Compare your new furnace to this photo

I’ve noticed a trend in several houses I’ve been through on home inspections and at friends houses lately (at least in Idaho). Sometimes I like to think there is a good reason for this trend, but unfortunately I have a hard time believing it.

Take a look at this photo. Compare it to your furnace, and check specifically for the white PVC tube that juts out of the top left of the furnace. If you do not have this, but instead have a metal vent, you most likely have a mid to low-efficiency furnace and are free to move on to my next post! (unless you’re considering upgrading) This is for people who have a high-efficiency furnace in their home (which is most often uniquely characterized by it’s PVC venting). Keep in mind I’m not talking about the two condensate lines in the right hand side of the photo. Now that we know what we are talking about, let’s move on!

Assuming you have one of these furnaces, let me explain how they work:

Typically, you should see two PVC vents sticking out of your high-efficiency furnace. One brings fresh, outside air in, and one lets it back out (once all the oxygen has been starved from combustion). This is an ideal setup. It is sometimes referred to as a “closed loop system”, meaning that it does not let inside air escape outdoors through the exhaust.

You may have noticed that the picture I gave you does not include that second PVC pipe. Instead it has a little port that was left without the PVC. This means that the installer for some reason decided to use indoor air to supply combustion air for the furnace. If yours is set up similarly, you can verify that this is the case by simply placing your hand for a brief moment over that port, and you’ll feel the suction caused by the amount of air it pulls through it.

Manufacturers of these high efficiency furnaces usually allow these units to be installed like this, pulling air from the interior of the home instead of outside. But why?

It doesn’t seem to make much sense when you look at the drawbacks:

  • you can lose 1-3% of your efficiency by doing it this way
  • you create what’s called “negative pressure” in the home, which can be dangerous if located near any natural draft appliance (pulling combustion products back into the home instead of out the flue) such as a water heater.
  • It can sometimes prevent bathroom exhaust vents from doing their job to their full extent, by preventing humid air from escaping as easily (if left long enough, causing mold or mildew).
  • If the area the furnace is located in is not dusted very often (many aren’t as they are often in an unfinished basement), it pulls that right into the burner area (which is sealed off in the H.E. furnaces), possibly lowering the lifespan of the unit.

While I may have overemphasized some of these drawbacks, and made you think that your furnace is severely compromised, you don’t really need to worry all that much. After all, manufacturers are ok with it, and 1-3% efficiency reduction is barely if at all noticeable on your bills. These problems are often very small problems depending on the size of your home, location of the furnace, and the way the house was designed.

The only reason I bring all this up, is because it doesn’t take that much work to pipe it to the exterior. Seriously! There’s already one that’s piped outside, and they even sell double wall pipes that function as both the exhaust and intake air (without mixing them), so that you don’t have to pipe two separate lines.

I guess what I’m getting at, is if your considering purchasing one, maybe confirm with the installer how you want it installed, as they may tell you that the manufacturer allows it be installed the other way, but now, you know better… Make sure it’s done best, not just right!

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