New condensing boiler sizing

20 Feb 2012
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United Kingdom
Hi all

Need some advice regarding the new condensing boilers, and sizing. I understand about the requirement that the return flow temperature is important to get the boiler to condense and this is why you need to size the rads more carefully these days but using the whole house calculator I am between the Worcester 18 & 24 RI in required watts. It states I need 20Kw but if I increase the roof insulation it falls to 18Kw and maxing the insulation down even lower to 16 Kw and this is with a 3kw water input. In the old days I would have just used the 24Kw and job done but new technology and higher efficiency boilers makes me ask !!!!

Now is it better to run an 18Kw condensing boiler at the high end of its output or run the 24Kw boiler turned down ? I will be increasing the loft insulation and adding extra wall insulation on some of the walls when I board them out internally.

As an overview, does an 18Kw boiler sound right for a detached three bed, one bathroom bungalow in the cold north ?

all the best & thanks Roy
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Improving the insulation should be a priority. It's a once-off cost which will be repaid by lower bills in the future (and with ever-rising prices the savings will get greater).

The whole house calculator assumes a maximum roof insulation of "more than 75mm" which appears to be about 150mm. Current recommendations are 250-300mm.

Oversizing the boiler is pointless, for heating, as it will automatically modulate down to provide the output required by the radiators. So, if you have 10kW of rads and a 20kW boiler, the boiler will modulate down to 10kW.

The modulation range of the boiler also needs to be taken into consideration. This is because the boiler spends 95% of its time running at less than full output. If you need 20kW when -1C outside, you only need 10kW when it is 10C outside. If the boiler can only modulate down to 12kW it will run in on/off mode when the outside temperature is more than 10C.

As for hot water, instead of allowing a nominal 3 kW for this, it is probably better to configure the system for HW priority, i.e the whole of the boiler output will be used to reheat the cylinder. You then do not need to allow the extra 3kW. Modern fast recovery cylinders will use the whole of the 20kW and take only 20 minutes to reheat.
Unless anything has been changed the EST advice is to allow 2 kW for water heating.

I agree that an 18 kW boiler and more loft insulation is the way to go.


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