Worcester 24i combi boiler settings & gas use?


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Jan 28, 2010
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This may sound silly, but we have recently bought a house with a combi boiler already installed and having never had one, I'm not sure what setting to have it on...though I'm not really that impressed so far...my last house had a separate boiler, separate hot water tank with immersion and gravity fed showers with a large cold tank in the roof...great hot water pressure...hot water 'instantly' and all day! But I digress!
Anyway, nobody I know actually has the answer to my main question, so can anyone help?

On the front of the boiler is a dial that reads 'Min' to 'Max' and inbetween it goes through settings I/II/III/IIII...I assume this increases the power/output of the boiler? The higher it is set the hotter the boilers 'btu' output? Am I correct?

I have set the boiler running on 'Min' and the rads are hot and it gently warms the house, if I turn it up to 'III' the rads are much hotter and the house actually gets too warm upstairs! OK, now assuming I am right about the output...I then assumed if it throws out less heat on a lower setting it will use less gas! So, I set it to 'Min' (which is less than a 'I' setting) and ran it for 3 hours and logged the gas meter reading, it used 1.557m3 of gas....I then ran it on the higher 'III' setting for (again) 3 hours and it only used 1.485m3 of gas???? The house was far warmer in my second trial on the higher setting, we have no thermostatic rad valves and the same radiators were on in both trials...how and why is the gas consumption lower on the higher setting????
Thanks in advance to anyone who replies and can explain this.
Thanks in advance to anyone who replies and can explain this.

We have a 30ci - it has a two digit LED display showing the return temperature.

On ours what you call the 'power' knob :)grin:), controls the return temperature.

Assuming your boiler is set for heating on:

The boiler works by pumping water around your house and through the boiler (effectively in a great big loop of pipe), whilst in the boiler the water gets heated by the gas flame. - so the water comes out hotter than it went in. (just getting this clear - I realise its obvious)

So the hot water comes out the boiler, is pumped through your plumbing, which leads to your radiators, which take some of the heat out of the water and dump it into the room. After all that the water ends up back at the boiler a lot colder than when it left it.

If the water when it comes back is colder than the boiler return temperature you have set, the boiler will stay lit. Once the return temperature is above the set level, the flame will cut out, but the pump will continue to run occasionally.
The boiler will only restart when the return temperature drops again.

So when you did you experiment I would guess the figures were like this.

In Stage 1 you had a lot boiler return temp set say 50degC(low on the dial)
1) The water left the boiler at say 80 degC.
2) Because the house was cold, more heat was absorbed by the radiators and the water came back at say 45degC on average over the 3 hours - this kept the boiler running the whole time.

In Stage 2 the house was warmer before you started and you set a higher return temperature say 60deg C
1) The water left the boiler at say 80 degC.
2) Because the house was warm, less heat was absorbed by the radiators and the water came back at say 65degC a lot of the time, this means the boiler was on for less time and used less gas.

Hope that helps.

PS The gas in my name has only a very tenuous link with the gas your boiler runs on.
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You say you are not impressed with a combi boiler, well wait until you get the bill. Modern condensing combi's are far more efficient than older systems and although you have no store of hot water which can be a pain, they WILL cost less to run.

Part of the difference compared with traditional boilers is they are fully modulating. In simple terms they can increase and decrease their output infinitely according to various parameters. You are not as such controlling the output by altering the control, just restricting the boiler's maximum output.

Of course, proper control won't happen unless thermostatic valves and a good room thermostat is fitted,

Remember, heating a living area to 20 degrees say requires a given amount of energy. What the boiler is set to is irrelevant. It still uses the same quantity of energy to hold the temperature.

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