Short Cycling

I would like to throw this question out there.
What causes short cycling in a gas forced air furnace?

besides a bad safety switch or component it can be that too many vents are closed off in the house or improper ducting.

Most common would be;

  • over sized furnace
  • restricted air flow (ie, dirty filter or ducting problems as Bruce mentioned)
  • faulty limit switch
  • faulty thermostat
  • theromostat improperly installed or located

One more thing, a dirty secondary heat exchanger normally located just above the discharge of the blower (if a 90% furnace) On most furnaces it’s a fin-tube device just like the A/C evaporator coil.

Short cycling on the high limit switch is caused by overheating. Normally caused by a lack of air flow. Clogged filters, faulty fan, restricted duct work. It can also be a cracked heat exchanger, both can be very serious and should be dealt with immediately.

The anticipator setting on the thermostat. On a mercury thermostat, there is an adjustment 0.2 to 1.4 which sets the cycling rate of the furnace. Lower numbers for shorter cycles, higher for longer cycles. Older standard furnaces are usually 0.2 to 0.4. Mid-efficiency and high-efficiency are longer 0.8 to 1.2 on mid-efficiency furnaces it is very important for the burner to run a few minutes before turning off. Short cycles will result in flue gas condensation and will prematurely rust out the furnace, vent connector and chimney.

Nice post David

Could be a blocked exhaust vent, safety shut-down on high efficiency.

Raymond Wand
Alton, ON

Not normally because those safety switches must be reset manually so it would only cycle off once and not come back on.

Most common cause I’ve found is heat anticipater set for too short of cycles, like David said.

I have not seen an anticipator setting on electronic thermostats. As to the safety shut-down for block flue there is a vacuum switch, where can the reset switch be found?

Raymond Wand
Alton, ON

Happened to me, the rain cap collapsed above the roof on one house and the insulation on the equipment panel came loose and lodged against the blower…both cause short cycling.

I have seen toy cars, balls pushed into ABS/PVC vents. One instance the furnace tech took the furnace apart (High Eff) to find a toy car ontop of the heat exchanger. No one knew how the toy got ontop of the heat exchanger. The toy created a hot spot and cracked the heat exchanger. The furnace was under warranty. The puchaser had to pay for a new heat exchange but no labour charge.

Raymond Wand
Alton, ON

The Most Common cause of short cycling, is “failure of the thermostst”.


Mercury thermostats or electronic? I have electronic thermostat at home here, and I can’t find any anticipator settings.

Raymond Wand
Alton, ON

I installed a new thermostat last week and it did have an adjustable anticipator. It was a 2 wire heat only thermostat.

The actual meaning of short cycle in the trade means that the furnace is turning on then off in a very short time frame from what might be normal due to current weather conditions. The determination is it normal or abnormal, on very cold night a short cycle could actually be normal as the house is loosing heat on a very accelerated level.

Abnormal cycling can be cause by lack of air flow from dirty evaporator coil, dirty air filter, dirty blower wheel and I have even seen the blower running backwards, too many supply’s closed, returns blocked off. Any or all or the above are very common, less common would be an oversized unit.

Short cycle may also be caused by a component failure, depending on the age of the unit modern units are loaded with things that can fail, 20 year old units no so many things. Example would be inducer motor proving switch defective, inducer motor not spinning to speed all the time, circuit board failure etc.

The thermostat heat anticipator too can cause this, they are best set using a heat anticipator meter, no guideline is correct since the length of the thermostat wire, the volt amps of the control transformer etc determine this. The stamped number on some furnaces is usually incorrect. But set incorrectly the furnace will short cycle. Most better digital stats will allow for anticipator adjustments, many cheaper ones will not and are pre set, how these are set is normally in the installation guide.

The purpose of the heat anticipator is to turn the heating unit off just prior to the house reaching the desired temperature, allowing the furnace to cool down with out over heating the house beyond the set point. Gas , oil and electric furnaces use them, heat pumps do not.

Thanks John thats good info.

Have come across a few induced draft blowers that sound like water is being retained in the bottom of the fan housing. One in particular would short cycle, while the other did not. Both were high efficiency models.

In those cases I put in the report - service furnace prior to closing of title.

Raymond Wand
Alton, ON

When I hear that it is normally a drain issue with the pvc rather then a clogged secondary heat exchanger ( I have only had one of those in 20 years), and the condensate from combustion backs up and you get that water sound in the inducer blower

:slight_smile: Like say your results may vary.

Sorry Raymond, when you said “safety shut-down” I wasn’t thinking about the pressure switch that proves flue draft.
However I’ve never seen a pressure switch that would cause “short cycling” but I suppose it’s possible. I have seen many that kept the burners off though for various reasons.

The manual reset vent safety switches/back draft switches are normally in or near some part of the flue system and detect flue draft problems VIA high temperatures. They could also be just a rollout switch since a flue not drawing will cause burner “rollout” depending on the type of furnace

A heat anticipator on an electronic digital thermostat is not something you can physically* *see, it’s adjusted by certain procedures called out in the installation instructions. (as in press/hold this button and that button to adjust heat cycles, press/hold this button and that bitton to adjust cooling cycles.)

*Just speaking in general terms since there are so many different brands with their own individual characteristics. *

Thanks for the info.

Raymond Wand
Alton, ON