Works out a household's daily hot water demand and the cylinder a heat pump needs to meet it.
Most of the form is filled in already with figures that give a sensible answer. Adjust what you know and press the button.
Work out the daily hot water demand of a dwelling and the size of cylinder a heat pump needs, using the formula in MIS 3005-D 3.5.2 and the BS 6700 guidance reproduced in MGD 007.
The demand is 45 litres a day for each of N, where N is the greater of the number of bedrooms plus one and the number of people actually living there. For a new build, where occupancy is unknown, the bedroom count is used.
The cylinder does not have to hold a whole day's demand, because it reheats as it empties. What it must cover is the largest draw-off before the heat pump catches up, which is why a larger heat pump needs a smaller cylinder: reheat time is volume times temperature rise, divided by 14.3 times the kW.
The result gives the BS 6700 minimum, a recommended size rounded to a volume cylinders are made in, and the reheat time. Note that cylinder volume is not the whole story: a heat pump cylinder needs a much larger coil than a boiler cylinder, because the water arrives cooler and there is less temperature difference to drive heat across the surface.
The same calculation answers on
POST /api/calculators/hot-water-cylinder
and as the MCP tool hot_water_cylinder.
How to call them.
A heat pump cannot make hot water on demand, so a house that has lived without a cylinder gets one back. How it is sized follows a formula in MIS 3005-D and a table in MGD 007.