Double-pane windows routinely have a date - a specific year such as an 88 - stamped on the seal between the panes of glass.

Architectural styles and neighborhood age can give a general idea, without an exact year. This may take a bit of training and research, however.

Other places to look for date stamps are the backside of drywall, the hinge sides of doors, occasionally cabinets, and on electric, water and gas meters.

If the original furnace and hot-water heater are installed, they have serial numbers with the last two digits of a given year hidden in a series of other numbers. Don't confuse this with the ANSI date, which represents the year applicable standards were adopted. That date is always a year or two earlier than the age of the actual equipment.

If you are looking at a really old house, you can pretty well be assured that if it has a brick foundation (as opposed to concrete block or poured concrete) it was built before 1906. The San Francisco earthquake of that year drastically changed the way foundations were built.

Q: Why does my off-white carpet have a brown stained area almost a half inch wide along the perimeter in many rooms, especially the stairs? The carpet cleaners can't figure it out, but say they see it all the time.

A: Those stains are dust from the air moving through the edges of the carpet because of pressure changes inside the house. These pressure changes in essence turn the interior of your house into a low powered vacuum cleaner that sucks dust in and out from the exterior, attic or crawlspace. The reason could be the exterior building shape, surrounding buildings, or trees and land, but more likely the cause is the way your heating and ventilating system is set up.

Interior pressure is induced by roof shape and prevailing wind, influenced by terrain. This phenomenon also occasionally can make proper venting of fireplaces and gas appliances difficult. Attic fans can create negative pressure inside a home, as well. Almost every house with forced-air heat has unducted return-air plenums. Joist and stud cavities often are used to transfer return air from interior return grille to furnace. No actual piping is used, just a cavity.

Don't believe me? Take a flashlight and peer down your return-air grille; you'll likely see pan metal or drywall sealing off the cavity. These cavities can be large or small and theoretically are sealed off to be airtight. In reality, the cavities are not airtight because of poor seams, unsealed electrical and plumbing holes and so forth (insulators will sometimes stuff them full of insulation, rendering them ineffective!).

The strong blower fan on the furnace will pull air from whatever source is easiest; many times it is through these openings and gaps. If you have a return grille under your stairs, it is likely the entire cavity under the stairs is a plenum; air is pulled from around the edges of the carpet, under the stairs and down into the furnace. Every floating dust particle in the air is filtered through your carpet on its way back to the furnace.

A partial solution is to stop the air flow around the edges of your carpet. The best way to do this would be to seal your sill plates at their wall junctions with foam or caulking after pulling the carpet back. Leaving a couple of windows slightly ajar in the house would also reduce interior pressure changes.

Ask The Expert answers readers' questions every Saturday. Send questions to Ask The Expert, Seattle Times, P.O. Box 70, Seattle, WA 98111, or call 206-464-8514 to leave your questions on Ask The Expert's recorded line. E-mail address is dhay@seattletimes.com

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