Ten characteristics of successful stove and fireplace systems

A perfect woodburning system builds strong draft quickly so kindling a new fire is always easy, not frustrating. Smoke never spills into the room, and cold smelly air never comes down the chimney when no fire is burning. In short, a perfect system is a pleasure to use. It is the kind of system you want in your house.

Very few things in this world are perfect, but it is valuable to define perfection because until we know what it looks like, we'll never know what to strive for. By defining perfection in stove and fireplace systems, we become less accepting of flawed systems because the flaws tend to stand out more.

The following list of design characteristics looks simple enough, but underlying these ten elements is twenty years and over a million dollars in research effort. I have spent part of my thirty year career in the hearth industry with research scientists trying to understand the theoretical basis for successful venting and with chimney sweeps and retailers reaping their insights from thousands of hours of observations in the real world. Only by combining the science with the practical knowledge was it possible to devise such a simple ten-point list.

When choosing the design of your hearth system, consider the extent to which it strays from perfection. That is, assign a demerit point for each characteristic that does not conform to perfection. Think of each item on the list as a "driving" characteristic that induces the flow of air and gas up the chimney rather than down the chimney into the house. Think of each flaw as an "adverse" characteristic that compromises the system's ability to perform successfully. Here is the list of characteristics that defines perfection.

ONE The chimney runs inside the building envelope (inside the heated space) so air and flue gases stay at least as warm as the air in the house until they are expelled outside.

TWO The chimney penetrates the highest part of the building envelope so the chimney always functions better as a chimney than the house does, even when there is no fire burning.

THREE The chimney is tall enough and its top is clear of obstacles to wind flow so it can produce stable draft and it has a chimney (rain) cap because without one any chimney is vulnerable to adverse wind pressures.

FOUR The chimney flue is insulated and is the correct size for the appliance so flue gases are kept warm and flow quickly through the system.

FIVE The flue pipe (if there is one) runs straight up from the appliance to the chimney and the chimney has no offsets because each change in direction presents resistance to flow.

SIX The appliance and venting system are reasonably well-sealed because leaks introduce cool air and big leaks make the system more vulnerable to adverse pressures.

SEVEN The stove or fireplace is EPA/CSA certified for low smoke emissions or has equivalent characteristics (like masonry heaters do) so it is unlikely to smolder because smoldering appliances are much more likely to spill smoke.

EIGHT The system is installed in a house that may be tightly sealed but has a balanced ventilation system. The alternative, exhaust-only ventilation, cause houses to be constantly depressurized and is disastrous for chimney vented systems.

NINE There is no large exhaust fan (like a downdraft kitchen range exhaust), or if one is present, it is electrically interlocked to a fan-forced make-up air system to prevent the house from becoming excessively depressurized when it operates.

TEN The appliance is operated by an informed user because the best of designs can be disabled by improper operation and a lack of maintenance.

You may have noticed that an outside air supply does not appear on the list. There is a good reason it was not included. While there is some anecdotal evidence that providing combustion air from outdoors can help in certain situations, there is no solid scientific evidence to suggest that outdoor aired systems are any less likely to spill smoke than are appliances that take their combustion air from the room. In developing the list, I have only included characteristics that consistently and reliably contribute to successful venting, so outside air does not qualify.

With houses becoming increasingly well sealed, it is more important than ever that hearth systems consistently produce strong draft so that, even when not operating, air will flow up the chimney rather than down. As you plan your appliance and chimney system, strive to reach perfection by meeting all ten design objectives. Perfection is neither easy nor common, so it is an achievement worth celebration. In fact, you will probably celebrate its perfection each time you use it because your woodburning system will work flawlessly.

Aiming for perfection is a worthy goal because it results in systems that do not spill smoke when operating and do not spill odors when not running. If every system were perfect, people would love their hearths even more than they do now.