furnace exhaust vent / combustion air


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Old 11-24-06, 10:30 AM
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furnace exhaust vent / combustion air

Had a problem this morning with my furnace. My CO alarm went off. I immediately opened some windows and turned off the furnace and hot water heater.

So with everything off, I inspected the furnace and water heater. Everything looked OK, but there was a draft coming in from the vent. The vent comes down the chimney and connects to both the hot water heater and forced air furnace. Draft was coming in at both water heater and furnace.

Went up on the roof and removed the cover from the vent (cover to keep debri out - lots of air flow). Could look straight down, no issues with vent pipe.

I opened a basement window and got a nice strong updraft... I should mention that before I bought the house, a radon mitigation system was installed. The fan runs all the time. Also, I have done a fairly good job of sealing around the basement windows and misc. areas. None of these improvement are new, but I think that perhaps I have made my house too air tight? Even worse, with the radon fan, I am pulling a vacuum?

So, I replaced one of the basement window panes with plywood and put a 4" pipe through. I have two elbows - the one on the inside turned up and the one on the outside turned down. I also put very course screen (1/2" mesh) on the outside to keep critters out.

1. Does my theory make sense?

2. Does the vent I have meet code? There are no local codes. Everytime I call the building inspector his response is that there are no local codes - use the internation building code.

3. Anything I'm missing or should do?

4. I did a quick check - with everything off (hot water heater, furnace, radon fan) I get pretty good airflot from the new vent. Why?

I am going to have my setup looked at - I just want to do my homework. Oh and by the way, the CO alarm is not registering anything now...

Thank you!
 
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Old 11-24-06, 11:16 AM
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CO alarm

It is entirely possible the house is too tight. With the increases in energy cost, this is something I encounter more & more all the time. You should have at least one square inch of opening to the outdoors for each 2000 btu/hr input to the furnace & water heater. These ratings can be found on the furnace & water heater data plates.
An alternative is something like Field Controls' Fan in a Can. You might find this interesting: http://fieldcontrols.com/cas.php
 
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Old 11-24-06, 03:16 PM
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Thank you for the reply. I do have one comment/question though...

In doing a little research, I question how my house ever passed inspection... Any way, I am short on meeting the requirement for combusion air using the standard calculation of 1 cubic foot of volume per 15 btuh. I have 140,000 BTUH = 9334 cu ft and my basement is about 7980 cu ft. So, I can calculate how much I am short = 1354 cu ft. This is equivalent to 20,310 BTUH. Using 2000 BTUH per 1 sq in, I need 10.16 sq in = 3.6 in dia. This is less than th 4 in that I put in, but I did use wire mesh, so I think I'm just about right.

I have been considering finishing my basement also - when I do, I'll make the furnace and water heater area into a "mechanical room" and install a fan to bring air from the outsie.
 
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Old 11-24-06, 05:54 PM
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Inspection

Don't know about where you live but in my county, there is no inspection of the heat & a/c except for the City of Dover.
 
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Old 11-25-06, 05:24 AM
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How old is your furnace and H2O heater ?? You could always replace them with high eff and then you would be fine.
 
 

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