Ducts – How bad are a few leaks?

Air return duct - wide open

Let me give you a hint – leaky ducts can kill you.

No joke. Duct leaks are serious business, and yet, most people have no idea just how important ducts are. I’ve had arguments with HVAC contractors over poor duct installation practices. Frankly, it’s criminal when ducts are installed like they are in the picture above. I mean, what were they thinking?

Ok, let’s take a step back. What is the purpose of ducts? They’re there to move air around your house – distributing heating and cooling as uniformly as possible to keep your house comfortable. Most homes have two types of ducts – supply and return

Supply Ducts

The purpose of supply ducts is to supply conditioned (heated or cooled) air to various rooms in the house. Each room has specific requirements, so proper duct design is a science. How big is the room? Does the room get a lot of sun or is it shaded? Is it upstairs or down? Does it have lots of windows? How much insulation does it have in the ceiling and walls? How far is the room from the furnace?

All of these factors affect the supply duct design. Unfortunately, most installers take a “cookie-cutter” approach, and just put one or two supply registers in per room and call it a day. If you’re lucky, they’ve installed dampers so that they can adjust how much air gets to each room.

Assume that they do all these things right. What happens when the ducts are leaky? Now all the calculations go out the window because the needed air isn’t reaching each room. In extreme situations, I’ve seen rooms receiving *no* air! Often this occurs when the ducts run through the attic and one has become detached. So now, all the air that’s supposed to be supplying heat to a room is spewing into the attic. How much do you think that costs you in wasted energy?

Return Ducts

Return ducts are used to “return” air back to the furnace or air conditioner. Think about it for a second – every room should have a balanced air supply, with the same amount leaving as is entering. Otherwise, it’s like blowing up a balloon. If the door is closed, the system is trying to blow air into a closed space. What happens then? The excess air tries to make its way out. Sometimes it will go out into the hallway and makes its way back. Other times, it leaks outside, so again, you’re wasting energy.

Think about what happens when the return duct leaks in the attic. Any leak in the attic sucks air into the ducts from that nasty space, usually filled with dust, mouse droppings, and fiberglass, as well as the obvious outside air that might be cold or very hot. So, return air leaks are bad, really bad. They’re unhealthy and really hurt your energy efficiency. How much? The example shown above where the filter port is open will decrease the system efficiency 30%-50%!

Now, what happens if you have a duct leak in the basement, like shown at the top of this article?

Well, first off, the room upstairs that’s supposed to have a working air return doesn’t, so the air flow won’t be proper. That’s the good news. The bad news is that a return air leak this big can suck the air out of the basement, creating a low pressure zone in the house. So low that it can literally suck the exhaust fumes out of the furnace or water heater or make it malfunction so that it generates carbon monoxide. In fact, it can kill you. 

This problem is so serious that the Building Performance Institute (BPI) requires all certified energy auditors to test for these potentially lethal conditions. And remember, you might have several things working to create dangerous conditions – the leaky return ducts, the clothes dryer (which might be expelling large amounts of air from the basement), the furnace (which also uses air for combustion), and the water heater. There are also natural forces at work that make basements lower pressure.

So when an installer takes shortcuts and doesn’t seal your ducts air-tight, not only are they robbing you and maybe making you sick, they could kill you. If you’re having duct work done, or a new house or addition built, insist on having the ducts tested. I will cost several hundred dollars more, but what price do you put on your family’s life? Don’t take shortcuts – make sure it’s done right.

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