The right size boiler

The right size boiler

WBosch-Greenstar-396x466When the salesman or heating engineer comes to your home, you hope he will advise you on the type of boiler you need. Some advisors will give you genuine ideas based on your requirements but others may try to persuade you to have products based on what they want you to have.

Firstly, in the back of their mind is the price and profit margin they get for a particular product. Secondly, they could be looking at the ease of doing the job and always they will almost certainly over size your boiler.

Now the reason for this is that they will use your old boiler size as a guide to what they should be using for your replacement. So if you had an 18 kilowatt boiler they will replace it with an 18 kilowatt version at the very least. But 9 times out of 10, they will encourage you to step up a size, perhaps to a 24 kilowatt boiler. You will be advised that it is good to have a little bit of extra capacity in case you extend or whatever. The real reason may be slightly different. A bigger boiler will mean a little more profit and by putting in a larger boiler, it will mean them avoiding that dreaded phone call where you may say “my house is not hot enough”. Over sizing your boiler ‘just in case’ is a little like buying a Ferrari to take the kids to school, just in case one day you are late and need to get there quicker!

Traditionally old boilers were sized using a formula that worked out that if the outside temperature was minus 1 degree c, it was still capable of raising the temperature of your home to 21 degrees. Now without being too scientific, with the average winter temperatures in Southern England being 6.33 degrees it meant that the boiler had plenty in reserve and realistically was already too big for your home. When the boiler was on and the rooms got hotter the heating water would return to the boiler hot and shut the boiler off. Simple. But a complete waste of heat.

New high efficiency ‘condensing’ boilers rely on the heat required being closely matched with the heat produced. The idea is when the boiler is on all the heat it produces  is given out to the air in heating your home via your radiators and when the heating water returned to the boiler to be re-heated it is quite cool allowing the boiler to remain in the condensing mode at all times, thus being highly efficient.


If, however, the water returns to the boiler hot, the boiler no longer remains in its highest efficiency mode and it will operate in much the same fashion as your old one. So much for massive fuel savings there! Because of this it is ESSENTIAL that the boiler is sized correctly for your home as the whole operation of replacing it will be a complete waste of time. There will be hardly any savings on your bill even though the boiler will modulate its heat being produced.

So when your sales man attempts to upsize your boiler, stop him there and ask him to justify it. I bet he will struggle to talk his way out.

Avoid being oversold – or you’ll pay through the nose for it…

Dave Murphy
1 Comment
  • Jonathan
    Posted at 13:47h, 18 February

    Major thanks for the article.Thanks Again. Cool.