How to Estimate Chiller GPM Using Pressure Drop

A field guide for HVAC technicians

The Problem

When you are logging or troubleshooting a chiller, flow rate is one of the first things that has to be verified. In most cases, if you do not know the flow, you cannot judge chiller performance with confidence.

It is the backbone for understanding load, heat transfer, and whether the machine is performing the way it should.

And this is not just a yes-or-no question. The real question is whether you have enough flow.

So how do you know if the chiller is actually moving enough water?

One of the most practical field methods is to use pressure drop across the heat exchanger. It uses the relationship between flow and pressure drop to estimate flow from the machine's design point.

The image below shows the basic idea: measure pressure at the entering and leaving water connections, calculate the pressure drop, then compare that pressure drop with the design pressure drop to estimate flow.

Diagram showing how to estimate chiller water flow using entering and leaving pressure on a two-pass chiller barrel
Pressure drop is measured across the heat exchanger. On a common two-pass barrel, the entering and leaving water connections are usually on the same end of the machine.

The Formula

GPM2 = GPM1 × √(PD2 ÷ PD1)

Where:

  • GPM1 = the design flow rate from the chiller's selection sheet, in gallons per minute
  • GPM2 = the estimated actual flow rate, in gallons per minute
  • PD1 = the design pressure drop across the heat exchanger, in feet of water
  • PD2 = the pressure drop you measure in the field, converted to feet of water
  • Quick conversion: For water at typical HVAC conditions, 1 PSI is approximately 2.31 feet of water. Multiply PSI by 2.31 to estimate feet of water, or divide feet of water by 2.31 to estimate PSI. Fluid density and temperature can change the exact relationship.

Where the Numbers Come From

Design GPM and design pressure drop come from the chiller's original selection sheet, submittal, or other equipment-specific design data. Those are fixed reference values for the original selection. The actual pressure drop you measure in the field can change as flow, tube condition, fluid properties, and the hydraulic condition change.

If you do not have the original design data, check the equipment records or applicable OEM documentation for the exact heat exchanger and pass arrangement. A rough estimate is not the same thing as a verified design point.

Actual pressure drop is what you measure in the field. You need two trustworthy pressure readings, one on the entering water side and one on the leaving water side of the heat exchanger, whether evaporator or condenser. Subtract leaving pressure from entering pressure to get the pressure drop in PSI, then convert that differential to feet of water.

Example: If your entering water gauge reads 42.3 PSI and your leaving water gauge reads 30.1 PSI:

  • Pressure drop = 42.3 - 30.1 = 12.2 PSI
  • Convert to feet of water: 12.2 × 2.31 = 28.2 ft H2O

Why This Matters

Flow is one of the biggest variables affecting chiller interpretation. If flow is wrong, everything downstream gets harder to trust.

  • Water-side tonnage calculation depends on flow. For water, the common field approximation is Tons = GPM × ΔT / 24. If your GPM estimate is off, your calculated tons are off, and any kW/ton result built from those tons is weaker too. Glycol and other fluids require correction for their actual properties.
  • Approach interpretation can change with flow. Water velocity affects heat transfer, and operating conditions can move the approach number. Do not judge tube condition from approach without considering flow and the rest of the operating condition.
  • Operating stability can change when evaporator or condenser flow moves outside the machine's expected range. Knowing whether flow is reasonable can help explain operating behavior before you start blaming refrigerant, controls, or heat-transfer surfaces.
  • Fault interpretation also needs context. A flow-related condition, a questionable proving device, or unreliable pressure readings can point troubleshooting in the wrong direction if they are treated as verified facts.

Common Mistakes

  • RULE #1. Do not blindly trust the gauges mounted on the piping. If flow matters, use known instruments and verify that the two pressure readings agree when exposed to the same pressure before trusting the differential.
  • Using the wrong design values. Make sure your design pressure drop and field pressure drop are expressed in the same units before using the ratio. Selection sheets may list feet of water, PSI, or both.
  • Using the wrong pressure locations. Take the readings across the heat exchanger path, as close to the chiller connections as practical. The farther the pressure points are from the heat exchanger, the more external piping and fitting losses can get mixed into the measured differential.
  • Ignoring instrument accuracy. Differential pressure is built from two readings, so error in either instrument affects the result. Check whether the gauge accuracy specification is based on full scale or reading, use an appropriate pressure range, and use calibrated or verified instruments when accuracy matters.
  • Comparing to the wrong baseline. The selection pressure drop represents the equipment at its stated design condition. Fouling, scaling, different fluid properties, or other changes in hydraulic resistance can increase the actual pressure drop at a given flow. If resistance has changed, the square-root estimate can overstate or understate actual flow.

When to Use This

Use this method when direct flow measurement is not available and you have trustworthy design data and pressure readings. Once you have those values, it takes two pressure readings and quick math, or you can use the free Chiller GPM Calculator.

If you're calculating water-side tonnage without a defensible flow value, the tonnage and any kW/ton result built from it are not defensible either.

Get the flow estimate as trustworthy as you can before building conclusions on top of it.

Disclaimer

This method has limitations and should be treated as a field estimation tool, not a direct flow measurement.

Confidence depends on the accuracy of the design data, pressure-port locations, instrument quality, fluid properties, tube condition, and whether the hydraulic resistance still resembles the design condition.

In general, the farther the pressure points are from the heat exchanger, the more external piping losses can be included in the differential and the less confidently that differential represents the heat exchanger alone.

This method is especially useful when the same chiller is checked repeatedly with the same process. Under repeatable conditions, it can help show movement over time even when direct flow measurement is not available.