Honeywell cm927 thermostat

[davy999]

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.

this is one reason I asked the question as sometimes we felt a little cold even though the room temp was at the set temp.

 

[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.

this is one reason I asked the question as sometimes we felt a little cold even though the room temp was at the set temp.

Not sure of the answer then - i'm getting the same thing - the room feels warm initially at 20c and then starts to feel cooler - even though the room temp is actually constant at 20c (confirmed by a second thermometer). I think perhaps 20c is a little on the low side for a living area.

 

[davy999]

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

yes the house is warm enough, another reason why I asked the question is that the old boiler was in the kitchen (as is the new one), the old boiler use to heat the kitchen but new boilers do not physically heat up like old ones so I asked for a radiator to be fitted in the kitchen but the installer suggested a plinth/kickspace heater working off the system which does look a lot neater because this switches off when the water temp drops the kitchen tends to feel cold.

Thanks for all your answers, can I assume that everything is working as it should.

 

[davy999]

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

Ignore the flame thingy. It only confuses people.

Has optimization been enabled?

optimization has not been enabled.

 

[davy999]

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'

is it possible to alter this cycle setting

 

[ALEC1]

thats the nature of on-off controls I am afraid...


lowering the flow temperature in mild weather is good... why not try 50c... it will run longer but the boiler will always be in condensing mode... and you will find it more comfortable...

when it gets colder turn up the temperature...

 

[D_Hailsham]

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).

I will agree with this.

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.

But not with this. The number of cycles/hour is fixed, although it can be changed in "installer mode".

The minimum on-time can also be increased, which might be a good idea if the relay is controlling a motorized valve which can take up to 30 seconds to change from one position to another. So a one minute on or off time is effectively reduced to half a minute on or off. Danfoss recommend turning TPI off if the stat is connected to a motorized valve.

 

[mikely]

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'

is it possible to alter this cycle setting

Yes, have a look at the Installer mode set-up in the Installation guide: http://www.honeywelluk.com/Documents/Full-Install/pdf/1069.pdf

 

[mikely]

The number of cycles/hour is fixed, although it can be changed in "installer mode".

I have never seen an explanation of actually what it does when the output demand is low, but from monitoring my own CM907 it is clear that, with a low demand, the output will stay off for a considerable time, so it is either skipping a TP cycle or using a variable cycle period. Or maybe it drops out of TP control in this situation?

 

[dormermike]

For each period, if the target temp is higher than the actual temp, the A/B contacts are closed and the boiler fires. Thats with the learning whatsit switched off (factory default setting). I can't see anything more advanced going on with my CM927 tbh... does what it says on the tin, or in the 1-6 settings

 

[D_Hailsham]

I have never seen an explanation of actually what it does when the output demand is low, but from monitoring my own CM907 it is clear that, with a low demand, the output will stay off for a considerable time, so it is either skipping a TP cycle or using a variable cycle period. Or maybe it drops out of TP control in this situation?

You have to take the "Proportional Bandwidth" (PB) into account, which is initially set to 1.5°C.

If the Actual temperature is below [Target temperature - PB], the boiler runs continuously. When the Actual reaches Target-PB, it starts cycling depending on how much the Actual is below Target. If 1.5C below, it runs 100% on, 0% off; if 1C below, it runs 66% on, 33% off; if 0.5C below, it runs 33% on, 66% off; etc. If it is only running 10%, the Actual is 0.15C below Target.

When Actual is the same as Target, the boiler is off permanently.

The "learning whatsit" is nothing to do with TPI; its part of the Optimization feature, which automatically calculates when the boiler should come on first thing in the morning so the room is up to temperature by the set time.

 

[mikely]

When Actual is the same as Target, the boiler is off permanently.

If, when actual is the same as target, the boiler is off permanently, then the set-point will not be maintained; the temperature will start to drop. The TPI controller works to achieve the output required to maintain the target temperature, which is when system heat output is equal to the heat loss from the building. This will be at a certain point on the proportional band ‘curve’, for instance when an output of 20% exactly balances the heat loss. This is similar to a weather compensator that sets the system heat output in response to the difference between target and outside temperature – effectively a direct measurement of the building heat loss.

The integral action (the ‘I’ in TPI) of the controller integrates the temperature error with time and pushes the proportional band up or down around the set point in order to achieve the correct constant output to maintain the target temperature. The question is what happens when the heat output required is very low and drops below say 5 or 10%. I assume that in this case the controller will drop out of the proportional band and the TP output will cease to function.

 

[Dan Robinson]

I suggest the OP turns on his Optimization so that all this banter can actually mean something to his comfort

 

[mikely]

The optimisation function is well worth using but, as D_H has pointed out, it won’t affect the temperature control. It is a separate function that calculates what time to start a heating period to achieve a certain temperature by a certain time.

 

[D_Hailsham]

When Actual is the same as Target, the boiler is off permanently.

If, when actual is the same as target, the boiler is off permanently, then the set-point will not be maintained; the temperature will start to drop.

On reflection, "permanently" was not the correct word.

The TPI controller works to achieve the output required to maintain the target temperature, which is when system heat output is equal to the heat loss from the building. This will be at a certain point on the proportional band ‘curve’, for instance when an output of 20% exactly balances the heat loss.

The integral action (the ‘I’ in TPI) of the controller integrates the temperature error with time and pushes the proportional band up or down around the set point in order to achieve the correct constant output to maintain the target temperature.

That does not agree with Honeywell's description of how it works.

If the boiler is being turned off, even for a short time, there can't be a constant output, so there will inevitably be a change in temperature in the house. The question is: what is the maximum allowable change that the system is designed to provide? The answer is provided in the Honeywell literature:

Temperature Control Accuracy : ± 0.5K at 20°C (50% load, 3K/hr)

The question is what happens when the heat output required is very low and drops below say 5 or 10%. I assume that in this case the controller will drop out of the proportional band and the TP output will cease to function.

I don't see why you should make those assumptions. There is a minimum firing time (normally one minute). So if there is a 10 minute cycle and the ON period is less than 10% the boiler should still fire for one minute.

I suspect that if the ON period is between, say 5%, and 10% the boiler will fire for one minute and below 5% it will not fire at all. This is based on the fact that my system can go for over 10 minutes without the boiler firing at all.