Here is one of my favorite quotes;
“Lord, grant me the strength to accept the things I cannot change,
The courage to change the things I can,
and the wisdom to know the difference.”
One of the things that I cannot change in existing homes is the location of the heating and air conditioning equipment and ductwork. In Southeast Texas, Most of the homes have all of the equipment and ductwork located in the attic, this is the very worse place possible. There is no debate on this, it is a FACT. The attic is the hottest place possible. Even with a perfectly sealed system, you still have considerable heat gain around the equipment and ductwork.
When all of that equipment and ductwork is located inside the living space, or “conditioned space”, the duct leaks are not as big a deal. The big problem is when all of it is located in the attic, any leakage in the system is either drawing air in or blowing it out inside the attic.
A recent report I read published by Berkeley Labs stated that, on average, duct systems leak 10% from the supply side and 12% from the return side. Let’s put that in perspective. On a hot summer day in Houston the average temperature in the attic is around 160°. Let’s say we have a typical 4 ton air conditioning system producing 400 cubic feet minute (CFM) of air. For a 4 ton system that would be 1600 CFM total.
1600 CFM x .12 = 192 CFM of air is drawn into the return from the attic.
1600 CFM x .88 = 1408 CFM of air is drawn into the return from the conditioned space.
Let’s say the room is set at 74° the return air is about 2° warmer or 76°.
We have 192 CFM of air at 160° and 1408 CFM of air at 76°.
192 x 160 = 30,720
1408 x 76 = 107,008
Total = 137,728
137728 / 1600 CFM = 86.08°
Average return air temperature will be 86°. This is the air that gets heat (and humidity) removed. (For the sake of this article we are only discussing the temperature and not the humidity. That’s a complete subject in, and of, itself.)
The coil in the attic is what removes the heat. (This to me is counter intuitive. It feels like it’s cooling the air but is technically removing the heat). The coil removes enough heat to leave the air 20° cooler than it was when it entered the coil. The air entering the house (or supply air) is now 66°.
86° – 20° = 66°. If the room temperature is already 76° in the room it’s only coming out 10° cooler at the register. If it were all in the conditioned space the air coming out would be 20° cooler or 56°. With that said, you are using twice as much energy to cool the same amount of space under the same conditions.
In this scenario you could probably cut your daytime usage on hot summer days by nearly half by just fixing the duct leaks.
There is a system we use called a Duct Blaster®. We seal off all of the registers in the house and connect the Duct Blaster® to the return filter grill. It pressurizes the system and tells us how much air is leaking. That will tell us how bad a system is leaking but won’t tell you where. We then use a theatrical grade smoke machine to force smoke into the duct system, then we can easily identify where the leaks are located. A few guys in the attic work through the system sealing all of the leaks where they see smoke. Where there is smoke there is money lost.
I am amazed when I watch the meter dropping as the guys work through the system sealing all the leaks. Ok, that’s a geek kind of thing but you can appreciate it when you see lower electric bills in the middle of summer.
The Berkeley study shows duct leakage to be, on average 22%. The department of energy says its 20% to 30%. For many homes in this part of the country, the air leaks are far greater. I have observed duct leakage in excess of 60%.