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Formula For Radiator Size For A Room

Dear Everyone,

Does anyone know the formula to calculate the proper radiator size for a room?

I am currently making changes to my gas steam heating system to resolve an uneven heating situation. But, I am still suspicious that the radiator in my coldest room is too small in size to adequately heat the room. If the room is still too cold after all the changes are made, I am seriously considering adding another fin to the radiator. It is usually 5 degrees colder than the room where the thermostat is located; and, it has been as much as 8 degrees colder.

The room in question has many additional variables that the other rooms in my home do not have in terms of heat loss. And, I think that this may need to be compensated for by adding to the radiator size.

This is the situation: The Master Bedroom is located in the front of the building facing west. This radiator is the furthest from the boiler on the second floor. This room is isolated from the other rooms on the floor by a hallway that measures approximately 3 feet wide by 16 feet long. Therefore, the heat from the other rooms cannot reach this one. All rooms on this floor are off this hallway. The Master Bedroom is located under the roof vent, which vents the roof of the building. The problem is compounded by the fact that there is two (2) outside walls (the building is semi-attached), four (4) windows, and a large closet with two (2) 48 inch-wide bi-fold doorways, which adds another 2 ½ feet to the width of the room to heat. If the door to the room is left closed for any length of time, it becomes even colder. Additionally, a neighbor I’ve talked with has the same exact problem with the room being too cold, especially when the door is closed.

Here are the specs on the bedroom:

Room Size - 12 feet (14 ½ feet with closet included) by 17 feet

Closet Size - 2 ½ feet by 11 feet (with two (2) 48 inch-wide bi-fold door openings)

Radiator Fin Size Installed - 47 ½ inches Wide x 6 inches High x 3 inches Deep

Air Vent Size Installed - Gorton No. D

I would like to know if I need to add to the radiator size? I can add another fin on top as a double-decker. The plumber told me it would be the same width, but not the same depth and/or height. The current fin is running along the center of the cabinet.

Many thanks. Take care.

Best regards,

~ Marguerite ~
From Brooklyn


  • joe_14
    joe_14 Member Posts: 138

    you should have a proffesionl come in and do a heat loss of your room. it sounds like you may need a larger raditor there are many factors to take into consideration when increasing raditor size for example. is the exsisting piping large enough to supply a larger radiator. if you get a copy of dans book THE GOLDEN RULES OF HYDRONICS, it gives you the formular for figuring out the BTU output of a raditor.

    you will find this in the books and more section.

    hope this helps

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