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Old 10-03-2009, 10:20 AM   #1
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Combustion air from the attic


I'm installing a 199k btu tankless water heater in a utility closet in the basement. I need to supply makeup air from the outside for a unit this size. I'll be drawing this air from the attic, so I guess I will need seven round 3" ducts running up my exterior wall to the attic:

Required free area of duct:
199,000 btu/4000 btu/inē ≈ 50 inē

Area of 3" duct cross-section:
1.5ē*3.14 = 7.077 inē

What are the consequences of my 2"x4" bottom and top plates to that degree? Would I be better off punching a 12" hole or two 8" hole through my foundation? My only concern is the potential to freeze my plumbing if I put that kind of hole in the wall. Our lows in winter regularly hit the teens. We even go negative once in a while.

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Last edited by benjamincall; 10-03-2009 at 02:05 PM. Reason: math error
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Old 10-03-2009, 10:52 AM   #2
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Combustion air from the attic


I think the hole in the wall will be safer. Just having enough duct area is no guarantee it will work in ALL conditions. If you get a negative pressure situation the water heater will not be able to draw air properly and may backfire and have flamerollout. In my 31 yrs I have seen all kinds of strange drafting situations/flamerollouts/spillage of fumes etc. I would buy a small electric wallmount electric heater for the utility room and set it for 40degF. It will only work when needed and is a very good backup system to protect your plumbing. Combustion air pipes are ALWAYS best done with a minimum of bends/elbows/restrictions. If not done properly CO poisoning becomes a serious concern. If you do the hole in the wall you need to have a hood outside w/o a screen or it will get plugged with spider webs/leaves and create another problem.

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Last edited by yuri; 10-03-2009 at 11:00 AM.
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Old 10-03-2009, 11:58 AM   #3
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Combustion air from the attic


What about an automatic intake damper hooked up to the 24v on my Rheem water heater?
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Old 10-03-2009, 12:50 PM   #4
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Combustion air from the attic


Are you putting in a commercial powervent waterheater or a true tankless heater? If it is the latter and you have the "closet" fairly well sealed off from the basement interior put in a blower on the incoming 4" induction air line from the side of your home. Run the 4" line down to about 12" off the floor in the closet. Run a second 4" "closet exhaust line" up and out the side of your home next to the incoming line. Check the schematic for your tankless unit to see if there is a point where either the control 24VAC or the induction fan (in the heater) 120VAC line is switched. The safest way would be to put in a DPST contactor of the appropriate voltage (control) and wire your Induction Blower AC power to the contacts. This way the blower won't over load the circuit in the heater and the burn chamber will have all the air it will need whenever the heater calls for heat. Insulate around the pipes coming through the wall out to about 2' from the wall if you are worried about freezing things. After that the basement room temperature should be sufficient to keep frost from forming inside the pipes.

Last edited by Grampa Bud; 10-03-2009 at 12:58 PM. Reason: forgot something
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Old 10-03-2009, 01:35 PM   #5
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Combustion air from the attic


Whenever you start adding "accessories"/dampers/homemade fans to any gas burning appliance you stand the chance of voiding any AGA, UL safety ratings and having a problem with house insurance if there is a fire or other problem. All appliances are only certified for safety as they are designed standalone. Interlocks can fail and I have seen furnaces and a rooftop unit catch on fire because a centrifugal switch failed to shut off the burner. NO safety device is foolproof.
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Old 10-03-2009, 01:36 PM   #6
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Combustion air from the attic


It is a tankless. Wouldn't a fan take that room down to 5° during an exceptionally cold winter while someone's filling my 215 gallon tub? I think my expansion tank and my water softener might have some trouble at those ambient temperatures. What do you think about the damper idea? I believe 100 inē is the required unosbtructed cross-section area for the duct in this application.

Last edited by benjamincall; 10-03-2009 at 01:54 PM. Reason: Math error
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Old 10-03-2009, 01:39 PM   #7
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Combustion air from the attic


Check my post above yours. You should get a licensed gas fitter to check the gas code book to be sure for that specific type of appliance. I always used 1 sq.ft for every/100,00 BTU's in a sealed room for natural draft appliances.
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Old 10-03-2009, 01:41 PM   #8
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Combustion air from the attic


Quote:
Originally Posted by yuri View Post
Whenever you start adding "accessories"/dampers/homemade fans to any gas burning appliance you stand the chance of voiding any AGA, UL safety ratings and having a problem with house insurance if there is a fire or other problem. All appliances are only certified for safety as they are designed standalone. Interlocks can fail and I have seen furnaces and a rooftop unit catch on fire because a centrifugal switch failed to shut off the burner. NO safety device is foolproof.

I wouldn't actually add anything to the heater itself. I thought Bud was talking about a fan directly into the utility room. *edit* I just realized that you're referring to the electrical connection.

Last edited by benjamincall; 10-03-2009 at 02:01 PM.
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Old 10-03-2009, 01:53 PM   #9
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Combustion air from the attic


Quote:
Originally Posted by yuri View Post
Check my post above yours. You should get a licensed gas fitter to check the gas code book to be sure for that specific type of appliance. I always used 1 sq.ft for every/100,00 BTU's in a sealed room for natural draft appliances.
At 1 inē/2000 btu for horizontal vents, I would need a single 12" vent or two 8" vents. If I followed your first suggestion, do you think the cost to run the electric heater all winter would be exorbitant? Would I be better off just selling this one and buying the direct vent version?

Last edited by benjamincall; 10-03-2009 at 02:20 PM. Reason: math error
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Old 10-03-2009, 02:06 PM   #10
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Combustion air from the attic


Your computation for the size of the pipe is incorrect. You computed the area for a 3 inch radius pipe, which is a 6 inch diameter pipe. So based on your computation, you need two 6 inch diameter pipes, not two 3 inch diameter pipes.
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Old 10-03-2009, 02:25 PM   #11
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Combustion air from the attic


Yes that is true for normally aspirated units; however a tankless heater has a powered air induction/exhaust system for the "sealed" combustion system. If this heater is in a small closed off closet with closed door the sub-zero temps will be warmed by the radiant heat of the heater and/or the bulk of the air blowing into the closet could be consumed by the heater or blow right back out of doors. Look at your power-vent water heaters of hot air furnaces. Normally aspirated units ALWAYS have need for larger air/vent/exhaust openings. Power vent heaters and furnaces are most often found with a 2" exhaust. For longer runs both vertical and horizontal (say 30' or more you may require 3" or 4" exhaust and the manufacturer may require intakes of the same size). As for combustion air being blown into the small room, it's being blown right back outside and through the heater fire box, plus it only comes on when it is requested by a call for heat. This is an accepted procedure for most heater/furnace installations and has been done for years all over the country.
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Old 10-03-2009, 02:25 PM   #12
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Combustion air from the attic


Quote:
Originally Posted by Daniel Holzman View Post
Your computation for the size of the pipe is incorrect. You computed the area for a 3 inch radius pipe, which is a 6 inch diameter pipe. So based on your computation, you need two 6 inch diameter pipes, not two 3 inch diameter pipes.
I was actually correcting as you wrote. That error trickled through everything.
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Old 10-03-2009, 02:30 PM   #13
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Combustion air from the attic


The trouble is, this isn't a sealed combustion unit. Maybe I should just try to resell this unit and buy a direct vent model.
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Old 10-03-2009, 02:36 PM   #14
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Combustion air from the attic


That may be a better and more efficient and safer unit and idea. I mentioned getting the info for a "specific" appliance from the code book as the newer direct vent and other units have different venting requirements. Plus the gas inspectors are very picky about that and actually read the install manuals. US code may be slightly different than Canada and your local codes prevail also.

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