boiler temperature to 75°C and the burner automatically modulates to maintain that themperature. That's fine when the outside temperature is -1°C, because you need the full output of the radiators to maintain an internal temperature of 21°C.

When the weather is warmer, the rads do not need to be giving out as much heat (half as much when the outside temp is 10°C), so the flow temperature does not need to be 75°C. But the boiler does not know this. It carries on maintaining the temperature at 75°C. The house will warm up very quickly as the rads are now producing twice as much heat as necessary. Result: an even bigger overshoot and more fiddling with the room thermostat in an effort to make the room "comfortable".

The only way a modulating boiler will work correctly is if it is controlled directly from an external source. This means Opentherm, or similar, and weather compensation.

 

[David937]

I agree with you D_Hailsham, our Vaillant 937 is set at 16kw and the flow temperature could be set at 75C for example, the boiler will fire at the full 16kw and modulate down as it approaches 75C. You'd have to adjust the kw regularly in d.0 if you wanted the boiler to fire according to how cold it is outside......that's how I see it and many of you know better

 

[D_Hailsham]

I agree with you D_Hailsham, our Vaillant 937 is set at 16kw and the flow temperature could be set at 75C for example, the boiler will fire at the full 16kw and modulate down as it approaches 75C. You'd have to adjust the kw regularly in d.0 if you wanted the boiler to fire according to how cold it is outside......that's how I see it and many of you know better

And you had Vaillant weather compensation, which had to be removed because it would not cope with your single rad upstairs zone! Do you live in a bungalow by any chance?

In the OpenTherm system the controller tells the boiler what flow temperature is required. The beauty of the system is that the controller (master) does not need to know how the boiler adjusts the temperature, and the boiler (slave) does not need to know how the controller determines the flow temperature. This is where the "heating curve" setting on the controller comes into play. There has to be some experimentation when the system is installed to arrive at the correct heating curve for the house. One day this will all be done automatically - the controller will learn what setting to use. A bit like the Optimization feature on the CM900 series, which works out when to turn the heating on.

That's taken us full circle.

 

[David937]

Nah we live in a 3 floor semi detached house.

We had the Vaillant VRT360f which is not weather compensated but can tell the boiler what flow temperature to use.

Now some of the Vaillant controls can be used with multi zone systems because the new VR61 mixer module etc is out.

http://www.vaillant.co.uk/installers/heating-solutions-1/controls-and-diagnostics/controls-2/

 

[mehran]

forgot to post this:

 

[martinogden]

Well I spoke to Honeywell and the open therm stats are not avaiable untill September so I guess we need to watch this space.
My understanding of the "open them" principle is that the device (either a room stat or weather compenstor) simple tells the boiler what the actaul temp is and the boiler works out the rest based on a programmed curve, this curve can be modified by adjusting other parameters within the boiler.
Again I hope i've got this right any more info would be appreciated.

 

[glasgowgas]

Well I spoke to Honeywell and the open therm stats are not avaiable untill September so I guess we need to watch this space.
My understanding of the "open them" principle is that the device (either a room stat or weather compenstor) simple tells the boiler what the actaul temp is and the boiler works out the rest based on a programmed curve, this curve can be modified by adjusting other parameters within the boiler.
Again I hope i've got this right any more info would be appreciated.

open therm is a brand of weather compensation.
in essence, full weather compensation needs an outside sensor, an inside sensor and a return temp sensor.

after setting your htg curve, weather comp dictates what the flow temp from boiler NEEDS to be, it overrides the flow temp you have set boiler at.

so as room gets warmer, flow temp is aoutomatically reduced, keeping boiler in condensing mode longer

to note ALL condensing boilers are meant to work this way,few do.

when the boiler is condensing, the actual fluid gives combustion chamber a constant wash!!

 

[namsag]

Why are yous going over something that was found out 3 weeks ago.

 

[D_Hailsham]

My understanding of the "open them" principle is that the device (either a room stat or weather compenstor) simple tells the boiler what the actaul temp is and the boiler works out the rest based on a programmed curve, this curve can be modified by adjusting other parameters within the boiler.
Again I hope i've got this right any more info would be appreciated.

Here is the OpenTherm Protocol. It's not the latest as that has not escaped onto the internet yet. It's very technical but the important bit is on page 24 item ID 1 - Control Setpoint - and the note at the bottom, which says:

There is only one control value defined - data-id=01, the control setpoint. The control setpoint ranges between a minimum of 0 and maximum of 100. It represents directly a temperature setpoint for the supply from the boiler. The slave does not need to know how the master has calculated the control setpoint, e.g. whether it used room control or OTC, it only needs to control to the value. Likewise, the master does not need to know how the slave is controlling the supply.

 

[Fleeting Glimpse]

Interesting reading.

I have a Ariston Genus 30 HE combi that is 2 years old (very good boiler so far, touches wood!)

I have fitted the 'weather comp' sensor on north facing wall....this works a treat after experimenting with the curves!

I have 'range rated' the output down to about 18.

At the moment I have a Honeywell CM907 which performs well. The only prob I have sometimes is when its mild outside but cold indoors, it takes ages for the house to heat up unless I shut off the weather comp and crank the temp right up.

My question is, if I fit a modulating stat like Aristons own, would that also tell the boiler to output at a greater rate till the interior warms even if the outdoor temp was higher?

I'm interested in that Honeywell modulating stat but so far can't find any info on it.