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Boiler Piping Flow & 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 (Δ) T Calculations.


Flow Rates – Boiler Piping Flow & Pipe Sizing Chart  
Pipe SizeFlow Rate for CopperFlow 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)

Boiler Piping Flow & Pipe Sizing - Characteristics and Considerations of Piping in Hot Water Boiler Loops

Boiler Piping Flow & Pipe SizingBoiler Piping Flow & Pipe Sizing - 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. If the size of the piping is not correct or the flow is too much or too little you will not have an efficient system.

Follow circulator pump manufacturers curve based on flow rate/delta T needed. Aside from the heat loss characteristics in the piping and boiler jacket (including up the flue) the system efficiency works in two ways:

1) Too little flow through the piping will not deliver the correct amount of BTU’s for the system therefore, comfort will be effected. Therefore the system will be inefficient because you are using energy but not properly delivering the results of expending that energy.

To correct this either a change in piping or a change in the circulator pump will need to be done. Perhaps both, however, a closer look at these two components need to be done.

2) Too much flow will result in (not Laminar flow issues - see comments below). Not-Laminar flow is when the velocity of the water is too fast that the water will form thermal layers with the hotter water forming in the center leaving the cooler water to cling to the inner part of the pipe. So all the hot water is flowing through the pipe but there is very little heat being radiated from the pipe (baseboards or radiators) because the heat contained in the water is stuck in layers in the middle of the pipe.

To increase the efficiency of the system from piping heat loss make sure the piping is insulated.

Laminar flow causes system inefficienciesBoiler Piping Flow & Pipe Efficiency and Proper Sizing

Typically these issues occur when the pipe is over-sized/under-sized or the pump selection is incorrect and provides the incorrect velocity for the system. A good example of this is as follows. Two brand new boilers were installed in a basement. One boiler served the first floor and another served the second floor. Both systems were identical including the pump size and piping size. The owner lived on the second floor and the mother-in-law lived on the first floor.

The owner noticed extremely high gas bills as compared to the first floor. A system survey found that the pump size for the second floor was incorrect. A recalculation was made and a new pump was installed. The gas bill for the second-floor unit actually began to be less than the first-floor unit. By correctly sizing the pump and delivering the proper amount of BTUs to the second floor the system ran less and saved money on the utility bill.

Boiler Piping Flow & Pipe Sizing - Flow Rate Calculation

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

Boiler Components: Boiler Circulator Pump
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. That is based on a heat loss calculation for the room. 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 steel or copper.

Delta Δ T

Boiler Piping Flow & Pipe Sizing - It is very important this is correctly done as too much flow rate reduces efficiency as described above and it can have detrimental effects on the boiler. All manufacturers of cast iron boilers want the Delta Δ T of the boiler with a certain range. Some are 20°F while others are 30°F. Exceeding this range of temperature will cause problems with any boiler as you are replacing hot water with colder water with a greater temperature range than any engineering of cast iron boilers can overcome.

It is the same as heating a piece of cast iron or steel to an extreme temperature and then pouring cold water on it. Eventually, it will crack. If the cast iron boiler cracks it is not good and will need to be replaced. There are ways to overcome a high ΔT - 1) Add a boiler system bypass loop between the supply and the return in the near boiler piping 2) Change the piping arrangement to a primary/secondary type of piping with a decoupling loop. 3) Add a variable speed circulator with a ΔT control to maintain a maximum ΔT for the boiler. 4) Add a diverting loop to the boiler piping. Some of these methods are better than others and it will take a professional to tell you which is best for your particular installation. The best method may not always be the cheapest method.

Boiler Piping Flow & Pipe Sizing Valves

Triple duty valve

a triple duty valve controls flow in commercial piping.

High Performance HVAC

Boiler Piping Flow & Pipe Sizing 

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