The reason why it’s common to have oversized boiler is that there is a common misconception among the public and many gas safe registered installers that the bigger the boiler the better. As Ecotechnicians we find this alarming, as the consequences of oversizing your boiler is poor efficiency and poor reliability!
Even the builders in Medieval Britain knew that the more draught up a chimney the better the fire in the hearth. To this day manor houses and castles throughout the UK have an oversized chimney, to allow for a better and hotter fire in the fireplace. But back then fuel (wood) was almost free and builders hadn’t quite got round to draught proofing houses to make them comfortable
Eventually, of course, the fireplace was superseded by the boiler, and modern hydronic heating systems were installed.
Boilers with cast iron heat exchangers became a popular choice but these were basically just more controlled fires, with gas burners sitting under a cast iron heat exchanger. The flue was slightly smaller to enable dispersion of the products of combustion, but even so, the challenge on these boilers was to be able to force a draught up a chimney quickly when starting from cold. One way of doing this was to have an oversized boiler so that the flue temperature rose quickly and the gases pulled away safely and quickly.
In the 1980s there was increasing concern for environmental issues, and electric fans were added to the boiler flue to ensure the flue could be down sized, which kept more heat in a controlled fashion where it was needed, inside the boiler to heat the radiators.
Curiously though the practice of oversizing has become a general practice, and this is costing the public dear.
From an Ecotechnicians perspective, what determines boiler size is the heat loss of a building at an out door design temperature, usually -3c. This is the maximum amount of heat that a boiler has to generate to heat a house at indoor design temperature usually 20c. Typically in the UK this is about 16kWs!
The astute readers will see a problem too its not -3c very much in the UK. That means that the 16kWs in the average UK house is too much at temperatures that are higher than -3C which is most of the time. So that correctly sized boiler is now oversized.
A rough guide to ensure your boiler is not oversized, is take the number of radiators in the house and multiply by 0.75-1.5kWs per radiator (depending on size), add this up and add 1kW for pipe losses.
From this you can see that most of the time even a correctly sized boiler is oversized! But clever manufacturers have designed boilers that modulate, so put simply the heat output of a boiler lowers the less the demand for heat.
Indeed any Ecotechnician can tell you that most boilers sit most of the time at their minimum output and rarely use the full output.
Put simply, excessively large boilers generate too much heat that the system cannot dissipate. For example a 30kW boiler needs a flow rate of 1400 litres per hour through it. But an 8 radiator system, in a house with a heat loss of 14kWs, may only need 600 litres per hour.
Then there is the lower output of the boiler, a 30kW boiler may drop down to 8 kWs, but the 14kWs has ability to drop down to 4kWs. This will result in longer burn times and higher efficiencies.
But not just lower efficiencies, temperatures in the boilers themselves get too high and components fail, the slowed velocity of the water on the smaller system, results in leakages and strain that the boiler is not designed for.
To size a boiler correctly go to Institute of Domestic Heating & Environmental Engineers
Many thanks to Alec Morrow for writing this piece. Alec is one of the countries leading specialists regarding modulating controls and also one of the founders of Ecotechnicians, you can get in contact with him by visiting the Find an Ecotechnician web page.
Copyright Ecotechnicians 2016