Gas usage sensible?

[D_Hailsham]

We got the 937 purely for the hot water performance.

Thought so!

The old boiler was a Baxi WM ... the thermostat never cut off the boiler because the house didn't get much above 18C.

The WM came in several versions, ranging from 6-16kW. Yours was probably undersized.

The interesting thing is that, now you have better temperature control with the CM927, you can live with a temperature of 19.5C. This is because the temperature is not fluctuating widely, and the boiler is bigger.

When the boiler fires first thing in the morning regardless of the flow temp it never cycles unless of course the Honeywell CM927 tell it to.

It may not be cycling but is it modulating? My guess is that, apart from the warm up of the house in the morning when the boiler will initially run at 14kW and then modulate back to 12kW, for most of the day the boiler is running at 12kW output and is being controlled purely by the CM927.

Have you checked out what the Sedbuk calculator says you need? You could be in for a surprise.

 

[David937]

Yes I checked and it is about 12kw and I also have the Vaillant technical CD- ROM for 2005 and that also gives 12kw.

Frankly I think I went a bit OTT by raising the flow temperature to 80C in cold weather, today there is snow on the lawn and the flow temp is set to 65C and when the CM927 fires the boiler it goes to about 55C.

If I was to balance the system would I balance it for a 70/50 or something else?

 

[snb]

Unless your house is very well insulated and your rads are definitely big enough to be run at the temps you have chosen then you should not be using optimization on the cm927.

I fit these all the time on my installations but never switch on optimization unless it it's a completely new system in a house with the insulation etc.

You usually find that the boiler is on up to 40% more as the flow temps are not high enough to achieve the required heat. ( losing heat quicker than be put in). optimization results in the heating being switched on 3 hours before the set time just to try and achieve the required temp. it will also override the cycling option as it can never achieve the desired temp.

I would try running it for a couple weeks with it switched off and see what happens. You will notice that the house does go colder at times when it is asked to do so.

Also it worth noting that even in non condensing mode (flow greater than 65 degrees) the efficiency of the boiler will still be around 86 to 88.

 

[David937]

I switched on the optimisation a few weeks ago, by doing that the boiler achieves the correct temperature at lower flow temps.

Is it worth turning it off and having the boiler on full blast in weather like this?

 

[D_Hailsham]

If I was to balance the system would I balance it for a 70/50 or something else?

70/50 would be fine - the output will be very slightly higher than 65/53 - but can you achieve that differential at the boiler? This is where the pump speed comes into play - the higher the speed, the smaller the differential. I asked what d.19 was set to. Have you found out yet?

This is my Balancing Procedure

 

[D_Hailsham]

Unless your house is very well insulated and your rads are definitely big enough to be run at the temps you have chosen then you should not be using optimization on the cm927.

So you would rather guess what time to turn the heating on so the house is warm enough when you get up?

You usually find that the boiler is on up to 40% more as the flow temps are not high enough to achieve the required heat. (losing heat quicker than be put in).

If heat is being lost faster than it is being generated, there is something wrong with the system design. It has nothing to do with Optimization.

optimization results in the heating being switched on 3 hours before the set time just to try and achieve the required temp.

The CM900 series limit the time to two hours not three.

it will also override the cycling option as it can never achieve the desired temp.

It does not override the cycling (PI) feature. If the house does not reach the desired temp, that will be because the heating system has not been designed properly - rads and/or boiler too small.

 

[David937]

70/50 would be fine - the output will be very slightly higher than 65/53 - but can you achieve that differential at the boiler? This is where the pump speed comes into play - the higher the speed, the smaller the differential. I asked what d.19 was set to. Have you found out yet?

The pump is set to Automatic which is speed 2 over half of the part load and speed 1 if half. I am sure that the boiler would not have a problem with a 20C drop as all the specs are for a 20C drop, however there are stated at 60/40 and 80/60.

 

[D_Hailsham]

The pump is set to Automatic which is speed 2 over half of the part load and speed 1 if half.

As you boiler is set to give 14kW max output, the pump will always be on speed 1, which is the slower setting.

I am sure that the boiler would not have a problem with a 20C drop as all the specs are for a 20C drop, however there are stated at 60/40 and 80/60.

They also quote outputs at 50/30 and 40/30. That does not mean those are the only temperatures at which the boiler will run. If you want to run at 70/50, then go ahead.

The problem, as I see it, is that you are running the pump at the slowest setting and getting a differential of 11C; to achieve 20C you would need to run the pump slower.

 

[snb]

Unless your house is very well insulated and your rads are definitely big enough to be run at the temps you have chosen then you should not be using optimization on the cm927.

So you would rather guess what time to turn the heating on so the house is warm enough when you get up?

You usually find that the boiler is on up to 40% more as the flow temps are not high enough to achieve the required heat. (losing heat quicker than be put in).

If heat is being lost faster than it is being generated, there is something wrong with the system design. It has nothing to do with Optimization.

optimization results in the heating being switched on 3 hours before the set time just to try and achieve the required temp.

The CM900 series limit the time to two hours not three.

it will also override the cycling option as it can never achieve the desired temp.

It does not override the cycling (PI) feature. If the house does not reach the desired temp, that will be because the heating system has not been designed properly - rads and/or boiler too small.

Yes you are correct about the time now being 2 hours not 3 as it used to be with the older 67 series.

However to to split up what i had written regarding the effects if the rads are not sized correctly etc does not help the readers.

other than the 2 hours all that i have written is correct and i can assure you that until the stat has been able to run through a time segment and have reached the target temp the cycling feature remains inactive.

 

[David937]

They also quote outputs at 50/30 and 40/30. That does not mean those are the only temperatures at which the boiler will run. If you want to run at 70/50, then go ahead.

The problem, as I see it, is that you are running the pump at the slowest setting and getting a differential of 11C; to achieve 20C you would need to run the pump slower.

But can you explain why the we use so much gas?

This new boiler is about 91.5% efficient, apart from better DHW, warmer house and saving the environment this new boiler doesn't seem to be lowering the gas usage !

 

[D_Hailsham]

However to to split up what i had written regarding the effects if the rads are not sized correctly etc does not help the readers.

But you were suggesting that, if the radiators were not sized correctly, the CM900 series would cause all sorts of problems so the house would never get up to temperature. I would only say that, no matter what type of thermostat was installed, if the radiators are undersized, the house will never get up to temperature; it's not the fault of the thermostat.

I can assure you that until the stat has been able to run through a time segment and have reached the target temp the cycling feature remains inactive.

By 'time segement' I assume you mean the time between one setting temperature setting and the next. It's obvious that, if the house takes two hours to heat up from 15C to 20C, but you only allow an hour and a half before you drop the temperature back to 18C, the set temperature will never be reached.

Cycling does not start when the set temperature is reached, but at 1.5C below the set temperature; though this can be increased up to 3C below.

 

[D_Hailsham]

But can you explain why the we use so much gas?

OK, you win! Back to the original question.

This new boiler is about 91.5% efficient, apart from better DHW, warmer house and saving the environment this new boiler doesn't seem to be lowering the gas usage.

What was the consumption like over the same period with your old boiler?

You will only save gas if you keep the conditions the same. But if you heat the house more or have the hot water at a higher temperature or have more showers or baths, you will either use the same amount of gas or more.

The time and temperature settings might have something to do with it. I know you have two zones, one controlled by the CM927, the other by a Salus. If you could post the time and temperature settings for both programmers, it might help - one day's settings would do.

 

[David937]

Ok, here goes.

Attic:


1800 @ 17.5C, 2230 @ 15.5C then 0600 @ 17.5C and finally 0800 @ 10.0C.

Downstairs:

0600 @ 19.5C, 0900 @ 16.0C then 1800 @ 19.5C and finally 2300 @ 14.5C.

The downstairs heating has never had to be activated in set back mode to date so it effectively remains off. The attic is not activated after 0800.

 

[D_Hailsham]

I have rearranged your figures to make comparison easier:

[code:1]
Attic Downstairs

0600 @ 17.5C - 0600 @ 19.5C
0800 @ 10.0C - 0900 @ 16.0C
1800 @ 17.5C - 1800 @ 19.5C
2230 @ 15.5C - 2300 @ 14.5C
[/code:1]

The only thing I can think of is that, during the day you have the attic set to 10C and the Downstairs set to 16C, presumably because everyone is out. Unless you have a door to the attic which is kept closed during the day, all the heat downstairs will rise upstairs into the attic. The attic will tend to stay above 10C but the downstairs could easily fall below 16C, which will bring the heating on unnecessarily.

Get a Min/Max thermometer and hang it next to the CM927. Set the CM927 0900 and 2300 temperatures to 10C. Then monitor the Minimum temperature for a few days and see how low it actually goes. Also check that the temp gets up to 19.5C at 0600 and 1800. You may find that the minimum temperature does not drop all that much and the house still gets up to temperature OK. You could be heating the house unnecessarily during the day.

 

[David937]

It is rare for me to come home from school to find that the heating is still off and hasn't been tampered with, last month only once out of 31 days did the heating stay on Automatic. My Mum is at home during the day and she usually opens the windows at 1000 to ventilate the house and then switches the heating back on at 1200 @ 19.5C as usual.On very cold days she won't open windows and she will normally put the heating on at 1000.

If she has to go out she will lower the temp to 18C.

It so happen that one cold weekend we were out so the heating was left at 16C, we got back just before 1800 and the temperature had dropped to 17.0C which was comfortable for me but not anyone else.

The attic has no door.



The house usually gets to temperature quite quickly, do you think it would be more efficient setting the flow temperature even lower and waiting a little longer for the house to warm up?

Thanks for making the timings easier to compare