gpm

STEVEN MARKS

How many GPM thru old cast iron column radiators?
What is the optimum GPM to get listed btu ratings?

Mike T., Swampeast MO

Another \"It Depends\"

But here's an example:

75 sq.ft. EDR cast iron radiator.

180° average temperature (the "standard")

Output @ 180° average = 170 btu/hr * 75 sq.ft. = 12,750 btu/hr.

With "standard" temperature drop across the radiator (delta-t) flow requirement is:

gpm = Btu/hr / (500 * delta-t)

12,750 / (500 * 20)

12,750 / 10,000

1.275 gpm

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Is this an existing system being re-worked? If so, be VERY cautious about using 180°. The radiation is generally extremely oversized and a boiler sized properly to the heat loss of the structure will most likely be utterly incapable of heating all of the radiation to 180°!

Chuckles_3

With lower gpm there are fewer gallons passing through, but each gallon spends more time losing heat in the radiator. The output does drop a bit with lower gpm, because as the water cools in the radiator it loses less heat, but it's a rather small effect. Gravity systems did fine with very low gpm. You can use the lowest gpm that will do the job. That way you'll also get the largest delta-t which is good for a condensing boiler. Multispeed pumps are great for finding out how low you can go.