perftest_asyncCSS: 0 NRBTHS Home heating systems that aren't properly maintained may be less than 50 percent efficient. Learn more about the importance of periodic furnace maintenance. critical styles end critical styles StartFragment How it Works critical styles end critical styles StartFragmentWhen a home system's thermostat is turned up, an electrical signal is sent to the heating equipment, where a valve or relay-switch starts the fuel-burning process. When the furnace