Cold "short cycling"

I have an older Bard NG furnace. It’s in super shape, or so I was told. With this cold winter it started short cycling with no burner. Specifics:
Limit at 220
Fan on at 110
Fan off at 80
pilot light works fine
flu clear
no preheater

I can not tell if the thermostat is actually sending a signal demanding heat, but the fan will start with no flame, run about 30-45 seconds and shut down. Next cycle is normal.
I removed the upper cover to see the controller and watched the cycle. When the short cycle starts, the controller shows a decreasing temperature, then shuts down.
Is there a “magic” setting of the controller for the fan on & off temps? If so, I have not found it yet.
Oh, I cleaned the blower fan and blew the dust out of the motor before the heating season started. Filter is only 3 weeks old. Problem started earlier this week.

Please help. I know this is not good for the contacts, motor or my electric bill.

This one is easy your fan switch the one with the dial with the two pointers cut in and cut out is the only control that brings the fan on. Try setting the on pointer to 100 and the off at 90 and if it continues to cycle change out switch very common for that type of switch to get out of calibration it is just a bimetal coil that expands and contracts

This would cause the unit to cycle more!! Presently the differential is 30* allowing the fan to stay on longer over a temperature drop of 30* in the hot air supply heat exchanger/plenum area. By setting the differential to 10*, a drop in temp of only 10* will cause the fan to cycle off and then back on with a temperature rise of only 10*.

For more heat transfer/energy efficiency,the fan on-off settings seem to have been lowered from the manufacturer’s settings, usually on at 150* and off at 120*. These temps ensure that air delivered at registers is warm enough to not cause discomfort from drafts…90* air moving at 300-400 fpm feels cool! By setting the on-off to 110*/80*, the 30* differential is maintained; heat is removed from the exchanger and delivered to the house earlier in the heat cycle and the fan remains on longer at the end of the heat cycle until interior furnace temps lower to 80* and not the higher standard 120*. The recommended fan on-off settings in my area to get more heat efficiency out of each cycle are 120*/90*.

Do you have an electronic thermostat or the older bi-metallic coil type?

Brian you will notice I stated TRY a 10 degree differential the gentlemen already stated that the fan was cycling with a 30 degree differential pretty obvious to me the fan switch is in need of replacement

I believe the controller is the bi-metal type. It has a slotted indicator wheel with 2 movable posts, (well maybe 3 cuz I have not tried to change the max setting).
Since I got home at a decent hour today I sat and watched the controller some more. I noticed that when I open up the span between the fan off & on posts (on:110 & off:70) that something strange happens. During the cool down cycle after a normal cycle, the dial holds at 80 for a minute or so, then snaps right past the off post to about 60-65. First time I’ve seen a bi-metal device do that. Maybe not a switch controller like this, but the bi-metal devices I’ve worked on just quit working altogether.
From what I have read from all of the contributors in this thread, and thank you very much I might add, I’ll need to call a pro in to replace the controller switch.

I tried that Charley. While the fan shut down a little sooner, the short cycle started up sooner but shut down sooner also. Like a hammer-on and then hammer-off in 5 seconds or so. I opened the span up and described the change in my reply to Brian.
Thanks for the suggestion.

The controller is bad.

Make sure that you don’t have a cracked heat exchanger in the area of the pilot.
The heat from the pilot can heat things up and make the fan come on if there is a break.

Joe I have changed out a truck load of those fan switches in my life time and it is always the same story the Bimetal gets distorted over time from the heat you can close the span or widen it with the same result its not going to perform properly. A majority of those switches do have the third pointer within the cam base and it is the high limit side of the switch. The proper term is combination fan and limit switch. And as David suggested on a furnace of that age I would also recommend having it checked for a cracked heat exchanger just as a precaution. A good idea is when ever a furnace exceeds 15 years of life the exchanger should be checked for cracks at the beginning of the heat season


Just interested in the 110* “on”/ 80* “off” settings. I have never seen settings this low. How are the air delivery temps in the house? Did you lower the settings from the standard pre-set temps?

Back in the early days of the first higher efficiency oil furnaces (with flame retention head burners), had a client that set on/off to 120/90 from the factory settings of 150/120. The furnace began short cycling at the end of the cycle when it had not before with the higher settings…figured it must have been due to the lighter mass of the thinner heat exchanger and fire chamber walls (stainless steel drum)…not much heat storage capacity.

Brian (and alll other contributors to this thread),

I’ve been in this home since 1994, and it was built in 1936. The serial on the furnace starts with a “72”, so I assume it’s from that vintage.

In 1998 the fan was making a horrendous noise at the start of the new heating season that struck me as an off balance type of situation so I looked into it. I found clumps of dust in the fan cage, around the bearings and in the motor case (open drip proof case rather than totally enclosed fan cooled). I removed entire the blower assembly (on this furnace it’s just 6 screws, disconnect wires to motor and slide out of the box) and totally cleaned it up. I made a quick inspection of the unit downstream of the blower and it was quite dusty. Not an area I am familiar with so I called in a pro to make sure it was done right. (He was the first to tell me that the entire furnace was in great shape for it’s age.) He cleaned the entire duct system for us, and changed the controller settings so I could squeeze every degree out of the gas that I could. I figured he knew better than I since he was a pro. Harrumph! He turned out to be a total moron that ended up hacking my ducting system. Maybe that should have been an indicator to me.
Since then, I have cleaned the blower assembly as I did the first time and have the ducts cleaned every 2 years. By a different contractor no doubt!
I did not start adjusting the combination fan controller until the short cycling started this year. In fact, the last setting I tried has stopped the short cycle. It’s now at 110* on, 70* off. The indicator wheel still does that “hold-then-jump” routine around 80-85*, but the short cycling has stopped.
Oh yeah, we have not noticed any inconvenience in the air delivery temps since this is the most cold blooded home I have EVER lived in, and the floor stays colder than the rest of the room. Since the registers blow the air across the floor, it always feels warmer than what’s there already. Yeah I know, long past time to move! :wink: Doggone economy.
I’m setting $$ aside for the service call to have a pro come in, and I’m going to insist NACHI cert this time, for a full inspection and switch replacement if warranted. I just hope the cert means something (no dorks or sheisters!) and that I still have some time on the life of the unit. When it works normally, you only hear the gas solenoid click on and then the fan motor hammer on.
Once this unit demands replacement I plan on relpacing with entire HVAC unit.

Oh! There was a question on my duct system further up this thread so:
The intake used to be a panned floor joist cavity that ran from the edge of the foundation to what I assume may have been a coal fired furnace. In fact there are still holes in the wood flooring in that cavity at the outer wall. Good Call! I suspect once this Bard unit was installed, the intake and heating ducting was changed to accomodate the learnings over the preceeding 36 years or so. That joist cavity is now 3 feet away from the furnace and no longer connects to it. Each heating duct has an adjustable flapper in it to control the delivered air flow. I keep them all open, but have the ducts that dump into the living room with the thermostat closed slightly to ensure other rooms heat up before the thermostat sees it’s set temp. I would estimate these 2 duct flappers to be about 15* from full open.

Thank you again to all of the fine gentlemen that have contributed to this thread. I have learned from you.
Best regards to you all,
Joe Beland

Older bi-metallic thermostats have a ‘recovery’ setting. If you remove the cover, there is usually a small sliding metal part with some numbers on it. Often people don’t like the big dead zone with older thermostats so they slide it for a very narrow band. I would check that before calling it bad.

Regardless, I tell my clients the new programmable ones have zero moving parts, and can save hundreds a year in heating and cooling costs. Often they don’t care about the older thermostats, and plan on scrapping them.



Have you changed that fan controller yet! ???

Changing filters or cleaning fans is one thing - trying to 2nd guess your heating problem by emailing people is a different story.

Call a service tech and get done with it.