Question about Furnace Short Cycling..

[groundhogy]

In a previous post, someone mentioned that if your battery voltage is low, you will get short cycling.

Short Cycling: (as i understand it) where the flame cuts off and back on during one heating cycle. fan continues to stay on the whole time of the heat cycle until the thermostat decided temp is high enough.

Question
What is the mechanism by which a lower battery voltage affects the flame of the burner?

[Gdetrailer]

theoldwizard1 wrote:
Home furnaces have an over temp sensor that will force a shutdown

Maybe the newest home furnaces made within the last 10 yrs will shut it down completely, but generally on the home furnaces I have had they overtemp only shuts down the burner only. Once the heat exchanger cools down to normal temps it will automatically relight the burner. Basically repeating the process unit the T stat temp is satisfied..

Excessive burner cycling can waste fuel (importantly with RVs and dry camping will considerably waste a lot of battery capacity) plus takes considerably longer to get things up to temp and if you are dealing with very cold weather the furnace will never satisfy the T stat temp.

My home furnace uses hot surface ignition which is basically a resitive layer of material deposited on a ceramic structure.. Excessive short cycling of the burner severely reduces the life of these ignitors.. At $30+ per ignitor you really want to make sure you are not having issues with short cycles.. Stinks to have one blow out in the middle of the night or on weekends.. I keep a new spare at all times..

[time2roll]

JimK-NY wrote:
This issue has come up numerous times in the past. Some members believe that cycling of the heat with the blower running constantly indicates some sort of blockage or defect. I am in the group of those who don't see a problem. My furnace did this 17 years ago when the unit was new and has continued to the present time. My furnace heats up a cold camper pretty quickly so I suspect the furnace was just oversized for the size of the camper and ductwork. The cycling doesn't seem to cause any issues.

Yes mine did since new and for ten years before I added 2x 4" additional ducts. Now the burner runs continuous until the thermostat is happy and the whole unit cycles off. Tons more heat too.

[JimK-NY]

This issue has come up numerous times in the past. Some members believe that cycling of the heat with the blower running constantly indicates some sort of blockage or defect. I am in the group of those who don't see a problem. My furnace did this 17 years ago when the unit was new and has continued to the present time. My furnace heats up a cold camper pretty quickly so I suspect the furnace was just oversized for the size of the camper and ductwork. The cycling doesn't seem to cause any issues.

[groundhogy]

Thanks guys for the great explanations.

[time2roll]

RV furnaces have a high limit switch that will cycle the burner.

Usually overheat is from lack of air flow. Probably minimal ducting with restrictions due to poor installation.

[theoldwizard1]

Home furnaces have an over temp sensor that will force a shutdown

[BFL13]

Long thin wire to the furnace means a voltage drop. So if the furnace needs X volts to run right, your batts have to be above that by at least that voltage drop, so that can mean you can't do it when the batts get low and then the voltage drop takes the voltage to the furnace too low.

Cure is change to fatter wire or a higher AH battery bank or to a battery type with higher voltage per SOC.

You can also put your meter at various points along the furnace parts to see if there is a big drop that should not be there from one part to another. I had that and found a wire was not tight on one of the spade terminals so the next part had its voltage too low.

[Gdetrailer]

Furnace is built around the idea of a 12V battery.

Typically battery voltage encountered for 12V systems will be 11V-15V.

10.5V is considered a "dead battery" and if you routinely run your battery down to 11V you are killing all the life out of your battery..

15V when your plugged into shore power and converter is charging at bulk votage.

12.8V is nominal fully charged Lead Acid battery resting voltage.

If you are discharging your battery to 11V or 10.5V to get through your camping session, it is time to change your habits. More battery capacity would be number one on the list followed by adding some solar and then if nothing else a small gen and recharge your battery before it gets to 11V..

Battery prices are not cheap now days and taking much better care of your battery will save you tons of money in premature battery replacements you didn't have to do.

[groundhogy]

GOOD EXPLANATION

When I purchased the furnace a couple years ago, the factory lady sent me another sail switch which had a bigger paddle. I bet thats why.

I plan to pull it apart now that we are not using the furnace and give that fan a good cleaning.

I noticed if my voltage goes down just a bit, I get the short cycling.

Maybe they should regulate the fan voltage down to say, 10 vdc and use a 10 volt fan. Might fix the problem. But that would cost more. lol

[Gdetrailer]

Lower voltage means the fan motor will turn much, much slower.

Slower fan, more heat buildup in the furnace heat exchanger..

More heat in the exchanger and you can reach over temp cut off which turns off the burner until the temp goes below the trip point.

Slower fan can also mean that the other "safety" can trip which is the fan "sail switch" and that can also trigger burner off.

Slow fan also affects the burner flame directly, the fan motor is double sided, it has a fan on for the inside air and a fan on the outside air for the burner (inducer fan). The inducer fan helps move the heat and combustion byproducts through the heat exchanger..