How Duct Sealing Can Reduce Heating and Air Conditioning Costs

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Duct Sealing SuppliesOne of the projects that we recently tackled was sealing the heating ducts in our home for the BooneyLiving Energy Conservation Challenge.

Since the only source of heating in our new project home is a forced air electric furnace, we decided to take a close look at the heating ducts to see if we could make any improvements to them that would reduce our heating costs.

When my husband crawled under the house, he made some shocking discoveries! Our home is a double wide manufactured home and if you know anything about these types of homes, you know that when they are transported on the highway they are separated into two halves to make them narrow enough to fit on the road.

When they are delivered and set up, the two halves of the home are connected to each other and a big flexible air duct called a “crossover duct” is used to connect one half of the home’s heat ducts to the other. When my husband inspected our crossover ducting, he was shocked to find two bowling ball size holes in it. I’m afraid to even guess how much of our heating dollars have been going towards heating the crawl space under the house. I’m sure it was a lot though.

After poking around under the house for a bit longer, he discovered that the HVAC plenum (the metal duct that connects the furnace to the homes ducting system) had some big holes in it! Well, they weren’t actually holes. The problem was shoddy workmanship of the person who originally installed the HVAC air duct system.

My husband said that it looked like they measured the plenum incorrectly when they were making it and instead of correcting it, they just installed it the way it was figuring that no home owner would be crazy enough to actually crawl under the house to discover the defect. The way it was set up, there wasn’t an air tight connection between the plenum and the duct work. This meant that when the furnace blower kicked on, a significant portion of the heated air didn’t make it into our heating ducts. Instead, this air was leaking under the house and heating the crawlspace.

After he made these discoveries, we headed straight to the hardware store to stock up on some duct sealing supplies. In the list below, you’ll find the supplies that we purchased as well as their costs.

  • One gallon of HVAC mastic – $11.95
  • One roll of fiberglass duct sealing tape – $6.45
  • One roll of foil duct sealing tape – $11.99
  • One paintbrush for applying the HVAC mastic – $1.48
  • One pair of heavy duty rubber gloves – $4.09
  • One 10″ x 25′ flexible crossover duct – $42.99

The total cost for the supplies to seal the heating ducts only came to $35.96. If you add the cost to replace the crossover duct, the total cost was $78.95.

Why Sealing Ductwork Is Important

In a forced air heating or cooling system, the furnace blows air under a considerable amount of pressure into the heating duct system. This pressurized air flows through the HVAC ducting looking for a way out. If your home’s duct system is built well, the only place for the heated or cooled air to escape is through the heating vents inside your house. If it has leaks in it, the air that you are paying to either heat or cool will escape through any hole it can find. Since heating and cooling ducts are often located in crawl spaces, basements, and attics, you are paying to heat or cool areas of your home that you don’t normally use. This results in less treated air coming out of your vents and increased heating and cooling costs.

Using The Right Tools For The Job Is Important

I know that it seems perfectly logical that if you have leaky ducts, you should buy some duct tape to seal them up but this just isn’t the case. When most of us think of duct tape, we think of the gray cloth backed tape that is famous for being able to fix anything. I find it quite ironic that, duct tape isn’t really that great when it comes to actually fixing leaky ducts. If you use it to seal your leaky ductwork, it’s inevitable that it will dry out after a few years and fall off or start leaking again.

That being said, there is some premium aluminum foil duct tape that is actually designed for sealing ducts. You can use this specialty foil tape for sealing small leaks in heating ducts. Keep in mind that if you choose to use this tape to seal small leaks in your ductwork, make sure that you clean the area well and that it is completely dry in order to get the best adhesion from the tape.

This foil tape is better than the cloth backed duct tape but duct seal putty or mastic is even better. This is especially true when you have a gap of over and 1/8th of an inch that you need to seal. This is where the fiberglass duct sealing tape comes into play. This stuff looks like the fiberglass mesh tape that is used to hide the seams when applying drywall

To seal these larger gaps, you’re supposed to spread out a layer of the mastic, embed the fiberglass mesh tape into the wet mastic, and then apply a top coat to completely cover the fiberglass mesh.

Applying duct sealing mastic is a bit messy but it’s water based so it washes off easily. My husband actually preferred it to the foil tape because he felt more confident that the repairs he made to our ducts would last longer by using it. One of the characteristics of this product is that mastic never actually hardens so when your metal ducts expand and contract, they are less likely to develop leaks than if it actually dried solid.

Summary of The Repairs We Made

We replaced the damaged crossover ducting, we sealed the leaky plenum, and we sealed the joints that connect the ducts to the registers that deliver treated air into the house. I’m not sure if this was the correct way but to seal the heat register joints, my husband just took handfuls of the goopy mastic and smeared it across the joints. This is the only way that we could find to seal these areas because we had to remove the register and snake our arms down inside the ducts to slop the mastic on the joints.

Hopefully taking the time to seal our leaking heating ducts will pay off in the form of a significant reduction in our homes total energy costs. I can’t imagine that it wouldn’t since there were some areas that were obviously leaking pretty badly. They’re not leaking now so that expensive electrically heated air will no longer be heating up our crawlspace.

Disclaimer: We’re certainly not experts in HVAC. The steps outlined in this article are the steps that WE took to seal the ducts in OUR house. If you are concerned about making sure that this kind of repair is done perfectly in your home, hire an HVAC professional to do the job. The purpose of this article isn’t to teach you how to seal ducts but rather explain why it is important to do and what supplies we used to seal ours.

Want Some More Of The Good Stuff?

Thanks for taking the time to read this article. Be sure and check back on the progress of the BooneyLiving Energy Conservation Challenge. In the meantime, if you’re interested in conserving electricity, you might want to read this article: How To Reduce Your Alternative Energy Costs




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One Response to “How Duct Sealing Can Reduce Heating and Air Conditioning Costs”

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  1. Gerald Eagy says:

    Your advice to call the HVAC technician is very well taken. Many people do not have the ability to crawl under their home to do the job you described. They must trust to the professional. I want to say that not all who call themselves HVAC contractors are qualified, but the majority are and will do you a good job.

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