A loose furnace gas valve can cause a fire. Gas is extremely flammable. The amount of gas used to heat a room by gas appliances such as a stove, oven, or a radiator, is controlled by the valve. If this valve has been broken, melted, rusted, or altered, then more gas can escape, causing a gas leak. A gas leak can be dangerous for people to inhale, especially for children and for people with asthma and other breathing problems. If you light a candle or turn on a gas stove in a room that is filled with gas, you can cause a deadly explosion.