Gas Fireplace Venting

There are five types of gas fireplace venting:

B Vent:

Fireplace vents vertically through the roof and will use room air to burn (may have a separate combustion air vent from outside to the fireplace or the fireplace cavity).

Direct vent:

Sealed combustion chamber that exhausts outdoors and uses outside air to burn. Usually incorporates a Co axial or Co linear venting system.

Outdoor Ventless: 

vents to the outdoors without a formal vent system.

Vent Free:

Draws room air to burn and vents all products of combustion directly into the room Incorporates an ODS (Oxygen depletion Sensor) which monitors the oxygen level in  the room and if the Oxygen level drops below a certain point (usually 18 - 20%) The pilot will cease to function and shut down the fireplace.

Minimum free air requirement and instructions usually suggests opening a window.

(Not approved for use in Canada)

Power vent:

When conventional venting is impossible or ability to comply with listed instructions are not practicable there may be an optional power vent system.

Some fireplaces are designated power vent systems that can not be vented conventionally. 

Power vent systems are designed to establish a draft, and once proven, will signal the ignition system to start the fireplace.

Standing Pilot vs electronic ignition:

There are three distinctly different ignition systems for gas fireplaces and log sets:

Standing Pilot: 

The pilot stays lit in a standby mode and is always running (24/7)

A call for heat will ignite the burner using the pilot flame

Intermittent pilot ignition:

There is no gas being consumed until there is a call for heat.

Upon call for heat, the pilot will light, and prove a flame is present (safety circuit)

When the safety circuit has been satisfied the main burner will ignite using the pilot flame.

Direct spark ignition:

There is no pilot light and instead there will be an ignitor electrode which will create a spark directly to the burner and once the burner is lit the flame sensor will continue the flow of gas. (burner flame rectification)

Gas Fireplaces and High Altitudes

When installing a fireplace at high altitude, the fireplace may need to be derated.

All gas appliances are certified and tested for use at Sea level where the oxygen level is 20.9 %

Most fireplaces are rated for up to 2000 feet above sea level without any duration.

Some are tested for up to 4500 feet above sea level without duration.

For anything above 2000 feet the manufacturer's instructions should be consulted for appropriate duration based on the install elevation. 

Also some jurisdictions (Colorado and some other areas) derate the calorific value of the gas for given areas to compensate for lower oxygen levels and may affect the manufacturers listed orifice specifications.

Generally, duration is 4% per 1000 feet above 2000 feet unless otherwise listed by the manufacturer.

FYI: typical BTU rating of Natural gas is 1030 BTUH per cubic foot 

       typical BTU rating of Propane is 2500 BTUH per cubic foot 

BTU: British Thermal unit which represents the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit

Mantle temperatures:

Mantle temperatures are established during the research and development stage of fireplace design to ensure 100% compliance with testing requirements and are based on strict testing and certification standards. 

The allowable temperature on any combustible on the face wall of the fireplace is 117°F above ambient/ room temperature. An average room temperature is 68 -76°F; therefore a surface temperature of 185-193°F can be expected. On a solid surface, 150°F is very hot to the touch (for example, water boils at 212°F and bath water average temperature is 110°F)

FYI: Allowable temperatures within the fireplace enclosure/ Framed wall cavity are 90°F above ambient/ room temperature.

Thermostat/ switch wire gauge

 Wire Size
 Max Length
14 Ga
50 Feet
16 Ga
30 Feet
18 Ga
20 Feet
20 Ga
12 Feet
22 Ga
9 Feet

Gas Pressures:

Always follow the information on the rating plate as provided by the manufacturer.

Natural Gas Minimum Inlet (Supply) Pressure is usually 7” W.C. (W.C. = Water column)

Natural Gas Manifold (working) pressure is usually 3.5” W.C. (may have a hi and low manifold setting)

Propane Gas Minimum Inlet (Supply) Pressure is usually 11” W.C

Propane Gas Manifold (working) pressure is usually 10” W.C. (may have a hi and low manifold setting)

Clocking the Gas meter (BTU Rating)

The complete formula is Cubic Feet per Hour (CFH) = (3600 x Dial Size) / Time (seconds)

#1 – Make sure all other gas appliances are completely shut off other than the one you are clocking. 

#2 – Make sure the appliance you are checking is running at high fire (max output)

#3 – Get a stopwatch

#4 – Watch the smallest unit dial on the gas meter, it will often be 1/2 cubic ft

#5 – Time how long that dial takes to make one full revolution with the stopwatch

#6 – Multiply the dial size by 3600 (3600 is the # of seconds in an hr) so if it's a 1/2 cu/ft dial it would be 1,800

#7 – divide that # by the # of seconds it took. So lets say it took 22 seconds that would be 1,800 / 22 = 81.82

#8 – Multiply that # by the BTU heat content of 1 Cu/Ft of gas provided by the utility. If it is 1,000 (which is common for NG) the total BTU per hr would be 81,820

Mercury Switch

Before fast drop out thermocouples existed mercury switches were used to create a safety circuit for a standing pilot fireplace.

Also known as a pilot flame safety switch. (example of RobertShaw Mercury Switch) 

Mercury Switches, as you may know, are no longer available and in order to repair a fireplace that is equipped with one you only have two options:

There may be a conversion kit available from the original manufacturer if they are still in business.

There are pilot assemblies and gas controls that would perform the same function and be safe to use however, if the manufacturer or local authority does not approve the conversion to a fast drop out system there could be a liability/ insurance issue.

Candles and their effects on direct vent fireplaces

Any kind of candle can leave soot on a wall if it's too close, left burning for too long, or in the direct line of a draft. When there is soot present from a candle it is possible that the soot will be collected by the convection chamber of the fireplace when heated up, concentrate the soot and charge it with positive ions, which in turn will deposit on the face wall of the fireplace in a uniform heat pattern. 

The customer will say that their direct vent fireplace is sooting.

Is it possible that a direct vent could soot? Yes

Is it possible the soot could deposit in the room? 

Yes but there would need to be a breach in the firebox or the door is not sealed properly.

When investigating do not mention candles but observe whether or not candles are present. 

The large three wick candles can burn very dirty.

Customer feedback

In order to best serve our customers, we are always interested in hearing your feedback.

If you are in need of specific parts we do not carry please let us know and if we can accommodate your request we certainly will.

We do have direct relationships with certain manufacturers and if needed can custom order parts upon request.

This process involves checking the price and availability with the supplier, then sending you a draft order to complete.

We then order in the product and ship as soon as it arrives.