Thread: Return air

Is this design acceptable? The furnace is housed in the closet a few feet from the return air vent. Does the door have to be sealed? I though the return air had to be 10 feet from the furnace.

Thanks

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You're right Mathew, refer IRC 1602. What your photo shows is non-code complying.

Originally Posted by mathew stouffer

Is this design acceptable? The furnace is housed in the closet a few feet from the return air vent. Does the door have to be sealed? I though the return air had to be 10 feet from the furnace.

Thanks

Originally Posted by Jerry McCarthy

You're right Mathew, refer IRC 1602. What your photo shows is non-code complying.

Huh, How ?

Is there a supply vent , plumbing vent , exhaust fan outlet, I'm not seeing?

Originally Posted by Fritz Kelly

If it is a Cat.IV furnace getting its combustion air from outside, it should be fine.

Matthew - Can't see from your photos where the furnace is getting combustion air from. As Fritz says, if it's direct vent from outside, that return should be ok.

This is not an uncommon problem and does provide a hazard for occupants. Yes, sometimes it's OK if it meets the exceptions to 1602.

There is a combustion air vent in that little closet. It comes down from the attic. The furnace is a mid efficiency 65,000 btu.

Mat

Originally Posted by mathew stouffer

There is a combustion air vent in that little closet. It comes down from the attic. The furnace is a mid efficiency 65,000 btu.

Mat

And Why can't the return air vent be at the bottom / sides ?

You got pictures of the inside setup ?

Billy,
That is what I am trying to figure out. The code is a little confusing. I thought the return air vent was prohibited from being within ten feet of the furnace.

Matthew,

The 10 foot rule would be if the furnace and return is in the same space.

From what you described I would be more concerned about sufficient combustion air supply.

I see J. L. is replying as well.

lets read his take.

IRC 2003
M1602.1 Return air
M1602.2 Prohibited sources. Outside or return air for a forced air heating or cooling system shall not be taken from the following locations:
...
5.
2.3 Return-air inlets shall not be located within 10 feet of any appliance firebox or draft hood in the same room or space.

While I see no direct requirement for weather stripping, common sense would dictate the need to separate the indoor air from the outdoor air that is inside the furnace closet.

Mat,

Do you have a photo with that door open?

"How" is that return air inlet connected to the furnace? Is the furnace on a plenum platform, with the return going into the plenum and the furnace setting on the plenum? If so, every one of those installation I've seen leak like a sieve around the furnace/plenum joint.

Is it a return air duct connected to the furnace in the manner in which ducts are connected, and sealed, to other things, such as with metal collars, sealed, and mechanically attached together?

Is that return air intake simply cut through the wall?

Jerry,
It is the first and third scenario. The return is cut into the wall and the furnace is sitting a plenum platform made out of OSB i think. I will try to get some pics.

Mat,

If that return air plenum is not lined in duct board, taped, sealed, etc., it will be sucking combustion air in all over and around it.

Plus, that puts the furnace setting on the plenum with little way to seal it up tight, further sucking in combustion air.

And, unless the opening cut through the stud wall is lined and taped, sealed, etc., it will likely suck air from the wall stud cavities and maybe even up to the attic, and potentially even more combustion air.

Along with combustion air comes unwanted other contaminates.

Try this: put a strong scented oil, cinnamon or something, real strong scent, in the closet with the furnace, turn the furnace on, if you smell that in the return air plenum or coming out the supply registers, the return air is sucking that along with combustion air into the return air stream.

Originally Posted by Jerry Peck

Mat,

Try this: put a strong scented oil, cinnamon or something, real strong scent, in the closet with the furnace, turn the furnace on, if you smell that in the return air plenum or coming out the supply registers, the return air is sucking that along with combustion air into the return air stream.

I did that trick with oil of clove in a crawl space last week. I placed four cotton balls soaked in Oil of Clove in a crawl space. We were trying to prove that the return air was sucking air from the crawl space. It took about three minutes before the entire 4,200sf home smelled like an apple pie!

Word of caution when using any scented oils like this..... Wear disposable gloves and do not touch your eyes or any other sensitive areas of your body!

The backdrafting, or even better description by EC Jerry's "sucking" of return air into the required combustion air required by a FAU is perhaps one of the most common defects in mechanical systems............ in my opinion.
Scott's method is a good test and even a couple of moth balls work well. The only problem is catching the little bastantes.

Here are some addtional photos of the return air configuration.

Matt,

Did you remove the return air grille and look in?

From the photos it has all the defects I expected (regarding this one issue), and, with the door not being weather stripped, no threshold, etc., the combustion air is going to be sucked out around the door and into the return air grille, even is it does not get sucked into the return air plenum.

I don't see a sediment trap either.

Matthew,

IRC 2003 figure M 1703.3 says you need another combustion air duct within 12 inches from the bottom.

That said some jurisdictions allow only 1 at the top.

http://www.muni.org/iceimages/bsd/handoutm03.pdf

Also, it won't take long before there's a dark strip in the carpeting just below the door to the furnace closet where it will act like a filter to all that sucking air. This phenomenon also occurs at closets containing automatic clothes dryers.

Guys,
Thanks for the imput. All the items you noted were in the report. However, I had that one question about the return air. Thanks for your help.

Mat

Word of caution when using any scented oils like this..... ' do not touch any sensitive areas of your body!'

And you know this because .......??? No pics please!!