In this episode, Ben Lagrange of Blue Ridge Heating & Air explains what HVAC short cycling looks like and why it can happen when equipment is oversized or ductwork restricts airflow. He discusses humidity and mold concerns, room-to-room comfort problems, equipment damage, common issues after home additions, and the measurements used to diagnose and correct the cause.
John Maher: Welcome to Clearing the Air, the Blue Ridge Heating and Air podcast where we help homeowners in upstate South Carolina and Western North Carolina make smarter, more confident decisions about heating, cooling, indoor air quality, and whole home comfort. Today I’m with owner Ben Lagrange, and our topic is HVAC short cycling in Brevard, North Carolina. Welcome, Ben.
Ben Lagrange: Hey, thank you, John. Glad to be here.
What HVAC Short Cycling Means for Brevard Homeowners
John: Thanks. Ben, what is HVAC short cycling? What does that term mean? And what signs might homeowners notice when their furnace, their air conditioner or heat pump is maybe turning on and off too frequently?
Ben: Sure. Yeah, I think there’s kind of two flavors of short cycling that happens, but first what it is – basically the system goes on, turns on and turns off again after a short period of time. This happens over and over. And oftentimes it happens before your system reaches the desired set point on the thermostat.
As I mentioned, the two flavors, one flavor is the system is not sized correctly. Typically, it’s oversized. And that’s very common around here where you notice it a lot in the summertime where the system comes on, it hits the set temperature on the thermostat right away and it turns back off and it just does it all the time, on and off. And an unintended consequence of that is it doesn’t dehumidify the house quite as much. But the other one that happens…
John: So that’s because it’s turning on, it’s dumping a lot of cold air into the house quickly. It’s hitting that temperature point, but because it doesn’t have a lot of time where it’s running, it’s not able to take the time to remove the moisture from the air. Is that how I understand that?
Ben: That’s exactly right. And we could kind of go down a whole rabbit hole on that. That scenario basically turns that HVAC system in that home into a mold factory. And that can be a pretty bad situation.
John: Because you’re going to have cold, wet air and humidity in the house.
Ben: Yeah. And so if you think about it, when the system turns off, all the air that’s not just in the house, but still inside that air handler and that duct system is still pretty high relative humidity. And so it sits there and it heats back up again over the next say 10 minutes until the system turns on again. And when it turns on, that really cold air shooting through there makes the lining of that duct system and the cabinet, the air handler gets really cold and you get condensation immediately. And that condensation lines that whole duct system.
And my belief is, in the Carolinas, once the summer starts and that happens the first time, those duct systems pretty much stay wet all summer because it’s too humid in the air for that air to ever really evaporate. And the system’s not on long enough to take the moisture out and it just stays there. And then it only takes a few days before molds can start growing. So it’s a real problem and it’s a very common problem.
Ductwork and Airflow Problems That Cause Short Cycling
John: So that’s one flavor, if you will, of short cycling. What’s the other?
Ben: The other is there’s something going on with the duct system that’s not allowing the proper airflow to go through the system. And in those cases, the system will actually start and stop oftentimes without ever achieving the set point of the system. And you can see these kind of things with, it could be weird noises with the startup.
In that situation, you could see noticeable temperature changes from one room of the house to the other because there could be a blockage on one part of the system that’s not allowing air to get to certain parts of the house or certain rooms as effectively it needs to. And then other rooms may be getting extra air than what they were designed for. So you get even a more dramatic effect.
You can see weird things happening with your energy bills. Depending on how the duct system problems are manifesting around where the thermostat is, it could be providing extra conditioning around the thermostat. So your system is not running as much as it should and therefore the house feels a little bit under conditioned or in some cases the area near the thermostat is not getting enough of the conditioned air and so it kind of runs the system more than it should. So the other parts of the house end up being over conditioned.
So there’s a lot of weird things that can happen in the HVAC system. And it could be something as simple as a pinched duct where someone put too much stuff in their attic or someone set a box down on something and it shifted a little bit and what was a nice round duct is now like a little pancake and it’s not letting the air get through.
John: Or it could be that the whole system was kind of misdesigned from the beginning, that the other ducts are undersized or there’s inadequate return air openings or things like that.
Ben: Yeah. So the design is definitely a thing that could happen as well or design issues. So the thing that we like to do is to make sure that the system is designed to manage a certain amount of air, cubic feet per minute of air. And then you want to evaluate the supply side where the air comes out of the system, that the duct system is designed to manage and deliver all of that air effectively to the house.
And then on the return side, that the size of the returns, whether they’re one room or one place in the house or multiple places in the house, all those returns at the walls or the ceilings or the floor, wherever they are, and those duct systems bring the return back to the unit are adequately sized. And the most common problem we see is the return duct system being undersized. I’m not saying it’s typical, but that’s the most common problem that we see.
Why Closing Supply Vents Can Disrupt Airflow
John: My parents had a house with a central air conditioning system and heating system and the vents that were in the floors of each room had a little lever on them to open and close that vent. Are homeowners doing anything wrong by opening and closing those to control the heat or air going into each one of those rooms or are they causing potential problems by managing those vents themselves?
Ben: Yeah. No HVAC professional likes to see those being used. It kind of disrupts how the system was designed to work. The best thing to do if you’re wanting to be able to reduce the flow to a particular room or area of the house is to have a damper installed in the duct system, whether that’s in the crawl space or the attic. Now, if you’ve got one that’s kind of trapped between floors and you no longer have access to it, there’s not as much you can do there unless you take apart some ceilings and stuff like that. But the best thing to do would be have your HVAC company install a damper in that line of your duct system and then adjust it to your desired airflow.
Home Additions and Ductwork Capacity Problems
John: Okay. How often do you find that a replacement system or maybe a home addition was installed without determining whether or not the existing duct work could handle that required airflow? And can that cause these types of short cycling problems?
Ben: Yeah, that’s extremely common. Sometimes not even just an additional room being added if the homeowner decides that, “Hey, this room needs more air, so I’m going to hire some guy to run another duct to this room.” And it kind of disrupts the rest of the house without realizing it. But certainly with additions, whether they’re a finished garage that now we want conditioned or a finished attic that now we want conditioned, or a sunroom, a screen porch that’s now enclosed, it’s very common that we’ll just run two extra supplies to this room and your system should be good enough.
Even if the manual J calculation is done, which is looking at the load, the energy load of the house, can your equipment handle the load of this additional room? The duct system needs to be evaluated for that as well. So the duct system has to be designed to handle that additional room as well.
And that would include the return side of that as well. So we find that quite often, and then down the road, something more dramatic has to happen to address the addition than just adding duct work.
How Short Cycling Damages HVAC Equipment
John: Okay. Can short cycling as an issue actually cause damage to compressors or blower motors, heat exchangers and equipment like that? Is it more than just a comfort issue and it can actually cause damage to the equipment itself?
Ben: Yeah, the short cycling, the best situation for your equipment is for it to either be off or to just run continuously. You think about your car when you’re buying a car and someone says, “it’s got high miles”. “Well, it’s all highway miles”. We kind of intuitively know that starting and stopping something over and over, like bending a paperclip, drives fatigue, which will crack things. And so starting and stopping of your HVAC system is going to increase the failure rate of the system and cause your motors and compressors to burn out.
The heat exchanger, you think about the heat exchangers constantly heating up and cooling off. And every time it heats up, those metals expand and they push into each other and they kind of drive these thermal stresses in the heat exchanger. And that drives thermal stress induced low cycle fatigue cracks. So you’ll get a crack in the heat exchanger much sooner than you’d prefer. So definitely short cycling is not good for equipment.
Diagnosing and Correcting HVAC Short Cycling
John: What measurements and diagnostic steps can you use as an HVAC contractor in North Carolina and South Carolina to help determine whether or not the duct work is causing the problem or whether or not it’s the system that’s oversized and how that can be corrected through adjustments and repairs rather than maybe a complete replacement?
Ben: Sure, yeah. Typically the first step will be a visual, particularly when the duct system’s located in a place that’s undesirable for the homeowner to go look at. If it’s in a crawl space that’s not really nice or in an attic that’s hard to get to, there could be something pretty obvious that can be addressed just by looking at it or something became detached.
But then beyond that, if everything looks visually okay and you want to start doing some diagnostics, we’ll take some pressure measurements. We’ll look at temperature change across the system, across the duct systems, across the equipment, look for anomalies there. We’ve got the ability to measure air velocities coming out of the vents going into the return vents, measuring the air volume, cubic feet per minute coming out and going in, can find where maybe the system is delivering all the air it needs to. It’s just imbalanced. And so one area of the house is getting too much air, another area is not getting enough. There’s things that can be done to address that.
John: All right, well, that’s really great information, Ben. Thanks again for speaking with me today.
Ben: Sure. Thank you, John. Thank you for having me.
John: And for more information, you can visit the Blue Ridge Heating and Air website at blueridgeheatingair.com or call us in the Greenville, South Carolina area at 864-485-6509 or in the Brevard, North Carolina area call 828-415-3339.
