Water heaters use cold water from the water main and heat it using electricity or combustible gases such as propane or natural gas. Some units use an insulated tank to store hot water for later use while others produce hot water on demand. Gas water heaters with tanks burn a combustible gas to heat water and store it inside an integrated tank for later use. Gas tankless water heaters burn combustible gases using a heat exchanger to provide hot water on demand with no storage tank required. Electric water heaters with tanks use electricity to heat water and store it in their integrated tanks. Electric tankless water heaters use electricity to heat water on demand rather than storing it in an internal tank. Replacement parts and accessories are used to install, maintain, and repair water heaters.