Direct Vent versus Non-Direct Vent


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Old 12-05-07, 01:17 PM
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Direct Vent versus Non-Direct Vent

All opinions are appreciated,

I recently moved into a new construction house that has a Goodman GMS9/GCS9 gas fired furnace along with a Heat pump.
The furnace is set up for non-direct vent (only has the one vent flue for the exhaust). The intake vent opening is just has a 90 degree elbow sitting on it drawing air in from the unfinished basement that it is in. This is how the builder put it in and that is all that the code in my area calls for, I live in Southern Indiana if that makes a difference.
My question is, What is the pros and cons between Direct Vent to the outside and the way my current setup is? Does it make a difference the climate you are in? Would it make a difference if I finished off my basement and enclosed the furnace room?
Also it would be very easy for me to direct vent my furnace because the builder roughed in a 3" intake vent pipe that ends at the furnace but it is capped off outside currently and not in use. I asked the builder why this intake was not used and he said that it was not needed for this particular furnace but that I might need the intake for another type of furnace if I ever replaced it.

thanks,
Matt
 
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Old 01-09-08, 04:07 AM
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As per Mfr's installation manual, this furnace has an FSP Category IV vent and could be set for either direct "or" non-direct venting. If set for non-direct vent, all the installer is required to do is add a 90 ell on the combustion air inlet. Too bad the installer did not take the time to explain to you the reasons why he did what he did...but he's right...all that is needed is that elbow fitting.

There are a variety of pro's and con's on each procedure.
non-direct vent:
(+) the combustion air used by the unit is warmer than outdoor air [that's always a help]
(-) you may develop a negaive pressure in this space [where the unit is installed]
to name but just two.

When and if you decide to finish your basement and enclose the unit in a small mechanical room, calculations will then have to be made to see the options available to draw sufficient amounts of combustion air. This is to determine if your furnace would be in what is called "a confined space"
 
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Old 01-09-08, 07:01 AM
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You know, I've never seen a calculation of the efficiency difference between a CAT IV furnace supplied by internal/external air in a typical heating climate.

If anyone is aware of one, I've be interested in reading it...
 
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Old 01-09-08, 07:42 AM
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I'd also add:

Often, in new construction, the furnace has been used as a "construction heater" in violation of the manufacturer's installation instructions, which may void the manufacturer's guarantee.

At least here in Chicago when doing home inspections of new construction and condo conversions we frequently find that cat IV furnaces drawing combustion air from the interior have construction debris in the intake path - sometimes you you fire them up there is so much loose material in the inducer fan that it sounds like a handful of loose change tumbling around in a dryer.

Sometimes such material gets in in spite of the elbow, or because the elbow as installed as an afterthought - one more reason I'm suspicious of this combustion air configuration.
 
 

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