As we enter the coldest months of the year, many of us are looking to feel the burn… from a fire, that is! One of the questions we are most often asked is: Can I put a fireplace in my high-performance home? If careful conditions are met, the answer is yes.
As is true for all Net Zero buildings, the energy efficiency of a BrightBuilt home is promoted through an air-tight, well-insulated envelope. The amount of warm air lost from inside the home to the outside is minimized, thereby significantly reducing heating demand and its associated energy consumption.
Air leaks and heat loss in a building occur in two ways because of indoor air pressure differentials. Most homes have a high-pressure zone in the upper half and low-pressure zone in the lower half. Warm air is buoyant and rises through the structure until it is trapped by the uppermost ceiling and walls. When enough warm air accumulates against these barriers, it will exert pressure on vulnerabilities in the envelope and force air to escape outdoors where possible. As this warm air moves upward and outward, negative pressure is then created in the lower half of the home. Cold air pressing on the outside of the home will enter via vulnerabilities in the lower half of the envelope. Through this cycle of hot air leaving and cold air entering simultaneously, the temperature indoors begins to drop.
A traditional masonry fireplace punches a hole in your home’s thermal envelope, and worse, usually in the lower half of it. It becomes easy access for all of the cold air outside to make a swift entry through the chimney. When a fire is burning, airflow in the flue is reversed. The hot air produced by combustion creates a draft that rises up the flue, and 80-90% of the heat generated is lost. Releasing this heat while the fireplace is in use is a good thing, as a well-sealed home will overheat much more quickly than a poorly insulated one.
Surprisingly though, heat loss is not actually the biggest concern when considering if, and how, to include a fireplace. Limited airflow in and out of a Net Zero home means that an ill-planned installation can also become extremely dangerous.
The issue is two-fold. Fire requires oxygen, and an open-hearth fireplace will pull the oxygen needed to burn from inside your home. In an airtight space, no new oxygen flows in to replace what is used during combustion, causing levels to drop quickly.
At the same time oxygen is being consumed, carbon monoxide is produced as a by-product of the incomplete combustion of whatever fuel is being burned (usually wood or natural gas). Some of this gas is inevitably released back into the enclosed room, but the rest tries to depart via the chimney. If working properly, this hot, polluted air will flow up and out. But if the fire is not burning hot enough, the chimney is obstructed, or air pressure differences are not in the correct balance, a backdraft can form and reverse the flow of air inside the flue.
High-performance homes are especially susceptible to this phenomenon because of their carefully controlled airflow. Most of the air exchange in a truly airtight structure is the deliberate movement of air out of the space via a high-powered exhaust or vent (stove vent, bathroom fans, etc). The movement of air out of the home, coupled with the depletion of indoor oxygen during combustion, creates such low air pressure inside that the pressure outside is actually greater. As such, the flow of air is reversed and blows down the chimney, even when a fire is lit. The result is a host of harmful pollutants – including particulate matter and carbon monoxide – moving into your space.
So what, then, is the answer? In the same way that Net Zero building is a reimagining of a home, just better, so are the options for clean, safe, and efficient fireplaces.
In a Net Zero home, instead of the standard open-hearth installation we normally envision, fireplaces need to be airtight with closed chambers that draw the oxygen needed for combustion from the outside – not the inside air you breathe. A room-air independent fireplace produces the same glow and warmth we know and love without changing indoor air quality. Intake and exhaust vents into the combustion chamber, as well as the chamber itself, are sealed as thoroughly as the rest of your home, making the fireplace less likely to serve as an unwanted point of air exchange.
These slow combustion, high-performance fireplaces come in many iterations and can run on fuel ranging from wood, to wood pellets, to solar-generated electricity. Airtight wood stoves are a stylish and easy addition to a home and offer all of the sensory delights afforded by burning wood. If you want to be truly green and operate with no emissions, opt for an electric fireplace. By far the most energy efficient option, electric fireplaces only lose around 1% of the heat they produce because they are in no way connected to the outside. They have also improved dramatically in the realness of the “flames” they produce and can be installed to assume the same visual presentation as a more traditional fireplace. Using electricity generated by your home’s solar array, this option can also operate entirely free of fossil fuels or the input of additional resources.
A final thought to consider: though burning wood may seem like an environmentally friendly way to heat a home, the process of combustion releases a significant amount of carbon dioxide - a greenhouse gas responsible for climate change - into the atmosphere. Trees sequester carbon dioxide from the air while living, but once felled and burned, that carbon is once again released. This process emits 75% more CO2 than burning natural gas, but raises the question: is the tradeoff of burning a high-emission renewable resource better than burning a cleaner non-renewable one? Certainly a much deeper discussion to be had…
For those of you stoked about fire but not about stoking the flames, we offer one final suggestion: there are some pretty convincing fireplace videos online and on most streaming services that can turn any computer or TV into the hearth you’ve always dreamed of. Just pretend the steam rolling off your hot toddy is actually from the flames.