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# Rough estimate of water content in a hydronic radiator

Member Posts: 1
How do you estimate the water content in a hydronic radiator ?

I have 15 radiators and have measured their length, width, and height plus the number of sections ("fins").

I need the water content to size an expansion tank and add the correct amount of chemical additives.

Thanks.

Steve

• Member Posts: 41

Go to the Burnham home page, Burnham.com. Click on the heading "Technical". Click on "Tool Box". Use the Burnham Heating Helper for finding warer content in almost any type of radiation. Very useful publication that's been around a long time and is occasionally revised.
• Member Posts: 9
25 to 30 percent

Here is an alternate method I was taught years ago:

An old-timer, now Dead Man, Ray Stevens, told me to punt, not waste too much time on figuring volume. (This after I, as a rookie engineer in training, took way too many hours to take off every piece of pipe in an old museum being renovated.)

Once I was out of the woodshed his advice was to take the cubic volume of an imaginary box around the radiator and use 25-30 percent of it as the internal volume. It would still be likely high.

Thus a 38 inch high column type radiator, 8 inches wide and 30 inches long (5.28 cubic feet at 7.49 gallons per) would have a gross volume of 39.5 gallons. 25 to 30 percent of this would be 10 to 12 gallons. Ray even suggested to throw in the runouts as included in that number, it would be that conservative.

Expansion tanks are inexpensive especially in the #30 size. A #60 and larger seem to cost more than twice as much. So, my habit has been to manifold two, three or more tanks on a common line.

Forget sizing based on the BTU output; those are averages based on small diameter copper pipe as you probably well know.
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