A hot water baseboard heater uses radiated heat from heated water to warm a room. Essentially, hot water baseboard heaters work the same as older cast iron radiators, except they take up less space. When you size hot water baseboard heaters, you are not determining the physical dimensions of the equipment; you are determining the required heat output. Heat output is measured in BTUs, or British thermal units. Once you know the BTU requirements for your home, you can easily determine the size and number of baseboard heaters you need to heat your home.
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Measure the height of the room, the width of the room and the length of the room. Multiply each number together to obtain the cubic footage of your room.
Multiply the cubic footage of a room by 25 BTUs to determine the rough estimate of BTU requirements for the room. Each room requires 25 BTUs of heat per cubic foot.
Add 25 BTU per cubic foot if your home has moderate to fair insulation. Add 75 BTU per cubic foot if your home has poor or no insulation. An example of no insulation would be stucco on brick exterior walls. If your home has excellent insulation, you do not need to add any additional BTUs.
Divide the total number of BTUs required for the room by 170. Hot water baseboard heaters are rated in EDR, or "equivalent direct radiation." Hot water baseboard heaters must put out 1 EDR per every 170 BTUs required to heat the room.
Select a baseboard heater that produces 1 BDR per every 170 BTUs required to heat the room.
Repeat Steps 1 through 5 for each room in the house.