Thread: Is ducting required for central furnace?

Small 1986 home (around 1,000 sq ft) over closed and insulated crawlspace (not vented). 50,000 BTU downdraft central furnace (older style drafted exhaust) installed in closet near center of home. Heat registers installed in floors of rooms, but *NO* HVAC ducting from furnace to heat registers in rooms. So, the furnace pumps warm air into the closed crawlspace, which then passivly rises up through floor registers. I can feel the warm air at the registers OK, though without much force.

Is this allowed by code? I see lots of descriptions about how ducting should be installed, but nowhere do I see it clearly stated that ducting HAS to be installed (i.e. old floor furnaces from the 30's and 40's had no fans or ducting). With the furance located pretty much in the center of the house, I would think that each room gets a pretty even distrubution of the heat. But is there some loss of effectiveness or efficiency?

I suppose the good point is that the closed and insulated crawspace (a permanent wood foundation) is fully conditioned.

Similar Threads:

• Floor Covering Required?
• Sweeping Required?
• Furnace Age
• Temple man badly burned fighting house fire - Central Maine Morning Sentinel
• Vapor barrier not required w/ Hardi Plank?

Terry that is a new one on me, but it seems the entire crawl space just became a plenum and would have to be constructed as such, which would mean no wiring, plumbing, etc.

Jim, you must be up late.

Yeah, good point. There of course is a gas line running through the crawl to supply the furance and gas water heater.

Yep, up way too late. I have to go get my beauty sleep. I need it!

Terry - How is the return dealt with? Is there a return plenum, or is it just open at the side of the furnace? How about combustion air in the closet?

Our code requires ductwork, it seems out of date that there's nothing in your area for this.
It sounds like they should have installed 1 or 2 wall units to heat the place not a furnace. Having a crawlspace act as a plenum could pose serious health risks. Any contaminants (dead rodents, radon, gas leakage, mold, etc) would get distributed throughout the home. Beyond the stupidity of installation, I see the health risks being a great problem.

To answer John's questions, the air return passes through the living room wall directly to the furnace (no, it's not drawing return air from the closet or crawlspace). Yes there is a combustion/dilution air supply vent from the attic. I am not sure if it is a sufficient size for both the furnace and water heater (one 6" vent), but there is a supply.

As Jim and Marcus reminded me, yes, the crawl essentially becomes a plenum, and even though the crawlspace is unusually clean and well sealed with a plastic moisture barrier, any contaminants in the crawl will enter into the household air. The homeowner could potentially develop a chronic plumbing leak that would go unnoticed for some time, and could pass mildew or mold into the house. I recommened he talk to a HVAC pro, along with a reminder to include at least one vent for conditioning the crawspace (since it is sealed).

I had one of those a couple of years ago. It is called a plenum crawlspace. Besides the concerns with some dust, the crawlspace was the nicest one I have seen in some time. All of the framing looked new on the 20 year home, and everything seemed to work great. My crawlspace had a layer of plastic down with washed pea gravel on top. I researched it back then, and could not find anything wrong with the set- up. I just wrote it up as an atypical installation and noted the reason.

From the 2006 IRC. (underlining is mine)

- M1601.1.1 Above-ground duct systems. Above-ground duct systems shall conform to the following:
- - 1. Equipment connected to duct systems shall be designed to limit discharge air temperature to a maximum of 250°F (121°C).
- - 2. Factory-made air ducts shall be constructed of Class 0 or Class 1 materials as designated in Table M1601.1.1(1).
- - 3. Fibrous duct construction shall conform to the SMACNA Fibrous Glass Duct Construction Standards or NAIMA Fibrous Glass Duct Construction Standards.
- - 4. Minimum thickness of metal duct material shall be as listed in Table M1601.1.1(2). Galvanized steel shall conform to ASTM A 653.
- - 5. Use of gypsum products to construct return air ducts or plenums is permitted, provided that the air temperature does not exceed 125°F (52° ) and exposed surfaces are not subject to condensation.
- - 6. Duct systems shall be constructed of materials having a flame spread index not greater than 200.
- - 7. Stud wall cavities and the spaces between solid floor joists to be used as air plenums shall comply with the following conditions:
- - - 7.1. These cavities or spaces shall not be used as a plenum for supply air.
- - - 7.2. These cavities or spaces shall not be part of a required fire-resistance-rated assembly.
- - - 7.3. Stud wall cavities shall not convey air from more than one floor level.
- - - 7.4. Stud wall cavities and joist-space plenums shall be isolated from adjacent concealed spaces by tight-fitting fire blocking in accordance with Section R602.8.

Seems to me that M1601.1.1.7 and 7.1 fit and describe what the crawlspace *IS NOT* allowed to be used for - "supply air".

Originally Posted by Jerry Peck

From the 2006 IRC. (underlining is mine)

- M1601.1.1 Above-ground duct systems. Above-ground duct systems shall conform to the following:
- - 1. Equipment connected to duct systems shall be designed to limit discharge air temperature to a maximum of 250°F (121°C).
- - 2. Factory-made air ducts shall be constructed of Class 0 or Class 1 materials as designated in Table M1601.1.1(1).
- - 3. Fibrous duct construction shall conform to the SMACNA Fibrous Glass Duct Construction Standards or NAIMA Fibrous Glass Duct Construction Standards.
- - 4. Minimum thickness of metal duct material shall be as listed in Table M1601.1.1(2). Galvanized steel shall conform to ASTM A 653.
- - 5. Use of gypsum products to construct return air ducts or plenums is permitted, provided that the air temperature does not exceed 125°F (52° ) and exposed surfaces are not subject to condensation.
- - 6. Duct systems shall be constructed of materials having a flame spread index not greater than 200.
- - 7. Stud wall cavities and the spaces between solid floor joists to be used as air plenums shall comply with the following conditions:
- - - 7.1. These cavities or spaces shall not be used as a plenum for supply air.
- - - 7.2. These cavities or spaces shall not be part of a required fire-resistance-rated assembly.
- - - 7.3. Stud wall cavities shall not convey air from more than one floor level.
- - - 7.4. Stud wall cavities and joist-space plenums shall be isolated from adjacent concealed spaces by tight-fitting fire blocking in accordance with Section R602.8.

Seems to me that M1601.1.1.7 and 7.1 fit and describe what the crawlspace *IS NOT* allowed to be used for - "supply air".

I lived in a house in NC that had an enclosed ( Air Tight Door ) crawl space.

Can't remember if the Joist Spaces were covered with a sheathing material so they would be enclosed & not part of the Supply Air Plenum.