Sizing Circulators for Old Gravity-Hot-Water Heating Systems

resize Clagett T 308 Overhill 001s

When servicing old hot-water heating systems that originally circulated by gravity, but now use circulating pumps, I began to notice a lot of oversized circulators. In some cases, the circulator was so severely oversized that the boiler could hardly get any heat out to the radiators. The water couldn't pick up much heat in the boiler or shed it in the radiators because it was moving so fast. There is very little resistance in the large pipes of a gravity system, so the consequences of oversizing the circulator are more severe than in a newer system with smaller pipes.

Looking for some way to determine the proper circulator size, I got hold of a copy of the Bell & Gossett Handbook from the 1940s and a Taco catalog which has no date on it, but appears to be a bit newer than the B&G book. Both tell you to use the next size larger circulator on a gravity conversion. Doing the math, I determined that both manufacturers were essentially saying to use a circulator about 50% larger on a gravity conversion than on a newer system designed for forced circulation. This is because of all the extra water contained in the larger pipes of a gravity system.

The resulting chart gives the circulator size in Gallons per Minute (GPM) for a hot-water system according to how many thousands of BTUs (MBH) or radiator square feet (EDR) the system is designed for. It gives the results for the usual forced-circulation system as well as for a gravity conversion. Besides using the chart to select circulators for a new system or boiler replacement, a technician can take this chart into the field to help troubleshoot a system that's not heating properly.

When replacing a boiler, don't hesitate to not use the circulator that came with the boiler if it's not the right size for the job. Packaged boilers are nice, but one size of circulator can never fit every job. Size your circulator properly and the system will run much better.


How to Use the Chart

You will need to have the performance charts, also called pump curves, of the circulators you're going to use. These should be available from your supplier or manufacturer's representative. If you have trouble getting these performance charts, or if you're looking for a chart for an older circulator, contact Dan Holohan.

  1. Determine the capacity of the system in MBH or EDR, and whether or not it's a gravity conversion.
  2. From this chart, determine the circulator capacity required in GPM.
  3. Select a circulator from the manufacturer's performance chart which will supply that amount of water. On a gravity conversion, you need a circulator which will pump the needed amount of water at a 3-1/2-foot head (back-pressure). On a newer system the head may be higher.


  1. When sizing a circulator for an old gravity system where some radiators have been removed, use a slightly larger circulator than the chart calls for. The radiators might be gone, but the pipes are still there, and you have to deal with all the water in these now-oversized pipes. If you can determine how much radiation was removed, add that amount to the remaining radiation and size the circulator from the result.
  2. If a heat-loss calculation indicates using a smaller boiler than the amount of radiation would seem to indicate, size the circulator (but not the boiler) to the radiation. Again, even though you're not generating as much heat, the radiators and pipes are still there with all that water in them.

Choosing the Proper Circulator for a Converted Gravity Hot-Water System

The following information is taken from the various circulator maker’s performance charts showing the flow rates for their products at a head (back-pressure) of 3-1/2 feet. This accounts for the resistance in the boiler, near-boiler piping, flow-check (if used) and air separator. The system piping was designed to facilitate gravity circulation so we don’t need that much help here from the circulator. All we need is to approximate the rate of gravity flow when the original boiler was at its maximum design temperature of 180 degrees F.

After you have measured the existing radiation (and allowed a bit extra if radiators have been removed) take the EDR total to the chart below. Try to pick a circulator that is closest to your radiation without going under or too far over the total radiation. This will assure that the system will have at least 10 degrees delta-T (temperature differential) between flow and return.

I’ve included standard iron-body, flange-mount, lower-head units from the three most popular circulator manufacturers in America, in alphabetical order: Bell & Gossett, Grundfos and Taco. If you prefer to use another brand, just check its performance chart for flow rate at 3-1/2 feet (or 1 meter) of head. If you’re using metrics, multiply gallons per minute by 3.78 to convert to liters per minute.

Circulator Make & Model GPM at 3-1/2 Feet of Head EDR on Gravity Conversion

(Shown as Circulator Model Number, GPM, Maximum EDR)

Bell & Gossett NRF series

NRF-9/LW 7.5 320

NRF-22 17 725

NRF-33 27 1151

Bell & Gossett PL series

PL-30 27 1151

PL-50 47 2004

Bell & Gossett LR series

LR-20BF 16 682

Bell & Gossett three-piece Boosters

100 27 1151

HV 37 1578

2-inch 62 2643

Grundfos UP series

15-10F 5 214

15-42F 13 554

15-58F (high speed) 17 725

26-64F 27 1151

43-75F 41 1748

50-75F 41 1748


Circulator Make & Model GPM at 3-1/2 Feet of Head EDR on Gravity Conversion

Taco “00" series

005 17 725

007 19 810

0010 30 1279

0012 45 1919

Taco 110 series, three-piece

110 27 1151

111 45 1919

120 60 2558

(You'll find Steamhead HERE.)


Leave a comment

Related Posts

Circulators or Zone Valves

Back in my rep days, half a lifetime ago, I sold Bell & Gossett circulators. We didn’t have a viable zone valve available at the time, and when the factory finally showed...

Published on 01/25/2023 1:02 PM by Dan Holohan
Posted in Hot-Water Heating
Adventures in Pumping

I read somewhere that the most prevalent machine in the world is the electric motor. I thought about that for a while and it made sense. Just look around.

Published on 12/07/2021 11:47 AM by Dan Holohan
Posted in Hot-Water Heating