What makes a home seem "drafty" in the winter? Most people will say it's the cracks in the home, but how do they make a home drafty? Does the wind blow right through them? Hardly.

In most cases, especially homes having an older furnace located within the conditioned space, it's their fossil fuel furnace itself causing the problem!

How? Try this simple experiment: With the furnace "off," hold a lighted match in front of an electrical outlet, then have someone else turn up the temperature on the thermostat to make the burners light. In a matter of seconds, even before the blower kicks on, the flame will begin to flicker. That's due to combustion air for the furnace rushing through the cracks in the walls.

It takes approximately 15 cubic feet of air to produce 1,050 BTU of heat. That calculates to approximately 1,429 cubic feet of combustion air required to run a 100,000 Btu furnace for one hour.

If, during a typical winter day, a 100,000 Btu furnace runs 50% of the time, it will run for a total of 12 hours over a 24 hour day. That would make the amount of outside air required for combustion 1,429 cubic feet per hour X 12 hours, or 17,148 cubic feet.

That means that, in a home where the furnace is located within the conditioned space, on a typical winter day, the furnace is drawing 17,148 cubic feet of outside air into the home, and that's the source of a lot of complaints about "drafty" homes. The furnace is actually drawing 17,148 cubic feet of below-freezing temperature air into the home every day and expelling warm air, that the homeowner has already paid to heat and humidify, out the vent pipe!

I've often gotten a laugh when, after explaining this to the homeowner, I've said, "One way to cool down your during the winter is to turn on your furnace!"

Sealed system

A sealed combustion chamber combined with the two-pipe venting system of the super high-efficiency furnaces reduces draftiness by bringing all of the combustion air to the furnace through a sealed pipe.

The larger the furnace and the looser the home, the more it's needed and the greater the improvement.

This brings us to another major benefit of the "two-pipe" sealed combustion system - better humidity control.

The recommended indoor relative humidity level is approximately 35%. A high quality, correctly size humidifier can provide that, which helps to protect the home and furnishings from drying out, as well as providing health benefits.

Ten degree outside air with a 70% relative humidity drops to 6% relative humidity when brought indoors and heated to 728.

Thus, the sealed, "two-pipe" combustion system of the super high-efficiency furnaces eliminates the drying effects of infiltration by using outside air for the combustion process.

Occasionally, you'll run into an installation calling for a new double-wall vent system where it is not practical or downright impossible to install one.

Typical examples are older homes having old furnaces with unacceptable venting, hydronic or baseboard heating or even no central heat (there may be floor furnaces or wall heaters), where the furnace must be located in the basement and there is no chimney to use for venting.

Often, the price of a super high-efficiency furnace with a horizontal venting system is comparable to the cost of installing a lower cost furnace (that provides the homeowner with fewer benefits) requiring a double-wall venting system.

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