Monday, August 13, 2012

HVAC Intake Pipe Information

At our pre-drywall meeting, I talked about the fact that our furnace only had one pipe coming out of it for exhaust and no "intake pipe".  Here's a general overview of how the heating system works and why it's better to have an outside air intake pipe:

Basically, your furnace needs two types of air supplied to it in order to work properly: 1. Air to condition, heat, then re-circulate through the house and 2. combustion air, which is needed to burn the oil or gas that is used for energy in your system (just like a flame needs air to burn).  The re-circulation air is typically pulled from the rest of your house and seeps in from the outside through cracks and windows.  Some of the vents you see around your house are for blowing conditioned air out, but others are for sucking air inward to run through the system, re-condition and then blow back out.  (Ask your PM which vents are for which purpose... he should be able to tell you.  Typically the larger ones are these "return vents" that suck air in). 

The second type of air needed for your system, the combustion air enters directly into the furnace either through a pipe (typically PVC) or simply a hole in the unit.  The diagram below shows a basic furnace setup with an intake pipe.  Without this pipe, there would just be a hole in the unit where the pipe shown is entering.

Without an intake pipe, your furnace must have adequate space around it to ensure that it has enough air to pull in.  With an unfinished basement (or attic if that's where your furnace is), this is typically not a problem because there's plenty of air in the basement, and it will also suck from the upstairs through the door, windows or voids in the floor.  However, if you decide to finish you basement and want to wall off the furnace area, you would have to leave a certain amount of space within the enclosure to provide enough combustion air to the unit.  The amount of space needed is based on the size of your furnace and apparently the manufacturer provides this information, but it's also usually regulated by township code.  For instance, a typical requirement is that 1 square foot of space be provided for every 100 BTUs, so if you had an 80,000 BTU furnace, you'd need to provide 800 square feet of open space around it.  So this would basically be wasted space in your finished basement.  If however, you add an intake pipe from the outside directly into your furnace, you would not need to provide this open space since the furnace then has unlimited space to pull from outside.  (Note that in our area, Ryan only provides the intake pipe when you upgrade to have them finish the basement).

Another benefit of this intake pipe is that you are pulling much cleaner air from the outside, so it increases the efficiency and life span of your furnace.  Just like our bodies, cars, etc. work better with cleaner fuel, the same is true for an HVAC system.  In addition, the furnace needs the same amount of combustion air to work regardless of whether you have an intake pipe or not.  If you do not have one, it needs to somehow suck new air though the house to replace the air it is using to burn, so it will pull it from the outside through every little crack or crevice in your home, which can create drafts.  Having the intake pipe reduces the need to pull in air from the outside through doors and cracks, therefore making your whole system more efficient.

Side note: The energy star number that is listed on your furnace and advertised by Ryan is simply the referring to energy required to run the actual furnace, not the efficiency of your home as a whole system.  There's a big difference.
This is our actual furnace.  The pipe on the left is the exhaust and the one on the right is the intake. 
Before they intalled the second pipe the one on the right was just a stub sticking out the top.

Hopefully this helped and wasn't too confusing!  Your PM should be able to explain all of this to you in more detail.  Or if you're having an independent inspector, it can't hurt to ask their opinion too.


  1. Thank you so much! I'm emailing this to my SR right now. :)

  2. great post I did confirm that I have the outside pipe since we did finish our basement

  3. I finally got to talk to the PM about it (did it really take a month??), and apparently the way they do it here is that if there isn't enough room they would put vents into a larger room to pull air from inside the house. I'm not sure how happy I am with that answer, but his explanation jived with what you put here, so at least I know we're talking about the same problem.

  4. quick question, I have a 2" PVC pipe for my intake that is currently in my basement and I want to extend this to the outside. Can I just extend the 2" pipe all the way to the outside or do I need to make this a 4" pipe and then extend it out?


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