Add Combustion Air Intake to Gas Furnace

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  #1  
Old 07-24-19, 07:46 AM
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Add Combustion Air Intake to Gas Furnace

I recently had my older 80% efficient gas furnace replaced with a new 98% efficient furnace. The furnace is located in an unheated 18' x 30' crawl space. The crawl space is vented to the outside with two 6 x 14 vents at opposite corners of the crawl space. The old furnace used combustion air from the crawl space and exhausted through a type B vent routed up through a chase, thru the attic and out of the roof.

The new furnace was installed and, I think, unfortunately was set up to continue to draw it's combustion air from the crawl space. It seems to me that drawing cold air from the outside, into the crawl space, is not as good as drawing only the cold air needed, thru a pvc pipe connected directly to the furnace. IOW for overall home energy efficiency and heat conservation I think drawing cold air into the entire crawl space to supply the furnace is not as desirable as having the furnace intake air piped directly to the outside.

I have included some photos of the furnace and piping. The HAVC contractor ran the new pvc exhaust thru the existing type B vent. There is plenty of room inside the type B vent for another equal size pvc pipe. The photos show that, due to how the exhaust pvc was attached to the furnace, there is not enough room for an intake pvc 90 degree elbow to attach.

My questions are:
1) Am I correct that piping the intake to the outside will help enough with overall energy efficiency to make it worthwhile to do?
2) If I add the fittings needed to make room for a 90 elbow to attach to the furnace for intake, would those additional fittings unduly compromise the exhaust flow?

I have a lot of home building and repair experience. Expertise as carpenter and ok with simple plumbing, electrician, tile setting, drywall etc work.

Thanks in advance for any advice or comments.

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Last edited by PJmax; 07-24-19 at 08:48 PM. Reason: resized pictures
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  #2  
Old 07-24-19, 08:43 PM
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Welcome to the forums.

Why don't you discuss it with the installer ?
I think that pipe thru the roof needs something..... possibly a right angle to keep the water out.
Every fitting added to the system changes the operation.
 
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Old 07-25-19, 07:44 AM
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Thanks for your reply Pete.

If I can get a hold of him which is near impossible, the installer has a stock reply, "I wouldn't bother." That's why I'm seeking a second opinion here.

I'm not asking about the roof exhaust, that was installed by the HVAC contractor. Since you mention it, most that I have seen and also plumbing vent stacks don't have an elbow at a roof termination. My understanding is that it doesn't matter if a little rain happens to fall down in there occasionally. Plus, as you point out" every fitting added to the system...."

My questions above remain, much appreciated if anyone can weigh in
 
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Old 07-26-19, 04:03 AM
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If the crawl space is open to the outside atmosphere then there is no reason to have a dedicated pipe to the wall edge. Combustion cold air is combustion cold air, The furnace does not care where it's coming from. And it does not need to be "cold" per se. It's needs to "oxygen" rich. And any air drawn in from the outside atmosphere is.
Not that many years ago most furnace were drawing combustion air from the house hold air with little or no problem. True not quite as efficient as today's furnaces.
 
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Old 07-26-19, 06:00 AM
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Thank for your reply Norm. I know the furnace can function effectively drawing outside air from the crawl space through. My concern is that it is pulling cold air into the crawl space and making the crawl space generally colder than it would if the furnace combustion air intake was piped directly to the outside.

The crawl space is getting cold air drawn into it making that part of the house envelope colder than it would otherwise be.
 
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Old 07-26-19, 06:30 AM
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The crawl space isn't within the house envelope and it should be vented to allow airflow from the outside. This not only feeds your furnace but keeps moisture levels down as well.
 
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Old 07-26-19, 10:18 AM
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I doubt you'll see a difference. The crawl space is open to the air and will be the same weather you draw air thru the furnace or not
 
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Old 07-26-19, 04:26 PM
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I would install a 2" bay vent (also available in 3") at the location you marked as air intake. Cut the existing pipe at the furnace and use it for intake air. Run a new exhaust pipe from the furnace exhaust up into the attic and connect it to the bay vent along with your air intake pipe. Cap the remaining pipe as it enters the b vent up high. Someday when you have to get on the roof you can remove the b vent and pretty up the house. Don't know what your max snow depth is, but you might be able to install the bay vent (or modified piping, see your furnace manual) through the rim joist and get rid of the whole vertical flue. They do have concentric vents too, but I don't care for the look of them.

https://www.supplyhouse.com/DiversiT...SABEgK9CfD_BwE
 
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Old 07-26-19, 04:53 PM
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interesting idea and thanks for your input!

(max snow depth is way too deep fro terminating out of the floor rim joist location)
 
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Old 07-26-19, 07:29 PM
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You're going to do what you're going to do. But I question, why would you want to put another hole on the side of your house when it really does not accomplish anything? If anything it tends to diminish the look of the house and reduce it's resale value.
 
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Old 07-26-19, 08:42 PM
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Hi Norm,

i am not trying to accomplish increased furnace performance, I want to close off the crawl space vents and better seal up the crawl space. I don’t want all the cold air being drawn into the crawl space all heating season.

My house is a very unique situation. It is a very small house on a very valuable lot in a high end ski resort. The furnace is in an addition I built. There is another part of the house that is historic and listed as part of a National Historic Landmark District.

When I get to old to live in the house and sell it the addition will be torn down. The historic part will be lifted up. A full basement and max allowable expansion will occur. Many, even most, houses here have already this done and 1 million dollars on the improvements is typical. Nothing I do to my house either increases or decreases the re sale value, which is now approaching 20x the purchase price.
 
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