Radiator Question

amhplumb_2

Fellow Wallies, We are re-piping a gravity conversion system to smaller tubing with P/S circulatin, small zone circulators, and keeping the radiators. I have 2 questions on radiators for sizing: 1) Is there any "rule of thumb" for calculating the the fluid volume for old radiators? I need this information for sizing an expansion tank. 2) Any "rule of tumb" for head loss through old radiators? This needed for the piping and circulator sizing.
Thanks, Al

Brad White_194

Some thoughts for you, Al

Their are tables available, so many gallons per SF EDR per type of radiator. I do not have that information handy right now.

But there is some advice I was given by a favorite mentor and now hallowed Dead Man, Ray Stevens, 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 run-outs as included in that number, it would be that conservative.

Remember though- Expansion Tanks are Relatively Inexpensive. A pair of 30's might cost less than a single 60 so install two, identically charged, for redundancy and capacity. Also, if the house has been insulated as it should be, the water temperatures needed and hence the expansion volume required, will be much less. The expansion volume will be as much as 25 to 40 percent less than if you were running 180 degree water.

Pressure drop? Negligible, really. You could not measure it. If you use the old piping rather than direct copper or pex runouts, even lower than you think. Imagine the PD through a 3/4" or even 1/2" copper runout pipe at less than 1.0 GPM per radiator. That goes into a positive cathedral of cast iron- so little resistance that I daresay the internal convection will probably assist flow more than your circulator. (My conjecture only but the forces are in that range.)

I did our house (1922 gravity system) with new 1" and 3/4" copper mains in a continuous loop around the basement. By "continuous loop", I mean that the mains go around, both S and R, and connect at the opposite side like a race-track; Depending on which TRVs are closing, flow can go in both or either direction for very low pressure drops in small pipe.

Runouts from the mains of 5/8" Pex-Al-Pex to connect to the older 1" and 1.25" risers and TRVs on all but two radiators.

This is all hooked up to a Vitodens 200 via a low loss header so there is little "system-side" head loss.

My total flow rate is about 4.5 GPM and my circulator (Wilo Stratos Eco) is at the lowest setting, about 4 feet of head. I have more than enough flow. Don't forget, I am not even using the older 2" mains.

Point being, very little pump is needed.

My $0.02

Brad

amhplumb_2

THANKS BRAD!

THANKS BRAD! Most of my work involves the older systems, and all of the programs and literature are geared for the newer systems. The job that I described resulted from taking over a boiler replacement done by a homeowner a few years ago, who did a pretty decent job, with the exception of the expansion tank sizing! Long story short, he called me because the relief valve kept discharging. He only installed a 15, I had to install a 60 to make it work! After that he called me for everything. This year he wanted to get rid of the low piping in the basement! After redoing all of the basement piping, eliminating 2-1/2" down in trunk piping, (and using the info you gave me) a 30 would be plenty with "room to expand!" no pun intended! Thanks again!

Mike T., Swampeast MO

Typical Water Content of Iron Rads

In PINTS and PER SQ.FT. EDR

Column Radiator: 1.5 pints/sq.ft. EDR

Flue Radiator: 1.75 pints/sq.ft. EDR

Fin Tube Radiator: 0.807 pints/sq.ft. EDR

These from a very good old book. I have no data on the typical content of "slenderized" radiators. If I had to guess, I'd say about 0.6 pints/sq.ft. EDR

Jack_47

Can I ask why you're replacing the old steel pipe with smaller pipe? Is there a problem with it? Is this a mod/con boiler - because I have heard that lots of water works well with them.

Thanks
Jeff

Jeff_122

The reason I'm asking is that I am planning to convert a hot water systems with 16 huge radiators and black steel pipes (and two converted coal now oil boilers) to a Knight mod/con gas system. The basement piping ranges from 2" to 4". Is there a reason to replace the supply and return pipes - I thought it wasn't needed?

amhplumb_2

There were several reasons. The piping was way "oversized" because it was converted from a gravity coal burning boiler years ago. The trunk lines began with 2-1/2" and reduced down to 1-1/2" on the mains. After a sizing the largest size that I needed was 1" and that was close to 3/4", and now using small circulators! Why have to heat all that water in the big lines? Quicker distribution of the heating & better control of it. Some energy savings. Also, the homeowner wanted more headroom in the basement!