Converting a wood burning prefab...     
            Gas Logs or Gas Insert?

A customer writes:
I have a prefabricated wood burning fireplace. I wish to use a gas unit to eliminate the need to use wood. What do I need to purchase to give me the look of a wood fire while creating the heating capacity at the same time with natural gas at a conservative cost? Gas fire log kit or something else? The unit is vented and has a gas line tube knock-out.

Reply
Hi, I'm afraid that just installing gas logs won't give you any appreciable heat gain. Gas logs consume excessive gas ( 1/2 gallon equivalent or more per hour) and 90% to 110% of the heat produced goes right back up the chimney because the damper must remain open for safety reasons to vent the fumes.

Vent-free gas logs, where the damper can stay closed, is not an option on a pre-fab fireplace because the fireplace, installed adjacent to combustible framing in your house, can become overheated and cause a fire where you don't want one. NFPA code standards now disallow vent-free appliances in a pre-fab, and we don't like them under the best conditions any way because of the fumes and excessive moisture they put into your house.

The good news is that you can install a high-efficiency gas insert into your pre-fab. This is a stove that has gas logs in it, and a glass window to prevent the chimney from sucking the heat back out of your house. These average 75-80% efficiency, a rating similar to a gas furnace. They consume much less gas (1/3 gallon equivalent or less per hour is average) and can heat 1000-1500 sq. ft. in most homes as a sole source of heat. They're non-electric so they're a great back-up emergency heater. They can be thermostat controlled for consistent, comfortable heating and to further save on gas consumption. The greatest efficiency will be had with a DIRECT VENT gas insert; these use a double vent system inside your existing chimney  that brings in outdoor air for combustion. Instead of using heated household air (which you have already paid to make warm) to provide combustion, you're using outdoor air.

Gas logs cost $500-$1,200 average, consume the most gas and offer no real heat gain. Due to their high fuel consumption and low efficiency, some localities do not allow them in new installations.

Gas inserts cost $1,500-$3,000 average but consume the least gas at the highest efficiency and offer the greatest heating capability. Plus, backup heat becomes priceless during a power outage as it can allow you to stay in your home in comfort while also helping protect water pipes from freezing. Some government localities and/or gas companies, from time to time, even offer rebates or other incentives for installing a high efficiency gas appliance for a new installation or an upgrade of an existing fireplace.

NOTE:
*If your woodburning prefabricated fireplace does NOT already have a gas line knockout then it CANNOT be converted to gas. Most prefabs installed since the early 1980's will have a gas line knockout, generally a circular indentation in the "firebrick" shaped refractory panels lining the inside of your fireplace.

*LP gas is sold by the gallon and is an easier point of reference. There are 91,500 btu's in a gallon of LP gas, so the input of the appliance will provide the consumption rate; a 30,000 btu input appliance consumes about 1/3 gallon of gas per hour.  Natural gas is sold by the therm or by the cubic foot. There are 100,000 btu's in a therm of natural gas.