Selecting the best-suited HVAC system depends on factors such as: cost and availability of the energy source; appliance or system efficiency; cost to purchase, install, and maintain the appliance or system; and environmental impacts associated with the fuel. Energy prices vary because of type of fuel, market pressures, supply availability, production capacity, weather, season, and politics. They tend to vary somewhat by geographic location. A common way to compare heating energy costs at a geographical location is by determining the delivered heating cost, expressed in consistent units (i.e., cost per million Btu, $/MMBtu), based on the cost of the fuel ($/unit), heating value of the fuel (HV), and seasonal efficiency of the system or appliance (η_hs), expressed in decimal form (i.e., 80% = 0.80):

Compute the delivered heating cost ($/MMBtu) for a wood-fired heating unit based on a heating value of 21 000 000 Btu/cord (pine) and a cost of $225/cord:
a. For a fireplace with glass doors with a seasonal efficiency of 15%.
b. For a modern, furnace, or boiler with a seasonal efficiency of 80%.

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Delivered Heating Cost ($/MMBtu) = ($/unit 1 000 000 Btu/MMBtu)/(nhs. HV)