Installing a gas fireplace

Right-sizing

The height, width and depth of the existing firebox opening can limit your choice of insert, simply because altering the size of the firebox is a big masonry job. Fortunately, most fireplace manufacturers offer models in several sizes.

A gas insert also has to be right-sized-not too big and not too small-for the space it will heat. Too little output, and it can’t push out enough heat; too much and it will overheat the room, or cycle on and off too often to be efficient. While there are rough rules-of-thumb to estimate output needs, Steve Haagmans, eastern division manager for Miles Industries, manufacturer of Valor gas fireplaces, recommends against relying on them: “There are just too many variables-ceiling height, party walls, windows, doors and insulation levels.” Calculators such as the one at the Valor site, or at Hearth, include some of these variables and are therefore more accurate. For even more precision, a heating contractor can calculate a customized “modified heat loss” for your home.

But no matter what size the fireplace, don’t count on it to heat all the rooms in a typical house; it simply can’t move that much heat around that far.

Convection and Radiation

It’s not just heat output that counts, but how the fireplace transfers heat-by convection or radiation-to you.

Convection is heat transfer when air warmed by the fireplace insert mixes through the cooler air in the room. It happens naturally because warm air is less dense, so it rises and gets into a flow.

Radiation is what you feel at a campfire when your face is hot and your back is cold. A fireplace, like any heat source, beams out infrared rays. The rays don’t heat air much, but do heat the solid objects they “shine” on. For that reason, you shouldn’t put a thermostat where those infrared rays hit directly: the rays will warm the thermostat and fool it into thinking the room is warmer than it is.

Manufacturers often hype one method of heat transfer over the other. The inserts that are better at convection are promoted as providing an even heat throughout the room, like a forced-air furnace. Radiant inserts are promoted as feeling more like a real fireplace and warming you faster. In fact, it is not really an either-or choice between convection and radiation. All fireplaces deliver both; different designs just produce one more efficiently.

Look for features that help heat transfer, such as the materials in the glass front. Tempered glass doesn’t let infrared rays pass through nearly as well as pyroceramic glass (similar to Pyrex ovenware material). Channels around the firebox and the exhaust system encourage heat exchange from the fireplace to the air around it and convection into the room.

Fans also speed convection by pushing that hot air out and, according to Don Fuegler, senior researcher at Canada Mortgage and Housing Corporation (CMHC), do tend to improve efficiency. CMHC suggests you look for a quiet, “squirrel-cage” fan-the kind that looks like a hamster’s exercise wheel and is used in a forced-air furnace.

An insulated outer casing is another of the CHMC’s recommended features. It reduces heat transfer to the walls on the back and sides, so that more heat goes out the front.

Opportunities to Vent

Combustion needs an air supply, and almost all combustion produces waste gases that need to be vented safely.

Natural venting is like a traditional open fireplace-room air is used for combustion, with exhaust vented outside. This simpler venting system is fine for older, drafty homes, because there’s enough fresh air coming in to replace air going up the chimney. But in a tightly sealed, well-insulated home, combustion that uses room air can create negative pressure, sucking dangerous gases back down exhaust vents into the house.

The safer choice in tightly sealed homes is direct venting, where outside air goes into the firebox through one vent, while combustion gases escape through a second. The fireplace insert has a dedicated air supply, so it won’t suck air out of the home.

Ventless (or room-vented) fireplaces use room air for combustion and vent exhaust back into the room. They are not approved for use in Canada.