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 tend to vary somewhat by geographic location because of availability of fuel type and supply, market pressures, production capacity, weather, season, and politics. A common way to compare heating energy costs is by determining the delivered heating cost, expressed in consistent units (i.e., $/MMBtu or $/GJ), based on the cost of the fuel ($/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 ($/GJ) for coalfired heating units (i.e., furnace, boiler) based upon a heating value of 21 000 kJ/kg and a cost of $0.205/kg.
a. For an older, low-efficiency unit with a seasonal efficiency of 65%.
b. For a modern, middle-efficiency unit with a seasonal efficiency of 80%.

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