Copyright Goodheart-Willcox Co., Inc. Chapter 3 Boilers 53 approximately 66% of the maximum Btu output capacity at low stage. Two-stage controls are governed by the boiler’s electronic circuit board and are controlled based on the sup- ply water temperature in relation to the actual supply water temperature set point. Although this type of control does not have the full efficiency capability of a fully modulating boiler, it provides adequate water temperature control and is much more efficient than conventional lower-efficiency and medium-efficiency single-stage boilers. Both two-stage and fully modulating controls are governed by the outdoor air temperature, which is also known as outdoor reset control. This concept is covered more thoroughly in Chapter 8, Boiler Control and Safety Devices. The application of mod/con boilers as dual-purpose appliances has generated much interest over the past several years. By dual purpose, we mean using the boiler for both comfort heating and heating water used in domestic appli- cations. A fair share of these boilers is of the “tankless” type. A complete explanation and application of dual-purpose boilers will be covered in Chapter 18, Domestic Hot Water Production. 3.6 Electric Boilers In areas where other fuels such as natural gas are not readily available and electric utility prices are competi- tive, the use of an electric boiler may be attractive. Just as gas and oil are used to generate heat for heating the water, electricity is used to heat an element submersed inside the boiler. Electric boilers are available in many sizes and can be used for residential, commercial, and even industrial applications, Figure 3-22. One of the biggest selling points for electric boilers is their efficiency. Because they do not use a conventional heat exchanger or flue venting, there is no heat lost through wasted gases. They are virtually 100% energy efficient. Electric boilers are typically more compact in size than gas- or oil-fired boilers, which results in greater flexibility when locating and positioning the boiler for use with comfort heating applications. Because electric boilers do not require a separate flue for combustion gases, initial installation costs can be cheaper than for a gas- or oil-fired boiler. Furthermore, there are no costs involved in running gas piping throughout the building or any requirements for setting an oil or propane tank on the premises. Because they do not contain as many mechanical parts, electric boilers are durable and require very little maintenance, with only the electric heating elements needing replacement. Electric boilers come with certain disadvantages, how- ever. There is a limit to the amount of water they can heat at any one time. This means that larger buildings may need multiple boilers in order to meet heating demands. Although Goodheart-Willcox Publisher Figure 3-21. An inside view of a modulating boiler. Hot water Cold water Water Heating element Control panel Goodheart-Willcox Publisher Figure 3-22. This illustration shows an electric boiler used for an industrial application.