Using and Maintaining HVAC Systems in Rental Properties

How Long Is Too Long — A Guide To Long Furnace Cycles

Cycles are an important concept to understand when dealing with HVAC equipment. Standard residential furnaces and air conditioners operate as single-stage devices. This design means they run at full power until they satisfy your thermostat and then shut down and wait for the temperature to fall (or increase).

While very short cycles are a problem, it's normal for your furnace to run a few times per hour. You might also notice the opposite problem: that your furnace runs too long. A continuously running furnace can be problematic for its own reasons, so it's important to understand when your furnace might be experiencing long cycles.

How Do You Know If Your Furnace Is Running Too Long?

Unfortunately, there's no easy answer for what qualifies as a long or short cycle. Your furnace needs to run long enough to reach your thermostat setpoint. The cycle length will depend on your particular heating preferences, outside conditions, and the quality of your insulation. If it's very cold out and you like your home very warm, you can expect your furnace to run in longer cycles.

On the other hand, your furnace should eventually reach your thermostat's setpoint. If your furnace runs for long periods and the temperature stubbornly refuses to climb, that's a large warning sign. You may also notice that your furnace will eventually shut down without satisfying your thermostat. This situation can occur if the long-run cycle causes the furnace to overheat.

Why Are Long-Run Cycles a Bad Thing?

Long-run cycles can be good if you have a modulating or multi-stage furnace. In these cases, your furnace will operate in a lower power state to maintain consistent and efficient temperatures. For a typical single-stage furnace, long-run cycles generally aren't beneficial. Single-stage furnaces try to heat your home as quickly as possible, so running too long usually means wasting fuel.

More importantly, a long-run cycle forces the burners to operate for extended periods, which can cause the heat exchanger to reach dangerous temperatures. In addition to potentially shortening the lifespan of your heat exchanger, this situation will eventually trigger your furnace's limit switches and cause it to shut down.

What Causes Long Run Cycles?

Long-run cycles can have a few causes, although airflow restrictions are among the most common. A clogged air filter or blocked vent can cause your furnace to run too long since not enough warm air will reach your home's rooms. Issues with your fuel supply can also potentially result in long-run cycles since the burners may not burn cleanly or hot enough.

When your furnace seems to be running for too long, you can always start by checking for obstructions and (if necessary) replacing your furnace filter. If these steps don't resolve the issue, contact a furnace repair contractor to investigate your system and look for issues restricting your system's airflow or reducing its heat output. 

For more info about furnace repair, contact a local company.