Author’s note: the following article on home heating is the third in an eight-part series. This article addresses climate conditions found in the San Francisco Bay Area, but may have applicability elsewhere. Forced air systems are the most common heating systems in California and are used in most new construction elsewhere. They have two big advantages: they are cheap to install, and they provide heat at a moment’s notice. Having “instant-on” heat is vital for intermittent use spaces like ski cabins. Otherwise, forced air is the least energy efficient and least comfortable way of heating a typical home. Why?
Ventilation and Heat Loss
For the health and well-being of its occupants, a home must exhaust stale air and refresh it with new air drawn from outdoors. Forced air systems heat and blow this air, via ducts, throughout your house. Since new air is continually entering and leaving, you are heating the outdoors.
There are ventilation systems specifically designed to make HVAC less wasteful. Heat Recovery Ventilators (HRVs) transfer heat from warm outgoing air to cold incoming air. (A comparable device called an Energy Recovery Ventilator can cool incoming hot air in hot-humid climates.) Running for some fraction of each hour, HRV fans typically exchange enough air with the outdoors to keep inside air fresh. However, HRV is rarely considered cost effective in our climate.
Even in the most air-tight houses, air is lost via unsealed combustion devices, bath or range exhaust fans, and the opening and closing of doors.
Ducts, Mold, Mildew and Rot
The ducting that moves the heated or cooled air is a network of hidden spaces that can collect and redistribute debris and moisture. When the heat cycles off, warm air cools and condenses inside the ducts, forming liquid water. The moist, protected environment is an ideal space for mold and mildew to breed. Broadcasting any dust, mold, mildew or pathogens growing in the duct work, forced air is often at the heart of Sick Building Syndrome.
Ducts also leak. PG&E estimates that the ducts in a typical home leak as much as 30% of the air they move. This warm, moist air can escape into an unconditioned space (for example, an unheated attic or crawlspace), cool, condense, and deposit water where you don’t want it and can’t see it, like your roof or floor framing. Subjected to enough regular wetting, this wood rots. When heated or cooled air escapes into unconditioned space, it heats or cools the outdoors, not the house.
Dry Sinuses, Skin and Eyes
Many consider forced air inherently uncomfortable. Since the blown air needed to heat the rest of the air in a room must be substantially warmer than the ambient air temperature, many people find that it dries their eyes, skin and sinuses.
As a homeowner, you have alternatives to forced air. See Thermal Mass and Radiant Heat.
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