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HVAC Hot Water Boiler Piping Flow Characteristics and Pipe Sizing

Copper and Steel Pipe Flow Rates and Characteristics of Piping in Hot Water Boiler Loops

Hot water boiler pipe sizing is important and based on many factors including BTU and Delta T Calcualtions.

Flow Rates
Pipe Size	Flow Rate for Copper	Flow Rate for Steel
½”	1 ½ GPM (Gallons per Minute)	2 GPM (Gallons per Minute)
¾”	4 GPM (Gallons per Minute)	4 GPM (Gallons per Minute)
1”	8 GPM (Gallons per Minute)	8 GPM (Gallons per Minute)
1 ¼”	14 GPM (Gallons per Minute)	16 GPM (Gallons per Minute)
1 ½”	22 GPM (Gallons per Minute)	25 GPM (Gallons per Minute)
2”	45 GPM (Gallons per Minute)	50 GPM (Gallons per Minute)
2 ½”	85 GPM (Gallons per Minute)	80 GPM (Gallons per Minute)
3”	130 GPM (Gallons per Minute)	140 GPM (Gallons per Minute)
4”		300 GPM (Gallons per Minute)

Characteristics and Considerations of Piping in Hot Water Boiler Loops

It is important that the correct pump is selected in hot water boiler loops so that the correct velocity is achieved throughout the system. Too much velocity and the system will not be efficient because of laminar flow issues. Not enough velocity and the correct amount of BTU’s will not be delivered throughout the loops. Additionally, always consider expansion and contraction of piping in hot water boiler loops when installing piping. 

To achieve calculated flow rate when sizing the pump use the following formula: Take the measurement of the longest run in the loop in feet and add 50% to that measurement. Multiply that number by .04 to get the pump head. The pump head refers to the capability of the pump to move the water through the loop and all the resistance in the loop or friction of the loop. Another method for determining pump head is to measure the longest pipe run in the loop and then adding the friction loss for each valve, elbow, and fitting. This requires the use of a chart which can be obtained from your HVAC circulator pump supplier.

Hot Water Pipe and Baseboard BTU Calculations

A BTU is measured by the amount of energy required to raise 1 Lb of water to 1° Fahrenheit. The weight of water is 8.33 lbs. Each individual room should be calculated for required amount of BTU’s to satisfy demand and then this number added together. For example: A house with a 100,000 BTU heat demand and a 30°ΔT will need a flow rate of approximately 7 G.P.M. (slightly less based on the following formula: (8.33 * 60 * 30°ΔT) = X. 100,000/X will give you 6.7 G.P.M. This means the minimum size needed to deliver the appropriate amount of BTU’s to satisfy demand will be 1 inch pipe.  

Hot Water Boiler Pipe Sizing is Important

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