Author Topic: combustion air intake pipe  (Read 545 times)

Offline Darkman5

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combustion air intake pipe
« on: May 29, 2018, 02:06:09 PM »
Hi, i recently had a new high efficiency trane furnace and A/C installed.  My installer did not run the 2nd intake pipe for combustion air to the outside of the house.  Instead just leaving it where the furnace is installed in my unfinished basement.  I've had someone say that i am losing efficiency by the combustion air not being drawn from the outside as now air is going to seep into the house in the current set-up.  Can anyone confirm or deny whether i am losing this efficiency and if its a big percentage?   I'm just trying to figure out whether i should spend more money on having this intake run outside of the house.  I've heard technically its fine and the system will work fine.  Thanks. 

Offline Admin

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Re: combustion air intake pipe
« Reply #1 on: May 29, 2018, 02:55:13 PM »
It sounds like you have a separate outdoor air supply that just lets cold air dump into the mechanical room.  This is okay to do by Code.  There is not much difference in the furnaces efficiency when it’s either one piped or two piped.  You would lose less than 1% of the furnaces AFUE rating.  The house would lose some efficiency having a duct that allows cold air to dump into it directly, but it sounds like this has always been there.

What you are describing is a pretty standard practice and I wouldn’t worry about it too much.

Just be mindful not to store paint and things near that furnace intake as you don’t want vapors to get sucked into the furnace.

Offline tenletters

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Re: combustion air intake pipe
« Reply #2 on: May 29, 2018, 03:58:24 PM »
If you're going to seal your house up tight or if your house is tight, run the second pipe.

Offline walker

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Re: combustion air intake pipe
« Reply #3 on: May 29, 2018, 04:22:12 PM »
It sounds like you have a separate outdoor air supply that just lets cold air dump into the mechanical room.  This is okay to do by Code.  There is not much difference in the furnaces efficiency when it’s either one piped or two piped.  You would lose less than 1% of the furnaces AFUE rating.  The house would lose some efficiency having a duct that allows cold air to dump into it directly, but it sounds like this has always been there.

What you are describing is a pretty standard practice and I wouldn’t worry about it too much.

Just be mindful not to store paint and things near that furnace intake as you don’t want vapors to get sucked into the furnace.

I think what he meant is the air used for combustion that is exhausted is being replaced by infiltration, whether or not there is an air opening or not.

I wouln't worry about it either unless you finish the basement or if its an extremely tight house.

Offline Hvacpimp

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Re: combustion air intake pipe
« Reply #4 on: May 29, 2018, 08:24:13 PM »
All of these guys are correct. Next time go with a contractor that cares enough about his install to run 2 pipes regardless. The install is totally fine but why not just run the second??

Offline walker

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Re: combustion air intake pipe
« Reply #5 on: May 29, 2018, 08:30:13 PM »
Sometimes it comes down to the CX wanting the cheapest option.  And one pipe is cheaper than 2.

Offline Darkman5

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Re: combustion air intake pipe
« Reply #6 on: May 30, 2018, 08:23:38 AM »
Thanks everyone.  I'm still hoping to try and get him to come back and run the second pipe.  But i guess i got what i paid for.  I'm assuming my costs would've been higher if he had done it initially. 

Offline Porcupinepuffer

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Re: combustion air intake pipe
« Reply #7 on: May 30, 2018, 04:44:07 PM »
All of these guys are correct. Next time go with a contractor that cares enough about his install to run 2 pipes regardless. The install is totally fine but why not just run the second??

This may not directly pertain to this scenario(if his house is old enough and doesn't have a fresh air existing), but sometimes the situation of direct venting runs into conflicts with clearances that make single piping the only logical option. Particularly in certain townhome situations where there's very limited venting options. Sometimes almost everything in bunched up in one tiny section and certain things won't work without big changes.

Ex. Only one small area to run the vent next to a power vented water heater. Furnace manual asks for 2' clearance from combustion air vent of furnace to power vent exhaust. Now you need to vent elsewhere or make an octopus of venting terminations outside. Or not be able to do that because it then leads you to being stuck too close to windows, gas meter, dryer vent, inside corner, decks, ground, etc... The 1 foot clearance to an existing fresh air definitely makes things easier.
We've found ourselves stuck enough times where the only way to vent the furnace, within a logical budget, was for the customer was to single pipe it.