Expansion tank?

saxman251

I am trying to figure out what size expansion tank I need for my hot water (converted gravity) system. I think I need to know the system volume, and this is where I get hung up because I can't find any resources that tell me how to figure out how much water is in my cast-iron radiators. Does anyone know where I can look up this info?

Thanks

Brad White_24

System volume

I too struggled to find a chart for radiator volume. (I assume you have a handle on pipe volume, charts abound for that kind of information.)

An old-timer, now Dead Man, told me to punt, not waste too much time on figuring volume. (This after I, 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.

Gravity system tank under-sizing is a common problem. Several tanks on a common line with allowances for more if necessary, is short money and short time too, IMHO.