HVAC Duct Problems

I recently had a HVAC duct problem at one of my rental properties. My tenant called and said the AC did not work. Over the years, I have developed a plan of action where I first make contact with the occupant before sending out a service repair person. Most of the time, I find the electricity is off or the thermostat is set too high. Usually, it is something very minor and certainly not worth a service call.

This time, when I arrived, the AC unit was running, but the house was still hot. So, I called the service repair person. He went out to the house and found the unit to be working correctly and, of course, sent me a bill for the service call.

A couple of days later, the tenant called again and said “it is too hot”!

So, I went back to the house and it was the same thing. The unit was running, but the house was hot. Here is a picture of the unit that set outside.

NOTE: This house has vinyl double glazed windows, all brick construction, 12 inches of insulation in the attic and 8 inches of insulation under the house. The outside temperature was in the 90’s. This house should have been cool.

I called the AC service person again. This time, he disconnected the heat strips just to make sure they were not energized. Then, he looked under the house thinking a HVAC duct may be open. Instead of the duct being open, he found the drain from the washing machine had split and he would not go under the house until the problem was fixed and the water was removed. So we had to take a break from the AC problem so the plumber could take care of the drainage problem.
When the AC serviceman came back, he crawled under the house and again found no open ducts. He found no problems at all.

The house was still hot! The AC serviceman made the suggestion that the only area he could not check was where the ducts go from the unit to the crawl space. At this point I was starting to see red! I needed resolution to the problem.

I noticed the air flow out of the registers from the HVAC duct was not very good. The temperature was cold, but not much air flow.

I have a commercial building construction background and every time we would build a call center, an equipment room or an office space, I remembered the HVAC people would measure the air flow coming out of individual registers from the HVAC duct. They would cover the register with a blue canvas cloth that had a metal frame. It had a meter on the side that would indicate the exact amount of air flow.

So I went hunting on the web to find something that could do the same job measuring air flow. I found an air flow meter on e-bay.


In the meantime, I went back to the house and removed the metal cover that protects the ducts from the house to the unit. As you can see in the picture, this space is really packed with HVAC duct work.

Close examination revealed that both the intake and the return ducts were pulled away from the metal adapters going through the foundation. It was unusual as this area is not easily accessible. It was almost as if the unit had been picked up and pulled away enough to separate the ducts then shoved back in to place. I never saw that before.

I then removed the old ducts and repaired any openings in the collars and attached and sealed the flex ducts back together without any openings. I put everything back in place, fired the unit up, and hallelujah, we had cold air.

Several days after I fixed the leaks in the HVAC ducts, the air flow gauge I ordered on e-bay showed up. I went back to the house and measured the air flow. It was much better.

The air flow gauge is one slick tool. It is a two piece unit called a Thermo-Anemometer.

For those of you who want more information, here is a floor plan showing the values at every register.
The intake registers 910 cubic feet per minute of air flow in. The outlets total 827 cubic feet per minute output. The difference is how the ducts are designed and how much resistance there is in the ducts. My local HVAC tech thinks the unit was working at 90.7% air flow efficiency, this is considered good.

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