Honeywell CM927

10 Dec 2010
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United Kingdom
Just had a Honeywell CM9272 days ago but some functions do not make sense to me!

I have a Target temperature 17 degrees Room temperature 18.5 degrees
but as long as the flame icon is on the boiler starts every 8-10 minutes for only one minute making no alteration to room temperature.

Even so why come on when there room temperature is HIGHER than the TARGET temperature.

For what reason is this coming on every 8-10 minutes making no adjustments to room temperatures and no call for heating?

Is there a fault in the thermostat?

My last thermostat the flame icon & heating would only come on in an overide action or room temperature run lower than target temperature
never came on every 10 minutes.

can anyone help me on this ?
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The CM-series seem to be generating a lot of confusion at the moment! You might be interested in this recent thread as it discusses what's going on, however it does disappear down a few rabbit holes in doing so.

Putting it simply, your stat is of the type that doesn't wait until the temperature has dropped below (or risen above) the target set point before doing something about it. Rather, it attempts to calculate the heat response of the system and controls the firing of the boiler such that a steady temperature is maintained.

This will mean that the boiler is called to fire even if the actual temperature is slightly above the set point because if the temperature is dropping then due to the thermal lag in any response heat will be required before it drops too low. The stat will thus be observed to call for heat periodically even when at/above the target temperature.

In your case your figure are rather intriguing however. One of the main advantages of this control method is that under/overshoot is minimised - typically to within 0.5C - and so how your actual temperature got up to 1.5C above target is, to me, something of a mystery. It could either be an odd configuration setup or, perhaps more likely, a consequence of the stat not having learned the heat response of the system yet (did you say it was only fitted recently?).

Does that make sense? All in all I don't think it is faulty but rather acting as it is meant to, albeit not as well as it can/should.

All that said, if stax1000 is reading this (from the other thread) I am now wondering if, as he thinks might be the case, the CM-series always makes a call for heat every cycle even if that call is for the minimum period possible (but still more than is necessary)...?

One thing to be aware with the CM927, and this is a real bugbear of a design flaw in my view, is that the flame icon on the stat does not necessarilly mean the boiler will be firing (as per its wired brothers and most other stats, not to mention intuition). Rather, it means that a call for heat is being sent to the relay box caveated with with a proportional value. If this value is, say, only 5% then the relay box knows to fire up the boiler for only 1 minute in every 20 (or whatever the configuration parameters specify the duty cycle to be). Thus, flame on doesn't necessarilly mean boiler on at that point in time.

matthew Many Thanks for the reply & information, Yes i have had it installed 3 days ago. Find it strange the flame icon is on for so long and times when it is not!

Target Temperature 17.5
Room Temperature 19.00

Heating will come on every 8-10 minutes as long as the flame icon is displayed.

Following day

Target temperature 17.5
Room temperature 19.00

No flame icon appears, boiler does not come on and off as previous day.
Does not make sense at all, understandable if it is withing the range of 0.5.
It is odd that the actual temperature is overshooting by so much. Do you have any supplementary heating such a gas/electric fire, cooker or sun through the windows?

The CM927 is a very capable stat and overshoots of over 0.5C, unless due to other heat sources beyond its control, ought not to occur.

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At first i thought it may be too near the music centre but moved it 3 meteres away but made no difference, this still occured 10pm at night.
Electric cooker is 6 metres away and was off, gas fire 6 metres away and switched off too, 10pm at night there is no sunlight shining through LOL
I'm really not sure then. As mentioned, firings even when at/above target temperature are to be expected, but not when 1.5C over - and it shouldn't be getting so high anyway.

Perhaps you need to get the installer back in, although be careful they don't just fob you off with 'they all do that Sir' because they don't - not to this extent anyway - there's many a poster here who aren't happy with how this stat handle steady-state control but they're not seeing this sort of overshoot (i.e. it's more like 'does my stat really need to call for heat so often when it's reached the target?).

That is what i state, why call for heat when it is not needed?
But been in touch with Honeywell technical support and just had a few replies

The CM927 operates a CYCLIC boiler on/off mode when near (within 1.5C) setpoint ; this is to inject just the right amount of extra heat into the radiator system such that the radiators emit heat to EQUAL that being lost. If the boiler just went OFF the room would COOL. – see attached

The CM927 is a time/temp controller NOT ann ON/OFF type.

SO if you need the boiler OFF at a particular time; you need to set the TEMPERATURE to 5C at the off time - see also attached.

(if the temp is left at same as on time or any other temp eg 18C it will maintain the room to that new temp.)


What is TPI Control?
TPI (Time Proportional & Integral) control is an industry standard term for an On/Off control using an advanced energy saving control method that can be used by most heating systems in the UK. The controls simply switch current supplied to the boiler on or off at different times. Unlike a traditional thermostat, these rely on digital technology that matches the boiler firing to theload on the system.

How does TPI Control work?

A thermostat operates using a fixed number of firing periods perhour (normally selected to suit the appliance and system). It then calculates how many minutes are required within each firing period to satisfy the demand of the rooms. This allows the flow temperature from the boiler to fall as demand reduces, which helps the return temperature to stay below the Dew Point (55oC) so that the boiler can operate in condensing mode more frequently. This is called Load Compensation and currently accrues a 3% boiler efficiency increase under the SAP (Standard Assessment Procedure) calculations.

A TPI room thermostat will have 100% demand when the system first calls for heat. When the room temperature approaches the set point, it enters the proportional bandwhere the demand varies. The TPI thermostat will then reduce the firing time in that cycle period in proportion with the demand (i.e. if the proportional bandis 2oC and the room temperature 1oC from set point, it will fire the boiler for 50% of the cycle period).

This means that as the room temperature approaches set point, the boiler is fired progressively less. Further smoothing of this demand is achieved with the Integral part of the calculation, which, when combined, prevent the thermostat ‘hunting’at all. The control will be at a steady even temperature with little variation, as can be seen in Figure 1.

Can TPI controls be used on all boilers?

Yes. What is required of heating controls in the Building Regulations?Building Regulations require that every system has a room thermostat, whether old or new. This will form an essential part of the Boiler Interlock, providing the means to switch the boiler off when it is no longer required. This performance can be enhanced using a TPI thermostat. How can a system be improved from Building Regulation requirements?

By fitting a TPI, the boiler efficiency is improved, TACMA testsindicate that under steady state conditions this can be as much as 10%. This would be reduced by seasonal and operational losses. But in addition, because the control is so much more accurate, the set point can be reduced, without loss of comfort, introducing extra savings.

The thermostat can then also have an intelligent or self learning facility, so that when it first operates, it can recognise how the system responds to its demand signals and can adjust them to adapt to the fabric of the building and the system as it is installed to give better control.
This FAQ sheet is for guidance only and at the time of production represented the latest information available to Honeywell fromvarious sources.

Honeywell reserves the right at any time and without notice to change any product, specification or any other information contained in this publication and cannot accept any responsibility for loss or damage arising out of any errors that may inadvertently be contained herein.
Technical support: Ring 08457 678999 (local rate charge from within the UK) or e-mail [email protected]
Sales enquiries: Ring 01344 656591 or e-mail [email protected]

Sales support literature: Visit the Downloads section of, ring 0800 521121 (UK only) or e-mail [email protected]
EN1H-2472 UK01 R0209
Figure 1 Temperature variation with a TPI room thermostat
TimeSet pointProportional band
That is what i state, why call for heat when it is not needed?
That question, or at least the general principle, has hopefully already been answered.

However, sadly Honeywell's stock response (it doesn't surprise me they have one - this must be their #1 topic, that or optimum start) doesn't address why you've got so much overshoot (1.5C) in the first place.

This may sound like a bit of a cop out but see how it goes for a few days - it may need some settling in if it's assessment of the heat response is either out or not yet defined. That's not to imply it is particularly complex, but it does need to work out how quickly your house heats, and perhaps cools, in order to calibrate its demand output.

I shall see how it goes over the next week, i may even get another thermostat as it drives me nuts coming on every 10 minutes.

i would rather nothing come on until the temperature drop 0.5C below target temperature which makes sense.
coming on and off so regularly you may tend to think, the wear and tear the boiler has to it, may be cheaper in the long run to get another thermostat. but wish to avoid a thermostat that keeps coming on every 10 minutes.


many thanks for your posts!
do you have a combi boiler installed?

are you sure it's the heating coming on every 10mins or is it the hot water preheat?
I have a combi boiler!
it is the heating coming on every 10 minutes, the water heats up every hour lighting up all the red Neon lights.

I have found a away to figure out what is causing the problem!

1: Set the target temperature 19C

Once temperature is reached the heating will go off and flame icon appears!

2: Straight away reduce target temperature 16.5

Flame icon disappears and heating does not come on every 10 minutes so it has nothing to do with the Boiler.

It is the TPI control of the thermostat which at times can continue functioning withing 0.5C - 2.0C

I do not like the TPI control and have to search for an addittional wireless room thermostat without TPI control, suprising how many thermostats have them now! finding a good one without may be hard.

In long term with boiler on and off every 10 minutes suprising what addittional wear & tear it achieve!
i would rather nothing come on until the temperature drop 0.5C below target temperature which makes sense.
To you perhaps, which is fine, but not to those who's desire is comfort and stability of temperature. ;)

I do not like the TPI control and have to search for an addittional wireless room thermostat without TPI control, suprising how many thermostats have them now! finding a good one without may be hard.
That fact ought to be telling you something!

I have a Drayton Digistat +3 which acts in the same way, ie target temp 17.5, room temp at 19 falling so boiler fires. Current hot weather has the house up to about 20C, then in the evening when the temp is falling we get the boiler coming on, even though the whole house is already warm. I really like the TPI feature in winter, but is there any way to disable it in summer as its a waste and stops the house cooling to a more comfortable level? I want to leave the thermostat settings alone for the occasional cold day when heating still needed (Glasgow!)

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