Your furnace kicks on, runs a few minutes, shuts off, then starts again. Heating pros call that short cycling. It burns fuel, wears out parts, and sometimes it's a safety control telling you something is wrong. Here's what to check first, and when to stop and call someone.

Safety first. If your carbon monoxide alarm is going off, or you smell gas, don't troubleshoot. Get everyone outside, then call 911 or your gas company from outside the house.

1. Check the thermostat

Make sure it's set to Heat, the temperature is set above the room temperature, and the batteries are fresh. Then look at where it hangs. A thermostat in direct sun, over a heating register, or near the oven can read the room as warmer than it is and shut the furnace off early.

2. Look at the filter

A clogged filter is one of the most common causes. When air can't move, the furnace overheats and a safety switch (the high-limit switch) shuts it down, and then it starts again once it cools. Check the filter monthly during heating season. As a rough guide, a 1-inch fiberglass filter lasts about 30 days and a 1-inch pleated filter about 60 to 90 days. Pets, dust and renovation work shorten that.

3. Open the vents and returns

Walk the house. Supply registers and the bigger return grilles should be open and not blocked by furniture, rugs or curtains. Closing off lots of rooms to "save heat" can choke airflow and cause the same overheating problem as a dirty filter.

4. After a storm, check the vent pipes outside

Many newer high-efficiency furnaces vent out the side of the house through plastic pipes near the ground. Drifting snow can bury them, and the furnace will shut itself off when it can't breathe. After a storm, gently brush the snow away by hand. Don't use a shovel blade on the pipes, and don't try to fix or modify the venting yourself.

This one matters in Massachusetts. In 2005 a 7-year-old girl named Nicole Garofalo died of carbon monoxide poisoning after snow blocked her home's heating vent. The state's CO alarm law, Nicole's Law, now requires carbon monoxide alarms in most homes. If you're not sure yours are working, test them today.

5. The flame sensor (usually a pro job)

On a gas furnace, a small rod called the flame sensor confirms the burner lit. When it gets coated with grime, the furnace lights, can't "see" the flame, and shuts off within seconds. Some handy homeowners clean it, but only with the power and gas shut off. If you're not comfortable with that, it's a quick visit for a technician.

6. Is the furnace too big for the house?

If short cycling started the day a new furnace went in, it may be oversized. It heats the house so fast that it shuts off before running a full cycle. That isn't a do-it-yourself fix. Ask a contractor to do a proper heating load calculation before anyone suggests replacing equipment.

When to call a pro

  • You've checked the thermostat, filter and vents and it still short cycles.
  • You hear banging, smell burning, or see soot around the furnace.
  • The furnace locks out and won't restart.
  • The vent pipe is cracked, disconnected or damaged.
  • Your CO alarm goes off at any point. Get out first, call 911, then call a pro.

How to pick someone you can trust

Massachusetts licenses the trades that touch heating systems. Gas work is done by licensed gas fitters, and heat pump and AC work involves licensed refrigeration technicians. You can look up both on mass.gov (links below). Ask for proof of insurance, get a written estimate before work starts, and read recent reviews, not just the star average. A company that answers your first call is a good sign, but it isn't a license.

Common questions

Is it safe to keep running a furnace that short cycles?

It's hard on the parts and wastes fuel. If the cause is overheating or a blocked vent, safety controls are shutting it down for a reason, so get it checked soon. If your carbon monoxide alarm sounds, leave the house and call 911.

How often should I change my furnace filter?

Check it monthly during heating season. As a rough guide, 1-inch fiberglass filters last about 30 days and 1-inch pleated filters about 60 to 90 days. Follow your furnace manufacturer's recommendation.

Does Massachusetts require carbon monoxide alarms?

Yes. Nicole's Law (Massachusetts General Laws chapter 148, section 26F½) requires carbon monoxide alarms in most residential buildings.

This guide is general information, not a substitute for an inspection by a licensed professional. If something seems unsafe, stop and call a pro.

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