Proper pump sizing is important for a few reasons. We need to know the resistance the pump will encounter and size the pump large enough to over come the resistance and move the proper amount of water. The amount of flow will determine how efficiently we will heat. If the flow is right we will get the maximum heat output from the system and help eliminate air from the system. Most systems except the newer radiant heat systems will normally heat with 3/4" pipe. This pipe can carry about 40,000 btu/h at a flow of 4 gpm at 27" per second. If the flow is too slow, we will reduce the heat output. If the flow is too great it will also reduce the heat output and create a noisy operation. See a chart for the proper flow rates and amount of heat carried.
Although proper pump sizing can be a little time consuming but not hard to do and 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 pipe (see chart for 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 1-1/4" 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 1-1/4" 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:
plus 9 ft of head for DHW tank and 1 ft of head for boiler 10 ft Head
12.21 ft head @ 6 GPM Required flow for indirect
This would be a 0010 Circulator
The piping would change from this ..................................................................................................to this
Indirect Tank Flow would have to be close to boiler flow requirement Flow in Indirect tank is less than boiler flow to be piped off the boiler loop. therefore the indirect is piped off primary pipe so is not affected by boiler flow
This is a calculation which gets you close to circulator sizing. There is a more accurate way of sizing circulators which is using this formula.
HL=k x c x L x (f1.75)
Where HL 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 Rate Factor (f)
|Raised to 1.75|
|Flow||Pipe Size||Value of k|
|"c" Correction Factor for Fluid Temperature|
|Average Fluid Temp||100º f||140ºf||180ºf|
|30% Propylene Glycol||1.353||1.187||1.088|
|50% Propylene Glycol||1.582||1.349||1.225|
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