Most people do not think about a chimney sweep in connection with furnaces and water heaters, but oil and gas furnaces also have chimneys for ventilations that need to be kept in good working order.

Furnace—household heating appliance often fueled by gas, oil, wood, or coal.
Furnaces that burn fuel need to be properly vented to the outside to prevent toxic gases from building up in your living space and causing toxic gases from building up in your living space and causing health problems including death for people inside.

Water heater—a household appliances that heats water for washing dishes clothes, taking showers, etc.
Most water heaters are tanks that hold water and keep it hot so it is ready to use when needed. These water heaters generally run on electricity, gas, oil, or propane.

Gas or oil water heaters must be properly vented for safety.