Honeywell cm927 thermostat

[davy999]

Hi, I had a Vailliant ecotec plus 418 and Honeywell cm927 installed a few months ago, my old system was 30 years old and did not have a room thermostat. Now we have had the heating on I am not sure if it's working as it should.
Initially when the heating comes on the boiler stays on until the room temp matches the set temp even though the flame icon still shows on the display, after this the boiler does not come on again until the room temp drops which can be 2 hours or more which means the radiators go cold. Is this how modern systems work

 

[Dan Robinson]

Depends on the set temp and the location of the 927.

Ignore the flame thingy. It only confuses people.

Has optimization been enabled?

 

[ALEC1]

if you want a "sensible" heating system where by radiators reflect the amount of heat needed in the rooms by varying boiler temperature you need to use compensation controls, such as vaillant's 392, or the weather compensation 430, both avaiable in wireless versions.

both these vary the radiator temperature according to the amount of heat needed, maintaning the programmed room temperature. This invariable means radiators are warm not hot, enabling the boiler to run at much lower temperatures, enhancing boiler efficiency.

Whilst no trials of this technilogy have been undertaken, studies by independent installers confrim that actual gas savings can acheive 15-20% over simple controllers such as the honeywell, but actual savings will depend on how you set up the controller

 

[mrgas]

Whilst no trials of this technilogy have been undertaken, studies by independent installers confrim that actual gas savings can acheive 15-20% over simple controllers such as the honeywell, but actual savings will depend on how you set up the controller

15-20%? No way, youre having a laugh

 

[mrgas]

Radiators will go cold if your house is warm enough.
So, is your house warm enough?

 

[ALEC1]

ok, its a bit naughty to suggest that such savings are easily attainable..one thing is sure though, that compensation controls can save energy with out compromising comfort......

like for like tests have never been done...so it really is difficult to say, other than sharing peoples experience....

 

[gas112]

Why do certain people on this site struggle to answer the actual quedtions asked.
OP if the room is still within about 1 degree of what you have set the stat at then untill temp drops below this the heating will stay off wether that is 5 mins or 2 hours .
If you look at actual temp on stat and its way below target temp then there is something wrong with unit or not set up correctly

 

[ALEC1]

ok its what the op's system is designed to do, but not what a modern heating system is designed to do, lets be fair...


on-off controls have to both over and undershoot be definition, probably raising the question in the first place....but what do you expect when the specified controls are just variants of on-off switches!!

somtimes answering the question is only dealing with half the issue...

 

[D_Hailsham]

on-off controls have to both over and undershoot be definition, probably raising the question in the first place....but what do you expect when the specified controls are just variants of on-off switches!!

All controls have a switching differential, even weather compensation types; for the CM927 it is ±0.5°C. Compare that to the differential of mechanical stats which can be as much as ±3°C.

Provided the actual room temperature is being maintained at the target temperature, the CM927 is doing its job. The fact that the boiler only lights now and again, once a house is up to temperature, shows how well insulated the house is.

 

[ALEC1]

agreed D _ H, but compensation controls lower the flow temp and the return twmperature and thus flue temperature as the room approaches set point temperature... this is one of the contributing factors to the energy saving of these controls

 

[dormermike]

OP - i recently fitted the same unit to an old boiler (ideal mexico 2) and experienced the same behaviour.

I put a digital thermometer next to the CM927 display, and sure enough the stat was doing its job. Some days the rads barely get warm yet its sufficient to raise the temp of the room enough. What I find though, is that if the room is at 20c for a while I occasionally start to feel cold, even though the room is a constant temperature - odd! But its me I think, as the temp is constant.

One thing I did find was that a certain amount of balancing is needed, as the rest of the house didn't get a chance to get up to temp.

Also, make sure you have no TRV on the rad, or set it to 5, where the CM927 is located.

 

[ALEC1]

well what people over look is that if it is colder than set point there is heat loss to outside, or even the cooler spaces in the building.

compensation controllers overcome this, as they lower the flow temperature as set point is approached...and raise the flow temperature as the room temperature drops...indeed the rad temperature can settle as low as 30c..matching heatloss of the rooms..


on off thermostats only measure the temperature at one point so rooms can cool down. compensation controllers seemingly monitor heatloss by monitoring flow tempertue with the pump running for longer when programmed to be in heating mode...

 

[mikely]

The CM927 isn’t a conventional on-off thermostat and does not exhibit a switching differential as such. It uses Time Proportional control of the heating system to achieve a stable temperature. Around the set point temperature, the controller determines the % of system heat output required to maintain a constant temperature and then switches its output on and off so that the time averaged output of the heating system balances the heat loss from the building, and thus the set temperature is maintained.

In its default settings the CM927 cycles the heating system on six times per hour with a minimum on-time of 1 minute. So, for instance, if it is on for 2 minutes and off for 8 minutes the heating is providing 20% of its maximum output. The minimum output would be 1 minute on and 9 off (10% output). However, if the controller determines it needs a lower output than this, it will increase the cycle time, or drop to a lower number of cycles per hour. So in milder weather the radiators may stay off for lengthy periods.

Quoting from a Honeywell brochure:
'TPI allows the flow temperature from the boiler to fall as demand reduces, which, in turn, helps the return temperature to stay below the Dew Point (55oC) so that the boiler can operate in condensing mode more frequently. This effect is referred to as Load Compensation and currently accrues a 3% boiler efficiency increase under the SAP calculations.'

 

[D_Hailsham]

agreed D _ H, but compensation controls lower the flow temp and the return temperature and thus flue temperature as the room approaches set point temperature... this is one of the contributing factors to the energy saving of these controls

The TPI control algorithm of the CM927 also has the effect of lowering the flow temperature.

I have had a CM927 for over four years controlling an Apollo Fanfare boiler and the flow temperature rarely gets above 70°C, even in the coldest weather. At the moment the boiler is still on the LO setting, which is about 65°C, and that is providing more than enough heat in this unusually mild weather.

 

[ALEC1]

yes thats right, but it only gives an on-off command to the boiler, so the boiler fires to a fixed set point. It does not allow the boiler to take advantage of the condensation effect of a lower return temperature that a demanded lower flow temperature would create.

In fact the controls industry, probably through TACMA, organised some trials at BRE, against non TPI on-off controls and the saving was non existent.

Openther, ebus, or km bus compensationcontrols have yet to be trialled in the UK.. these give a continuous signal to the boiler, varying the flow temperature, enhancing both comfort and fuel efficiency...