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Like a Bull in a China Shop – An Oversizing Story
This article was written by Genry Garcia of Comfort Dynamics, Inc. Thanks, Genry!
This is a piece about oversized air conditioners.
Though the symptoms and consequences of oversized heating equipment are similar to those of air conditioners, you’ll notice that the focus throughout the article will be on the cooling side—specifically from the perspective of climate zone 1 (hot and humid).
I’m going to skip right through the lecturing about proper equipment sizing, selection, and duct design. There are trained professionals for that, and I’m not one of them. Instead, we are going to riff from the perspective of a system that has already been installed and is doing damage.
We will go over some of the symptoms, their characteristics, and why making improvements to oversized HVAC is a slippery slope.
So, what does an oversized system looks like?
It looks like any other system you’ve worked on. However, you can expect one of these systems to have the following issues:
- It can’t keep the occupants comfortable throughout various rooms in the house.
- Comfort complaints are intensified at night.
- It short cycles periodically, but it specifically does so when it’s less than 94° outside and still feels warm inside—even when the thermostat shows 67° as the room temperature.
- The relative humidity is consistently high (over 55%) or, at best, goes through big swings throughout the day. These swings will normally track with the operation cycles.
- Light films of condensation might be visible on supply vents.
- Ductwork sweating.
- Excessive noise from vents—returns, supplies, or both.
- The temperature feels (notice that I said feels, not reads) significantly warmer around the perimeter areas of the space (larger exposure to exterior walls) than on the interior ones (hallways and such).
If you pull up to a service call, and any meaningful combination of these symptoms are the reason you’re there, put your gauges back in the truck. There is no need to worry about subcooling or superheat. I promise.
But why? What’s so wrong with oversized equipment anyway?
Runtime is the obvious place to start—the lack thereof, that is.
Oversized equipment will naturally result in larger and colder air volume being moved throughout the space. Invariably, the wall control will reach its setpoint faster, and the system will cycle off before it had the chance to do its job.
What is its job exactly?
Let’s start with the mean radiant temperature. This linked article explains it very well, but in short, the temperature of the surfaces around us has as much to do with human comfort as the temperature displayed by the thermostat. (Here's another article specifically about mean radiant temperature.)
Our body temperature is normally 98 degrees; our skin is closer to 94. So, if we were to stand by a wall with a surface temperature of 75 degrees, our bodies will cool off by radiating heat to it at a more comfortable rate than if we were to stand by a wall at 85 degrees. The same goes for couches, beds, kitchen counters, etc.
An A/C system must run long enough to keep a cooler and consistent temperature on all the surfaces of a home. If the outdoor temperature it’s in the 90s, but the system runs for only 10 to 15 minutes each cycle, this won’t be enough to keep the mean radiant temperature of the surfaces in your home under control. Therefore, you’ll be uncomfortable despite the thermostat reaching and “maintaining” an indoor temperature in the 60s. This phenomenon is worsened at night when the outdoor temperature drops and the A/C runs even less.
Apparatus dewpoint (ADP) is next. ADP is the effective surface temperature of the cooling coil—or, as we call it, coil temperature. I will use these three terms interchangeably.
While a system is off, the evaporator coil will be at a temperature close to that of the return air path and its surrounding surfaces. This temperature is much higher than it is when the system is running. Once the system cycles on, the return air temperature will dictate the evaporator saturation temperature based on the DTD (design temperature difference), and it will reach the ADP.
But just because the refrigerant entering the evaporator is at 40 degrees, that doesn’t mean that the entire coil will immediately drop to this temperature. This process takes time. The cold refrigerant has to make several passes before it can absorb the heat from all of the evaporator’s body mass and then come down to the design ADP.
If we have average run cycles in the 10-15 minute range, that won’t be enough to ensure that the whole evaporator surface reaches its design operating temperature and dehumidifies the air before the system cycles off. Therefore, the dehumidification capacity of the system will be consistently and greatly compromised, resulting in poor relative humidity control in the space.
This phenomenon severely worsens when dealing with high-efficiency systems. Manufacturers have found ways to drop the compression ratio—therefore, power consumption—to increase SEER ratings. To achieve this, they have increased the suction saturation temperature through the use of larger coils. So, not only does the evaporator start out warmer, but it now has more surface to bring down to temperature. The shorter run times of oversized systems will accentuate the otherwise negligent consequences of having a larger and warmer cooling coil surface temperature.
Did you just say SEER?! There is no other time when an A/C system is more efficient than when it is not running, right? Because it is not using any energy. So, wouldn’t it make sense to provide the consumer with a system that cycles off more often then? Nope. To begin with, the up-front costs of having larger equipment installed are normally more than that one of a smaller capacity.
Also, more importantly, the single highest point of energy consumption for an A/C system is when it turns on. Once a system cycles on and off more times than necessary throughout the day, the presumed savings of not having it run for a given amount of time go out the door.
And to top it all off, the clients are ticked off! Not only did their electric bill not go down much, if any, but now they are also uncomfortable.
So how can we fix it?
Well, to fix it, we would have to replace the system with one of the appropriate capacity. But that’s probably not going to happen right away, is it? Not until the consumer has enough pain to motivate the expense, anyway.
Before we invariably end up talking about extending runtime or lowering airflow, I want to make a quick stop on static pressure, particularly when an oversized system is connected to existing, older ductwork. As soon as you start diagnosing the issue, you’ll run into a high external static pressure reading. At this point, a light bulb will go off in your head. “It’s the ductwork!”
You’ll carry on to quote duct improvement solutions that will drop the TESP, maybe even throwing in some return air path upgrades. Let’s say the customer agrees, and once the work is done, you perform a complimentary (or not) test and balance, and you ultimately confirm that the TESP is now within acceptable levels.
“I’m going to be a hero,” you may say to yourself. Well, if you did, in fact, improve the duct system to a point where the equipment is now moving more air than before, then the problem just got worse.
I get that it’s a controversial stance, but next time you realize you are in front of one of these situations, ask yourself:
More, colder air. Do I really want to make this oversized system run better?
About extending run time
If the envelope doesn’t change, then the remaining alternative would be extending runtime. There are several ways to achieve this:
- Strategically place remote temperature sensors on the warmest areas of the house that report to the thermostat, therefore tricking the system into running more. The thermostat may feature dehumidification-specific algorithms.
- Purposely de-balance the airflow distribution throughout the house so that there is more air hitting the exterior surfaces and as little as possible on the interior areas where the wall control may be located; the ceiling on this strategy is pretty low, in my experience.
- All of the above plus reducing the airflow to its minimum possible setting to run a colder coil temperature and run a lower sensible heat ratio (SHR). Therefore, the dry bulb temperature as sensed by the wall control won’t drop as fast—maybe.
It doesn’t sound that bad, does it? Maybe not at first, but these will also result in colder supply air temperatures. Cold supply air is the leading cause of sweating ducts and vents in these scenarios, but that’s not the worst part.
It will directly result in localized, colder surfaces throughout the envelope as well. You can notice condensation on vents and ductwork fairly early—before they become a problem. But what about the condensation you can’t see? For example, there's one that had been forming on building materials for a while and wasn’t a problem until now that a coconut tree sprung out of one of the walls. A “moisture” remediator gets called next, and what follows it’s an unfortunate tale of lawsuits and bad reviews.
I am not saying that you shouldn’t aim to improve the ductwork and runtime for an oversized system, but…
Have you ever heard of the bull in a china shop metaphor?
The china shop is the house, and the oversized HVAC is the bull.
Comfort Dynamics, Inc.