February, 2014
Inspection News and Views from the American Society of Home Inspectors

Gas Fireplaces and Gas Logs


Gas Fireplaces and Gas Logs

Although these are typically decorative appliances, some gas fireplaces do provide heat for rooms. Fireplaces with heat circulators and direct-vent fireplaces are more likely to be heating appliances. They may be rated as space heaters as a result. The heat circulators can be convection or fan-forced. The burner systems can be pilot- or electronic-ignition systems. There are three basic types of gas fireplaces. Inserts are designed to be fit inside existing masonry wood-burning fireplaces. This can be a full fireplace or gas logs. Factory-built fireplaces or “zero-clearance” fireplaces are typically built-in and may have a chimney or sidewall venting system. These units may be direct-vent, taking combustion air from outdoors. There are also freestanding units, reminiscent of woodstoves. In some areas, unvented gas fireplaces are permitted.

More like a Gas Space Heater than a Wood-Burning Fireplace
Gas fireplaces and gas logs may be installed in wood-burning fireplaces. However, gas fireplaces have more in common with gas furnaces and space heaters than with wood- burning fireplaces. Gas fireplaces designed to fit into masonry fireplaces are often referred to as fireplace inserts.

Glass Doors
Some systems have glass doors that cannot be opened. Others have glass doors that can be kept open or closed.

Listed, Certified or Approved
As with all gas appliances, you should be looking for a label that suggests that the system has been tested by a recognized agency.

Combustion Air
Combustion air for these devices may be taken from house air or from outside.

Combustible Clearances
Combustible clearances are typically smaller than for wood-burning fireplaces and determined by the manufacturer’s instructions on the label of the units. Combustible clearances around the openings of fireplaces can often be smaller than the six inches required for wood-burning fireplaces.

Gas vent terminating directly under the soffit is a poor location

Location Restrictions
Generally speaking, gas fireplaces are not permitted in bathrooms unless they are direct venting systems.

Vented gas fireplaces and gas logs must be vented either through a chimney or through a direct-vent system. Direct-vent systems often go out through a side wall rather than up through the roof. This reduces installation costs.

Damper Sealed Open
Where gas logs are installed in an existing fireplace, the fireplace damper must be permanently opened or removed so that the exhaust can go up the chimney.

Gas logs must be rated for use in a masonry fireplace. The fireplace itself must be a noncombustible unit properly designed for burning wood. Ignition Systems

Ignition Systems
Gas fireplaces and gas logs can either have electronic ignition or continuous pilots.

Remote Controls
Remote controls are available to turn the fireplace on and off and to control circulating fans. Fans may be manual or thermostatically controlled.

Venting Options
These systems can be vented through a conventional B vent or can be direct-vent systems with special venting. Single-wall metal liners in masonry chimneys are also allowed in some areas. Some systems with B vents have an automatic vent damper, similar to the efficiency upgrades on gas furnaces. These vent dampers automatically open when the fireplace comes on and close when the fireplace is shut off. Some systems are power-vented. The power-vented systems can be sidewall=vented.

Outside Air Options
Direct-vent systems have an integral outside air intake. Those with conventional venting systems may also have outside air provision. This is an option on some systems.

Combustible Clearances For Vents
Combustible clearance requirements for vents vary with manufacturer. In many cases, the required clearance is one inch or less.

Direct Venting With No Fan
Most central furnaces that are direc-vent systems use an induced draft fan to remove exhaust products from the home. Many direct-vent gas fireplaces rely simply on convection. There is no fan to induce exhaust gases to leave the building. Similarly, there is no fan to draw combustion air in. This all takes place on a convective basis

Heat Exchangers Built In
Some gas fireplaces contain a heat exchanger to help transfer heat into the house air. Heat is transferred into the house by a combination of radiation and convection. Gas fireplaces that act as room heaters claim efficiencies of 75% or even higher.

Typical Combustible Clearances
A gas fireplace might require a one-inch combustible clearance behind it, six inches at the sides, 36 inches out in front and 36 inches above. A typical clearance to a six-inch wide mantel shelf above the fireplace is four inches. Watch for evidence of overheating above the firebox.

Heating Capacity
Gas fireplaces often have input rates of 5,000 to 40,000 BTU/hr. Outputs are typically 75% of this, although some direct-vent units claim to be more than 90% efficient. Care should be taken with efficiency numbers since there is more than one way to calculate these. Steady-state efficiencies tend to be higher than seasonal or Annual Fuel Utilization Efficiencies.

Vented gas logs put into an existing masonry fireplace can have ratings up to 90,000 BTU/hr. These units are typically the least-efficient gas fireplace systems. Unvented gas logs can be more than 90% efficient. They are typically smaller capacity, topping out at roughly 30,000 BTU/hr. Unvented logs are not allowed in all jurisdictions. These units generate a lot of heat and may require special protection around them, including a hood to contain the heat.

There is also a hybrid called partially vented logs. They require keeping an original fireplace damper open about 10% of the way. They are typically available in sizes up to roughly 50,000 BTU/hr.

All of the standard problems with gas appliances apply here. Here are some specific issues that we won’t get into detail about.

1. Not suitable for use in a bedroom or bathroom Where you see a gas fireplace in a bedroom or bathroom, it should be a direct-vent system or approved for use in bedrooms/bathrooms. This only applies to some jurisdictions; you will have to check with your local authorities

2. Damper in existing fireplace not fixed open Where you have vented gas logs in a masonry fireplace or an approved factory-built fireplace, the damper should be permanently fixed open. Damper clamps are often provided with the log set so they can be attached to the damper.

3. Glass door problem Some direct-vent natural gas fireplaces with permanently sealed glass fronts have had problems. There have been some cases of gases accumulating inside the firebox and exploding, shattering the glass doors.

This gas fireplace insert has inadequate combustible clearance on the top, sides and bottom
This is a gas log system that is inserted into an original wood burning fireplace - this unit is missing a non-combustible hearth

We have introduced the topic of gas fireplaces and gas logs, and have outlined a few of the conditions that are typically found during home inspections. More information on the causes, implications and strategies for inspection can be found in the ASHI@HOME training program.