Circulator Sizing
Although proper pump sizing can be a little time consuming but not hard to do it is imperative that we do it. We must have the proper flow for the amount of resistance the water will encounter. We have to gather some pertinent information first. We must determine the equivalent feet of copper pipe, see chart for ( EFP)or iron pipe ( EFP). Everything the water passes through creates a resistance. We measure all the piping and note the pipe size. Next count all the fittings. We need to know how many elbows there are, the number of tees and if the flow is straight through the tee or, if the flow turns and exits the branch of the tee. How many valves and the type of valves we have. The pressure drop of the heat exchanger and indirect water heater coil if applicable. Include any other system components the water passes through like zone valves, flow control valves, air separators, differential pressure bypass valves, ESBE valves, 3 or 4 way valves etc.
Once we know the resistance of flow we can determine the Feet of Head which will help us size the circulator. Next we must know the gpm flow rate. Once we know the feet of head and gpm flow rate we can look at a manufacturers pump curve and choose the proper circulator.
Step 1  Measure all the pipe from the boiler all the way through the system and back to the boiler. We normally are only interested in the longest loop. If a circulator will move enough water through the longest loop in theory it will move water through the shorter loops.
Step 2  Make note of all the fittings the water passes through. Calculate all the resistance to get EFP.
Step 3  Determine flow rate required. This will normally require multiple calculations. If we pipe an indirect with primary/secondary (p/s) off the primary loop and have three (3) heating zones we need to size the boiler circulator, the indirect circulator and the longest heating zone. We need to do three (3) calculations.
Let's look at an example;
View the piping diagram below for the piping to an indirect hot water tank piped off of a cast iron boiler. To keep it simple the boiler flow rate required is 6 gpm and the flow rate through the indirect coil is 6 gpm. The pressure drop through the indirect coil is 9 ft head and the pressure drop through the boiler heat exchanger is 1 ft head. So far we have a flow of 6 gpm with a resistance of 10 ft head. Now we need to figure our EFP in all of the fittings and other devices.
Let's assume the boiler is piped in 11/4" copper pipe and the indirect is piped in 1" copper pipe.
The water leaves the boiler and goes through some elbows, into a tee and exits the branch. Then the water passes through some 1" copper fittings, valves, tank coil, purge valves and into the branch of a tee. It now flows through 11/4" pipe back to the boiler, through the boiler and back out again. Once you know the EFP multiply that times 0.04 to convert to resistance or Ft of head. Here is a list of fittings the water will pass through on a call for domestic hot water tank. Let's count the fittings:
2  11/4" Unions
1  11/4" Tee (straight through)
2  11/4" Tee's out branch
8  1" elbows
1  1" swing check
and we will assume the piping equals
11/4" pipe 15'
1" pipe equals 25'
1" pipe 25'
2  11/4" Ball Valves 7 x 2 = 14 EFP
2  11/4" Unions .5 x 2 = 1 EFP
1  11/4" Tee (straight through) .6 x 1 = .6 EFP
2  11/4" Tee's out branch 5.5 x 2 = 11 EFP
8  1" elbows 2.5 x 8 = 20 EFP
1  1" swing check 4.5 x 1 = 4.5 EFP
and we will assume the piping length is as follows
11/4" pipe equals 15' x .042 = .63
1" pipe equals 25' x .042 = 1.05
Total 52.7 EFP (x 0.04)
plus 9 ft of head for DHW tank and 1 ft of head for boiler 10 ft Head
Required flow for indirect 12.21 ft head @ 6 GPM
If you want a more accurate formula, See below
Let's figure this out to include a boiler circulator and an indirect circulator.
The piping would change due to two different flow rate requirements. If the flow rates are close you can pipe like diagram 23A and if the flow rates are much different use diagram 23C.
Indirect tank flow would have to be close to boiler flow requirement to be piped off the boiler
Flow in Indirect tank is less than boiler flow therefore the indirect is piped off primary pipe so is not affected by boiler required flow
Above is a calculation which gets you close to circulator sizing. Below will add more accuracy in sizing circulators which is using this formula.
Where H_{L} is head loss
k = Tubing size  the size tubing being used
c = Correction factor for fluid type  what type of fluid you are using
l = Equivalent length of piping including all the valves, fittings are separator, zone valves etc. converted to Equivalent length of pipe.
f(1.75) = Flow rate through pipe raised to the power of 1.75




Flow  Pipe Size  Value of k  
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