Optimum CM927 settings for fuel economy with WB 28Cdi?

[SteveZodiac]

I'm about to fit a Honeywell CM927 programmable thermostat to my Worcester 28Cdi. We have 10 radiators (includes 2 large doubles and 4 smaller doubles), all fitted with TRVs except in the bathroom. I know the system is out of balance because the rads heat up at different rates. It's a 3 bedroom 30s built house with 9 inch solid walls, double glazing and loft insulation.

I want to run the system as efficiently and economically as possible and would be grateful for advice on the following:

1. On the CM927 are the default settings of 1 minute minimum boiler on time and 6 cycles per hour OK for a 28Cdi?

2. We've always found that the front living room room, which tends to be the coldest in the house, takes a long time to get to 21c in the evenings if the heating is set to come on in the early morning and at about 4.30pm only. What's the most economical way of dealing with this? Should the CM927 be programmed to keep the house at, say, 18c between 8am and 6pm and 16c overnight and at 21c early mornings and evenings REGARDLESS of whether anybody is at home during the day? Or would it be more economical to have the heating come on earlier at, say, 3pm if nobody is in during the day? My gut feeling is that letting the system maintain a minimum temperature of 18c during the day in a house with solid walls would use gas more economically than heating up from 15c in the early evening.

I should confess that we haven't got a room thermostat which is why I'm fitting the CM927, and the boiler thermostat knob has always been set to about 3 quarters but I understand it should be set to max. My wife gets home at 4.45pm by the way and although I'm at home during the day I can cope with 15-16c.

I hope I've explained things clearly or am not asking too much!

 

[bengasman]

The less the boiler is on, the less gas is used; in the end it is as simple as that. The warmer the house is, the more heat is lost to outside. The longer the house is warm, the longer you loose heat.
The most economical way long term is to have the system corrected by a good RGI and let him maintain it from there on. If the installer did not bother to install a roomstat, or to balance the system, he probably did not carry out other important bits.
As for the boiler stat; set it at whatever you find pleasant. The control is there for the user to adjust; if it was meant to be set at a certain value, there would not be a big knob on the outside, but an adjuster on the inside.

 

[David937]

You might want to get your house used to a lower temperature, lower the temperature by 0.5 degrees every week or so. I managed to lower it from 21.5 degrees down to 19.5 degrees.

 

[kevplumb]

the optimum setting for economy is switch it off and put more clothes on

 

[Statia]

The most economical way long term is to have the system corrected by a good RGI and let him maintain it from there on.

The most economical way would be to do-it-yourself. Learn and understand your own heating system and save the expense of paying an RGI to do work you can do yourself!

Paying someone else is rarely economical.

 

[bengasman]

The most economical way long term is to have the system corrected by a good RGI and let him maintain it from there on.

The most economical way would be to do-it-yourself. Learn and understand your own heating system and save the expense of paying an RGI to do work you can do yourself!

Paying someone else is rarely economical.

My tools and test equipment cost more than the boilers you will wear out in your lifetime; to state it would be cheaper to do it yourself proves you don't have a clue what it takes

 

[Statia]

My tools and test equipment cost more than the boilers you will wear out in your lifetime; to state it would be cheaper to do it yourself proves you don't have a clue what it takes

My Dad's bigger than your Dad

You dont need a van full of tools to work on the rest of your CH system that doesn't require an RGI to do it. You can balance the rads, change your pump, valve, switch to a sealed system and so on and so forth.

You don't need to talk your job up. It's just plumbing, not an elite brigade of specially chosen individuals. There were many many years before Corgi came in that people installed their own heating systems from scratch, including their boilers, and they were competent, intelligent and handy peoplen and their installations were probably better than a lot of plumbers would do. Not everyone who picks up a screwdriver besides a tradesman is an idiot.

Give DIYers some credit.

 

[WDIK]

//www.diynot.com/forums/viewtopic.php?p=863266#863266

 

[Statia]

//www.diynot.com/forums/viewtopic.php?p=863266#863266

Y-A-W-N

If you took the time to read my post properly, you'd note that I never mentioned working on the gas side of things. Everyone is quick to jump on people about unqualified people working on gas, and I don't condone it. But that doesn't mean people can't work on the other parts of their heating systems themselves.

I'm sure if someone was stupid enough (and some are), they could do enough damage to themselves with just a hammer and nothing else. Some idiots wouldn't know which end of the hammer to hold. But that doesn't mean everyone who picks up a hammer is an idiot.

This may be a bit too subtle for some to grasp, but I think most can get the jist of what I'm gettting at.

 

[D_Hailsham]

1. On the CM927 are the default settings of 1 minute minimum boiler on time and 6 cycles per hour OK for a 28Cdi?

Don't change it

The CM927 allows you to set six different temperatures over a 24 hour period, so you could set it for 21°C when you get up, reduce it to 16°C when your wife leaves and then back to 21°C for when she returns at 4.30 and the overnight temperature could be set to 10°C. That would still leave two unused settings. You can have different settings for each day of the week and over-riding a setting is easy.

I would put the CM927 in the living room and, if you don't have them already, put thermostatic valves on all rads except in the living room.

The CM927 controls temperatures very accurately so you will find that you can tolerate lower temperatures.

The other thing it has is "optimum start". When this is turned on, you just have to set the time you want the house to be up to temperature, say 7am when you get up, and the thermostat works out when the boiler has to come on, adjusting the time according to the heat loss overnight. means you don't have to keep resetting the clock so the boiler comes on earlier/later.

the boiler thermostat knob has always been set to about 3 quarters but I understand it should be set to max.

That's essentially correct, you leave the room thermostat to control when the boiler goes on and off.

The CM927 has another trick up its sleeve. Traditional thermostats kept the boiler running until the set temperature is reached and then turn off. The radiators carry on giving out heat, so the room gets warmer than required. The temperature then drops until the thermostat cuts in, this can be as much as 5°C lower, which is noticeable.

The CM927 senses when the temperature is getting close to the required setting and starts cycling the boiler so there is a gradual build up over the last couple of degrees. In this way the set temperature is not exceeded. The same thing happens when the temperature starts dropping. The net result is that the room stays at a virtually constant temperature.

 

[SteveZodiac]

the optimum setting for economy is switch it off and put more clothes on

And they say Americans don't understand irony! Assuming you are of course. Unfortunately I've got a wife who should really be living in a tropical climate. Hmmm, now there's a thought for saving gas................

 

[SteveZodiac]

The most economical way long term is to have the system corrected by a good RGI and let him maintain it from there on.

The most economical way would be to do-it-yourself. Learn and understand your own heating system and save the expense of paying an RGI to do work you can do yourself!

Paying someone else is rarely economical.

My tools and test equipment cost more than the boilers you will wear out in your lifetime; to state it would be cheaper to do it yourself proves you don't have a clue what it takes

Now now boys, let's not fall out! I don't know when the regulations changed, but when I installed central heating in my first house in 1981 (never again!) I got a gas fitter to connect the gas to the boiler and commission it even then. I'd never touch gas, but I suppose it depends on how you define "gas work" - that is, is it anything to do with the boiler/system or just gas parts like the gas valve and burner? I'd guess that opinions differ, hence the get out of a "competent person" but only in your own home.

 

[SteveZodiac]

The CM927 allows you to set six different temperatures over a 24 hour period, so you could set it for 21°C when you get up, reduce it to 16°C when your wife leaves and then back to 21°C for when she returns at 4.30 and the overnight temperature could be set to 10°C. That would still leave two unused settings. You can have different settings for each day of the week and over-riding a setting is easy.

That's exactly what I've done! It hit me last night when I was playing with it. However, what I'm trying to work out is what is most energy efficient in a house with uninsulated 9 inch sollid bricks walls. I'm sure I've heard somewhere that because the low specific temperature of solid walls means a room will heat up more slowly than one with insulated cavity walls, in theory some background heat during the day will result in lower average gas usage because you're not constantly warming the walls up from a lower temperature. Or is that a myth?

The other thing it has is "optimum start". When this is turned on, you just have to set the time you want the house to be up to temperature, say 7am when you get up, and the thermostat works out when the boiler has to come on, adjusting the time according to the heat loss overnight. means you don't have to keep resetting the clock so the boiler comes on earlier/later.

I wondered about that, but won't you end up using more gas in the long run? Or is it cleverer than that?

The CM927 senses when the temperature is getting close to the required setting and starts cycling the boiler so there is a gradual build up over the last couple of degrees. In this way the set temperature is not exceeded. The same thing happens when the temperature starts dropping. The net result is that the room stays at a virtually constant temperature.

Interesting, because although I've only had it installed for a day, I noticed last night that the room tempertaure had got to 21.5c with a target of 21c and the boiler was still on for some reason. Also the temperature did seem to drop noticeably (but not drastically) before the boiler kicked back in. Time and experimentation with locating the thermoststat will tell I suppose.

Anyway, thanks for the input.

 

[D_Hailsham]

The CM927 allows you to set six different temperatures over a 24 hour period, ...

That's exactly what I've done! ... However, what I'm trying to work out is what is most energy efficient in a house with uninsulated 9 inch solid bricks walls. I'm sure I've heard somewhere that because the low specific temperature of solid walls means a room will heat up more slowly than one with insulated cavity walls, in theory some background heat during the day will result in lower average gas usage because you're not constantly warming the walls up from a lower temperature.

You've got me there! Also, I think you mean specific heat of the walls.

The other thing it has is "optimum start".

I wondered about that, but won't you end up using more gas in the long run? Or is it cleverer than that?

You shouldn't end up using more gas - the purpose of the feature is to save gas. Say you want a temp of 20°C when you get up. If the temp drops to 18°C overnight you may only need the heating on for half an hour, but if the temp drops to 10°C overnight you may need the heating on for two hours. The CM927 has a max of two hours before the set time.

The CM927 senses when the temperature is getting close to the required setting and starts cycling the boiler so there is a gradual build up over the last couple of degrees. In this way the set temperature is not exceeded. The same thing happens when the temperature starts dropping. The net result is that the room stays at a virtually constant temperature.

Interesting, because although I've only had it installed for a day, I noticed last night that the room temperature had got to 21.5c with a target of 21c and the boiler was still on for some reason. Also the temperature did seem to drop noticeably (but not drastically) before the boiler kicked back in. Time and experimentation with locating the thermoststat will tell I suppose.

There are two other settings which may help here: Temperature Offset (12:t0); and Proportional Bandwidth (13b).

Temperature Offset allows you to make the thermostat display the same temperature as a separate thermometer. Useful if you can only put the unit in a warmer/colder room.

Proportional Bandwidth determines the temperature range over which the controller cycles the boiler to keep the temperature constant. As the temperature has risen above that set, you could still be getting some overshoot. Increasing the Proportion Bandwidth may help to reduce this. It's a case of experimenting.

 

[David937]

1. On the CM927 are the default settings of 1 minute minimum boiler on time and 6 cycles per hour OK for a 28Cdi?

Don't change it

The CM927 allows you to set six different temperatures over a 24 hour period, so you could set it for 21°C when you get up, reduce it to 16°C when your wife leaves and then back to 21°C for when she returns at 4.30 and the overnight temperature could be set to 10°C. That would still leave two unused settings. You can have different settings for each day of the week and over-riding a setting is easy.

I would put the CM927 in the living room and, if you don't have them already, put thermostatic valves on all rads except in the living room.

The CM927 controls temperatures very accurately so you will find that you can tolerate lower temperatures.

The other thing it has is "optimum start". When this is turned on, you just have to set the time you want the house to be up to temperature, say 7am when you get up, and the thermostat works out when the boiler has to come on, adjusting the time according to the heat loss overnight. means you don't have to keep resetting the clock so the boiler comes on earlier/later.

the boiler thermostat knob has always been set to about 3 quarters but I understand it should be set to max.

That's essentially correct, you leave the room thermostat to control when the boiler goes on and off.

The CM927 has another trick up its sleeve. Traditional thermostats kept the boiler running until the set temperature is reached and then turn off. The radiators carry on giving out heat, so the room gets warmer than required. The temperature then drops until the thermostat cuts in, this can be as much as 5°C lower, which is noticeable



The CM927 senses when the temperature is getting close to the required setting and starts cycling the boiler so there is a gradual build up over the last couple of degrees. In this way the set temperature is not exceeded. The same thing happens when the temperature starts dropping. The net result is that the room stays at a virtually constant temperature.

I noticed this at my Uncles house, once the room has reached the set point the radiators continue giving out heat which makes the room quite hot, then the temperature falls and people start to feel cold which causes the thermostat to be turned up. The temperature swings from hot to cold.