Table of Contents
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 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) |
Boiler Piping Flow Pipe Sizing - Characteristics and Considerations of Piping in Hot Water Boiler Loops
Boiler 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 BTUs 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 BTUs for the system therefore, comfort will be affected. Therefore the system will be inefficient because you are using energy but not properly deliver 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 needs 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.
Boiler Piping Flow, Pipe Efficiency, and Proper Sizing
Boiler Piping Flow, Pipe Efficiency, and Proper SizingBoiler Piping Flow Pipe 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 the 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 a 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 the 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 that 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 the required amount of BTUs 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 BTUs to satisfy demand will be 1-inch pipe steel or copper.
Delta Δ T | Boiler Piping Flow Pipe Sizing
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
a triple duty valve controls flow in commercial piping.
Boiler Piping Flow Pipe Sizing
Resources for Gas Heating
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Is it desired then to create a system that is laminar through the feed pipes thereby creating a parabolic thermal profile and allowing the hottest water to travel down the center? Then, once it reaches a rad, there should be a pipe diameter reduction to cause the velocity to increase and become turbulent to circulate the hot water out to the walls?
In your example, what did you do to the pump on the second floor….increase or decrease flow?
increases it above the recommended pump curve.
increased
I think you need to do a little research on laminar flow.
Please explain it to everyone in laymans terms as it would be applicable in this situation. You are welcome to write an article on it to be published here for our visitors.
FYI
https://en.wikipedia.org/wiki/Laminar_flow
Gary B is right.
Okay, I give up……………I took off my lab coat, stuck my enlarged head under steam to shrink it, and am going to sit on the beach and feed the sea gulls. Instead of contribute to society and share knowledge I’m gonna be a beach bum. It must be a campaign from the Laminar Flow Society because I get these comments way too much. Okay, so I am wrong about how I describe it or put it. I invited Gary B and others to explain it differently in better terms. BTW, I’ve done this experiment in real life more than once and increased comfort while decreasing the cost to produce that comfort (heating). I try to be objective and NOT be snarky in any of my replies, really I do but this single subject has caused many people indigestion and all they can do is send me the Wikipedia link (which is linked in the article BTW) and not explain anything. I’m a licensed HVAC Master and have worked with pipe, engineers and plumbers et al so I know piping has turbulence in it as this is a factor in sizing and the TEL and head (friction) calculations. Velocity is also a factor. So please, someone, say it in different terms or explain it differently. I’m a big man and can admit I am incorrect when I am incorrect. So please, everyone, stop telling me I’m wrong. I know I am wrong in my description and theory but it is a way to help someone come to a better understanding of the fluid dynamics and the flow in piping. It may not fit exactly into academe’s description scientifically but is not science a theory in and of itself? I beg academia’s forgiveness. I bow out and allow the eggheads their due (even though no one has explained it in a better illustrated way).
I deal with real world scenarios all day so when I have a frustrated customer who has called me to solve an issue I look at the problem from different angles. I’ve seen it with my own eyes and experience and not just in a lab. Two identical installations (boiler and piping arrangement) with two different pumps where installation B had a more than double gas bill from installation A. I changed a few things and slowed velocity and everything came into congruence and everyone was happy. BTW, what I changed was outside of the curve chart.
I have edited the article as you may see above because admittedly and clearly I am wrong about the true definition of the term that caused me to come to enlightenment and make many positive contributions to my customers.
I think that the article is backwards…increasing flow beyond laminar stage will increase turbulance and break up the boundry layer effect somewhat, thereby creating a higher pipe wall temperature which will result in a higher heat output of the fin tube. If you look at fin tube data sheets they caution you about using the higher heat output ratings unless you know for certain that you have the higher flow rates.
Yes, I agree……………but this is my entire point…………stratification of the temperature of the water in certain flows. Especially with an irregular piping system like hydronic hot water heating. In some sections you have lots of els and in other sections lots of straight through piping. In the section with the els (where there is lots of turbulence) the piping is hot where as in the straight runs where there is no turbulence the piping is not as hot. It is my theory that laminar flow causes this problem to occur. When you change the flow rate in the straight piping or ad something to cause turbulence the problem goes away.