No Chimney? No Problem. Alternatives To Unvented Gas Heaters

Jan 9, 2002

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The Bottom Line Direct vent gas heaters are sealed combustion systems bringing in fresh combustion air through an outer pipe and exhausting through an inner pipe. Tidy, safe, and low cost.

As a very vocal opponent to the idea of installing unvented gas space heaters in homes, I'm often asked what are the alternatives, if you don't have a chimney or an easy way to run a gas vent system through the house.

Let's make one thing clear, right from the start, adding a sidewall vent system to your home does not cost an arm and a leg. They are extremely cost effective, they are safe and they can be made to look attractive or completely hidden. Those who promote unvented gas heaters would like consumers to believe they are saving a lot of money and trouble by avoiding them.

The most common sidewall gas vented system is known as a coaxial vent system. Simply put, a 4" diameter pipe within a 7" diameter pipe. It can be either rigid or flexible pipe depending on the make and the installation.

I'm referring to these as sidewall vented products, even though most of them are also approved for rooftop venting if desired. For example, if the unit were going into an upstairs bedroom, it may be simpler to extend the vent to the roof, rather than through the sidewall of the home.

The truth is, the added cost of the installation is usually no more than one to two hours of labour and material costs to the contractor are less than $100.00. Many manufacturers include everything required for sidewall venting in their adjustable vent kits which are supplied with the appliance. I mean everything, from vent pipes to the outside wall mounted vent termination kit, including spacers to keep the inner and outer vents equally spaced, screws and a tube of high temperature sealant for any joints in the vent system. In the end, the inner exhaust pipe and the outer air intake pipes are sealed tight, so that all the homeowner gets into the home from the heater is the heat. All the contractor needs to provide are the tools and the labour.

There are essentially two types of gas appliances that use coaxial vent systems. For those who either don't have a chimney or don't want the added expense of building one from the ground up. Freestanding and Zero Clearance.

Both of the following types are designed and approved as room heaters and are required to carry certified output capacity ratings on the nameplate. Although there are some that use this vent system, and are specifically designed to be decorative units, producing almost no heat, only effects flames, if that is the homeowners preference.

1.) Freestanding Direct Vent wall furnaces come as either a simple space heater or designed to resemble freestanding wood burning stoves.

a.) If all you want is heat, there are literally hundreds of makes and models to chose from. Wall heaters with top or rear, sealed combustion vents, with input ranges from 15,000 Btu's to up to 100,000 Btu's. Often used in mobile homes, trailers, cottages, etc., where floor space is limited.

b.) Freestanding gas stoves, designed to look like a wood stove. Most come with input rates of between 25,000 and up. The heat output on these many of these products can be very similar to a wood stove, only more controlled.

The great thing about a gas heater that simulates a wood stove is that the black pipe coming off the top looks like an authentic wood stove chimney. However, it's simply black pipe, sealed at each joint and the outer rigid black pipe is conducting air into the firebox. Inside the outer pipe is of course, the 4" diameter flexible aluminum exhaust pipe.

With one of these products the installation locations within the home are almost limitless. It can be simply mounted near an outside wall and vented up and out. It can be located in a corner or in the center of the room and the vent system can almost always be strung to the nearest outside wall. There are a few limitations, but I've found 90% can be fit in where the homeowner wants it.

2.) The other is a Zero Clearance built-in type wall furnace/fireplace. Following the manufacturers instructions and installing the gas unit against an existing wall and framing it in. The vent system is usually disguised by a wooden mantel or even a bookshelf above the fireplace. The final product looks like it came with the house.

Within this category the sky is literally the limit with regards to whether a large or small fireplace is desired. Btu input is again, available from low to high. All come with adjustable gas valves and circulation fans with rheostats that give the homeowner total control over the heat output, wall thermostats and or remote controls can be added or just connect it to a simple wall switch.

They all come with easy to operate, safe, push button ignition systems. The newest ones on the market have doors that are so easy to open for a quick wipe down, no tools are required. Often just a lunch box style clasp to unbuckle and swing the door open. If the unit has been set up properly, you probably won't have to do this more than once or twice per year.

These products are ideal if you have an outside wall that you don't mind having a small vent box protruding a few inches from it. The variety of designs available from rear to top vent, from narrow to wide, and with respect to the outer casing designs will suit any living room, bedroom or family room. The only limitation is your own imagination as to how this unit may be finished off, once it is installed. I've seen plain walls turned into massive, field stone finished fireplaces and the effect is remarkable.


Most direct vent heater units today are 80% or higher in steady state efficiency and since they don't consume air from within the home for combustion, or communicate with the air within the home in any way. They are considered among the safest gas products on the market today.

Another misconception, that is promoted by the "unventless-free" mob, is that a vent in a fireplace means there will be a constant heat loss from the home up the chimney. With the sealed combustion, coaxial vent systems, there are no affects whatsoever on the heat loss or the air quality of the home. They are completely neutral with regards to the home environment. You certainly can't claim that with an unvented space heater.

The fact that they are isolated from the home envelope means a higher annual fuel utilization efficiency (AFUE) is achieved, as required by the U.S. Department of Energy (DOE). All direct vent gas fireplace room heaters on the market today, "exceed" the DOE efficiency requirements for a minimum AFUE. That's a fact.

Another aspect that the unvented proponents tend to ignore is that the outside air supply requirements for "unvented" space heaters, installed in a room, will require more fuel consumption to offset any openings that may be needed to ensure all of the oxygen isn't consumed.

These direct vent products are subjected to some of the harshest safety tests in the world. Any sealed combustion system that has a glass front door (I.E. a fireplace heater) is subjected to pressure tests to ensure the entire firebox and air supply system are sealed as tight as a drum. They are also subjected to severe delayed ignition tests. By severe, I am referring to a series of progressively worse tests allowing gas to accumulate within the firebox for from a few seconds to up to a minute and purposely igniting raw gas. These tests are designed to ensure product integrity, and no portion of the appliance may be damaged or projected into the living space, including of course, vent system and the glass door.

Having witnessed some pretty powerful explosions under controlled lab conditions, I beleive that most of these products are built tough and are safe. If they're not, they simply don't pass national standards set for them.

They are all equipped with pressure relief mechanisms which will safely relieve pressure within the firebox before allowing damage from delayed ignition. With fast acting thermocouples in the pilot flame, the pilot will drop out within four seconds of flame failure. The odds are very slim that you will ever see it used. It's just nice to know it's there. It's also a good thing to check the relief door is properly shut and sealed after the installation is complete.

Almost none of these systems require electrical power for operation, with the exception of the circulation fan. They use standing pilots for flame ignition and millivolt gas controls. Millivolts are generated by the pilot flame and open and close the main gas valve on demand from either the thermostat, remote control or the wall switch controlling the fireplace. These products will generate quite a bit of heat in a power outage, even without the circulation fan on.

There you have it. A basic outline of some pretty nice, safe, and cost effective alternatives to slowly suffocating your family with an unvented gas space heater.

To vent or not to vent, I don't see the question?


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