Help to calculate water boiler BTUs required for radiators

Robert_22

How do I determine the amount of BTUs required on a new munchkin water boiler to sufficiently heat my single family home? I know I will need 600 BTUs per linear foot of baseboard with fin tubing, but how do I calculate the amount of BTUs for the radoators? I have 7 radiators on the first floor of my home, and they vary in size, as follows (approximate sizes follow):
-40 inches high X 6 feet wide (1)
-40 inches high X 24 inches high (5)
-20 inches high X 5 feet high (1)

You may post your response on your website and email me at my address provided.

Thanks
Bob

Brad White

Radiators are not Heat Loss

In hot water systems, boilers are sized by the heat loss of the house. (Steam is sized by connected radiation, a different thing entirely.)

Go to the "Resources" tab above and go to "Heat Loss Calcs" and start there.

Here is where you can go horribly wrong taking radiation as an indicator of heat loss:

Say your house was built before insulation was the norm. Since then your windows were replaced with high-grade styles, the walls and attic were insulated and you re-sided with a Tyvek windbreaker underneath. The previous 75,000 BTU heat loss is now down to 40,000.

If you have a 100 MBH boiler you are double the size you need to be and can maybe turn down to 25%. Even on the coldest days though, you will cycle on and off.

If you took your even formerly over-sized radiation (who knows, maybe it totals 85,000 and at 180 degree water?) you would buy a 100 MBH boiler when a 50 or 80 MBH boiler would do the trick. (The 50 might cover the heat loss but be slow in the domestic hot water category. If no indirect, the 50 is a good bet for a 40 MBH load, if that is what you have).

What is GOOD about all this is, suppose you DO have 85 MBH worth of radiation (when at 180 degree water with a 170 degree average water temperature) against a 40 MBH heat loss? You could run 140 degree water (130 average assuming a 20 degree drop) and be condensing on the coldest day. You would also have more modulating range on your boiler for when the load lessens in milder weather.

Your 50 MBH boiler if it modulates down to 25 or 30% (12.5-15 MBH on a 40 MBH load) that is not a bad thing at all.

Do yourself a favor and calculate a heat loss. It's free. Energy is anything but.