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With colder weather here in Northern Colorado (down to 22° F) my Goodman 90% efficiency furnace (GKS90453BX) came on for the first time in the (ca. 2005) home that I recently moved into.

The furnace was blowing only very slightly warm air. I checked the filter and it is fine.

However, I did notice that the closet containing my gas furnace and gas water heater was very, very cold. Upon further inspection, I noticed two 4" vent pipes that terminate in the ceiling on one end and to the exterior of the home on the other end. (See photos.)

Are these possibly artifacts from an optional 80% efficiency install? Can I close them off? The closet doors are bi-fold with a gap at the bottom, so I imagine there would be enough fresh air supply, but that is what I want to double check. Thanks!

UPDATE: I found the manual for the 40,000 BTU Water heater. It included the following instructions regarding combustion air availability. Provided I don't have a "tight construction" it appears that I can add combustion air via two additional 10" x 4" openings in the closet doors to communicate with the sufficiently volumetric adjacent inside air space.

Confined Spaces Confined spaces are spaces defined as having less than 50 feet³ (1.41 m³) of space per 1,000 BTU (.29kw) per hour of input.

Unconfined Spaces In unconfined spaces in buildings, infiltration may be adequate to provide air for combustion, ventilation and dilution of flue gases. However, in buildings of tight construction (for example, weather stripping, heavily insulated, caulked, vapor barrier, etc.), additional air may need to be provided using the methods described above under CONFINED SPACES: All Air From Outdoors or SPECIALLY ENGINEERED INSTALLATIONS.

All Air From Inside the Building: The confined space shall be provided with two permanent openings communicating directly with an additional room(s) of sufficient volume so that the combined volume of all spaces meets the criteria for an unconfined space. The total input of all gas utilization equipment installed in the combined space shall be considered in making this determination. Each opening shall have a minimum free area of 1 inch² (6.45 cm²) per 1000 BTU (.29 kw) per hour of the total input rating of all gas utilization equipment in the confined space, but not less than 100 square inches (645 cm²). One opening shall be within 12 inches (31 cm) of the top and one within 12 inches (31 cm) of the bottom of the enclosure.

All Air From Outdoors: The confined space shall be provided with two permanent openings, one commencing within 12 inches (31 cm) of the top and one commencing within 12 inches (31 cm) from the bottom of the enclosure. The openings shall communicate directly, or by ducts, with the outdoors or spaces (crawl or attic) that freely communicate with the outdoors.

  1. When directly communicating with the outdoors, each opening shall have a minimum free area of 1 inch² (6.45 cm²) per 4000 BTU (1.2 kw) per hour of total input rating of all equipment in the enclosure.

  2. When communicating with the outdoors through vertical ducts, each opening shall have a minimum free area of 1 inch² (6.45 cm²) per 4000 BTU (1.2 kw) per hour of total input rating of all equipment in the enclosure.

  3. When communicating with the outdoors through horizontal ducts, each opening shall have a minimum free area of 1 inch² (6.45 cm²) per 2000 BTU (.6 kw) per hour of total input rating of all equipment in the enclosure.

  4. When ducts are used, they shall be of the same cross-sectional area as the free area of the openings to which they connect. The minimum dimension of rectangular air ducts shall be not less than 3 inches (7.5 cm).

vent pipe photo 1

vent pipe photo 2

utility closet doors

closet overview

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    Does the new furnace have an air intake of its own? Are there any other fuel burning appliances in the closet? I see water lines, so I suspect there might be a water heater. – Tester101 Oct 15 '18 at 16:56
  • @Tester101 Yes, there is a water heater. Yes, the furnace appears to be vented. But dang is that closet cold. That can't be great for keeping the hot water hot, either. I added an overview pic. Thanks for your comment! – jboeke Oct 15 '18 at 17:14
  • You could treat the closet as unconditioned space. Insulate the closet, to separate it from the rest if the home. Insulate the water heater, and furnace duct work, to reduce heat loss. – Tester101 Oct 16 '18 at 0:33
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The furnace has the intake and exhaust of a sealed unit but the water heater is an old style open burner. The water heater requires the outside vent. The size of the vent is based on the BTU rating of the water heater. I would not close off the vent because this provides the fresh air for the water heater.

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  • I agree, but it seems like the total vent area could be reduced. Any idea how to make the calculation? – isherwood Oct 15 '18 at 19:05
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    Also, is a low-resistance flap an option? I faced the same dilemma with a rental home I had in Minnesota. The tenant kept blocking the vent as they didn't want to pay to heat the universe. – isherwood Oct 15 '18 at 19:06
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    The water heater usually comes with the instructions on how many square inches it requires, I haven't installed a tanked unit in years, in older homes they did not require external venting but now do, I want to say that the external fresh air source had to meet the size of the flue 3 or 4" in most cases but it has been two long to remember for sure. – Ed Beal Oct 15 '18 at 19:17
  • Thanks, @EdBeal. I'll try to find the install guide for the water heater. Will update and mark answered when I find out! – jboeke Oct 15 '18 at 20:32
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    This answer describes how to calculate the size of the required duct. But the references are old, and may have changed. – Tester101 Oct 16 '18 at 0:30
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The best way to determine if your home is "tight construction" is to close all the windows and outside doors in the house, hang a sheet of newspaper over that vent under the bifold door, and another over the gap at the bottom of the door, then watch the papers while somebody opens and slams the front door.

Also, in the more general case, for anyone else reading this, you can tell how tight your house is by closing all the windows and watching the water level in the toilet bowl as somebody slams the front door.

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  • Thanks! Your answer is helpful. However, I muddied the waters on this question with my update regarding tight construction, so I'll keep the original accepted answer to the original question. – jboeke Oct 18 '18 at 19:38
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    I concur - I did not answer your original question. – A. I. Breveleri Oct 18 '18 at 20:59

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