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Securing Safe Forced-Air Heating


Question: Before buying my home, I hired a home inspector. While his report listed various defects that were attended to, he found nothing wrong with the forced-air heating system.

After closing escrow, I called the gas company to resume the service and light the pilots, which had been turned off.

When they came by the house, instead of turning the furnace on they "red tagged" it, claiming that since the furnace is installed in a hall closet, its base needs to be sealed to the platform due to "combustion air." Could you please explain why the base of a forced-air furnace needs to be sealed?

Answer: When gas companies activate service to a home, they routinely consider a list of critical safety-related conditions, including (but not limited to) possible gas leaks, proper venting of exhaust, physical damage to fixtures, safe clearances between furnace components and combustible materials and compliance with combustion air requirements.

The safety implications of combustion air requirements are so vital that the Uniform Mechanical Code (the portion of the building code that regulates gas-burning fixtures) devotes an entire chapter to the subject.

In essence, combustion air requirements can be broken down into three basic categories:

* A gas burner must have an air supply sufficient to enable total combustion of the fuel being burned.

* The air supply should be drawn from a location that will not create a hazard to building occupants. For example, forced-air furnaces should not draw combustion air from a bedroom.

* When gas-burning fixtures are installed in confined spaces, such as closets, it is possible for exhaust fumes to mix with the combustion air supply. In such locations, it is essential that contaminated air not be allowed to mix with the heated air, which circulates into the dwelling. Contamination of the circulating air could expose the occupants to deadly carbon monoxide.

A simple way to isolate exhaust fumes from the circulating air is to seal the base of the furnace to the platform, as the gas company recommended.

The air blower beneath the platform creates a vacuum. If the base of the furnace is not sealed, air from the closet can be sucked through any unsealed cracks. If exhaust is present in the closet, the result could be fatal to occupants of the building.

Fortunately, prevention entails little in the way of effort and expense.

Simply apply a bead of caulking at the entire perimeter of the furnace base.

Use Tape, Not Screws, On Dryer Exhaust Duct

Q: A home inspector recommended repairs to the exhaust duct on my clothes dryer. He says the duct is too long and that the fittings should not be attached with screws. These recommendations seem unnecessary because the dryer works just fine. What are the legal requirements for exhaust pipes on clothes dryers?

A: The Uniform Mechanical Code sets some basic guidelines for the venting of clothes dryers. Among these are limits on the overall length of the duct and a prohibition on the use of screws at the duct connections. Both of these rules are intended to minimize airflow resistance, thereby assuring efficient drying, while preventing overheating of the appliance.

The maximum allowed length of a dryer exhaust duct is 14 feet, or shorter if the duct has more than one 90-degree turn. But strict enforcement of this rule is not always possible, because some floor plans do not enable venting to the outside of a building with only 14 feet of ducting. The layout of your home may preclude the possibility of strict code compliance.

The restriction against screws for fastening duct connections is important. When screws are used at the fittings, lint can accumulate on the screw tips, and this can eventually block the airflow. The best material for securing dryer exhaust fittings is duct tape.

Blinds Will Cut Solar Heat From Skylight

Q: After purchasing our new home, we found the living room too dark, so we installed a large skylight. Now the room is well lighted, but the skylight seems to act as a solar heater. Every afternoon, the room becomes unbearably hot when the sun glares through. We'd like to correct this and enjoy our room rather than wait for a home inspector to point it out when we get ready to sell. How can we attain a tolerable balance between heat and light?

A: A skylight can be an efficient and desirable source of passive solar heating, assuming you need the heat. If not, it can be a genuine nuisance. Fortunately, there is a practical and inexpensive means by which to moderate the heat input from your skylight.

Simply install a set of Venetian blinds or another adjustable window covering over the skylight opening. By varying the angle of the slats, you can minimize heat gain in your living room.


If you have questions or comments, contact Barry Stone through his Web site at

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