The plans are laid out for your next energy efficient house. You will spend thousands of dollars on insulation, tight windows, insulated doors, air sealing and the latest heating equipment. And there in the living room sits the fatal flaw. It’s the open hearth fireplace. Whether it’s masonry or metal, an open hearth fireplace is the Achilles heel of an energy efficient home. It’s a health hazard, too.

Negative Efficiency

At best, an open fireplace is no more than 20 percent efficient. While operating, open fireplaces draw from 90 to 400 cubic feet of air per minute. In a 3,000 sq. ft. house, this massive flow would force the heating system to re-heat all the air in the house three times every hour. That’s why operating fireplaces may actually have a negative efficiency.

The situation isn’t much better when the fire dies down, because you must leave the damper open until the fire is totally dead. Dampers often stay open all the time. You might as well leave a window open. Even when closed, dampers could have as much “leakage area” as all the windows and doors combined. Energy analysis standards often ignore these losses. So your house really isn’t as energy efficient as you think.

Health Hazards

Combustion generates a number of toxic gases, including carbon monoxide, benzo-a-pyrene and respirable particles. The biggest danger is from “backdrafting.” This happens when the flue gases from a combustion appliance back up into the house instead of going up the flue. It’s like breathing car exhaust.

In a tight house, the suction created by a typical exhaust fan can be strong enough to cause a fireplace to backdraft. Clothes dryers and downdraft cooktops can pull a suction on any house. Even when there’s no fire, air often flows into the house through the fireplace, carrying with it emissions from the flue deposits, such as creosote. To protect the occupants from these harmful gases, you must totally isolate the combustion chamber from the living space. This simply can’t be done with an open hearth fireplace.

Attempted Solutions

Over the years features have been added to open fireplaces to reduce these impacts. Dampers are installed, but seldom used. Outside air inlets to the firebox reduce the amount of heated indoor air used for combustion. Glass screens separate the firebox from the room.

Glass screens never fit tightly. In fact, manufacturers build them leaky to keep the glass cool. They purposely avoid tight screens that would force them to comply with federal regulations for “airtight” wood burning appliances. Loose fitting screens can’t stop harmful gases from leaking in and heated air from leaking out. Without tight screens, the combustion air inlet is just another air leak in the house.

Off-Cycle Problems

Since fireplaces tend to be used so little, dismal heating efficiency isn’t the big issue. The real problem with fireplaces is what happens when they’re not burning. A fireplace could generate air leakage and hazardous gases anytime. Even worse, non-operating fireplaces could cause other combustion devices to backdraft. These problems should make you carefully consider putting an open fireplace in your next house.

Solutions

In a relatively tight house “make up air” is in short supply, and backdrafting is going to happen. The safest, most sensible course is to avoid the problem. Here are several options.

• Don’t install open fireplaces.
• Never install a combustion device unless it is “sealed” combustion. That means all the air used for combustion and draft comes from the outside. Don’t install “gas logs” and gas log starters in open fireplaces.
• Never install an open fireplace in the same house with a “negative pressure” ventilation system. (Most general ventilation systems fall into this category, because they use a centrally located exhaust fan with fresh air inlets located around the house.)
• Look into electric fireplaces that use Hollywood-quality illusion to create a convincing fire.
• If you must have an operating fireplace look into one of the new models certified by the US Environmental Protection Agency for efficiency. These are specially designed “air-tight” woodstoves that fit in masonry or wood-framed walls. They look very similar to traditional fireplaces.