Heating your home is accomplished using natural gas, which also heats your hot water tank.
The diagram below provides a great overview of how the furnace works.
HOME OWNER'S PREVENTIVE MAINTENANCE TIPS
1. Replace furnace filter (every one to three months depending on the type of filter and the amount of usage of your furnace. Furnace filters for OGT is 16" x 25" x 1". It helps to reduce the amount of dust in home and even smoke. Recommend using the 3M Filtretes products.
2. Have your furnace inspected and serviced every three to five years by a professional.
3. Ensure there is no obstruction around the fresh air intake.