I am in desperate need of advice on how to handle the concept of using the crawl space and space between 1st and 2nd floors for a plenum eliminating all HVAC ductwork. The home is insulated concrete forms, steel bar joists under steel decking and concrete floors. There is no wood in the home except for cabinets and mouldings. We are in an extremely mild climate in Sonoma Ca. I am going to try to eliminate AC. The issues that are leading me toward the rubber room, center around the inability of the Fire Marshal, Plan Checker, Inspector, Electrical sub, Plumbing sub, and the HVAC sub to arrive at definitive answers. What can go in a crawl space that is used for forced air?
The main issues revolve around: Plastic drain pipe, armor cable/romex or conduit, plastic pipe and fittings for sprinkler system, insulation, plastic water supply pipe, and vent pipe. I’v gotten answers on both sides of almost every material mentioned.
Has anyone heard of a definitive publication anywhere on this topic?
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Replies
Based on experience in my area...
In a commercial plenum ceiling return application, all items in the ceiling must be plenum rated. This impacts lighting fixtures, the type of low voltage exposed wire, conduit connections (compression) and the type of plumbing used (no exposed PVC pipe). I don't know how much of this applies to your area or crawlspace, but food for thought.
Using a crawl space for the return plenum means that you will lose any control of how much air is supposed to come from a particular space.
There is no way a "plenum supply" would fly around here, but homes are built from wood.
Some commercial projects involve a plenum floor, but the materials are all fireproof and there are the same additional materials issues mentioned above. Airflow is directed by removing tiles in the floating floor design. A standard residential fan system can't develop enough pressure to make it work.
If ductwork elimination was a goal, I would have chosen hot water/steam heat or mini-split heat pumps in the design phase.
Whatever your code officials decide, better get it in writing.
It appears that you're using the wrong term (or at least using it in an unfamiliar situation). Normally the "crawl space" is the area under the lowest floor of a building, surrounded by the foundation.
You're basically talking about using the ceiling space above a "suspended ceiling" as a plenum, similar to many commercial buildings. But commercial buildings are built to a much more fireproof standard, and, in addition, generally only use this space for air return, employing more or less regular ductwork for the supply side.
You might be better off with some sort of a box plenum scheme where some portion of the overhead area is boxed off for ventillation duty. Then all of the unpleasant mechanicals can be kept out of this area.
Also, how do you intend to handle the air return issue?
happy?
Thank youI am planning to pressureize the crawl space. The insulated concrete forms go to slightly below grade. I want to insulate the crawl space with 2" foam over the earth, then cover with an inch or so of concrete, making the whole crawlspace insulated to the same level as the house. The area between the first and second floor is 30" which is taken up by the steel bar joists. The plans call for 4 - 75,000 btu forced air heaters. Two upflow and two downflow. The HVAC contractor has used this concept on numerious applications with great satisfaction. The two sites that I visited were in fact commercial applications, and the owners were in love with the systems.The main outcome of this project is to maintain a uniform temperature in a very large house. The climate is very mild, and we want to use the forced air in the crawl space and the space between floors to not only distribute the 90 degree forced air, but to use the 4"concrete floors as heat sinks to moderate the temperature throughout the home. We are hoping that this concept would eliminate the hot and cold spots that large homes are notiourious for and use the fans for circulating cool night air during the few hot days we have.
The two sites that I visited were in fact commercial applications, and the owners were in love with the systems
Then have your HVAC contractor submit plans and specification for approval to your code enforcement authority, including the fire protection plan.
In using such areas as supply or return plenums you need smoke detectors in those areas that are interconnected to the air handling units. That is addition to the above mentioned ratings for other mechanical, electrical and plumbing items that enter the areas.
Get you plan approved first, and the inspectors won't have a problem with them. Doing it by the seat of the pants will cause nothing but confusion and misunderstandings.
Dave
Dave