Furnace Short Cycles | What is Gas Furnace Short Cycling?

We recently had a question come in from our Furnace FAQ page, where the visitor wanted to know more about gas furnace short cycles, and what it meant to him with his brand new furnace in his home.

In this quick gas furnace article, we want to help you understand what gas furnace short cycle means, and why its bad for your home heating system. Most often on new systems, it’s due to not having the right size furnace in your home!

Furnace Short Cycles

Short Cycling is the process in which the following occurs:

  • Thermostat calls for heat
  • Gas furnace turns on and runs
  • Within just a few minutes (usually less than 5 minutes) the furnace turns back off.
  • Process repeats several times an hour.

This is commonly called a furnace short cycle.

Imagine if you will, sitting in your car on a cold morning. You get cold, so you reach up and turn the heater onto full-blast in your vehicle. Within 1-2 minutes, its more than you can take, so you reach back up again and turn it off. Now, repeat that process for the next 30 minutes on your way to work.

The reasons short cycling can be bad for you are many. Most importantly however are that short cycling prohibits the area from being properly heated, cost you more money to run your furnace and reduces the life of the unit!

My advice to you if the furnace is new:

  • First, call out the company who installed it, explain the situation and ask them to provide your load calculation. Along with that, ask them to correct the issue.
  • If they refuse, or cannot do so, call the Manufacturer Rep in your area, and ask for assistance.

Reasons for Short Cycling

While the reasons for your furnace short cycles can be endless, there a few that are most common, especially on a new unit installation. Assuming everything is working properly, check the following:

  • Thermostat too close to the vents or heat source (Relocate the thermostat) and when the heat turns on, the thermostat is immediately warmed to the set point, and shuts back off.
  • Gas furnace much too large for the area! Even a moderately oversize furnace is going to run OK 90% of the time. Oversize systems are not uncommon and can generally be adjusted down to eliminate short cycling. A grossly oversize system on the other hand is not going to do.
  • Several closed vents, forcing too much hot air into a confined space. (Open other vents)
  • Other, technical issues in limit switches, pressure switches etc. (You should call a technician)

Is Your Furnace Short Cycling?

Share the situation in the comment box below. Someone will surely jump in and help.

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Comments

14 Responses to “Furnace Short Cycles | What is Gas Furnace Short Cycling?”
  1. hanoverfist says:

    goodman furnace burners only stay lit for 3-5 seconds then shut off

    • Clarke Howatt says:

      I have exactly same problem.Burners light for about 5 seconds and shut down. New filter,new therm batteries. Any suggestion

  2. ddkdewy says:

    I have a 2 story house not insulated real good but OK with a full basement in Minnesota…new furnace 5 weeks ago…3 techs out to find out why it takes 40-90 minutes to heat up 2 degrees with outside temp at 0-20 degrees…we have 1440 sq. feet not including the basement and the furnace is on and off every 7 to 12 minutes thru out the day..the furnace is rated at 42,000 btu…any ideas???? DW

    • Mark says:

      @DDK – I would suggest calling back the installer and asking to see the heat loss calculations they ran. Its impossible to guesstimate what the issue might be, but it sounds like your furnace may be a bit on the small side. 40,000 btu just sounds small.

      If they tell you they didn’t run the Manual J… ask them to that day.

    • Steve says:

      The furnace sounds undersized to me. I am in the process of purchasing a new furnace myself. The heating business came out and did a heat loss calculation. My house is 2245 Sq ft. INCLUDING the full basement with r-11 in the attic and no sidewall insulation. the calculation showed that the 60,000 is probably the best size except that on 0 degree days, my heat loss is estimated at 54,000 so the unit could run almost constantly on 0 degree days. the contractor says go with the 60,000 verses the 80,000 in my case. FYI-the furnace is a Trane 90% AFUE. I’m going with the 60,000.

  3. ddkdewy says:

    oh yeah 40-90 minutes is from 6a.m. until it shuts off before the 7-12 minute cycles…DW

  4. ddkdewy says:

    Hey Mark…Thanks for the info…just we what suspect but they dont seem to want to get back to us about doing that…therefore we are thinking about contacting Manufacture….dosent seem we are saving much with new furnace…..DW

  5. ddkdewy says:

    Hey just thought of something…will the size of the heat and cold air duct work play a part in sufficiant heat in a house…heat ducts are 8×8 inches cold air returns are 7×15 inches…Thanks DW

  6. Angelika Hall says:

    Brand new 4 ton Trane AC and furnace – AC short cycled all summer, humidity unacceptable high, mold growth and rust on several registers. Unit was replaced with 3.5 ton – problems continued. Manual J determined 3 ton unit would have been sufficient. Company refuses to replace it. Now the gas furnace is short-cycling as well. Thermostat clicks – heat comes on, runs for 3 minutes, then shuts off. Air coming out of vents never gets really warm, just slightly warmer than room temperature. My 40 year old heat pump did a better jub than this.

    • Mark says:

      Angelika, where is the thermostat located, and how close it is to any other vents? If you are running into an issue like that for both heating AND cooling, it sounds like something other than a sizing issue.

      Mark

  7. dave d says:

    i have a 12 x 70 mobile home in the north woods. when i show up there and turn on the heat it short cycles until the house reaches temp. once it hits temp it seems to run normal. i noticed while short cycleing the burner stays lit, just the fan short cycles. the house can be anywhere from 0 degree to 30 degree when we get there. any help would be appreciated.

    • Mark says:

      Sounds like you have a 2-stage furnace in the home that is starting/running on high fire to meet the demands of the load, then settles into low-fire after its been met.

  8. JL says:

    Hello,

    My Lennox Oil Burning furnace with a Beckett AFII 85 Oil Burner ran out of oil the other night, so I got 255 gal and purged the line. Now with the thermostat in the house calls for heat, the blower starts and the burner ignites the oil just fine, but the flame is only lit for a few seconds (maybe 5 or so) then the flame goes out, a few seconds later (10-20) the flame comes back on for about 5 seconds then goes out. This repeats the entire time the thermostat in the house is calling for heat. It is not going into lockout mode.

    Also, the vents in the house normally get really hot and the air coming out should be hot, but it’s just warm. So the air is not too hot at all.

    Any help would be great.

    Thanks,

  9. jerry says:

    my sons furnace burners would come on for about 3 or 4 minutes and go off for about the same amount of time, meanwhile the fan never shut off. The temperature was -4 degrees. I jumpered the high limit safety switch and the furnace ran normal and the plenum was hot to the back of the hand, but no more than my furnace at home. A furnace man came and wired the high limit safety switch back up and said he thought the plenum had a crack or the A coil on top was dirty and restricting air flow. Now after saying all of that, the furnace is back to normal again, out side temperature is 20 degrees. What is going on here ?? I was thinking maybe the high temperature safety switch was opening and closing , causing it to cycle. ?

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