BTU Overview
What's BTU input?
The energy fed into a fireplace is measured in BTU/hr (British Thermal Units per hour). The BTU content for gas is fixed, and the cost per BTU can be found on your gas bill. For reference, 1 Therm equals 100,000 BTUs.
The BTU content for wood varies significantly depending on the type of wood and its moisture content—the more moisture in the wood, the lower the energy output. On average, dry wood provides about 8,000 BTUs per pound.
How many BTUs do you need?
Several factors go into determining heating needs, but as a general rule, homes in New Jersey require approximately 40 BTUs per square foot. For example:
- A room measuring 15 ft x 20 ft is 300 square feet and would require about 12,000 BTUs to be heated (300 sq. ft x 40 BTUs = 12,000 BTUs).
- A fireplace with a 75% efficiency and a 30,000 BTU input will provide approximately 22,500 BTUs of heat (30,000 BTUs input x 75% efficiency = 22,500 BTUs output).
What's efficiency and why is it important?
The efficiency rating of a fireplace or stove is crucial as it indicates how much heat is generated from the fuel consumed. The higher the efficiency, the more heat you get from the same amount of fuel.
For instance, if your fireplace has a 75% efficiency, you will receive $0.75 worth of heat for every $1 spent on gas. Efficiency also helps determine the size of the fireplace needed to heat a room effectively. A higher-efficiency unit uses less gas or wood to heat the space, whereas a low-BTU unit will require more fuel or may not be sufficient to heat the area at all.
How does efficiency impact the cost of operation?
Efficiency also affects the cost of running a fireplace versus the heat output you receive from the energy used. For example:
- Scenario 1: A fireplace with a 75% efficiency and 30,000 BTU input costs $7.56 per hour to operate (30,000 BTUs x $0.25/100,000 BTUs). The heat output is $5.66 per hour (7.56 x 75% efficiency), resulting in a loss of $1.89 per hour.
- Scenario 2: A fireplace with 20% efficiency and the same 30,000 BTU input also costs $7.56 per hour to operate. However, the heat output is only $1.52 per hour (7.56 x 20% efficiency), resulting in a loss of $6.05 per hour.
Although both fireplaces have a 30,000 BTU input, the high-efficiency fireplace is far more cost-effective.
What are the main types of fireplaces?
Decorative fireplaces and stoves
These fireplaces are purely designed for aesthetics and enjoyment and will not add significant heat to a room or area. They have a very low efficiency (0-10%), meaning that most of the fuel used is lost up the chimney, directly increasing the cost of operation.
- Most open-front fireplaces
- Vented gas logs installed in open fireplaces
- Cool-touch fireplaces and Commercial-grade fireplaces
Supplemental heat fireplaces and stoves
These units will increase the heat in an already heated area by several degrees but may not be capable of heating a larger room on their own. They have a low efficiency (10-30%), with most of the fuel going unused and escaping up the chimney, adding directly to the operational cost.
- Electric heaters (these have a high efficiency but are often too small for effective heating of large areas)
- Vented gas logs installed in open fireplaces
- Decorative fireplaces with large energy input (over 50,000 BTUs)
Efficient heating fireplaces and stoves
These fireplaces are designed to heat a room or area efficiently and may be rated as room heaters. They can effectively heat certain size rooms or areas with an efficiency of 50-75%, adding net heat to the home. The cost of operation is lower since they replace standard heating needs.
- Direct vent fireplaces an stoves
- Vent-free fireplaces and stoves
- Wood stoves and inserts
- Gas stoves and inserts
Significant heat source fireplaces and stoves
These fireplaces are rated as heaters and can serve as the main heating source for significant rooms or areas. They are highly efficient, especially when centrally located, making them effective primary heaters for larger spaces.
- Large wood stoves and fireplaces
- Direct vent (DV) fireplaces