Thread: Updraft furnaces

I don't usually see slab on grade up here, so I need some advice. I inspected two condos recently with different furnace configurations. They were both 75K BTU in small closets. Air supply was from underneath. Returns were noted in the rooms in both condos. One furnace closet had a 12 inch square grill in the closet door. The other didn't. The one that didn't had a small, 3" x 12" register inside the closet on the wall about 2 feet up. I couldn't see what function this grill was performing. Question is does the closet door without the grill need one?

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Greg, I don't follow what you have in full, but typically when I see a vent opening in a wall of the closet, it uses the stud cavity going to the attic to another opening there. Usually this will function as the lower vent with another high vent opening going through the ceiling to the attic. In this configuration, there is no vent on the door, all combustion air provision is from the outdoors via the attic.
Jim

Greg, sounds to me you have a problem with adequate combustion air for those FAUs.
IRC 2006:
M1701.1.1 Buildings of unusually tight construction. In buildings of unusually tight construction, combustion air shall be obtained from outside the sealed thermal envelope In buildings of ordinary tightness, insofar as infiltration is concerned, all or a portion of the combustion air for fuel-burning appliances may be obtained from infiltration when the room or space has a volume of 50 cubic feet per1,000 Btu/h (4.83 L/W) input.
M1702.2 Confined space. Where the space in which the appliance is located does not meet the criterion specified in Section M1702.1, two permanent openings to adjacent spaces shall be provided so that the combined volume of all spaces meets the criterion. One opening shall be within 12 inches (305 mm) of the top and one within 12 inches (305 mm) of the bottom of the space, as illustrated in Figure M1702.2. Each opening shall have a free area equal to a minimum of 1 square inch per 1,000 Btu/h (2201 mm2/kW) input rating of all appliances installed within the space, but not less than 100 square inches (64 415 mm2).
The Uniform Mechanical Code; Chapter 7 says basically the same for requirements of combustion air.

Thanks, Jim and Jerry!

West Coast Jerry,

That's if you are using 'all indoor air' for combustion.

If one vent leads to the attic, it all changes and there are more options available.

Then, of course, don't forget this sentence in there: "In buildings of unusually tight construction, combustion air shall be obtained from outside the sealed thermal envelope."

Then think about what new and newer construction is ... "buildings of unusually tight construction" ...

Therein lies the crux of the matter, regardless of the size of that area or combined adjacent areas, *it is still considered* "confined space" and, as the code states: "combustion air shall be obtained from outside the sealed thermal envelope."

East Coast Jerry
Not knowing the age of the condos is why I posted both codes addressing required combustion air in & out.
I've been retained on a case where a new 42 unit PUD has all its gas-fired FAU and WHs drawing combustion air from inside a really small mechanical closet in each unit and when the local bldg. inspector caught it doing a final she required the builder to cut a vent in the closet doors. I told the complaining party's legal team that all combo-air for the unit's gas-fired appliances needed to be drawn from the exterior. This is a three story building over post tensioned concrete slabs with 2 story below grade underground parking. The fix is going to cost the developer a bundle. Oh well?

Originally Posted by Jerry McCarthy

a new 42 unit PUD has all its gas-fired FAU and WHs drawing combustion air from inside a really small mechanical closet in each unit and when the local bldg. inspector caught it doing a final

Better late than never.

she required the builder to cut a vent in the closet doors.

Guess she has not been attending your code classes, otherwise, she would have know about it?

I told the complaining party's legal team that all combo-air for the unit's gas-fired appliances needed to be drawn from the exterior.

That's why I made my post above, to make sure that all HIs reading this thread were aware of it for new and newer structures which are "unusually tight construction".

Of course, then one much define "new and newer" as to a time frame, and, to me, that means when energy code became prevalent - in Florida, that would be back into the late 1970s to early 1980s, so figuring that anything from 1993 forward is "unusually tight construction" should be a good, solid, defensible date as 1993 is when Florida came out with a revised and updated Energy Code, one much more readable and understandable than the earlier versions, and, at the latest, 1997 which is when the fancy-smancy wording beating around the bush for air tight recessed lights actually came right out and addressed the recessed lights as being 'air tight' (that was always in there, but the wording left so much to be debated that it was air tight recessed lights was not within the thinking, and was almost unenforceable for those who were aware of its intent).

What time frame would you place for homes in California as being of "unusually tight construction"?

East-Coast Jerry
My 1985 UMC does not mention “Unusually Tight Construction.” Somebody borrowed and never returned my 1990 UMC, the 1994 mentions “Unusually Tight Construction” as does my 1997 UMC under Sec.701.3. Therefore, I suspect that the California Energy Codes kicked in the requirements for drawing combustion air for gas burning appliances from the exterior somewhere between 1990 and 1994. At least that’s my guesstimate.
Perhaps we have a few old West Coast code book collectors who could confirm?

Based on Florida's 'defensible' time period of 1993 (give or take) and California's 1994, I'll combine the two and say that *around the mid 1990's* is when energy codes 'most likely' began to apply 'unusually tight construction' methods.

Thus, a home of, say, 1995 and newer, should have all combustion air from the exterior ... as a rule of thumb ... which could be lopped off at any time upon proof to the contrary.

“Lopped off?” You must be a student of the French revolution of 1789? On second thought that may be an excellent solution to our on-going political problems. Let’s get rid of the entire entourage of political lackeys in DC including our president and appoint a benevolent king.