combustion air registers

[James Savage]

Can high and low combustion air vents (100 square inch free area) be installed in the same stud space as long each opening is seperated? Each vent opening would be framed seperately in the same stud space, within one foot of the floor and one foot of the ceiling.

[Jerry Peck - Codeman]

Hi James,

I will answer based on the IRC not knowing where you are located, and presuming this is a dwelling which qualifies for being constructed under the IRC and not the IBC (although under the IBC the IMC and IFGC would apply and they are basically, if not exactly, the same as the IRC in this matter).

The first problem I see is actually accomplishing what you are describing.

I will start off calculating the size the correct way with two ducts in one stud space with separate openings for each duct, then end with calculating the wrong way with one duct in one stud space with two openings for that one stud space duct

First, with the minimum opening (and hence duct size) of 100 square inches and 3 inches minimum dimension, the opening's and duct's other dimension would be 100 square inches / 3 inches = 33-1/3 inches, or an opening and duct of 3 inches by 33-1/3 inches. That would not fit withing 16 inch on center studs or 24 inch on center studs.

To accommodate a 24 inch on center spacing, which leaves 21-1/2 inches between the studs, then allowing 1/2 inch for being able to fit the duct in the stud space, that gives a dimension of 21 inches, making the size 100 square inches / 21 inches = 4.76 inches.

Fitting two duct in a single stud space would require the use of 4.76 inches + 4.76 inches = 9.52 inches, then allowing 1/2 inch for being able to fit the ducts in the stud space, the studs would need to be 10 inches minimum, or 2 x 12 studs for 11-1/4 inches.

If you used 2 x 12 studs, then, yes, the two ducts could be configured to fit into the same stud space and would be allowed.

Even one combustion air duct in one stud space would require the use of 2 x 6 studs (5-1/4 inches) spaced 24 inches on center.

From the 2006 IRC. (underlining and bold are mine)
-> M1703.2 Two openings or ducts. Outside combustion air shall be supplied through openings or ducts, as illustrated in Figures M1703.2(1), M1703.2(2), M1703.2(3) and M1703.2(4). One opening shall be within 12 inches (305 mm) of the top of the enclosure, and one within 12 inches (305 mm) of the bottom of the enclosure. Openings are permitted to connect to spaces directly communicating with the outdoors, such as ventilated crawl spaces or ventilated attic spaces. The same duct or opening shall not serve both combustion air openings. The duct serving the upper opening shall be level or extend upward from the appliance space.
- -> 1703.2.1 Size of openings. Where directly communicating with the outdoors, or where communicating with the outdoors by means of vertical ducts, each opening shall have a free area of at least 1 square inch per 4,000 Btu/per hour (550 mm2/kW) of total input rating of all appliances in the space. Where horizontal ducts are used, each opening shall have a free area of at least 1 square inch per 2,000 Btu/per hour (1100 mm2/kW) of total input of all appliances in the space. Ducts shall be of the same minimum cross-sectional area as the required free area of the openings to which they connect. The minimum cross-sectional dimension of rectangular air ducts shall be 3 inches (76 mm).

The above is presuming you are intending to use the required separate ducts.

If you were instead planning on simply using one stud space (still 2 x 6 studs (5-1/4 inches) spaced 24 inches on center to meet minimum dimension for one duct, and then wanting to put two openings in that single stud space acting as one duct, with one opening high and one opening low, the answer is no, that is not allowed.

One reason this is not allowed is you are only providing, at most, one-half of the required combustion air.

Another reason this is not allowed is you are required to get the combustion air for each from outdoors, and in this case the combustion air would be combined, thus you would be getting the combustion air from inside the same confined space.

Codeman

[James Savage]

Thanks Jerry I appreciate the feed back!

However we are not on the same page and it is my fault for not giving you enough information late last night, so I will try again. Just to be clear I will be using indoor combustion air with these two register vents. No duct work to the outdoors.

I want to provide combustion air for my mechanical room that my gas furnace and gas hot water tank are in. I have done the math calculations and the recreation room adjacent to the mechanical room is large enough to provide indoor combustion air for the hot water tank. The furnace is a direct vent, so it already gets combustion air from the outdoors.

I will be using two return air grills one mounted within 12" of the ceiling and one mounted within 12" of the floor. The size of the grills will be 12"x12" with 1.07Ak of free area to meet the code required minimum of 100 square inches of free area on each grill. I will be framing in each opening by installing two 2x4's 12" apart horizontally in a 14" stud bay. Basically making two boxed in openings in the same stud space so both registers won't be connected within the stud bay. Will this be legal? Can the registers be in the same stud bay if they don't connect with each other?

I did more research and found this thread which basically talks about my situation.
http://www.inspectionnews.net/home_insp ... ation.html

Thanks,

James from Butler,Pennsylvania

[Jerry Peck - Codeman]

Hi James,

I will be using indoor combustion air with these two register vents. No duct work to the outdoors.

Ah! Now I see, said the blind man.

You are providing the two openings to allow the otherwise confined space to communicate with the rest of the structure (I should specify 'the areas not separated by doors in the rest of the structure') and are installing one opening *through the wall* to the adjoining space within 12 inches of the ceiling and the other opening *through the wall* to the same adjoining space within 12 inches of the floor.

As you have already done the math and have determined that the minimum size of 100 square inches meets the size requirement for your equipment, as you stated below, there is no need for me to duplicate those calculations:

I want to provide combustion air for my mechanical room that my gas furnace and gas hot water tank are in. I have done the math calculations and the recreation room adjacent to the mechanical room is large enough to provide indoor combustion air for the hot water tank.

You will need to line the inside of your 2x4 "duct" with drywall and seal it to the wall's drywall, making the inside dimension large enough such that the 12 inch by 12 inch metal grill will fit over the opening and not be partially blocked by the "drywall duct" you constructed.

In which case my answer changes to "Yes, it is allowed to install two openings *through the wall* in the same stud space provided the openings are "ducted" through the wall and are not connected to each other.

Here, however, is a catch: When was your house constructed and is it tight and well insulated? If so, then this may apply and bring us back to my original presumption of using outdoor air.

From the 2006 IRC. (underlining and bold are mine)
-> SECTION M1702
- -> ALL AIR FROM INSIDE THE BUILDING
- - -> M1702.3 Unusually tight construction. Where the space is of adequate volume in accordance with Section M1702.1 or Section M1702.2, but is within a building sealed so tightly that infiltration air is not adequate for combustion, combustion air shall be obtained from outdoors or from spaces freely communicating with the outdoors in accordance with Section M1703.

Codeman

[James Savage]

The house was constructed in 2004. I don't consider the basement "unusually tight construction" for the following reasons;

1) No house wrap was used.(My mistake)
2) There are three large windows and a large walk out patio door in the basement.
3) The door at the top of the basement steps has a one inch gap at the bottom.
4) The HVAC contractors installed 4 supply registers in the basement space with no returns. ( I will be addressing the lack of a return in the basement at a later time)

Some additional information:

The water heater is power vented to the outside and needs 3800 cubic feet of air when operating. When the heater is not running the 3" pipe can allow outdoor air to freely enter or leave the mechanical room.
The recreation room by itself provides 8880 cubic feet of air,this is the room with the windows and patio door.

[Jerry Peck - Codeman]

Hi James,

Basically every house built since the mid to late 1990s were built under the various energy codes, especially houses built since 2000 when the ICC (International Code Council) published their first codes, which included an energy code, will likely be considered as being built of unusually tight construction.

This is because the energy codes not only included certain levels of insulation, but required maximum levels of air infiltration and exfiltration, which required sealing around windows and doors, and windows and doors which meet more stringent requirements for the same as were required in the past.

I recommend you ask your local building official (the AHJ - Authority Having Jurisdiction) whether or not they enforce Section M1702.3 of the IRC (or their adopted version of it). If your local AHJ does not enforce that section on homes built in the time frame as your house, then you are free and clear of that requirement.

The codes are established as "minimum requirements" and even "meeting code" does not guaranty that fuel burning appliances will operate properly and safely regarding combustion air and the air within the dwelling. Conversely, though, if the installation does not meet those "minimum standards" as set forth in the codes, there is a good chance that the fuel burning appliances will not operate properly or as intended. This could, and sometimes does, put the occupants at risk of elevated levels of carbon monoxide, which can lead to carbon monoxide poisoning:
http://www.cdc.gov/co/
http://www.epa.gov/iaq/co.html
http://www.bacharach-training.com/healt ... onoxid.htm

A qualified and competent HVAC contractor should perform test on the equipment to assure proper operation of the equipment for the safety of the occupants.

I am going to contact a qualified person and see if he has additional information for you.

Codeman

[DavidR]

Jerry is exactly right on the testing part of the installation.

In order to verify any combustion air grille is actually doing it's job the function of it must be verified through performance testing.

Many times passive combustion air grilles can do more harm than good depending on how Mother Nature impacts them.
Remember it's only a hole in the wall, a driving force must happen to create air movement through that opening.
Many times the pipes that are run vertically to an attic or outdoor area are called combustion air ducts but may actually be exhausting air.

Your appliances should be combustion tested by someone using a combustion analyzer, when read properly this instrument can tell you if your appliances are operating safely or not.
Contractors that have been certified by the National Comfort Institute www.nationalcomfortinstitute.com in CO/Combustion analysis are a good bet for getting this service performed.

The airtightness of the building can be estimated with the use of a blower door.
Many utility programs offer this service at no cost to the consumer where a more detailed approach can be found by using a HERS rater or a building performance consultant.

David

[Jerry Peck - Codeman]

James,

David is the person I contacted to review your question and provide additional information, David is a very knowledgeable person who also teaches on these and other issues.

Codeman

David,

Thank you for providing the additional information for James.

Thank you,

Jerry

[James Savage]

Gentleman,

I had the HVAC installer stop by to look at the enclosed mechanical room framing and the proposed use of the upper and lower grills for combustion air. He believes the grill set up will work properly esspecially because the water heater is power vented and will pull air into the mechanical room. I will have the room checked for proper operation when it is complete.

As a side note: I found a major flaw in the return air duct while framing for the mechanical room that definately affected combustion air in the basement since the house was built (2004). The top of the return duct had a 10" hole in it less than four feet from the hot water tank!!!! One side of the joist space was panned for the upstairs dining room return and on the other side of the duct the HVAC installers forgot to block off the joist space. So the dining room returns never worked and my furnace has been pulling air from the basement (next to the water heater)for five years!

Can you believe that?
If that didn't kill us I don't think my mechanical room with combustion air grills will. I will however have the room tested when complete.

I immediately installed the proper blocking in the joist space and will be panning in a proper return in the rec room area. The basement has four supply registers, but never a "designed" return.

Thanks you Jerry and David for your help,

James

[Jerry Peck - Codeman]

James,

We are glad you found and are correcting that problem and hope that all tests out well when your new project is complete and is tested.

Best of luck with everything going forward,

Codeman