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Estimating for the domestic hot water supply.

Where the central heating boiler also heats a tank of domestic water supply, the boiler needs to be sized to include keeping the tank full of hot water while not disrupting the hot water for the central heating. This does not apply where a Combi Boiler is installed as no hot water storage is involved.

Imperial calculations

The size of hot water tank will vary between houses, larger houses having larger tanks as it is assumed that the need for hot water is greater. For this exercise, a 26 gallon tank is assumed.

First a design reheat time needs to be established, this normal assumes that all the hot water has been used and the tank is filled with cold mains water. Three hours is probably reasonable in most cases although the 'life style' of those living in the house may cause a shorter or longer time to be chosen.

In the UK, mains cold water is generally assumed to be 40°F whilst 'hot water' for a domestic system is usually around 140°F (this will avoid scalding). So the cold mains water needs to be raised by 100°F within the hot water tank.

As 1 gallon of water needs 10 BTU to raise it through 1°F, a 26 gallon tank of cold mains water needs 26,000 BTU to raise its temperature to the required level. With a 3 hour design reheat time, this means about 9,000 BTU per hour.

Metric calculations

The size of hot water tank will vary between houses, larger houses having larger tanks as it is assumed that the need for hot water is greater. For this exercise, a 120 litre tank is assumed.

First a design reheat time needs to be established, this normal assumes that all the hot water has been used and the tank is filled with cold mains water. Three hours is probably reasonable in most cases although the 'life style' of those living in the house may cause a shorter or longer time to be chosen.

In the UK, mains cold water is generally assumed to be 4°C whilst 'hot water' for a domestic system is usually around 60°C (this will avoid scalding). So the cold mains water needs to be raised by 56°C within the hot watertank.

As 1 litre of water needs about 1.16 watts to raise it through 1°C in an hour, a 120 litre tank of cold mains water needs a total energy input of about 7800 watts-hours (120*1.16*56) to raise its temperature to the required level. With a 3 hour design reheat time, this means a power input of about 2600 watts.


The calculations are simply adjusted to take account of different tank sizes and/or the design reheat heat time but don't mixed up the imperial and metric units (i.e. gallons, degrees F with litres, degrees C). .

The final hourly energy requirement figure is then simply added to the figure calculated separately for the central heating energy requirement, the sum of these two figures, gives the required output for the boiler.


(Our thanks to Dr Stuart McGlashan for pointing out some confused thinking in the previous version of this page and suggesting some appropriate amendments)