How to Troubleshoot Old Furnaces

Timothy Sexton

An old furnace is probably not as efficient as a newer furnace, but as long as it is dependable in its supply of warm air, you can still put off upgrading. Maintenance for an old furnace really is not that different from the maintenance required for the newest high-efficiency furnace.

Replace an older thermostat with a new thermostat, one with an anticipator.

The basic template for forced-air heating of the home has not changed much over the decades.

  1. Start your troubleshooting of an old furnace by making sure that it hasn’t been running on old fuses. Go to the main service panel, and check for either a blown fuse -- most likely if you have a really old furnace -- or a tripped circuit breaker.

  2. Make sure that the thermostat for your old furnace is located in an area where the temperature is constant. Thermostats for older furnaces often are situated in locations where the temperature fluctuates more than in the rooms where you spend most of your time. If possible, place a new thermostat in an area of the house where the temperature is more typical.

  3. Replace an older-style thermostat with a new kind that is equipped with an adjustable thermostat anticipator. A furnace with a thermostat that predates the 1970s may well be unequipped. The anticipator needs to be adjusted to a number that is 1.4 times the amperage setting. The amperage figure should be located on the label of the thermostat.

  4. Inspect the layout of your duct system. In older houses, your efficiency may be compromised by ducts and risers that take a sharper turn than in newer homes. This will create an impediment to unobstructed air flow and result in less warmth in some areas of the house than in others.

  5. Replace the air filter or filters located inside the house. Dirty, clogged air filters can substantially affect efficiency. A furnace is only as effective as its direct air flow.

  6. Remove the damper registers on each duct entryway and shine a flashlight into the duct. An older furnace is much more likely to have problems caused by a buildup of dust, grime and fibers that impede efficient flow of air. Use a vacuum cleaner to suck up the dust and replace the older registers with new one if they are rusty or otherwise damaged.

  7. Tip

    Remove the cover of the thermostat and run a business card between its contact points to clean away debris that may be affecting the thermostat's setting.


    Turn off power to the furnace before attempting any repair. Use insulated tools when working with an electric furnace.

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