Thread: Mid efficiency furnace combustion air

So the mechanical room is located on the lower floor of a house constructed on a downslope lot and has 2 - 100,000 BTU, mid-efficiency furnaces. Two 4x14" screened vents opening into the foundation crawlspace provide combustion air. If each furnace requires a minimum of 100 square inches, these two vents are inadequate. Right? Thought so.

Now the real question. I was flipping through my code check and found an interesting note that I had not noticed before. "Only lower opening connected to underfloor area". This indicates that only one combustion air vent can go down into the foundation crawlspace area, the other has to go up into the attic or out an exterior wall. However, perusing the IRC, it looks like there is no restriction on both venting into the foundation crawlspace area, just cannot share an opening or duct. Anyone have an answer?

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Might want to recommend they seal the openings and have a combustion air system that will work under all conditions installed.

Field Controls makes some great stuff to address issues like this.

The openings in the floor will probably work better than high/low combination's due to the fact they will allow CO2 to escape the room if an appliance is spilling.

Originally Posted by DavidR

Might want to recommend they seal the openings and have a combustion air system that will work under all conditions installed.

Don't know about Field Controls. Forgot to mention, the lower room is on a concrete slab, so a floor vent is not practical. Also, natural gas, forced-air furnaces.

Originally Posted by Gunnar Alquist

Don't know about Field Controls. Forgot to mention, the lower room is on a concrete slab, so a floor vent is not practical. Also, natural gas, forced-air furnaces.

Being on a slab is even worse, no where for the CO2 to go.

FIELD CONTROLS, LLC - CAS, UV-AIRE, SWG, IAQ if you are interested Gunnar, they make great stuff that truly works.

Gunnar,

In our area, we need to have two separate, dedicated vents/ducts for combustion air into an enclosed room with non-direct vent combustion appliances. One vent down low and one high. Two high or two low vents won't perform as intended. I have talked myself silly trying to convey this to HVAC contractors.

DavidR - Are you referring to carbon monoxide (CO) or carbon dioxide (CO2)?

Originally Posted by Eric Barker

Gunnar, In our area, we need to have two separate, dedicated vents/ducts for combustion air into an enclosed room with non-direct vent combustion appliances. One vent down low and one high. Two high or two low vents won't perform as intended. I have talked myself silly trying to convey this to HVAC contractors

Eric,

There are 2 separate screened vents. They are too small and they are not within 12 of the floor or ceiling, but that is not my primary question. My primary question was; can the two combustion air vents both go to the same foundation crawlspace area? My Code Check seems to indicate no, but the IRC does not.

I can't address the code issue, however if there are two vents to a crawl there is the possibility that they can become occluded if flooding occurs (not likely but...), hence the high/low requirement. There is a requirement for 1 square inch per 1,000 BTU venting (in this case 100 sq. in. per unit) with one vent high and one low (12" from ceiling/floor, as you said). The rationale is that separate vents prevent occlusion. This is the way I understand it.

Note also the type of vents may have different restiction qualities. Wood slat vents have about 25% usable vent area; metal screen type vents 75% usuable vent area. That's the rule of thumb I use.

Originally Posted by Gunnar Alquist

So the mechanical room is located on the lower floor of a house constructed on a downslope lot and has 2 - 100,000 BTU, mid-efficiency furnaces. Two 4x14" screened vents opening into the foundation crawlspace provide combustion air. If each furnace requires a minimum of 100 square inches, these two vents are inadequate. Right? Thought so.

Gunner, if the crawlspace is vented, it is considered to be venting directly to the outdoors. "The inlet and outlet air openings shall each have a free area of not less than 1sq in per 4,000 BTU per hr of the total rating of all the appliances in the enclosure." NC Gas Code Book old 1988 but I think it is still the same.
If the crawlspace is not vented it is 100 sq in per 1000 and the 100 sq in minimum code requirement is just that "100 sq in min. total" Your 200,000 would still need 200 sq in though.
I can't find an answer to the question of both combustion and makeup vents to the crawlspace, but the absence of any diagram with this configuration suggest it is not allowed. Sorry for lack of positive ya or na.

Gunnar,

An unusual but great question. You can run both vents up but I really don't know about running both down. The question that I'd have is would the warm air in the furnace room sink into the cold (cooler) crawl space? I'd probably be asking my local gas utility their opinion. Personally, I don't think I'd want that arrangement in my home. But then again, the vent on my water heater is undersized and allows for a little spillage. (Shoemaker's children)

I'm sure that as old as this thread is Gunner has called the AHJ and knows the answer.... let us know Gunner!

The crawl space as combustion air has always made me wonder a bit.... So, what happens in the winter when all of the vents are blocked off with foam pieces?

Also, a lot of the crawl spaces around here don't really have all that much cubic space and I don't imagine the air exchange through cement and the ground is as much as it is through sheetrock and siding.

The IRC implies, based on the drawings, that when obtaining combustion air from a crawlspace that the lower opening is allowed to vent from the ventilated crawlspace, but that the upper opening is to be vented out through the attic - Figure G2407.6.1(1).

The code text, Section G2407.6.1 does not explicitly say that. The code wording simply treats "vertical" ducts, whether vertically into an attic or vertically into a crawlspace, as "vertical" ducts. The code does differentiate the difference between vertical duct and "horizontal" ducts as there is an obvious loss of air flow through a horizontal duct as related to a vertical duct.

The intent, whether through a vented attic or a vented crawlspace, is to provide at least one high and one low vent opening which will allow for air circulation within that confined space and provide more efficient combustion air flow. Either way, the combustion air will be drawn from the negative pressure being developed at the burner, drawing air from both vents.

Originally Posted by Eric Barker

DavidR - Are you referring to carbon monoxide (CO) or carbon dioxide (CO2)?

Carbon Dioxide is what I was referring to in my previous post Eric.

Originally Posted by Eric Barker

I'd probably be asking my local gas utility their opinion.

Eric,

I really don't have a lot of faith in my local utility company when it comes to questions like that.

Originally Posted by Vern Heiler

I'm sure that as old as this thread is Gunner has called the AHJ and knows the answer.... let us know Gunner!

Vern,

Actually, no. I had already deferred to an HVAC contractor because of the size and height of the openings and left correction to them. Since CA does not use the IRC, I would not be able to get a useful response. It was just something that I noticed when I was referring to my Code Check.

Originally Posted by Jerry Peck

The IRC implies, based on the drawings, that when obtaining combustion air from a crawlspace that the lower opening is allowed to vent from the ventilated crawlspace, but that the upper opening is to be vented out through the attic - Figure G2407.6.1(1).

Any comments on the crawl space vents being blocked during the cold months.... which, of course, is when the furnace is going to be used?

Up here in 'crawl space land' the design throughout the 80's was to insulate the perimeter walls and leave notes on panel box and furance that instruct the homeowner to block the foundation vents during the winter. On houses after the underfloor insulation requirement in the early 90's I typically tell my clients it's not of as much importance to block the vents.

Originally Posted by Matt Fellman

Any comments on the crawl space vents being blocked during the cold months.... which, of course, is when the furnace is going to be used?

The comment would be the same as my first thoughts: It's dumb to obtain combustion air from a crawlspace, hard to believe the code allows it, but it does, however, it must be kept in mind that code is minimum requirements - or it this case, not "requirements", but "allowances".

Then you immediately get back to reality and add: Remember, code is minimum, code does not address common sense or basic construction practices.

Common sense and basic construction practices would say something like: It's dumb to obtain combustion air from a crawlspace.

Crawl spaces are vented for a reason. Of course this discussion always gets nipped in the butt by many saying crawl spaces don't need ventilation. Personally my belief is all crawl spaces need ventilation, winter or summer. By saying that there would not be any blocking the crawl space ventilation in the winter. This would allow proper air flow for intake or combustion air. I also am a believer that pulling air from a crawl when almost all crawls are notorious for conditions that would not be conducive for drawing any air from is not a good idea.

Originally Posted by Ted Menelly

By saying that there would not be any blocking the crawl space ventilation in the winter.

Unless you consider the height of the vent and the snow build-up height. It may well be that, while the homeowner does not block the vents, mother nature will.

Originally Posted by Ted Menelly

Crawl spaces are vented for a reason. Of course this discussion always gets nipped in the butt by many saying crawl spaces don't need ventilation.

Ted, There is quite a bit of info that supports the incorporation of the crawl space into the conditioned area and I strongly support that. We have a great deal of humidity here in the summer and when it enters a cool crawl it can create significant problems. Perhaps in other areas of the country it's not so much an issue.


Back to pulling combustion air from the crawl: You can do this with an attic provided that the attic itself is well ventilated. Crawl spaces on the other hand will often have vents to the exterior that can be closed. Not that I would necessarily expect this to actually make much of a difference, but by the book, it's a consideration to make note of.

Originally Posted by Eric Barker

Ted, There is quite a bit of info that supports the incorporation of the crawl space into the conditioned area and I strongly support that. We have a great deal of humidity here in the summer and when it enters a cool crawl it can create significant problems. Perhaps in other areas of the country it's not so much an issue.


Back to pulling combustion air from the crawl: You can do this with an attic provided that the attic itself is well ventilated. Crawl spaces on the other hand will often have vents to the exterior that can be closed. Not that I would necessarily expect this to actually make much of a difference, but by the book, it's a consideration to make note of.

My big concern is that I have seen more than likely an 80 20 reason in most crawls for not drawing air from there. The 80 being the bad conditions, moisture mostly, smell, rodents, insects, plumbing or should I say sewer gas leaks and so on. I have not met to many folks in my life that ever go into their crawl to check for concerns until something shows up because it is so obvious they know something is going on down there. In a world of proper maintenance to a crawl space (that does not exist) I would have no problems with drawing air from a crawl.

As far as closing ventilation from the outside to the crawl. In a world of proper maintenance to a crawl (that does not exist) I would not have a problem with that either.

I lived in Neptune Beach Florida 3 blocks from the ocean. I never closed my crawl ventilation off and never had a moisture problem. Yeah I know there is only a week or so of days that starts out below freezing but the humidity is high most of the year especially being right next to the big puddle. I could see it in longer colder winters up north with high humidity and a long heating season. Of course my crawl was not soil. Pretty much sand so we had excellent drainage and not much moisture coming up thru the soil.

Anyway. Did I say how much I hate crawl spaces. Those dirty buggy, smelly, lions and tigers and bears oh my. Cannot stand the dam things.

Originally Posted by Matt Fellman

Any comments on the crawl space vents being blocked during the cold months....

Originally Posted by Jerry Peck

The comment would be the same as my first thoughts: It's dumb to obtain combustion air from a crawlspace...

Originally Posted by Ted Menelly

Crawl spaces are vented for a reason.

Originally Posted by Eric Barker

Ted, There is quite a bit of info that supports the incorporation of the crawl space into the conditioned area and I strongly support that.

Interesting selection of opinions. It is pretty clear to me that regional weather differences can influence many construction practices. For instance, we have relatively mild weather and there is no snow to block foundation vent openings. Foundation crawlspace vents are (almost) never intentionally blocked. I have never heard any homeowner suggest or query intentional closing of a vent opening. In this area, adequate ventilation is necessary and required. Then again, I never look for ice shields or hurricane clips either. This is a great forum. But for the diversity, I would blindly continue thinking that everyone inspects the way I do.

Ted,

Remember, the code is only saying 'you can are allowed to get combustion air from the crawlspace', it is up to contractor to understand the specific installation conditions present at that installation.

With a crawlspace, such as those I would see when in Gainesville, Fl, or even in South Florida, those crawlspaces would never be blocked off, either intentionally or unintentionally by mother nature. Yes, over time the vents may have become blocked by paint, but that also happens to attic vents - that is where the home inspector (if no one else) points out and explains the problem and the solution.

The code permits things which are allowed ... it is up to the contractors to - here we get back to what I mentioned earlier - use their common sense and construction basics (basic construction knowledge).

Originally Posted by Ted Menelly

Anyway. Did I say how much I hate crawl spaces. Those dirty buggy, smelly, lions and tigers and bears oh my. Cannot stand the dam things.

Ted,

Aren't there scorpions and such under Gulf coast houses? If so, I don't blame you.

Originally Posted by Gunnar Alquist

Ted,

Aren't there scorpions and such under Gulf coast houses? If so, I don't blame you.

Scorpions, centipedes, black widows, brown recluse, roaches, occasional snake, opossums, rats, bob cat once, wet clay, leaking sewage, lions and tigers and bears oh my.

Gunnar
If you have a copy of ther 2006 UMC or 2007 CMC check Chapter 7: Combustion Air. This diagram may also help as there has been some changes (many) in this chapter.

Originally Posted by Jerry McCarthy

Gunnar
If you have a copy of ther 2006 UMC or 2007 CMC check Chapter 7: Combustion Air. This diagram may also help as there has been some changes (many) in this chapter.

Jerry Mc.

No, I don't. Time to buy the CBC. I was just so frustrated when CA did not use the IRC that I refused to buy the latest code.

However, your post does not completely address the question brought up by the Code Check flip book It looks like I was not as clear as I had originally thought. In this particular home, the vent openings are into the wall that separates the lower floor from the foundation crawlspace. The lower floor is on a slab and the main floor is above part of the lower floor and partially above a sloped foundation crawlspace. No vents into under the lower floor since it is a slab. Vent openings are into the wall. As I said earlier, too small and not low/high enough. But the problem that I had was this note in the Code Check (IRC edition) book that indicated both vents could not go into the crawlspace area even though I could not find the reference in the IRC.

Originally Posted by Gunnar Alquist

But the problem that I had was this note in the Code Check (IRC edition) book that indicated both vents could not go into the crawlspace area even though I could not find the reference in the IRC.

Gunnar,

Nothing in the IRC or the Commentary to the IRC prohibits *both* openings from going to the crawlspace.

This is the code and part of the commentary, which lumps crawlspaces and attics into the same "allowed" category.

G2407.6 (304.6) Outdoor combustion air. Outdoor combustion air shall be provided through opening(s) to the outdoors in accordance with Section G2407.6.1 or G2407.6.2. The minimum dimension of air openings shall not be less than 3 inches (76 mm).

Commentary (underlining is mine)

- This section provides two methods for providing combustion air from the outdoors: the traditional method by means of two direct openings or ducts to the outdoors and a newer method by means of one opening or duct to the outdoors.

- Openings to spaces that are naturally ventilated with outdoor air, such as attic or crawl spaces, are considered as an acceptable alternative to a direct connection to the outside. Attic and crawl spaces can be acceptable sources of combustion air only where such spaces are provided with adequate natural ventilation openings directly to the outdoors. Attic and crawl spaces ventilated by mechanical means are not an acceptable source of combustion air.

So Gunnar. in our area we have this company ( clean crawl space ) when they come in and button up the sub-structure to a home to control all moisture. when they block off the air vents they are in turn blocking off the lower air to any type of (heater/water heater) units. Not good.

If its not one thing its another.

Best

Ron

Originally Posted by Ron Bibler

So Gunnar. in our area we have this company ( clean crawl space ) when they come in and button up the sub-structure to a home to control all moisture. when they block off the air vents they are in turn blocking off the lower air to any type of (heater/water heater) units. Not good. If its not one thing its another. Best Ron

Ron,

I agree. I see very few of these installations and have not seen one installed in a home that had an interior furnace. However, the more recent installations that I have seen have left the vents open. The plastic is still sealed to the foundation stemwall and mid-span support piers and a sump pump is located under the membrane. I still do not think that they are worth the money.

Gunnar Alquist;63431]Jerry Mc.

No, I don't. Time to buy the CBC. I was just so frustrated when CA did not use the IRC that I refused to buy the latest code.

Gunner, if you want to view the current code on line go to "www.iccsafe.org" click on the eCodes block at the left then click on the Free Codes block at the left of the page. Many of the states, inlcuding Cal. are listed on the site. You can't search, copy, or print, but you can look up as if you had the hard copy.