What is Short Cycling?

Everything around you is designed with a specific set of uses. Consumer lighting is generally designed to be in a warmer environment, be turned on for a few hours, then be shut off for a few hours. Your car is designed to start up, run for at most a few hours, reach a few thousand rpm, carry a given load, and then shutdown for a while. Everything is designed with these use constraints, that they’ll be on for a given time, achieve a given performance, and then shut down. We call this off, on, off pattern a Duty Cycle.


Getting Out of Cycle

Your car works wonderfully when you use it as its designed, doesn’t it? Short trips to the store, the commute to work, and the occasional long haul trip all work out just fine. What would happen if you didn’t use it as intended? Imagine if you ran it for short periods. Start up, drive to the stop sign, shut off. Start up, drive 5 minutes, shut off. Start up, drive around the corner, shut off. Something is going to break eventually. Either the starter will have overheated or the battery will die, but it’s going to work a lot shorter than desired because it’s designed Duty Cycle is being interrupted.

This disruption, a constant start-stop is short cycling. The system keeps going through cycles, but they’re shorter than they should be. Just as with the car, your heating, air conditioning, and most everything else HVAC related, was not designed to work like this. When this happens, something is out of place.


Malfunctioning Hardware

The most common culprits of short cycling are failing or failed hardware. Consider for example, your thermostat. If it is on its last leg, it may not read the temperature correctly. It could be constantly flipping between a need to heat and a need to cool the home. The furnace is going to receive the command for heat, fire up, and just moments later be shut down when the thermostat has suddenly detected a temperature spike.

Inside the furnace, malfunctioning limit switches can trigger a premature, emergency shutdown. Cracked heat exchangers can shoot fresh air onto hot flames, triggering roll outs and  overheating. A dirty intake air filter can cause the furnace to burn incorrectly and overheat, leading to an automatic shutdown. In any of these cases, once things cool down, the system will invariably try to return to its duty cycle and assuredly shutdown prematurely once more.

Air conditioners are just as susceptible to such failures. A blocked air filter here will cause the system to overheat trying to reach a target temperature. It’ll be forced to shutdown. A frozen evaporator coil, as we discussed before, may cause the system to become ineffective at cooling the building. It will run for an automatic cycle, achieve nothing, and be forced to start again when nothing has changed. Low refrigerant levels are just as bad, forcing the system to work harder for the same amount of cooling.

In any of these cases, you’re home or office will seem fine. It might well maintain the desired temperature, but you’ll notice higher energy bills and assuredly hear your equipment running far more often than normal. Generally speaking, you’ll need to have your HVAC Professional take a look around to see what’s wrong. Unfortunately, this isn’t as easy as replacing a leaky valve or swapping out a bad electrode.


Designed to Fail

There are however some cases where a brand new piece of equipment starts to short cycle. Every single component is brand new. You’ve got receipts, bills, and boxes to prove that everything is freshly manufactured. There’s certainly a chance for defective hardware, but that’s pretty minimal. If nothing is defective, what’s going on?

Your system was probably designed to fail. Someone wrote down the wrong figure, miscalculated, mismeasured, or otherwise built something too big for the building at hand. A massive furnace can fire up and send a roaring, roasting blast of heat into the home. The temperature change is near instant, but it’s only enough to warm part of the home. That sudden surge of hot air hits the thermostat and suddenly everything stops. So far as the thermostat is concerned, there is no longer a need for more warmth. After about ten minutes, that surge of warmth has passed, the cool air has mixed with the hot, and you’re back to square one as another brief heat wave surges into the room.

With an oversized furnace or air conditioner, you’ve essentially gone rabbit hunting with a battleship. You’ve gone out looking for Bugs, but you’re armed to take down Godzilla and his three best friends. It’s overkill to the point that it doesn’t actually work. Where a lesser furnace’s longer cycle would slowly heat the room and slowly heat the entire volume of air, the oversized behemoth is only able to heat a part of the air before it’s told to stop.

In some circumstances, this can lead to heat build up inside a furnace, or icing up an evaporator coil. The furnace may end up shutting down to a safety being tripped after it’s worked hard enough to heat Santa’s workshop, because it can’t force any more warm air into a small building.

The solutions here aren’t always great. It may be possible to adjust, relocate, or program the thermostat to account for the V8 sized power hours you’ve attached to a tricycle. For air conditioners, defrost controls can be used to tame the wild, icing beast. Outside of these options however, you’ll probably have to replace your new furnace, with a new, but smaller furnace.

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