How to calculate your home heating costs for different fuels using heat transfer equations and online calculators.
Most homeowners want to get a handle on their heating costs, but it is difficult to get a handle on what it should cost to heat your home. These calculations require general knowledge of how your home was constructed and the various thermal properties of each material or building envelope components.
Different parts of the country have differing heating requirements and to make things easier many online calculators divide the country into zones similar to planting zones.
Heat Transfer Basics
Heat transfer occurs by three primary mechanisms, acting alone or in some combination: conduction, convection, and radiation.
Conduction is the flow of heat through a material by direct molecular contact. This contact occurs within a material or through two materials in contact. It is the most important heat transport mode for solids; it is sometimes important for liquids, and it is occasionally important for gases.
Convection is the transfer of heat by the movement or flow of molecules (liquid or gas) with a change in their heat content. This is an important heat transfer mode between fluids and solids, or within fluids.
Radiation is the transfer of heat by electromagnetic waves through a gas or vacuum. Heat transfer by this mode therefore requires a line of sight connection between the surfaces involved. All objects above absolute zero radiate heat energy and is mostly of importance for heat transfer between solids and within highly porous solids.
Comparing Fuel Costs
One would think comparing the costs of different heating fuels and electric heat sources would be pretty easy. That’s not the case. For starters, while we purchase some fuels by the energy content of the fuel, we purchase others by volume or weight—and we use different units for different fuels. Heating oil, propane, and kerosene are sold by the gallon, natural gas by the hundred cubic feet (ccf) or therm (100,000 BTUs), firewood by the cord, wood pellets and coal by the ton, and electricity by the kilowatt-hour (kWh).
Listed below are the most common fuels used for home heating:
Natural gas - measured in cubic feet. Residential customers typically purchase natural gas in units of 100 cubic feet - or Ccf. (A Ccf is also referred to as a "therm.") Your monthly natural gas usage can be found in the Natural Gas Section of your bill.
Electricity - measured in watt hours. Residential customers purchase electricity in kilowatt hours - or kWh. A kilowatt hour is equal to 1000 watt hours. Your monthly electric usage can be found in the Electric Section of your bill. There are 3,413 BTUs per kWh.
Propane (LP) gas - measured in gallons. Residential customers purchase propane gas in gallons. There are 91,000 BTUs per gallon.
Fuel Oil - measured in gallons. Residential customers purchase fuel oil in gallons. There are 140,000 BTUs per gallon.
It appears that each fuel type has its own unit of measurement; however, there is a common unit of measurement that applies to all of these fuel sources. It's a BTU — or British Thermal Unit. If you were to strike a single match, it would put off the same amount of heat contained in a BTU.
Unfortunately these numbers represent the amount of heating if the fuel is burned at 100% efficiency. The efficiency of combustion appliances varies widely, from a low of about 40% for older woodstoves to over 95% for condensing gas furnaces. Electric-resistance baseboard heaters are 100% efficient, while heat pumps, which use electricity to move heat from one place to another instead of converting the electricity directly into heat, range in efficiency from 200% to over 300%. For electric heating the numbers don’t account for the losses associated with electric generation, nor do they begin to account for environmental costs. A typical fossil fuel electric utility may be 30 to 35% efficient.
As a rule of thumb contractors will use a value of 35 Btu per square foot to heat a home. This varies upon the type of construction, as newer homes may require on 25 Btu/sf.
Different systems have varying efficiencies, but they will be sized on BTU input and output. The output is the heat your home needs to maintain the temperature setting. By dividing the output by the input, you will come up with the net system efficiency. As the equipment ages, the efficiency will go down. Proper maintain and operation will keep heating appliances running close to their nameplate efficiency ratings.
Since water carries about 4 times as much heat as air, it can be run through piping or PEX tubing to deliver the heat to the rooms. Ductwork needs to be larger to carry the same amount of heat. Also keep in mind that there is a time factor with heating or cooling, water (hydronic) systems use gallons per minute (gpm) and forced-air uses cubic feet per minute (CFM).
A room may require 150 CFM of air for heating, but only 1 gpm of water to heat the room. While this sounds like a big difference you have to compare apples to apples. Fin-tube radiators, commonly known as baseboard heat, may produce 600 BTU/hr, but capacities vary depending on the gpm, entering water temperature, room air temperature and installation height. The heat transfer equation for water systems is BTUH = gpm x 500 x TD where TD is the temperature difference from the entering and leaving water temperatures. Air systems are normally based on the equation BTUH = CFM x 1.08 x TD where TD = temperature difference between the supply air and the room air.
Say it takes 10,000 BTUH to heat a room, let’s figure out how much air versus water it takes to transfer the heat to the room.
Water: 10,000 = gpm x 500 x (170 – 140)
10,000 / (500 x 30) = gpm Answer: 0.67 gallons per minute
Forced-Air: 10,000 = CFM x 1.08 x (140 – 70)
10,000/(1.08 x 70) = CFM Answer: 132 Cubic feet per minute
While this may seem like a great savings, it also takes more energy to heat the water to the design temperature than air. Savings are realized during very cold periods where the water temperature remains higher and less energy is required to heat the water. For forced-air system, the air cools to below the temperature setting and then must be reheated completely.
These calculations deal with system efficiencies and not fuel costs. It is very difficult to calculate the heating requirement of homes due to differences in construction type and climate. Fortunately there are a number of online calculators that can help determine how much you should be paying to heat your home. A calculator at BuildingGreen.com can perform this kind of analysis. Most compare fuel costs per million BTU (MMBTU) of delivered heat. Try it out here: http://www.buildinggreen.com/calc/fuel_cost.cfm
You must also account for normal fluctuations in fuel and electric costs as the various energy sources rise and fall depending on environmental conditions, regulatory issues, taxes, and primarily supply and demand. For example if the costs of fuel oil increases and thousands of oil users switch to natural gas, the demand for gas goes up and the demand for oil will go down. The price of each fuel will rise as demand increases since it will become more scarce and producers will be able to charge more for their product.
Typical Heating Requirements
The following list is an estimate for home much heating is required for homes in different parts of the country.
Climate: Size of Home: BTUs Required per Month
Mild Winter Zone: 1500 Sq. Ft.: 1.5 Million
Mild Winter Zone: 2500 Sq. Ft.: 2.5 Million
Moderate Winter Zone : 1500 Sq. Ft.: 3 Million
Moderate Winter Zone: 2500 Sq. Ft.: 5 Million
Severe Winter Zone: 1500 Sq. Ft.: 5 Million
Severe Winter Zone: 2500 Sq. Ft.: 8 Million
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