Replacement boiler size calculation

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Your new boiler must supply enough heat to warm your home comfortably when the outside temperature falls below freezing. In addition, it must allow you to run your dishwasher and washing machine while other household members are showering or bathing. Do not use your old boiler as a guide. An under-sized boiler won’t work and an over-sized unit will cycle on and off constantly. This reduces the boiler’s service life, while at the same time, it blows excess heat out of the flue and boosts your utility bills. Avoid multiple heat loss calculations by using an online boiler sizing calculator for the best results.


Use a boiler sizing calculator designed for the Crown Copyright “Whole House Boiler Sizing Method,” such as the one in the reference site listed at the foot of this article. Before starting, if your home is T-shaped or L-shaped, break the floor plan down to two or more rectangular sections. If this applies, run two or more separate calculations and enter the hot water allowance into the main section only. Change the allowance to zero in the remaining section or sections.


Add the output requirements for all radiators installed in a single storey extension and plug the figure into the box provided. Note: This only applies if it coincides with your particular floor plan. Once this is done, follow the steps and enter all the data called for, such as dwelling type, internal measurements, wall types, number of external walls and roof type. Plug in the window specifications; if you have a mixture of single and double-glazed windows, use the single glazed option for all your windows. In addition, if you have exceptionally large windows, calculate their total area and enter that figure into the box provided.

Hot Water

Enter the allowance for domestic hot water. The basic allowance for the average UK dwelling is 2,000 Watts. However, since some modern cylinders have higher wattage requirements and provide better heat up times, ask your boiler vendor for the wattage requirements of the boiler type you wish to install. Once you have that figure, enter the modified wattage if applicable. In addition, if the new boiler’s control system gives priority to hot water requirements, reset the number of Watts in the box provided to zero.


Finally, enter your home’s location. If you have broken your floor plan down to two or more sections and run separate calculations, add the results up to arrive at the correct boiler size for your home.

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