Why would the boiler be turned down to 10kW?

7 Mar 2024
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United Kingdom

I’m investigating lukewarm radiators and intermittent hot water in the house I’ve just moved into. We have a British Gas 330+ boiler. I note in the manual it says the boiler is factory preset to 18kW, but upon entering the diagnostics menu I see the “boiler output” is set to 10kW, and the “DHW part load” is set to 18kW.

So I’m wondering why the setting might be 10kW. Might it have been done as a result of finding a problem (e.g. low input gas pressure?)? An attempt at saving money?

I also note in the manual it says: “The boilers are fully modulating for central heating, and it is therefore not necessary to range rate the boiler” - which I don’t understand.

Changing it up to 18kW seems to be a matter of pressing the + button 8 times, but is this the kind of change I should be doing as a civilian, or should I get a gas safe person to do it?

Thank you!
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I have my 18Kw boiler turned down to 12Kw and our pump on the lowest speed. We never have all 10 rads turned on at the same time anyway - in the evening, a max or 6 are turned on at any one time. Runs quieter and I believe it uses less gas. Turn yours up if it’s not hot enough or taking too long to heat up.

If you drive a car, unless you are in a hurry, do you have your foot flat to the floor at all times?
1. If the boiler is too powerful, it will heat the radiators quickly, then shut off, wait until they cooled a bit and then restart. This is called cycling.
2. Modulating boilers don't use this on, off, on, off approach. As the radiators heat up, the return water temperature reaches towards the flow temperature. The boiler detects this and turns down the gas, "modulates", to keep a fixed difference between flow and return temperatures. There is a limit to how low the boiler can run before it turns off.
3. Different boilers have different modulation ranges.
4. For some non-modulating boilers, the output when in central heating mode can be turned down from maximum, called range rating. This can be used to match the boiler's output more closely to the heat requirements of the property, and thus minimise, or at least reduce, cycling.
5. Many modulating boilers can also be range rated. Not strictly necessary if the normal central heating heat requirement can be kept within the boiler's modulation range.
6. There is no reason why you shouldn't change the range rating set in your boiler. There would be no need to open the casing and it is a user function. Whether changing it will make your system better is a different matter, but you can always set it back to its current value.
A gas safe registered engineer isn’t needed to change the Kw output using the user display
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BG may have rated it to 10kw because that's what your house needs and because of the limitations of the gas supply.

I think with the boiler you have, this would be done by physically adjusting a potentiometer on the boiler's circuit board. They would then put a sticker on the PCB saying 'do not adjust'!
If this setting is changed and the boilers output is increased, the gas supply pressure at the boiler would need to be checked and you'll probably need to upgrade your gas pipework, at least some of it, to a bigger size.

(Instead of buying boilers with different maximum heat outputs, say 15, 18, 24, 30kw etc to suit different houses, BG would buy loads of 30kw boilers at a discount and then adjust there outputs as needed)

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