80% efficient furnace with 2 makeup air intakes?


Old 01-20-13, 07:18 PM
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80% efficient furnace with 2 makeup air intakes?

Hello everyone!

I recently purchased a new digital humidistat with external temperature sensor to replace the existing manual humidistat in order to try to improve the humidity level in my dry Colorado home without overflowing the humidifier on cold days. While looking for the most convenient route to run the sensor wire outside, I discovered that my 20 year-old home has two fresh air intakes. (While I can't find the efficiency rating of my RUUD UGDG-15EARJR furnace, it's appearance and installation method strikes me as an 80% efficient model.)

The first intake is what I would expect for an 80% efficient furnace installation... a 6" metal duct that comes into the basement from outside at ceiling height, runs along the ceiling until it gets near the furnace, and then takes a 90 degree bend downward toward the floor, stopping about 6" off the concrete.

The catch is that I also have a second 6" metal duct that comes in parallel to the first... and stops at ceiling height.

Has anyone seen this before? I really don't want to dump unnecessary cold air into my basement from ceiling height. (And I can't even implement the bucket trick up there, due to the tight quarters in which the duct ends.) I'm tempted to block off the end with some insulation and rely on the first duct only for fresh air and pressure equalization, though I suppose I could put dampers on both ducts.

Additional info: This is a 2 story ~2800 sq ft house plus partial basement that also has central AC, a gas water heater, gas fireplace (that I never use because the builder-grade unit looks like 3 Bunsen burners when lit) and a whole house fan (that I operate only after opening windows and/or doors before turning it on). Neither air intake duct has a damper in it. I also had a lot of insulation work and air sealing done on the house earlier this year, so its substantially tighter than it was originally.

Many thanks for any insight!
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Old 01-21-13, 07:25 PM
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It is doubtful that 6 inches is enough combustion air.
If you can find your installation manual, look it up. It is tough to find Ruud and Rheem installation manuals online.
Old 01-21-13, 10:03 PM
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Thanks for the response, Houston204!

My question wasn't so much about why there was a second pipe, but rather why the second pipe stopped at ceiling height instead of running down to within 6" of the concrete like the first.

That said, I still took the opportunity to check the math based on the installation manual. Making the assumption that the "15" in my UGDG-15EARJR model ID corresponds to the 150,000 BTUH size unit, then I need a 6.9" diameter inflow for vertical pipe (1 sqin per 4000 BTUH) or a 9.8" diameter inflow for horizontal pipe (1 sqin per 2000 BTUH). Ignoring the vertical run get from 6" off the floor to the ceiling, I have about 6 ft of horizontal pipe to get outside.

So that confirms the need for the second pipe and that blocking it off entirely would be a bad idea, but the crux of my question remains... why stop the second pipe at ceiling height? The horizontal sections of both pipes are next to each other, so it isn't like they couldn't run both verticals next to each other.

Just looking to see if there might have been a conscious decision behind the implementation...

Old 01-21-13, 10:14 PM
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That sounds like a combustion air setup. I would not plug them unless you have a contractor come in and verify otherwise.
Old 02-05-14, 07:57 PM
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Me too

I'm curious if you ever found your answer to why you have two intakes. I live in Lafayette, CO and have the same situation - low efficiency furnace with a high and low 6" intake. Any idea why I couldn't add a 180 bend to that low pipe to bring it up to 3' or 4' to create a cold air trap and reduce draft?
Old 02-05-14, 08:21 PM
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I have an 80% gas furnace unit that replaced an old 65% unit (too many problems with intake and exhaust of a 90+% unit). My make up air comes into the utility room from a foil flexible wrapped duct that takes in air from the uninsulated open attic area. It dumps into a 5 gallon plastic pail to create the siphon break. This is a very common system in our area and seems to work.

Old 02-05-14, 08:53 PM
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Bthomas and Dick,
the real function of the bucket and 180° up-turn is to add resistance to the path and to help mix the cold air with the warm to make it less objectionable. Neither actually acts like a trap because once the cold air starts flowing down the duct it is the 600 pound gorilla and will continue to flow.

As for the high low configuration, it is probably following the wording of the vent requirement. But, technically, the high vent has less pressure across the envelope and thus less air flow. The low vent will have a greater air flow because the cold air inside will make its effective envelope penetration at an equivalent floor level pressure. When the duct makes a 180 it adds resistance. Any vertical distance upward will simply reduce the effective pressure.

Electronically controlled dampers (not cheap) can open and close as needed. As for letting in cold air, far better to direct it into the furnace room than having is seep in all over the house. It has to come in one way or another.


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