Is my Honeywell CM927 wasting gas?

[staz1000]

I recently fitted a CM927 programmer into my house with a BDR91 relay box. I quickly noticed that the boiler seemed to be firing very often. I looked at the installer guide and, as I'm sure you're all aware, it's designed to fire up a certain number of cycles an hour so this is normal.

The default setting was to cycle 6 times an hour for a minimum of 1 minute, I've changed it to 3 times and hour now but I still have an issue.

Yesterday I stood in front of the programmer and the required temp was set to 18.5 and the room temp was 19 degrees and a demand for heat was still sent to the boiler! The relay clicks in for the minimum 1 minute and the boiler continues to fire until it reaches about 60 degrees and then it slowly shuts down.

I live in a new build and yesterday I tried a test and found it took 2 hours for the temperature to drop 2 degrees, even so my [noisey Potterton] boiler is still kicking in EVERY 20 minutes!

Can anyone explain to me what the idea is supposed to be here? I can appreciate that it keeps the house at the optimum temperature better than more but it seems like an awful waste of gas, especially when the default was set to every 10 minutes originally!

Thanks in advance

 

[staz1000]

42 views and no replies! Anyone?

 

[dumbrill]

42 views and no replies! Anyone?

The amount of heat your CH needs to put out is equal to what is lost through the walls, etc. Too much heat and the house gets hotter and too little it gets colder. This is irrespective of what thermostat you've got.

The CM927 has been made for years and prior to that was called the CM67. It's made by Honeywell who are hugely experienced in such controls and it's very popular. I think by now that any deficiencies in it's design would have been identified. If I were you I would press the 'reset to factory defaults' button and forget about it. The more you fiddle with it the less likely that it will work properly.

 

[ianniann]

Personally I don't like the whole design of the CM927 that "regulates" the boiler by cycling it. This is a cheap and nasty way of attempting to modulate down the water flow temperature as the interior temperature gets close to where the thermostat is set. Most boiler designs will fight this feature and attempt to maintain their set flow temperature by firing at full power whenever they are allowed. If you wish to modulate your flow temperature then find a boiler or controller that does it properly, otherwise just set a suitable flow temperature and let it get on with it.

Lots of people on here use and recommend the CM927. Perhaps some of them will comment further. Perhaps some of the people who hate boiler cycling of any kind will also comment? They have often stated that this sort of cycling multiple times an hour wastes a considerable amount of gas but I've never seen any statement of just how much.

 

[staz1000]

The amount of heat your CH needs to put out is equal to what is lost through the walls, etc. Too much heat and the house gets hotter and too little it gets colder. This is irrespective of what thermostat you've got.

Thanks for that, not sure what made you think this was a necessary inclusion to your reply though.

The CM927 has been made for years and prior to that was called the CM67. It's made by Honeywell who are hugely experienced in such controls and it's very popular. I think by now that any deficiencies in it's design would have been identified. If I were you I would press the 'reset to factory defaults' button and forget about it. The more you fiddle with it the less likely that it will work properly.

Highlighted deficiencies may have been passed up but we may just not be aware of them.

Personally I don't really want my boiler kicking in every 10 minutes (as is the default) when it takes 2 hours for the house to drop 2 degrees. It's a total waste of gas and is not good for the boiler.

Personally I don't like the whole design of the CM927 that "regulates" the boiler by cycling it. This is a cheap and nasty way of attempting to modulate down the water flow temperature as the interior temperature gets close to where the thermostat is set. Most boiler designs will fight this feature and attempt to maintain their set flow temperature by firing at full power whenever they are allowed. If you wish to modulate your flow temperature then find a boiler or controller that does it properly, otherwise just set a suitable flow temperature and let it get on with it.

Lots of people on here use and recommend the CM927. Perhaps some of them will comment further. Perhaps some of the people who hate boiler cycling of any kind will also comment? They have often stated that this sort of cycling multiple times an hour wastes a considerable amount of gas but I've never seen any statement of just how much.

Thanks for your reply. I intend to do a bit experimenting on how much gas it uses so I'll post results once I've done that, in a couple of weeks though.

I think perhaps Honeywell are trying to apply an electronic principle known as Pulse Width Modulation to regulate the temperature, it's normally an extremely efficient way of running things. I'm not convinced it works in this application though.

I may just change the programmer for one that uses a normal demand system.

 

[staz1000]

Okay I found a useful site that explains things a little:

http://www.homexpertbyhoneywell.com/en-DE/Support/FAQs/Pages/default.aspx

This is what it says:

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 the load on the system. TACMA tests indicate that under steady state conditions as much as 10% energy savings can be made with TPI. 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.



How does TPI Control work?

A thermostat operates using a fixed number of firing periods per hour (normally selected to suit the appliance and system). This can often mean a thermostat can fire the boiler even when the set temperature is reached.



A TPI room thermostat will have 100% demand when the system first calls for heat. When the room temperature approaches the set point, it then calculates how many minutes are required within each firing period to satisfy the set temperature. The TPI thermostat will then reduce the firing time in that cycle period in proportion with the demand. This means that as the room temperature approaches set point, the boiler is fired progressively less.

It does make sense and I can understand that thinking, but still I'm not convinced that my boiler switching on when the room temperature has already been exceeded is a very good idea. Perhaps it just fails on a modern house with a very high efficiency rating

 

[dumbrill]

The default setting was to cycle 6 times an hour for a minimum of 1 minute, I've changed it to 3 times and hour now but I still have an issue.

By changing the setting to 3 times an hour you have forced the control to be more coarse. How do you know that this isn't contributing to your problems?

The boiler will still fire above the set point to prevent undershoot. I don't see that as a problem.

 

[dumbrill]

Personally I don't like the whole design of the CM927 that "regulates" the boiler by cycling it. This is a cheap and nasty way of attempting to modulate down the water flow temperature as the interior temperature gets close to where the thermostat is set.

With an old boiler turning it on and off is the best you can get so I'm not sure it's fair to say it's cheap and nasty. I totally agree that it's not the right way to control a modulating boiler.

 

[dumbrill]

The relay clicks in for the minimum 1 minute and the boiler continues to fire until it reaches about 60 degrees and then it slowly shuts down.

Isn't this shutdown due to the boiler thermostat cutting in?

Also I assume that any TRVs on radiators that influence the thermostat are set to max?

 

[staz1000]

By changing the setting to 3 times an hour you have forced the control to be more coarse. How do you know that this isn't contributing to your problems?

The boiler will still fire above the set point to prevent undershoot. I don't see that as a problem.

Coarse?! It is still firing when the room temperature is HIGHER than the required temperature. Coarseness had absolutely no bearing on my problem.

Isn't this shutdown due to the boiler thermostat cutting in?

Also I assume that any TRVs on radiators that influence the thermostat are set to max?

I don't know what you mean by boiler thermostat, I'm not a heating engineer. All I can tell you is the relay clicks on, the boiler fires, the relay clicks off after a minute and the boiler still continues to rise to about 60 degrees before it starts to drop and eventually switches off. The boiler temp is set to max BTW, which is 80 degrees I think, is this what you mean?

The nearest radiator to the programmer doesn't have a TRV, however when I take it to another room the same thing happens anyway.

 

[MJN]

Hi Staz1000,

Try not to be too quick to dismiss the capabilities of the Honeywell CM; as the others have touched on it it has a fairly long pedigree and whilst some aspects of it may not have evolved particularly well (optimum start in particular) the general control functionality holds up fairly well all things considered.

Remember that with only an on/off control of the boiler no control system is going to be perfect. From what you have said yours seems to be doing okay if there's only a 0.5C overshoot/undershoot.

It is still firing when the room temperature is HIGHER than the required temperature.

You may need to consider that there is an inevitable lag between input (demand) and output (heat) and so in order to minimise undershoot the stat will indeed need to call for heat as the temperature drops, even if the temperature at that moment in time is still above the set point. If it were to wait until it dropped to the set point, or below as in the case of a mechanical stat with high hysteresis, then you'd get undershoot such that the boiler would have to work harder anyway to bring it back up.

So, whilst the reduced temperature swings increase comfort I can accept that the repeated boiler firings may be undesirable from parts wearing perspective, not to mention wasted energy from gas purging. If you currently have the maximum cycle rate to be 3/hr, and min on/off time at 1min, then you are currently able to provide an equivalent output rate of 5%. You cannot drop the cycle rate any lower however you could try upping the min on/off time - even 2 or 3 mins would raise your minimum output to 10% or 15% hence you may find if the calculated heat output is less than this then your firing rate will be reduced.

Mathew

 

[staz1000]

Hi Staz1000,

Try not to be too quick to dismiss the capabilities of the Honeywell CM; as the others have touched on it it has a fairly long pedigree and whilst some aspects of it may not have evolved particularly well (optimum start in particular) the general control functionality holds up fairly well all things considered.

Remember that with only an on/off control of the boiler no control system is going to be perfect. From what you have said yours seems to be doing okay if there's only a 0.5C overshoot/undershoot.

It is still firing when the room temperature is HIGHER than the required temperature.

You may need to consider that there is an inevitable lag between input (demand) and output (heat) and so in order to minimise undershoot the stat will indeed need to call for heat as the temperature drops, even if the temperature at that moment in time is still above the set point. If it were to wait until it dropped to the set point, or below as in the case of a mechanical stat with high hysteresis, then you'd get undershoot such that the boiler would have to work harder anyway to bring it back up.

So, whilst the reduced temperature swings increase comfort I can accept that the repeated boiler firings may be undesirable from parts wearing perspective, not to mention wasted energy from gas purging. If you currently have the maximum cycle rate to be 3/hr, and min on/off time at 1min, then you are currently able to provide an equivalent output rate of 5%. You cannot drop the cycle rate any lower however you could try upping the min on/off time - even 2 or 3 mins would raise your minimum output to 10% or 15% hence you may find if the calculated heat output is less than this then your firing rate will be reduced.

Mathew

Thank you very much for that reply Mathew. I fully appreciate where you're coming from and I understand that the system is designed to avoid the input-output lag but you've given me a better understanding so thank you.

It's clear that there will be a limit at which the programmer will skip a cycle correct? For example if I turn the system off it goes into frost mode and the boiler will not fire until it reaches that temperature (which it does as the house is empty for 5 days a week). What controls the limits at which this is determined? I guessed it could be done by the proportional bandwidth setting, and having spoken to a Honeywell rep he said the same but he was unsure how it worked. Can anyone clear it up? If I change it from 1.5 degrees to something higher and I going to force the boiler to skip a cycle or 2?

My boiler is pretty noisy when it fires (it's the lowest spec they fit into the houses on my new estate) and I really cannot live with it going off every 20 minutes, especially when I know 9 times out of 10 it's only going to switch back off again straight away.

 

[D_Hailsham]

Yesterday I stood in front of the programmer and the required temp was set to 18.5 and the room temp was 19 degrees and a demand for heat was still sent to the boiler!

Presumably those are what the CM927 displays as required and actual room temperatures. When you say "a demand for heat was sent to the boiler", do you mean that the green light on the relay box was lit or that the flame signal appeared on the controller?

Have you checked the actual temperature using a thermometer?

 

[staz1000]

Presumably those are what the CM927 displays as required and actual room temperatures. When you say "a demand for heat was sent to the boiler", do you mean that the green light on the relay box was lit or that the flame signal appeared on the controller?

If you read the line that's just after that one it says:

The relay clicks in for the minimum 1 minute and the boiler continues to fire until it reaches about 60 degrees and then it slowly shuts down.

Not sure what's confused you there.

Have you checked the actual temperature using a thermometer?

Yes.

 

[D_Hailsham]

If you read the line that's just after that one it says:

The relay clicks in for the minimum 1 minute and the boiler continues to fire until it reaches about 60 degrees and then it slowly shuts down.

Not sure what's confused you there.

Sloppy reading! I was in a hurry as I had to get to the station to pick someone up (more an excuse than a reason)

Have you checked the actual temperature using a thermometer?

Yes.

Have you use the Temperature Offset setting to correct the CM927 for any discrepancy between actual and displayed room temperature?