Copyright Goodheart-Willcox Co., Inc. 44 Hydronic Heating: Systems and Applications 2500 Btu/cubic foot, or approximately 92,000 Btu/gallon. As the name implies, LP gas will condense into a liquid state when pressurized. This makes it more easily transportable into rural areas, where natural gas may not be available. Pro- pane is shipped by truck and stored in above-ground tanks, where pressure regulators step down the pressure, convert- ing it from a liquid to a gaseous state as it enters a building, Figure 3-6. Liquefied petroleum is a derivative of crude oil, produced as a by-product of the oil refining process. LP gas is heavier than air, with a specific gravity of 1.52. (The specific gravity of air is 1. Gases with a higher specific gravity are heavier than air, and their vapor will drop when released into an open space.) For this reason, take care when working with propane in basements, crawl spaces, or low-lying spaces to guard against the buildup of dangerous pockets of explosive gases. 3.2.4 Electricity Although not as predominant as natural gas and LP, electricity can be used to fuel boilers. Electricity is almost 100% efficient because there are no heat exchangers or flue pipes used with electric boilers, as compared to boilers burning fossil fuels. However, because electricity has a much higher cost per Btu when compared to fossil fuels, it can have a prohibitively high operational cost. A heat exchanger is a device that transfers heat from one fluid to another without allowing the two fluids to make physical contact. The heat exchanger in a fossil fuel boiler works by converting the energy (heat in this case) from fuel over to water. The water that passes through the heat exchanger in the boiler is heated up in the process. The water is then fed into the central heating system via piping. A flue is a duct, pipe, or opening in the boiler’s chimney for conveying exhaust gases from the fire side of the heat exchanger to the outdoors. The heat content of electricity is about 3.41 Btu/watt. A watt is a unit of power and is calculated by multiplying the input voltage of the appliance by the amperage used by the appliance. Most commercial electric boilers are rated in kilowatts (kW) of energy. One kilowatt of electricity is equal to 1000 watts of power, or approximately 3410 Btu of energy. 3.2.5 Biomass Biomass is organic matter—such as wood, corn, and other biological materi- als—that is burned as fuel. The burning of biomass releases carbon emissions, TECH TIP Converting from Natural Gas to Propane Most residential and commercial boilers are shipped from the factory designed for use with natural gas. It is possible to convert these boilers to utilize LP gas with the proper changes. Note the requirements for converting a boiler from natural gas to LP: 1. Change out the orifices on the burner manifold. 2. Change the regulator pressure spring on the gas valve. 3. Set the proper operating pressure on the gas valve upon startup. INSAGO/Shutterstock.com Figure 3-5. Natural gas is distributed by a network of piping, from which it is delivered to homes and businesses. Noel V. Baebler/Shutterstock.com Figure 3-6. LP gas is shipped to homes and businesses by tanker truck and stored in above-ground tanks such as this.