This is one of the most frequently asked questions that we deal with when it comes to fireplaces. It is a mixture of the type of gas fireplace you have (decorative or heat producing) and also the type of gas you are burning (natural gas or propane/LP gas). You could easily do the calculation yourself once you find out the answer to the questions. If you know the manufacturer of your gas fireplace and model number you can easily look it up online. Also, your gas appliance has some kind of tag on it, which will give you the BTU of that model. To determine the type of gas you have is easier. Typically if you have propane, there is usually tank located outside the home which gets filled regularly by a propane company. Natural Gas is typically piped right into the house from the street.
There are really two types of gas fireplaces – decorative appliances and heat producing appliances. Decorative appliances are usually referred to as gas log sets and are designed to go into an existing masonry fireplace. In Massachusetts we have to remove the damper and then the fireplace will have a set of operable glass doors which must be open when the fireplace is in use. In other states you do not always need to have the damper removed or glass doors. They give you a large flame which looks just like a real wood fire, but most of the heat they produce goes up the flue, exactly like a wood burning fireplace does. See the picture to the right. So the fireplace really does not add a tremendous amount of heat to the room. they will feel warm if you stand in front of them, but usually they are sending the heat produced right to the outside. They use the air from the room for combustion, which is why the doors are open when in use, and then the doors are closed when it is off, acting like the damper would, to prevent warm air from escaping through the flue. This type of appliance typically uses anywhere from 70,0000 – 90,000 BTU/Hour when running on high and the efficiency tends to be lower than 25%.
The second type – a heat producing type – is a gas insert or gas fireplace. A gas insert is a metal box which installed into your masonry fireplace. A gas fireplace is built around new construction and there is no masonry chimney. Both are metal boxes with solid glass front that do not open and have a pipe that either goes out the side of the house or up through the roof. In both cases, as the fireplace is turned on, the exhaust runs through the exhaust vent and out the top of cap, which creates a negative pressure inside the fireplace insert, and causes/forces the intake air to come down the intake liner. As a result, this fireplace only uses outside air for combustion and does not take any of the air from inside the house and send it outside. About 20-30% of the heat is lost through the exhaust. The glass gets extremely hot and radiates a tremendous amount of heat back into the room. Most inserts also come equipped with fans which will force the warm air into the room at a faster rate. The largest of these types of fireplaces tend to be about 40,000 BTU and area anywhere from 65-85% efficient.
Current Natural Gas is the cheapest of the two types with the cost/therm being somewhere around $1.00/Therm. Propane is currently hovering around $3.00/gallon. Those numbers do not mean much unless you know how gas appliances work. Gas appliances are figured on BTU/Hour. In Natural Gas, there is 100,000 BTU in a Therm. In Propane, there is about 91,500 BTU in a gallon.
So based on those numbers, in order to operate a 70,000 BTU/hour decorative gas log set it would cost you roughly around $0.70/hour for natural gas and about $2.30/hour for propane. Most of the heat that is produced will go up the chimney in this case. For the equivalent in a gas insert or gas fireplace, a large could use about 40,000 BTU/hour and in this case would cost about $0.40/hour to operate for natural gas and about $1.31/hour for propane. But with the inserts you are gaining a majority of the heat back into your home, so the heating system should not be working as hard and in many cases it can actually offset the cost to normally heat your home. Now these prices are subject to what you are paying for you gas. So check your bill out and see what your current rate is, and use the simple calculation to figure it our for yourself.
There is one thing to note as well. Any fireplace that has a standing pilot is using about 1,000 BTU/hour to keep the pilot going. That means whether or not you turn on the main burner, the fireplace is using a therm, or gallon of propane, every 3-4 days. Make sure you factor that into your calculations. Newer units tend to be electronic ignition so there is no standing pilot all the time.