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Central Heating

Correct boiler size (kw)

Hi, currently having differing advice from my builder/plumber and a 3rd party heating engineer regarding the correct size and brand for my replacement system boiler!
My house is a 1960's 4 bed semi, cavity walls, uPVC double glazing, 2 bathrooms, 10 rads and a Megaflow tank. The current boiler is a Halstead Boss 60 (18Kw??). My builder is completing a rear extension (insulated cavity walls) with 3 new rads c.3Kw total.

When I complete the boiler size calculator it comes out at 12.08Kw but my builder recommends I overspec and get a 28Kw Main Eco Elite, whilst a heating engineer recommends either Worcester Bosch or Valiant with a minimum 30Kw.

Should I get another opinion or is the minimum requirement for my house a 28Kw boiler?

Any advice would be gratefully received.

3 Answers from MyBuilder Heating Engineers

Best Answer

It's always best to match a boiler to the output of the system, after all it is simple physics, radiators cannot give out more then their designed output (i.e a 1 kw rad can not output 2 kw's of heat).

A 600x1000 single convector rad outputs approx 1kw so a 30kw boiler could heat up 30 of these radiators (to help give you some perspective).

Therefore there is no benefit to over sizing and in fact it can cause problems with the performance of the system as larger output boilers use up a lot of the residual pump head inside the boiler which can lead to flow problems to the radiator circuit as there is not enough capacity to circulate water around the complete heating circuit, as well as causing short cycling (the boiler cannot dissipate the heat it generates fast enough and so cycles on and off) which increases gas consumption and warm up times, this also increase wear and tear of the heat exchanger and can lead to premature failure of components over the relative lifetime of the appliance.

Also don't forget that the heat loss calc is the maximum figure based on a outside temp of between -1 to -3 deg celcius as the outside temprature increases the heatloss of your property decreases in proportion, for example when it is 10deg C outside your heat loss may only be around 8kw's for the property. There are only a handful of days in any given year on average where the temp falls below 5 deg's.

klind regards ,Dean

Robinson Heating


Answered 1st Jan 2016

without a doubt minimum 30kw. trust your heating engineer not your builder in regards to your heating system. these do it yourself calculators leave out a lot of factors.
also i recomend fitting either a vaillant or a veismann boiler as both of these manufacturers offer up to 10 year parts and labour warranty if fitted by an approved engineer.

many thanks

nic jackson


Answered 30th Dec 2015

Hi, you say you are adding 3x radiators which will make your whole system 13 radiators in total plus a MegaFlow cylinder. I recommend you speak to the manufacturers and ask their technical advisors of the boiler that they recommend to suit your purposes.
If you ask Baxi (manufacturers of the Main Eco Elite range), Worcester-Bosch and Vaillant you will be significantly more knowledgeable and able to make a more informed decision.

For your information, Worcester-Bosch and Vaillant are more expensive boilers than the Main Eco Elite range.

I hope this is helpful to you.


Answered 29th Dec 2015

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