Air leaks in Tappan furnace.

[john_n]

The other day a friend of mine asked if I knew how much it would cost to have his ductwork cleaned. He said he and his wife were having serious allergy problems whenever they were at home. When I went over to look at the situation, I discovered that they had NEVER changed the filter! They have lived their about 3.5 years I believe. I got him an alergenic 3M filter to replace the old one. When I pulled out the old filter, I happened to take notice that the furnace was raised up on bricks at each corner. This raised the furnace 2.5" off the concrete floor. I believe the purpous was to raise the furnace to the hight of the reurn duct. The unit appears to have a 2.5" high unfiltered air leak around the entire open bottom of the unit. The house is slab on grade, and the unit is situated in a first floor closet. The closet has openings to the ceiling joist cavities and wall cavities With untold years of dust and possible mold ect.... The furnace is deffinetely drawing air from the closet, ceiling cavities, and probably the adjoining kitchen ae well. Now for my question, Should I seal the open space created by the brick risers, or is this space necessary for some reason? I think it is a big part of their problem, and I could easily cut some 2x4 blocking to fit between the bricks at the corners. Also thought about sealing any remaining small openings with some of that yellow handi-fill fiberglass insulation sold at the HD. Am I on the right track, or will this approach cause a problem with air flow or cause a fire risk? Air return is on the bottom, combustion chamber is in the middle about 2' from the floor, and hot air duct is up top. Btw, not sure of what size furnace it is, but the filter on the return is a 16x25x1. That gives about 400 sq in of filter area. I'd approximate the unfiltered opening into the closet, caused by the unit being raised off the floor to be close to 140 sq in. Just don't want to over strain the blower motor due to a lack of air flow. Thanks in advance for any advice you can offer.

edit for spelling

[ 02-01-2004, 02:59 PM: Message edited by: john_n ]

Wow.... The reason the furnace might have been raised is for safety. Gas furnaces need to be elevated 18-24 inches(depending on local building codes). This is in case of a gas leak the heavy gas will go to the floor and the furnace being elevated won't ignite it. You should have two sources of combustion air. One high and one low. It is against most building codes to take that air from the house, living space. Is there a underground return, is there a filter in the unit as well? If so it could have been raised to draw return air from two sides of the house. One from under the furnace and one from the underground duct work. Sometimes floor joists will be used as duct work. Either supply or return. A lot of those questions are hard to answer without seeing the situation. If you dont get any good replies call a HVAC company. A good one. Specializing in indoor air quality. If they can't explain all the circunstances good or bad call some one else.