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Combustion air in single car garages

Chris Bernhardt

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Every new house with a single car garage that I have seen lately with a gas furnace and hot water heater hasn't had any combustion air source vents from the exterior or anywhere else. The cubic feet of these single car garages wouldn't be sufficient under the 50 cu/ft per 1000Btu rule. Has something changed in the code to allow them to do an install in single car garages with out vents from the exterior?

I know there are some complicated looking air leakage formulas in the code. Are they some how using these?

Chris, Oregon

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Hi Chris,

I write them all the time and, so far, haven't had a builder call me back screaming about it. I also write up the use of single-walled exhaust vent material in unheated garages. I think it's a common screw-up among builders that, once they're educated about, they correct immediately and don't forget to do next time.



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Absolutely no change in the code

A rare problem in my area because of the AHJ's, but I would call that out as a significant/major safety hazard.

From the Carbon Monoxide Safety Analyses I do, including draft testing, I can tell you that in a two car garage, it takes (roughly, with a lot of variation) about 10 - 20 minutes for the lack of adequate combustion air to start showing up with insufficient draft and shortly there after, rising CO levels in the flue gases.

(Any rising CO in the flue gases after a warm up period is a major concern.)

Visual clues that this is happening are signs of condensation at the flue connections, and, in severe cases, evidence of condenesation draining back into the draft inducer (I'm assuming atmospheric draft and 80+ furnaces.

(Note, the condensation marks can also result from the lack of a post purge cycle in the draft system after the flame goes out - so the diagnosis isn't a slam dunk, visually)

Also check closely under the water heater bonnet for debris/rust.

I've attached two pics with evidence of flue condensation problems. Neither arose from this situation, and bioth are more severe than what this situation would typically show, but they point out what to look for.

(The water heater pic was a beauty - not only did the flue picth the wrong way, and have insufficient rise for the run, and run single wall through an unhconditioned space, but it ran the flue through a sidewall, discharging horizontally with a dryer vent flap over the discharge!)

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Hmmm, attaching pics, Let's see beard> navel> (little html joke there!)

Of course, the furnace pics 01 and 02 (overview and closeup) speak for themselves. Using the rule each 90 turn = 5', it starts with over 20 of effective horizontal flue - it's gonna take a stinking high chimney to provide sufficient rise/height for that run!

[On Edit] Thanks, Mike. I really like the resizer in powertoys, but I do most of my image editing in PsP

On second edit - hmmm, I have them down to 99k and 95k

On third/fourth or something edit: Voila!

Download Attachment: icon_photo.gif DraftProblemsFurnace01.jpg

29.84 KB

Download Attachment: icon_photo.gif DraftProblemsFurnace02.jpg

28.92 KB

Download Attachment: icon_photo.gif DraftProblemsDHW.jpg

32.42 KB

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Hey RJ,

Thanks for your input on this post. That's some of the most useful and relevant information I've come across in a long time regarding gas furnace stuff, assuming you're not blowing hot air (yes, pun intended).

I assume some of your background is in HVAC stuff?

Also, from what sources do you rely in diagnosing "proper" venting. My research always comes up with confusing, convoluted, complicated formulas and trade jargon with which I'm not familiar?

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My background includes (i) practicing int'l trade law etc for more than a decade (definitely gives me a hot air expertise [:-crazy] - I call myself an attorney in recovery...) (ii) various areas of residential construction before and after the aforesaid.

I've been doing inspections about a decade - 3200 + inspections.

I've taken the Carbon Monoxide Analysis course from the Building Performance Institute (twice, about 7 & 6 years ago - I don't know if they still offer the course - it took me two times to really start to get it) Bachrach's Carbon Monoxide testing course (about 5 years ago) and the National Comfort Institute's Carbon Monoxide Safety Analysis and Combustion Analysis course, and just did my biannual recertification with them.

I don't have numbers on the # of CO tests I've done - probably 400 +/-

My views on and knowledge about CO and drafting and combustion is based on that training, that experience, and careful consideration of what my testing results tell me. There are also a few good heating contractors in my area from whom I've learned a lot. (And too many who don't have a clue.) And careful reading of every technical and installation manual I can get my hands on

Almost all of my experience is with residential and light commercial natural gas furnaces and water heaters, as well as nat gas stoves and gas log sets and space heaters.

I have some experience with propane, and very little with oil burners.

Fortunately, the staute of limitations has run on some of my early inspections when I'd just stick the probe in the register, somehow thinking that an equipment catalogue photographer would be a good authority for CO testing procedures.

FWIW, a now somewhat outdated protocol for CO Safety testing can be read at http://home.att.net/~cobusters1/coprotocol.htm

Please note - reading that protocal without knowing the underlying principles will not qualify folks to really test properly, in my opinion.

And the protocol doesn't include combustion analysis, which adds significant information for understanding what's going on and what a combustion devices performace and problems are.

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Originally posted by randynavarro

Also, from what sources do you rely in diagnosing "proper" venting. My research always comes up with confusing, convoluted, complicated formulas and trade jargon with which I'm not familiar?

Rule 1: the GAMA venting tables are "guidelines:" the producer of the tables states the "designer" of the venting system is responsible for determining the effectiveness of the system as designed.

Specifically: the tables disclaimer:

LEGAL NOTICE: This information is, in part, a result of research performed by Battelle under the sponsorship of the Gas Research Institute (GRI). Neither GRI, members of GRI, nor any person acting on behalf of either:

a.) Makes any warranty or representation, express or implied, with respect to the accuracy, completeness, or usefulness of this information, or that the use of any apparatus, method, or procedures disclosed may not infringe privately owned rights;


b.) Assumes any liability with respect to the use of, or for any damages resulting from the any apparatus, method, or procedures disclosed

And the software used to develop the tables states:

"The user should realize that no safety factors have been included in the VENT - II calculation routines for vent capacities. The results will be reasonably accurate for new, well defined vent configurations under transient-cycling or steady - state conditions. However, the field performance of vent systems can be adversely affected by installation peculiarities, outside wind conditions, house tightness, leaky vents, other exhausting appliance, partial blockage of vent pipe, or other conditions. Therefore, appliance vent designer should use good engineering judgment in making allowances for the various perturbations which may occur in the field.

Have you ever run into a combustion appliance operating under "steady-state" conditions? Ever seen a vent that cannot possibly experience "perturbations?"


My venting diagnosis starts with the exterior, looking for rust on the B-vent. Any rust gets a mental note - "significant" rust will be called out. I will usually mention the rust to the client, saying "it raises the possibility of venting issues, but we have to see what's going on inside. I just wanted to mention it so if we see something inside you'll remember what I'm talking about." (I want to be careful to not freak people out unnecessarily. I do the outside first with the client -when rust is there, this is a great confidence booster, especially if/when the furnace or water heater show signs of venting problems, which they usually do if there's rust above the roof.)

Also, outside, orientation/clearances of course, but "meeting code" isn't a 'safe harbor'

In a development I'm also looking at the condition of protruding Bvent on neighboring houses

Or checking the inside of the flue on a chimney for signs of condensation.

At the combustion appliances, I'm looking for [any signs of condensation - on the flue, esp the joints, on and under any draft inducer, or "significant" rust inside or under a drafthood.

Depending on what I see, I might pull out the draft gauge (Dwyer #460 Air Meter. $35 -40)) But, if I do, I've sentenced myself to (I) creating winter conditions (all doors and windows closed) and (ii) a lot of running up and down stairs to operate exhaust fans, whole house fans, thermostatically controlled attic fans, and also to open and close the attached garage door and any down wind entrance door - all to see if the draft is affected.* (when "dad" is there or some other wanna be inspector or irritant, this is a great excuse to get him out of your hair!)


* As a general rule of thumb, if there's a whole house fan, a standard sliding patio door has to be at 18" open before the water heater stops back-drafting. A few need the door completely open


Also, once you pull out the draft gauge, be prepared for arguments with HVAC guys who think "smoke" is a way to test for draft. (Last year I had an HVAC guy - big company in my area - hold a match in front of a drafthood -which had no measurable draft with my gauge- blow it out, blowing towards/into the drafthood opening, and then telling me it was "drafting fine. Look at the smoke." It wasn't until afterward that I though of the comment I should have made: "I've heard of blowing smoke out of your ass, I’ve never heard of blowing it into a drafthood!")

I don't charge extra for simple draft testing - maybe I should.

In other cases, I might recommend adding a full scale CO Safety and Combustion Analysis.

That involves an additional charge.

Remember - the venting tables are guidelines. they are usually a good indicator, but not always.

I'm interested in performance.

My testing over the years has taught me that most times (but not always) when I see the visual indicators (rust and condensate stains) or just a stupid installation (like a water heater T'd (not Y'd) into the furnace flue) I'll usually be able to demonstrate a venting problem.

And even if I can't show a venting problem at the time, well, weather conditions change perturbations occur, and those changes and perturbations can affect performance.

There's a 10 yr+/- house that belongs to a woman at my church. 80+ furnace, b-vent flue. The flue meets the applicable venting tables. (Just barely, but it does) I've tested it on a number of occasions for my own education. Most of the time it's fine. When the wind is 'firm' but a bit gusty from the NW, however, it doesn't vent properly. (in fact, it barely vents.)

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I know that this is thread drifting into venting issues. Although I have never been hit with a complaint concerning vent issues after reading some things lately I have gotten nervous. This is certainly one area that you want to be careful about punting to an HVAC contractor with all due respect to the ones that know what they doing. I want to know the correct way things should be vented and what to look for. I see evidence of condensation on or in venting fairly often but its seems to never be addressed by the HVAC guys when I come around on a reinspect but what I have read is that you are really not suppose to have any if your venting properly.

Chris, Oregon

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I recommed taking the CO and Combustion Analysis course from National Comfort Institute. Jim Davis teaches it. He's the fellow who "invented" the whole filed. I don't agree with everything he says, but he is a character with tremendous knowledge.

There is an (incomplete) list of heating contractors at www.NationalComfortInstitute.com who have that training.

The "correct way?" The way which works. Start by following code, which means following the venting tables, which goes well beyond any SoP I know of.

Or start by knowing the basics and knowing what to look for and, perhaps, using a draft gausge. And use your judgment.

FWIW, I would consider the furnace visual indicators I posted above as mild, but needing attention. The water heater - definite problem which mucst be fixed.

When you see signs of such magnitude, though, you'll usually see also obvious mistakes, like the flue wrapping around the filter slot

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I should have posted these with the last post. I support my comments about the lack of sufficient air for combustion in garages etc. with the following articles that I e-mail to my clients along with the report.

JLC Article - Makeup Air For Combustion Equipment

JLC Article - Venting Gas Appliances

The first is free, but the second will cost you $2.95 unless you are a JLC Online Member (Cost - $59.95 a year without the CD, $99.95 with the CD). When clients are armed with these and hand them off to their builder/contractor, it always seems to work.



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Wow, RJ. Lots of questions for me, but so little time (and space).

One at a time. Rust on the b-vent above the roofline: from what I'm hearing from you, when you see rust, its a cause for concern?

Unless I see some obvious, visual violation (too many elbows, bad flue size etc) I just write it off as "wear and tear". IOW the moist, warm air from the vent hits the cold outside air. This will contribute to condensation and begin to rust any metal, regardless if galvanized or not.

Rust isn't simply a wear and tear issue?

Edit: perhaps we move this topic to begin another thread?

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Hi Randy,

I'll try and explain what's happening. If I screw it up, Rob will straighten my raggedy-ass explanation out. Just realize that I'm not much of a scientist, so I probably will screw this up.

That rusted vent above the roof line is speaking to you. It's saying, "Hey, look at this. The exhaust gases leaving me have cooled so much that they've practically lost all buoyancy by the time they've left me. You need to check this out when you go inside and check out the furnace and water heater, as well as all of the exhaust venting below the roof line that's attached to me. You need to try and figure out why this is happening or refer me to someone who can."

The exhaust gases are acidic. If those exhaust gases are hot enough, they'll have plenty of buoyancy when they leave that vent and will rise rapidly away from the roof and the portion of vent exposed above the roof will look great. However, if the exhaust gases in that vent cool too rapidly and lose too much buoyancy - either because the vent is too restricted, has too many bends, is a single-walled vent in an unheated location, There's a bad inducer, or something is causing the furnace to back-draft, etc., by the time they leave the pipe they are being forced out of the vent by gas below and are cooler than the air immediately around the vent. So, instead of rising well clear of the vent they immediately settle around it. That exhaust air, the vapor cloud one often sees hanging lazily around a vent like a halo in cold air, contains acid. That acid reacts to the zinc in the galvanizing on the vent pipe and that reaction causes the vent to corrode on the inside surfaces, producing the white mineral salts that you see filtering down into furnace or water heater from above or leaking out of joints in the vent. That halo cloud hanging around the outside of the vent does the same thing, but the mineral salts are rinsed away leaving the rusted vent pipe. That's why the vent pipe is speaking to you.

Hope this makes sense.



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If you are on the street and the gas vent is from a man larger than you, I would suggest: "Pardon me, but was that your shoes scuffing on the concrete?"

Or in a small dimly lit bistro and the gas vent is from an attractive woman in the next booth: "Isn't is delightful how aroma therapy lifts our spirit?"

"it" is all in the writing!

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