How to Maintain Minimum Pump Flow In HVAC Applications

By Chad Edmondson

Hydronic HVAC systems must maintain at least a minimum flow through centrifugal pumps, otherwise the pumps can be severely damaged. Operating at no flow (known as dead heading) or even extremely low flow should be avoided. The ASHRAE Systems and Equipment Handbook states that operating in an extremely low flow condition can cause severe impeller shaft deflection due to increased radial thrust inside the pump. Dead heading will result in constant recirculation of water in the pump volute, which causes a significant rise in water temperature. If either of these situations are left untreated, it can cause bearings, seals, impellers and even the pump shaft to fail. This article discusses how to maintain minimum flow through centrifugal pumps so that these problems are avoided.

Any hydronic HVAC system that has two-way valves installed on the coils (as most do) is at risk if the proper flow control measures are not in place. ASHRAE actually requires two-way valves on coils to reduce flow during periods of low demand, thereby reducing pumping energy. However, there is a way to have your two-way valves and maintain the minimum flow required by your pumps.

First, you must determine the minimum flow requirement for the pumps. This information should be readily available from the manufacturer. (Note: If you cannot obtain the minimum flow requirement, see the heading, “What if you don’t have the manufacturer’s minimum flow requirement for your pump?“ at the end of this blog.)

As an example, we have selected a Bell and Gossett pump for a design condition of 1200 GPM at 65 ft of head (Table 1). This pump, a model e-1510 5BD, has a 25 HP/1800 RPM motor. Notice that the manufacturer’s minimum flow for this selection is 263 GPM at full speed, which is 1770 RPM. Since most variable flow applications utilize a VFD to slow the pump down and save energy, we must determine what minimum pump speed is required to give us our control head – that is the minimum amount of head that must be present in the system at all times to establish full flow through the critical coil(s).

Table 1

Let’s say we have a control head is 20 ft. We can consult the variable speed control curves to estimate the minimum speed needed to give us our control head. In this example, it would be around 840 RPM (Figure 1).

Figure 1

We can apply the pump affinity law to calculate the new minimum flow at this minimum speed: RPM1/RPM2 = GPM1/GPM2

This tells us that we can operate at low flow for an extended period of time as long as we maintain 125 GPM through the pump. But how do we do that in a variable volume system with two-way valves? We simply swap the 2-way valve at the last coil(s) in the system with a 3-way valve (Figure 2).

Figure 2

With a 3-way valve installed as shown, we are always able to flow water thru the headers to the last coil and maintain our pump minimum flow if all the 2-way valves are closed. Another advantage of using a 3-way valve is the first three coils on the loop will always receive hot or chilled water at design temperature to the coil should a 2-way valve open. Keep in mind that multiple 3-way valves may be required, depending on the piping layout and minimum flow needed.

What if you don’t have the manufacturer’s minimum flow requirement for your pump?

What if you don’t know the minimum required flow through the pump? Perhaps it’s an older installed system and the pump specification and documentation can’t be found. In this case, you may consider a practiced “rule of thumb” that suggests having 1 GPM of flow per motor horsepower. In other words, this rule assumes that if you have a 25 HP pump, you will need at least 25 GPM of flow through the pump.

Now you may be thinking, “Isn’t that a lot less than what I might calculate based on a manufacturer’s minimum pump flow requirement?” and you would be correct. Obviously, if you apply this rule of thumb to the example described in this blog, it will give you a minimum flow of 25 GPM versus the 125 GPM we just calculated based on the manufacturer’s requirement. But before you toss this particular rule of thumb out the window, keep in mind that (1) most HVAC systems do not operate at low flow for extended periods time, and (2) manufacturers are generally quite conservative when assigning minimum flows to their pumps.

The basis for using 1 GPM per horsepower is that it will limit the temperature rise of the water in the pump volute to 5 degrees. The math (and physics) happens to bear this out:

1 HP = 2544 BTU/hour

BTU/hour = 500 X GPM X delta-T

If the delta-T is 5 degrees and we have 2544 BTU/hour of heat: 2544 BTU/hour ≈ (500)(5) X GPM

Therefore, 1 GPM per motor HP will limit the temperature rise to 5°F, assuming the system is unknown or unexpected to operate at low flows for an extended period of time.