Sunday, September 12, 2021

Short-cycling of the furnace at the Hunting Lease


Those of you who have been following this blog for a few years know that the furnace at the hunting-lease has been short cycling.

While my attempts to "fix" the furnace have been epic, they are by no means as epic as the dad on A Christmas Story

To make a long story short, I chased voltages through the hardwired control logic to the junction box that fed the motor. That junction box was on top of the heat-exchanger enclosure.

Popping the cover off the junction box, I saw the cause of our short-cycling problem.

A repair person with more imagination than money had substituted a snap-disc thermostat for the proper fan-limit-control. 

The proper limit switch (between $80 and $160 on-line and probably twice that much purchased locally) has a probe or a stalk that extends between 5-and-11 inches into the Stygian depths of the heat-exchanger. The $12 snap-disc merely senses the temperature of the outermost skin that is furthest from the fire-box. That skin is slow to warm up and quick to cool off.

I ordered a fan control limit with a 5" probe. I do not have a manual for the furnace and it is an off-brand that has been extinct for decades so I am rolling the dice.

This is the face where you can change the on-off settings or pointers.

One of the nice things about the proper fan control is that you can "program" it by setting the pointers.

The furnace DOES have a fan control unit but judging by the soot on the probe, it is in the fire-box and I am not sure it does anything.

Another puzzling thing about this furnace is that I don't have a mental picture of the air flow through it. I know the blower is at the top of the unit and it blows down. I know the ducting for the structure runs below the floor. 

I think the air around the control switch heats up via convection but then I don't know if the probe is in the air-stream after the majority of the heat-exchanger or before. If it is in the stream before it goes through the HE then the rush of ambient air it will very quickly cool-off the probe and turn off the fan.

Another weird thing about this unit is that approximately 24" of bare exhaust pipe runs inside the cabinet with all the exposed wiring and electrical doo-dads. I bet that wire gets pretty toasty.

That is the mystery of off-brands. You never know how much validation testing they did before selling units.

It may turn out that my best option is to by-pass the snap-disc thermostat and simply let the fan run the entire time we are heating the structure, regardless of whether the burner is producing BTUs. I don't see much downside. Over the 45 days of November and December that we are likely to heat the structure I doubt that we will use more than $5 of electricity.


  1. I had a problem like that long ago in my shop so I just bypassed the switches and wired the furnace and the fan together and hooked it up to a 110 volt thermostat. Problem solved and I used it for several years. ---ken

  2. That would work, so would Ken's idea. But the problem as you've said, is no name, off brand, no instructions... sigh

  3. Hey! Saw in the photo of the fan controller it was a Honeywell control. This might help for wiring: (This came from How to Install & Wire the Fan & Limit Controls on Furnaces Honeywell L4064B & All White Rodgers Fan .webarchive.
    By your description your furnace is a downdraft model. If you can find (more important: able to read) the data plate, InspectApedia has an ask a question box. I'm an old retired Commercial/Industrial HVAC/R guy... RARELY did any residential in my time. Thought I'd throw some info that might help... 1 thing to remember is that Limit controls are SAFETY controls NOT operating controls. What you have is a combination fan control (operating) and a limit control (Safety) on one device with a set of electrical terminals for each.

  4. As an ancient chiller mechanic, I concur absolutely with Seabear.... safety and operating controls make beautiful music TOGETHER. Fix it right and it is no longer a gnat to swat at.
    Another source for small equipment/controls, over in Wichita, is an outfit called Washer Specialties, and they will sell direct to you & ship. Vast catalog. Have used successfully and I don't have to drive into Denver/Colo Spgs cesspools.
    [] Little equipment sucks......


Readers who are willing to comment make this a better blog. Civil dialog is a valuable thing.