 Get a Quote How safe is my boiler? - British Gas Scorching or brown/black marks on the boiler. A musty smell or signs of soot. More condensation than normal on your windows. If you suspect carbon monoxide or a gas leak you should immediately switch off your gas supply at the meter open your doors Get a Quote Furnaces and Boilers | Department of Energy Gas-Fired Furnaces and Boilers (includes units fired with natural gas and propane) Oil-Fired Furnaces and Boilers . Other retrofitting options that can improve a system's energy efficiency include installing programmable thermostats upgrading ductwork in forced-air systems and adding zone control for hot-water systems an option discussed in 