I’ve had several requests via private e-mails in the past several months for recommendations on pipe sizing and/or circulator selection. The request is often difficult to answer because not enough detail design information is given to provide an engineered answer. In this “Toolbox Note” I’ll describe my design procedure.
In a closed loop hydronic system the circulator pump only needs to overcome the hydraulic friction loss created by the piping and plumbing components. The design approach is to calculate this loss and then match a circulator pump that will overcome this loss and flow the fluid at a rate needed to meet the BTU load.
Fluids have been conveyed in pipes for many, many centuries, everything from water to poop. Analysis and design engineering techniques have improved considerably from the dawn of modern civilization. Hydronic heating piping is a very special case of full fluid filled pipe with smooth inside walls. This simplifies the design considerably. While many will here will consider my design procedures complex, in the engineering world it’s quite simple.
Common hydronic piping is copper, PEX and steel. Copper has the smoothest interior followed by PEX and steel. You likely haven’t thought about it but the rougher the inside wall of the pipe the more resistance to flow it has. As you will see later the effect is significant. Schedule 40/80 steel pipe has the most resistance because of the way it is made. It’s rolled from flat stock and then welded lengthwise. The seam makes an interior bump, something not present in copper or PEX.
The design procedures I’ll show are promoted by John Siegenthaler, P.E., in his books and various hydronic heating magazine articles. The formula below calculates the piping head loss in feet. It’s a specialized relationship derived from the widely used Darcy-Weisbach equation. It only applies to hydronic heating type applications.
Even with this simplified formula there is some complexity. The fluid properties factor varies with fluid temperature. To simplify the equations’ use I have plotted the fluid properties factor(a) for water as a function of water temperature. Hotter water flows easier than cold water, so a conservative design procedure would be to use your design boiler return water temperature. Fluids other than pure water, i.e. some anti-freeze mix would require you to calculate a new graph.
The pipe size coefficient is listed in the chart below. Just select the type and size pipe you are using. Pipe fittings and valves have a resistance to flow. For analysis purposes these fittings have resistances expressed in equivalent lengths of pipe. For example the resistance of a 3/4" copper 90 deg elbow is equivalent to 2’ of pipe.
Let’s do an example for a boiler located outside a heated home in a shed or garage. Assume the equivalent total piping (supply + return + fitting equivalent) is 160’ and we want to use 1” PEX-AL-PEX. A 140 deg return water temperature gives a fluid properties factor of 0.0475. Plugging in the numbers gives:
This formula gives us the flow characteristics of our piping. The piping loss (feet of head) will be greater as we attempt to get more flow (f = GPM) through the pipe. The challenge is to find a circulator that will pump our water at the flow rate required to deliver the needed BTU.
A pump curve is provided by the pump manufacturer. It’s a plot of pump head vs. pump flow. The circulator will pump to over come the head resistance it sees. Said another way the head loss will equal the pumping head provided by the circulator. This is known as the intersection of the “pump curve” and the “system curve”. This intersection determines the fluid flow rate in the pipe.
Let’s take an example. The Taco 007 is a very common hydronic system circulator. It’s “pump curve” is given by the equation:
To find the flow rate f you set the two equations equal to each other:
and solve for the flow rate f. This can be solved by many methods, trial and error, pocket calculator, graphically, etc. A graphical solution is shown below. The curve intersection is approximately 9.6 GPM.
Is 9.6 GPM sufficient to flow the BTUs you need? Let’s find out. An approximation to the rate of heat transfer using water as the fluid is give by:
For a 20 degree temperature drop design our example heat flow would be:
Q = 500*9.6*20
Q = 96,000 BTU/hr.
This is likely too small a pump. Let’s try a Taco 0012
The equation for a Taco 0012 is:
Plotting the Taco 0012 on the system piping graph gives:
Reading the curve intersection shows the flow rate as approximately 11 GPM. Again for a 20 degree temperature drop design the heat flow would be:
Q = 500*11*20
Q = 110,000 BTU/hr.
Still not enough flow. Try another pump.
The equation for a Taco 0013 is:
Plotting the Taco 0013 on the system piping graph gives:
Reading the curve intersection shows the flow rate as approximately 17 GPM. Again for a 20 degree temperature drop design the heat flow would be:
Q = 500*17*20
Q = 170,000 BTU/hr.
Yes, it’s a winner. It can meet the heat flow requirement for a 130,000 BTU/hr. boiler.
There are other solutions. Increase the pipe size or change the pipe type. Copper pipe has less resistance, so with a given circulator copper of a similar size will flow more BTUs than PEX-AL-PEX. Schedule 40/80 steel pipe will flow more BTUs than PEX-AL-PEX but not as much as copper. As you have perhaps noted pipe coefficients are not listed for schedule 40/80 steel pipe. That’s because a different analysis technique is used. One that’s more complicated and difficult to calculate. What I do is use the copper based procedure to select a pipe size and if I want to pipe it in schedule 40/80 use the next larger pipe size.
To complete the example the student should repeat the design procedure using 1” or 1-1/4” copper pipe. It will show you can use a smaller pump and still get adequate heat flow. Long term, this would be the more economical solution. You are trading up front costs (more expensive pipe) for lower operating cost (less electric consumption). As you can see there can be an endless combination of choices that will work.
As the complexity of your piping system increases so does the analysis complexity. Parallel pipes split the flow and require more complex analysis. My intent here is to give you some simple analysis tools to select you piping choices wisely. Some straight forward analysis can save you money. In my own case when I first installed my boiler, I simply piped it in the same size as the tappings in my boiler. A much smaller pipe size would have done equally well. The larger pipe size was a needless expense. More design techniques are available in magazine articles authored by John Seigenthaler. Do a web search on his name to find them.