A Restricted Dual Fuel Heat Pump Gets a Much Needed Airflow Upgrade in Greeneville, TN

A family member of mine in Greeneville, TN got their home painted professionally and the painting team put 2 coats of latex paint on the return air grille. The return air grille was previously painted brown with latex paint before they purchased the home. So, we are talking about at least 3 coats of latex paint on this return grille now, maybe even 4 coats if the previous owner put 2 coats of brown on it. Painting a return air grille shrinks the surface area of the grille openings and creates an airflow restriction for your HVAC unit. After the paint job I noticed that there was a whistling sound with some resonance when their HVAC would run in cooling mode while visiting them at their home.

My family member has a 4-ton unit at their home and they only have 1 return air grille which is a 24"x30"x1". On a 4-ton unit you should always have at least 2 return air grilles that total a minimum of 800 square inches of surface area in order to slow the velocity of airflow through the return grilles and return air filters. At 24"x30"x1" this family member's home only has 720 square inches of return air grille surface area, so they are short on their minimum requirement. They do have adequate return air ducting under their home, but since they had the paint job done the whistling noise and resonance became more noticeable. I made a recommendation to change out the return air grille with our HVAC Duct Service and to install a new one and to seal up the inside of the grille as well as the outside of the grille.

Being that they are already short on return air grille surface area, I made sure to get a 1/2" spacing on the fins with the new grille. I also manually bent in each and every fin in order to open up the spacing in between the fins a little more than the factory set up of 1/2". I bent the fins open more in order to increase the spacing in between the fins, which helps to slow the velocity of airflow down entering in between those fins. Less air velocity means that their return air grille will be quieter while their unit is running and creates less of an airflow restriction for their HVAC unit. A slower airflow velocity helps the air filter do a better job of filtering the air inside the home as well.

I removed the old return air grille with multiple coats of paint on it rather quickly because it was not sealed up on the inside or outside of the grille. The old grille was not installed level either so there was always a wider gap on the side of the grille. The air filter had to be centered just right or the air would bypass the air filter all together. I secured the return air grille to the studs, sealed up the grille on the inside with duct mastic infused silver foil tape to help prevent air intrusion from the wall cavity. I also sealed up the outside of the return air grille with White Dynaflex 230 sealant in order to prevent air from bypassing the air filter and to give the return air grille a seamless look.

I tested the system before and after the return air grille installation with my Bluetooth probes and actually helped the performance of the system fall within factory specifications. Before the test we had a higher-than-normal temperature split and a lower indoor coil temperature than design because we weren't moving as much air as we needed to. After the return air grille installation, the temperature split and indoor coil temperature came back into the normal range of performance for this machine, which means that we improved the airflow capacity of this HVAC unit. If you have a whistling noise or resonance while your HVAC unit is running and would like some assistance figuring out the cause, please feel free to reach out. Stay safe and best wishes.