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HVAC Testing – When is a System Stabilized?

Feb. 17, 2017

When measuring and diagnosing the operation and performance of an HVAC system, the timing and measurement are critical. Ideally, the system should be calling for full heating or cooling, and fan speed so it can reach a stable operating condition. Let’s take a closer look at what constitutes stable operating conditions so your test data allows you to accurately diagnose the performance and operation of your systems.

Why is Stability So Important?

Say you’re measuring the temperature rise across a furnace. You set the thermostat to 85º F, and insert your temperature probes in the return and supply plenums. The return air temperature measures 70º F and supply air temperature measures 105º F. You subtract return from supply temperatures to find a 35º F temperature rise across the furnace. You record the Delta T on the service record and you’re on your way.

Good job, you measured. But let’s look a little deeper into the conditions surrounding this test to be sure your measurement is accurate and reliable by asking a few questions.

  • Since temperature rise is highly dependent on airflow, did fan airflow match the specified airflow? If the system specifies 1400 cfm to deliver a 35º F temperature rise, what if the actual fan airflow is only 980 cfm and the temperature rise is still only 35º F? This would indicate a far less efficient level of system performance.
  • What if the furnace is multi-stage or has a variable-speed fan that is programmed to ramp up over 12 minutes? If the fan was ramping up during first stage and delivering 700 cfm at the time of the test, this too would affect your ability to accurately interpret the temperature rise.
  • How about if you were delayed in taking your temperature readings and the unit has tripped the high limit? What if it is no longer producing heat with the temperature rise dropping when you took your reading? This condition would also affect your ability to interpret the reading.
  • If you took your measurements too soon and the heat exchanger and duct system hadn’t yet warmed up completely, could this cause an inaccurate reading? Sure it could.

Accurate testing requires the timing and test conditions to be right to accurately interpret the operation and performance of the system. If timing and conditions are off, your diagnosis may not be worth the paper it’s written on.

Is Runtime a Good Indicator?

If you look to installation instructions, a common recommended runtime for cooling or heat pump systems to stabilize is 15 minutes before testing and adjusting refrigerant charge.

Some single-stage gas heating equipment installation instructions call for as little as five minutes before temperature testing can be completed. The burner ignites and the heat exchanger warms up quickly. The fan starts at its programmed speed and because the delta T is so high, the duct system warms up quickly. Five minutes of run time is probably sufficient for this type of system. During spring and fall, the load on a building may be so low, that the system may cycle off before the testing can be completed.

System runtime gets much trickier when multi-stage equipment and variable speed fans are present. Some fans can be programmed to ramp up at 3 or more speeds over a 12-minute time period. The various stages of heating or cooling may take as long or longer to cycle through until the system reaches full capacity.

While testing any furnace, to ensure the fan is at full speed and temperatures have stabilized, we recommend a five-minute waiting period after the fan has turned on and fully ramped up. For the examples given above, the total wait time might be as low as six minutes for a typical single-stage furnace, or as long as 17 minutes for a system with a 12-minute fan ramp up time. Each system you visit may be different, so make an effort to understand the sequence of operation to determine when to test.

While minimum minutes of run time are a good rule of thumb, the expertise, and judgment of the technician are also essential for determining the right moment to test.

Steady State Operation

Steady state is used to describe ideal test conditions. Near steady state operation is only practical to achieve under controlled laboratory conditions. When a heating system is operating, the heat is on and the temperature is constantly rising. The temperature rises quickly at first, then slows down. Our goal is to test after the rate of temperature change slows down.

Consider your testing like a single frame of a motion picture when searching for a reliable test condition. Take and record test data from the frame that best represents the action in the segment of the motion picture you are describing.

This idea can be incorporated in the field by watching the readout on your temperature instruments as the system runs. When you start to see no more than two tenths of a degree variation in temperature change through the equipment over about a 10-second window, it’s time to begin capturing temperature data as quickly as possible.

Consider Test Conditions to Improve Diagnostics

In addition to test timing, specific test conditions of fan speed and equipment function must be present at the moment of testing.

When testing or measuring airflow, assure the fan is receiving a signal from the controls or thermostat calling for full heating or cooling airflow and that the fan is fully ramped up. This is also critical when measuring system static pressures. This guarantees the system is doing all it can to deliver required airflow under current conditions so you can test with confidence.

When testing and measuring temperatures verify the controls and control board are actively calling for full equipment capacity when testing. Verify all stages are calling at the moment you conduct the test. This test condition is also important when taking electrical and Btu measurements.

When measuring temperature in the spring or fall, building load may be low. Under these conditions, temperature will change quickly through the system and within the building. Under these test conditions your timing must be quick and accurate. Be patient. Wait to test until the rate of temperature change across the equipment slows, but not so long that indoor temperatures are outside of typical operating ranges, which may lead to a furnace shutting off on high limit or compressors beginning to unload.

In summary, testing at the right time is critical for accurate performance and diagnostic measurements. For cooling systems, we recommend testing after 15 minutes of full cooling operation with the blower at full speed. For heating systems, we recommend testing five minutes after the blower turns on and comes to full speed. Low load conditions may result in very short windows of time for testing, so be aware and plan ahead if you are testing during shoulder seasons. In situations that won’t allow for the full recommended runtimes prior to testing, watch your measurements to determine if temperatures stabilize quickly enough to permit testing before the unit shuts down or the space gets too warm or too cool to test.

Rob “Doc” Falke serves the industry as president of National Comfort Institute an HVAC-based training company and membership organization. If you're an HVAC contractor or technician interested in a free Temperature Testing Procedure, contact Doc at [email protected]or call him at 800-633-7058. Go to NCI’s website at for free information, articles, and downloads.