HVAC Performance Testing
HVAC performance testing is the process of properly measuring the BTU capacity output of a system to determine if it’s functioning at full capacity. Most people think that is what happens during a tune up but that just isn’t the case most of the time.
Many consider a twenty degree difference from the supply to the return (air in and out of the air conditioner) to be the determining factor for performance but that idea is missing a critical factor of measured airflow in determining the actual performance.
HVAC Performance Testing Calculation
The formula for calculating HVAC performance testing – sensible capacity of a unit is cfm x temperature difference x 1.08 = BTU
As an example, let’s say you have a 3-ton air conditioner that is rated for 25, 920 sensible BTU based on the current indoor & outdoor conditions and you have a twenty degree drop in the system . The only way to know if that is good or bad is to measure the airflow. For testing equipment airflow, we use a magnehelic gage to determine the static pressure. Then, we use the blower chart for your equipment that tells us what the airflow actually is. Based on our testing, about 90% of the equipment we’ve tested has restricted airflow.
A 3-ton air conditioning unit should have 1, 200 cfm (Cubic Feet per Minute) of airflow. The analysis formula would be 1, 200 x 20 x 1.08 = 25, 920 BTU which means you would be getting 100% out of that unit assuming a 20 degree drop in the system at time of testing. If that same unit was blowing only 900 cfm then the formula would be 900 x 20 x 1.08 = 19, 440 BTU. Based on the calculations, this unit would only delivering 75% of its capacity!
The examples listed above show how to determine if the equipment is performing correctly. That is important to know but that alone doesn’t tell the whole story. What really matters is how much of the cool air you are paying for actually gets where it supposed to go. The way to determine that is by using a flow hood at each register and grille to measure the air and temperatures. In the same examples above, we might find only 750 cfm is actually getting deliver to the space. There may be some fittings that aren’t properly sealed which could result in an 18 degree temperature difference. The ducts are in the attic where it’s hot and some of the insulation may not be up to par. The formula then becomes 750 x 18 x 1.08 = 14, 580 BTU and 44% of the cooling you are paying for never gets inside your home! You might think this case is extreme but we see it on a regular basis. This explanation was almost exactly the performance of my own upstairs system when I bought my home.
The proper course of action is to renovate the ducting system by properly designing the ducting. This usually involves adding or enlarging the ductwork to allow for proper airflow to each room at the manufacturers designed static pressure which also serves to balance the temperature throughout the space. Sealing and insulating the ductwork to the international energy code standards and then testing with a duct pressurization system to verify proper sealing. Then you have a properly running system that is delivering balanced temperature to each room, maximum capacity and the best efficiency it can deliver.
Unfortunately, we often meet customers that were told the old system was worn out and doing the best it can do. Then, they are advised to replace the system with a larger more efficient unit leaving the undersized ducting in place. This means that the ducting is even more undersized for the larger system. The ductwork will leak even more as the new blower is stronger than the old one. Many of these customers find themselves with even higher utility bills and still can’t get comfortable in their own homes. Others don’t want to even consider upgrading a system as they are already paying outrageous utility bills. To remedy their own problems, they close off the ducts to the rooms they seldom use to “push” the cool air where they want it. This might seem to work for a while but the ductwork was already undersized and closing off part of it only serves to raise the static pressure making the performance worse. As a result, the compressor in the unit will fail long before it should. Sadly, some contractors install replacement units without ever mentioning the ductwork problems. This causes the cycle to begin again.
Most people think the largest consumption of energy in the United States is to power vehicles when in fact buildings consume more energy than cars. One path to reducing energy consumption, cutting our dependence on fossil fuels, lowering your carbon footprint, saving money (and the world if you want to be dramatic) is to quit wasting 45% of the energy it takes to keep your home or business comfortable.
HVAC Performance Testing by Empire Heating and Air Conditioning
If you have high energy bills, poor comfort levels and dirty indoor air talk to us or another qualified HVAC contractor about your system and comfort needs. A few hundred dollars of performance testing from an NCI certified contractor could save you thousands of dollars in energy costs and keep your existing unit running in tip-top shape! Empire Heating and Air Conditioning is a NCI certified contractor. Call Empire at 404-294-0900.