How to measure your HVAC system’s performance


Pre-Test Process

The pre-test of your furnace or air conditioner consists of taking measurements of airspeed and air temperatures at the unit. This helps determine how much hot air a furnace is creating or how much heat an air conditioner is removing in a home. Think of going to the doctor for a checkup.

Temperature Rise

The first thing they do is take your vitals, temperature, weight and blood pressure. The technician will measure the system’s temperature the same way a doctor does. This reading will show how effectively a furnace heats air entering the filter in the air leaving the furnace after it’s been heated. This is called the temperature rise.

When the air conditioner is being tested in cooling mode, your technician will measure how much heat is being removed by the indoor part of the central air conditioner which is called an evaporator coil. An evaporator’s primary function is to absorb heat and humidity from the air in the home and then to transfer that heat through the refrigeration process to the outside. This is, of course, called temperature drop. There are several measurements that are taken from the furnace and air conditioner to determine how much heat is being removed or put into your home.

Static Pressure Test

The next measurement your heating and air technician will take is called a static pressure test. This measures how hard the fan is working to move the air throughout the home. Think of this as a doctor testing your blood pressure. This test will tell your technician how restrictive the ductwork is or if the fan is working harder than it should be to heat or cool the home. It will also show if the filters are too dirty or restrictive in the system. This measurement is taken for heating and cooling modes as the fan works at two different speeds most times depending on what mode the system is in.

Proper static pressures are absolutely critical in the performance of an HVAC system. Imagine running a marathon while breathing through a straw, how effective could you really be? That’s what happens with a heating and air conditioning system that does not have the proper airflow moving through it.

Restricted airflow can be caused by dirty or undersized filters, improperly sized ductwork and dirty equipment. The final calculation determines the amount of heat delivered in the home in comparison with the rated capacity of the furnace relative to the temperatures within the home during testing. For the air conditioner, we would be calculating the amount of heat from your home relative to the outdoor ambient temperatures.

How are the efficiency adjustments calculated?

Furnace Efficiency

A Home Comfort Hero will calculate the measurement of current efficiency by a series of measurements. Each system has a “rated” efficiency of what it is supposed to operate at. For example, if we were to test a 100,000 BTU furnace we also must calculate what the rated output is. If the furnace is designed to be 80% efficient it means we should see 80,000 BTU of heat produced by the furnace. If after our measurements we see that we are only producing 65,000 BTU of heat delivered from the furnace we can calculate 15,000 BTU is being wasted or not going to where it is supposed to.

At this point, it is obvious that the calculations indicate a significant improvement could be made to the system in performance. After completion of the furnace tune-up, we again measure the performance of the system to determine the improved output. For example, the new measured BTU delivery is 75,000 after performing the furnace calibration and adjustments so we have improved the efficiency of 12.5%.

AC Efficiency

Performing the calculations for the air conditioner’s improved efficiency is slightly different. The capacity to remove heat with an air conditioner changes based on the temperature outside. The cooler the air outside, the less heat a central air conditioner can remove from inside the home.

When a manufacturer builds an air conditioner the rated size that they classify, it’s based on a certain outdoor temperature that calculated during the engineering of the air conditioner. If we test at any temperature other than that number, we will get a different rated efficiency. Because of that reason, manufacturers have rated capacity charts for equipment based on outdoor conditions. We simply record the conditions outside and use that to determine the rated capacity at the time of testing.

For example, if a 36,000 BTU cooling unit is rated at 36,000 on an 85-degree day, it’s capacity may be 30,000 on an 80-degree day. This means if we test the system when it is 80 degrees, the most we can get for capacity would be 30,000 BTU. We use the same measurements of temperature change and fan performance to determine the delivered amount of cooling for the air conditioner as we did for the furnace.

Efficiency = Rebates

After we verify the operation, we are able to determine if we will be able to have the adjustments qualify for rebates. For example, if our rated BTU is 30,000, based on current outdoor weather conditions, we only measure 23,000 BTU delivered. We know we have 7,000 BTU that is not being delivered to the home. When we perform the System Rejuvenation on the central air conditioner, we make changes to the system efficiency and it improves the BTU of heat removed from the home to 28,000 BTU and we have now increased our efficiency by 23%.


At Service Legends, we take pride in heating and cooling Des Moines and the surrounding areas. Our HVAC services include furnace and air conditioner installation, repairs, maintenance, and tune-ups. Our lineup of Indoor Air Quality solutions includes whole-home humidifiers, air purifiers, and air filtration systems. We can even help with HVAC financing.

Call 515-657-6634 to contact our Home Comfort Heroes today. We’re standing by 24/7.

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