Combustion air duct install questions

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Old 09-22-03, 08:18 PM
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Combustion air duct install questions

I was told by a local HVAC dealer that I needed a duct for combustion air when I seal my furnace room. He wants to use 4" flex duct and run it through an unfinished soffit out the side of the house. He can't do it for 6 weeks and my sheetrock installers are coming in 2.

Is this something I can do myself?
 
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Old 09-22-03, 08:25 PM
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You should be able to do that part yourself. You will need to get the end of the duct to an area that is protected from rainwater and vermin. Other than that you can't go wrong. It may be more difficult to do what needs to be done at the furnace/boiler. If it is an oil system, there may be an accessory that needs to be added to the burner to accept the new duct. And then the burner needs to be re-adjusted to work with the resistance posed by the new duct. There is a maximum allowable length for the duct and a maximum number of elbows that can be used, but I will assume that your contractor has already considered that and your plan works within that limitation.

Ken
 
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Old 09-22-03, 11:07 PM
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Ken, what's the best way to terminate the flex duct on the exterior of the house?
 
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Old 09-23-03, 06:03 AM
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The best way is to use a dryer type hood with no flapper (of course) and put some kind of screen in it. Something with 1/2" openings. That will kep mice and birds out and not block air flow.
There are hoods made specifically for intake but you can do just as well with a vent hood. Look for the type called 'big mouth', the hood opening is a little larger and you would have more room to get the screen in there. Field Controls (no relation to me, unfortunately) makes a whole kit that also contains a vacuum breaker which would help if the hood ever got blocked. Look at this for example http://www.fieldcontrols.com/cas1-2.html

Good luck,

Ken
 
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Old 09-23-03, 07:12 AM
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Thanks Ken.
 
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Old 09-23-03, 08:08 AM
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vent

Like Ken said ,You didnt say Gas or oil ,how many BTU on this unit . You should have about 1"square for each 1k of BTU. So for a unit with 90,000 btu input you should have about 90"square of make up air if no blower on it. If that is uninsulated flex you use in the drop down ceiling wrap it with insulation so it dont sweat in the winter time,Or be sure and use insulated flex for this ED
 
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Old 09-23-03, 01:41 PM
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Ed, it is only for a 40,000 BTU gas water heater. So, according to your recommendation I would need 40" square - but I don't believe 4" flex duct will accomplish this. Am I correct?
 
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Old 09-23-03, 02:47 PM
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Only you and your contractor know how air tight your room is, but Ed's rule of thumb was correct from the past knowledge of gas & oil equipment. Newer burners will draw through 4" pipe or even 2" pvc. That doesn't necessarily mean that your water heater will be able to get enough air through the vent you plan on installing. I would rely heavily on your contractors recommendation and common sense. He shouldn't mind having you install the duct if he is as busy as you say he is.
 
 

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