Heater and air conditioner size for a room, in BTU and watts BTU
A heater is sized from where the heat goes, element by element and with the air. A room air conditioner is sized from the ENERGY STAR chart. Both are shown for what they are.
The room
Where the heat goes
List what separates the room from somewhere colder. A wall to a heated room loses nothing and can be left out; one to an unheated garage takes the garage’s temperature. The U-values come from the building’s drawings or an energy assessment: a solid brick wall is around 2, a new insulated one under 0.3, double glazing 1.2 to 2.8.
What the chart adjusts for
The ENERGY STAR chart for room air conditioners, with its own adjustments: 10% less for a heavily shaded room, 10% more for a very sunny one, 600 BTU an hour for each person past two, and 4,000 more in a kitchen. It assumes an eight-foot ceiling.
What size
The working
Heating is the steady-state heat loss of BS EN 12831 and CIBSE Guide A in its plainest form: U × A × ΔT for each element, and 0.33 × air changes × volume × ΔT for the air, with no thermal bridges and no margin for heating that goes off at night. Cooling is the ENERGY STAR sizing chart for a room unit, not a Manual J calculation. One watt is 3.412142 BTU an hour.
How it works
BTU works out how big a heater or a room air conditioner a room needs. Heating comes from the room's heat loss: U-value times area times temperature difference for each wall, window and floor, plus the air. Cooling comes from the ENERGY STAR sizing chart with its own adjustments. It gives watts and BTU an hour, and the measurements stay in this tab.
Doing this often? Foundry, the Engineering Suite's drawing half draws the rooms to scale and measures each area off the drawing, then prints the plan to scale. On its own, US$2 a month; with everything else, US$5 a month or US$48 a year. 7 days free.