Combustion air and/or exhaust air makeup
I have a home that was built this year – mostly just code standard for air tightness and insulation in Minnesota.
The question i have is that we have a fresh air inlet duct (~5″) that drops into our basement – seems unneeded with our appliances and windows/tightness.
Have a high eff furnace with piped combustion air, gas power vented water heater, no kitchen hood exh, gas drier and 3 small exhaust fans.
I can turn on all my fans and i notice no change in the amount of air coming from duct. Plus i read things like
https://www.greenbuildingadvisor.com/blogs/dept/musings/designing-good-ventilation-system
” Do I need passive air inlets?
If you do install an exhaust-only ventilation system, don’t bother installing passive fresh air inlets in the walls. Fresh air will find its way into the home through random cracks. “
Is there a recommended method besides a j-trap, bucket, large foam enclosure/trap? Had originally thought of a small backdraft damper… weighted to near neutral (so if dryer and some fans were all on…it would open…). Or a “cape” backdraft fabric – which wouldn’t allow quite as much passive/wind/stack airflow ….would allow flow if there was a high demand for makeup only? Or just close off almost entirely?
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Replies
Nick,
The type of outdoor air duct you describe -- a 5-inch-diameter or 6-inch-diameter duct bringing outdoor air into a basement -- is sometimes required for atmospherically vented furnaces or boilers (or a collection of atmospherically vented water heaters in a multi-unit apartment building), especially if the combustion appliances are located in a small, tight mechanical room.
If you are sure that your combustion appliances (furnace, boiler, and water heater) are sealed combustion units, it's a pretty safe bet that your outdoor air duct isn't necessary. If I were you, I would cap it -- or better yet, remove it and do a proper job of sealing the rim joist.
He does have a gas dryer.
I didn't think code required a make-up air vent when all the appliances were direct-vented or power vented. Is dedicated make-up air still required under those circumstances in MN?
There is a rationale for make-up air requirements when you have atmospheric drafted burners, but the same rationale is rendered moot in sealed combustion or power-drafted heating appliances, given that there is no combustion air to be made-up. IIRC you only need make-up air if the thing is pulling more than 200cfm out of the room and it's a fairly tight room. I doubt your water heater pulls anywhere near that much, but a gas fired clothes dryer might. You might still need make-up air for kitchen exhaust hood volumes too.
Aaron,
Trust me: just because you have a gas dryer, doesn't mean you need a 5-inch-diameter hole in your building envelope, funneling in cold outdoor air 24/7.
Some friends just moved into a new half-duplex that has those
intake ducts into the mech closets. It's nuts: the duct enters
the basement wall 15 - 20 feet away and runs along the "step"
of the foundation, uninsulated bare metal, and finally drops
to a screened "can" on the floor near the furnace and DHW.
It supposedly has a motorized damper on it that opens whenever
one of the appliances lights its fire, but how well sealed could
that be? And the mech closet has louvered doors, so while I
haven't had a chance to evaluate this lashup in cold weather
yet I expect it'll be a major contributor to stack-effect flow
into the place whether the appliances are demanding or not.
The idea was nice -- bring cold combustion air in near where
it'll get consumed instead of relying on infiltration -- but
the implementation is a bit of a forehead-smacker. Especially
considering the mech closet *is right on the exterior wall*.
Why did the builder choose to run that distance? Some
required separation between exhaust and effective intake?
_H*
Thanks for the input guys - i'll have to dig a little into the MN Code and play with amount it is open (zip tie) for a little while. Might even do some testing with a digital mag to see if my pressure even changes for the better with it open vs closed (with exhaust fans on).
My guess is my houses pressure won't change with it open vs closed.
Here is just a snipit about MN code...
Q. What are the highlights of the make-up air requirements?
A. Make-up air is necessary to replace air removed by exhaust equipment. Lack of sufficient make-up air can cause depressurization, which can result in increased garage infiltration, performance problems for other mechanical equipment and callbacks. Contractors will now be able to manage equipment compatibility, choices and costs during the planning process.
To easily compare multiple scenarios, use our Mechanical Code GuidelinesTM software, or refer to the worksheet and reference tables in the Minnesota Mechanical Code.
Q. What are the combustion air requirements?
A. Combustion air is often confused with make-up air, but they are completely independent of each other. Combustion air is the air needed for space heating, water heating, and for fireplaces to turn fuel into heat. The combustion air requirements reflect nationally accepted methods and the latest industry research. To simplify calculations, use our Mechanical Code GuidelinesTM software, or refer to the worksheet and reference tables in the Minnesota Mechanical Code.
http://www.centerpointenergy.com/services/energymarketing/contactces/3669d5e120074110VgnVCM10000001a10d0aRCRD/
seems old (2004...?)
Trying to make sense of some code info - training PDF... makeup/combustion air starting around page 283, example just like my house page 304.
https://www.dli.mn.gov/ccld/PDF/ResCodeTraining.pdf
They appear to estimate building leakage...then total exhaust (135cfm dryer, 80% of biggest exh fan)....
So in my case 80% of around 80-100cfm (broan cheapy) and a dryer...so 215cfm total minus 550leakage... giving a negative demand for makeup. So none needed.
So my house should be fine without.... wonder why they put them in regardless of conditions... ugh
Even if I get conservative and say two exhausts are on and the dryer; plus reduce the natural infiltration factor to the next lower (.09cfm/sf instead of .15) then I am still good on the makeup (295exh vs 333leak)
A house with 2700sqft finished and 1000 unfinished.... would .09cfm/sf be a fair number to use on a 2013 code built house? They did a decent job sealing from what observed, plus i added a few tubes of caulk and cans of spray-foam along the way.
thanks for any insight. Zip tied it most of teh way last night. Have a a year or two before I finish the basement, and get to decide if i want to remove/seal off the entire inlet at the wall.
Nick,
I'll repeat my earlier advice: since you have sealed combustion appliances, you don't need this outdoor air duct.
If you want to verify that your bathroom exhaust fans and clothes dryer are working, go outside and place your hand near the exit ports. If you can feel air blowing out of the building, they are working. By definition, if air is blowing out of the building, then an equal volume of replacement air is finding its way into the building, through various cracks.