Hot water flow rates and combi boiler size
This is another calculation on heating water, this time relating hot water flow
rates and boiler power. It too ends with a handy rule which can be applied
without reference to a manufacturer's manual.
Suppose we want a hot water flow rate of, say, 9 litres per minute, from a
combi boiler or instantaneous water heater - how much power do we need?
We'll assume for the moment a temperature rise of 50 celsius, so cold water
entering the boiler at 10°C leaves at 60°C.
1 litre weighs 1 kilogram, so we're heating 9 kilograms per minute, or 9/60 =
0.15 kilogram per second.
The energy to do this is:
mass of water x temperature rise x specific heat capacity of water
which in this case is 0.15 x 50 x 4.2 = 31.5 kilojoule per second. A kilojoule
per second is the same thing as a kilowatt, so we need a 31.5 kilowatt heat
source. As some heat from burning gas is lost, it would be wise to allow
another 5% when calculating the boiler net heat input needed. This is the
same as multiplying by 1.05, and 31.5 x 1.05 = 33kW.
(Note: if a boiler manufacturer specifies the boiler heat output, maybe as
“DHW output”, that is the heat directly available to heat the water and no
efficiency correction is needed.)
The usefulness of these calculations is in establishing the limit set by nature –
no amount of clever engineering can improve on this.
Let's now look at a few boiler specifications:
Ideal Logic Combi 30: specified flow rate 12.4 litre/min for 35°C rise,
requires 12.4/60 x 35 x 4.2 = 30.4kW. The specification states “DHW output
(kW) Max” = 30.3kW. That's slightly different from our figure due to their use
of a more precise 4.18 for the specific heat capacity.
Worcester Bosch 28i junior (combi): 10 litre/min for 40°C rise (notice how
the temperature rise specification differs between manufacturers – another
reason for being savvy with the calculation). This needs a heat source of
10/60 x 40 x 4.2 = 28kW, which tallies with the manufacturer's rated heat
output for hot water – the number appears in the name of the product.
Vaillant ecoTEC plus 837: DHW flow rate at 35°C rise: 15.2 litre/minute. So
the heat required is 15.2/60 x 35 x 4.2 = 37.24kW. The manufacturer states
37kW and the 37 forms part of the product name.
Handy rule: multiply the hot water flow rate in litres/minute by the
temperature rise in °C. Divide by 100 and multiply by 7 to get the
boiler DHW output in kW.
Example: 12 litre/min at 40°C rise: 12 x 40 = 480.
480/100 = 4.8.
4.8 x 7 = 33.6kW boiler DHW output.