HPAC Magazine

Getting it right

Sizing a pump is a relatively easy exercise.

February 1, 2016   By Steve Goldie

Troubleshooting makes up a large part of my job. When things do not seem to be working quite like they should be in a hydronic system, I am often the guy who gets the call. No heat calls, not enough hot water calls, too much noise calls; whatever the issue, I will visit the site to determine what the problem is.

Many of the calls start something like this; “that (boiler, pump, hot water tank, and so on) you sold us is a piece of junk.” I admit, even with the most stringent quality control procedures in place, every manufacturer will occasionally ship a dud, but the issues are most often the result of improper installation or application, rather than faulty equipment or components.

One of the most common components in a hydronic system is also one of the most often maligned: the simple circulating pump. The circulator is the heart of a hydronic system and it has the job of moving the heating or cooling fluid throughout the system. If you have no flow, you have no heating or cooling. As we will see however, no heat or no flow does not always mean the pump is no good.

Circulators in closed loop hydronic systems are typically centrifugal pumps. The centrifugal pump transfers mechanical energy from the motor to the fluid through the rotating impeller. The fluid flows from the inlet to the impeller centre and out along its blades. The centrifugal force increases the fluid velocity causing an increase in the fluid pressure from the pump inlet to its outlet.

They do not so much create pressure, rather they create a pressure differential and when this differential is greater than the friction resistance (head loss) in the piping, the fluid moves. How much fluid can be moved and how much head pressure is created is a function of many factors such as the rotational speed of the impeller, the size and diameter of the impeller and the horsepower of the motor. All of this data is calculated and plotted on graphs or pump curves.

I am not going to go into a detailed explanation of these as I am sure most, if not all of you, are somewhat familiar with pump curves. In addition, I do not believe the problems I see with pump sizing are the result of people misreading pump curves. Issues with pumps usually occur when they are sized without the correct information, or when something in the distribution piping, or the piping itself prevents them from doing their job properly.

Sizing a pump is a relatively easy exercise. Pumps are sized with two critical pieces of information: how much fluid do you want to move (flow) in gallons per minute (gpm), and how much pressure do you need to overcome, typically measured in feet of head. Flow and head pressure; that is the information wholesalers need in order to provide you with the correct circulator. This is the data represented on the two axis of a pump curve. As shown in Figure 1, the x axis (vertical) shows total head while the y axis (horizontal) shows flow capacity, typically in gpm.

All too often customers ask for a pump sized by horsepower alone. This is problematic as there can be quite a range of varying duty points with the same horsepower, and identical duty points can be achieved with multiple horsepowers. As an example, if you were in looking for a 1/6 horsepower pump, I could give you one that could supply eight gpm at 28 feet of head, or a different one that could supply 45 gpm at five feet of head.

Which one would be correct, if either? One can deliver relatively low gpm at a high head while the other can deliver lots of gpm at a relatively low head. If you remember nothing else, remember that pumps are sized by flow and head, not by horsepower or pipe size.

If you are working on a new construction project this information should have been calculated and appropriately-sized pumps should have been selected by the mechanical engineer or designer. If the pump selection is up to you then calculate based on how many Btus you want to move. Use the universal hydronic formula, GPM = Btuh/(500 x delta T). So, if you want to move 100,000 Btuh at a 20 degree delta T, that would be 10 GPM (100,000/(500×20)).

For the head pressure, if you have a reasonable estimate of the total length of piping and number of fittings in the piping, calculating the head pressure is also quite simple. If you choose a circulator with this information and it does not seem to be doing the job, you need to look for another reason. Is the pipe size too small? Are the lines fully bled of air?

I have lost count of the number of times I have been called to a job with flow issues and the contractor swears up and down that all the air is out. Sure enough, when I get there I discover the problem is an air lock. Remember, a closed loop circulator does not create a lot of positive pressure, it creates a pressure differential across its inlet and outlet. This differential is easily absorbed by trapped air and it does not require a lot of air to lock up a circuit. A bubble the size of a dime could potentially stop the flow in a ½” circuit.

It is somewhat understandable when you encounter what appears to be a flow issue to assume the circulator may be the culprit. I urge you not to be too hasty in installing a larger pump. I often see components that have boiler connection sizes that are much smaller than the pipe size required to deliver the amount of Btus they are rated for. Plate heat exchangers and indirect hot water tanks are two common examples. The undersized connection will only be a problem if the installer fails to upsize the connected piping to match the Btu load they are hoping to deliver. The pinch point at the connection will only create a slight increase in head pressure and will not significantly impair flow as long as the accumulated piping is sized correctly.

I wish I had a dollar for every time I have seen a scenario where the indirect tank does not deliver the expected recovery, the tank gets blamed, then the boiler gets blamed, and finally, the pump gets blamed. There is a lot of blaming going on until the pipe size is corrected, and low and behold, all of a sudden the pile of junk boiler, tank and pump suddenly perform just as advertised.

Getting it right can be as simple as following the manufacturer’s instructions, following good piping practice, and having all the correct information. Guessing on things such as pump and pipe sizing will ultimately cost you time and money and possibly business. Let’s take the extra time we need to do it properly the first time.

Steve Goldie learned his trade from his father while working as plumber in the family business. After 21 years in the field, he joined the wholesale side of the business in 2002. His expertise is frequently called on to troubleshoot systems and advise contractors. He can be reached at sgoldie@nextsupply.ca.