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Revisiting the Bull in a China Shop: Oversized Equipment
This article is an updated version of another HVAC School article on oversized equipment. This edition is a joint effort by Genry Garcia of Comfort Dynamics, Inc. and Kaleb Saleeby. Both of them are regular contributors to HVAC School, and we are grateful for their contributions. Thanks, guys!
The symptoms and consequences of oversized heating equipment are similar to those of air conditioners. However, the focus here will be on the cooling side—specifically from the perspective of climate zones 1-3 (hot and humid).
Of course, there are resources for learning how to select and size equipment properly and how to properly design a duct system (Manuals J, D, S, T, etc.). But what happens when you’re servicing a system that has already been installed and is doing damage? Making improvements to oversized HVAC equipment can be a very slippery slope.
What does an oversized system look like?
It looks like any other system you’ve worked on. However, you can expect an oversized system to have the following issues:
- It can’t keep the occupants comfortable throughout various rooms in the house.
- Comfort complaints intensify at night.
- It short-cycles periodically.
- Specifically when it’s less than 94°F outside and still feels warm inside—even when the thermostat shows 67°F as the room temperature.
- The relative humidity is consistently high.
- Over 55%
- RH goes through big swings throughout the day; these swings will normally track with the operation cycles.
- Films of condensation may be visible on supply vents.
- Sweaty ductwork
- Excessive noise from vents—returns, supplies, or both
- The temperature feels significantly warmer around the perimeter areas of the space (i.e., exterior walls) than in the interior areas (hallways and bedrooms).
- Notice I said feels, not reads. The occupant is a much better sensor than the thermostat, which reads only air temperature).
If you pull up to a service call and any 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 result in a larger and colder air volume being moved throughout the space due to its increased sensible capacity (the capacity to drop air temperature). Invariably, the thermostat will reach its setpoint faster, and the system will cycle off before it has the chance to bring the evaporator coil below the dew point of the occupied space, thereby failing to dehumidify.
What is the air conditioner’s job exactly?
Let’s start with mean radiant temperature. In short, the temperature of the surfaces around us has more to do with human comfort than the temperature displayed by the thermostat. (Here's another article on radiant heat transfer.)
The average body temperature is normally ~98°F; the average skin temperature is closer to ~94°F. If we stand by a wall with a surface temperature of 75°F, our bodies will cool off by radiating heat to the wall at a more comfortable rate than if we were standing by a wall with a surface temperature of 85°F. The same can be applied to furniture like couches, beds, kitchen counters, etc.
An A/C system must run long enough to keep a cool and consistent temperature on all the surfaces of a home. We typically accomplish that with runtimes of around 1 hour at a time. If the outdoor temperature is +90°F, but the system runs for only 10 to 15 minutes each cycle, that 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 of 68°F. Short-cycling is exaggerated at night. That's because the outdoor temperature drops and the A/C runs even less due to the low sensible load.
The next thing to focus on is the apparatus dew point (ADP). ADP is the effective surface temperature of the cooling coil when dehumidification is involved. In other words, ADP is the coolest possible temperature for an evaporator coil to reach under the current conditions in order to dehumidify adequately. We find ADP by intersecting the return air and supply air temperatures on a psychrometric chart using the sensible heat ratio (SHR) line appropriate to the current conditions. ADP is different from the internal saturation temperature of the refrigerant running through the coil.
While a system is off, the evaporator coil will be at a temperature close to the return air and its surrounding surfaces. Once the system cycles on, the return air temperature will determine the evaporator saturation temperature based on the DTD (design temperature difference—you can find more on that HERE). At this point, the indoor coil should reach the apparatus dew point.
But just because the refrigerant entering the evaporator is at 40°F, that doesn’t mean the entire coil will immediately drop to this temperature. This process takes time. The 40°F refrigerant has to make several passes through the coil before it can fully absorb the heat from all of the evaporator’s surface area. Only after this is complete will the coil temperature finally come down to the design ADP. This process can take up to 10-15 minutes.
If we have average run cycles in the 10-15 minute range, the full evaporator surface area will never reach its design apparatus dew point, and it will never dehumidify the air before the system cycles off. Therefore, the dehumidification (latent heat) capacity of the system will be consistently and significantly reduced, resulting in poor humidity control in the space.
This phenomenon severely worsens when dealing with high-efficiency systems. Manufacturers have found ways to drop the compression ratio of their equipment. This allows them to reduce overall power consumption, which ultimately increases SEER ratings. To achieve this, manufacturers have increased the surface area of both indoor and outdoor coils. One of the unintended consequences of this, however, is that the suction saturation temperature will now be warmer than if the coil were smaller. So, not only does the evaporator start out warmer, but it now has more surface to bring down to temperature. Shorter run times of oversized systems will accentuate the consequences of having a larger and warmer cooling coil surface temperature. (Note: Higher-efficiency equipment is not inherently bad. Variable air speed and variable capacity methods are generally used to compensate for the larger surface areas.)
But there’s no other time when an A/C system is more efficient than when it isn’t running, right? Because it isn’t 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 installing larger equipment are normally more expensive than a smaller, properly sized system.
Also, more importantly, the single highest point of energy consumption for an A/C system is at startup. That's when all the motors first energize. 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 of it all, the client is pissed off! Not only did their electric bill not go down, but now they are also uncomfortable.
So, how do we fix it?
Well, to fix it correctly, we would have to replace the system with one of the appropriate capacity and airflow design. But that’s probably not going to happen right away, is it? Not until the customer has enough frustration and headache to motivate the expense, anyway.
Before we talk about the most popular methods of dealing with this problem (e.g., extending runtime), I want to focus on static pressure, particularly when an oversized system is connected to existing, older ductwork. As soon as you start diagnosing the issue, you will likely run into a high external static pressure reading. At this point, a light bulb might go off in your head. “It’s the ductwork!” You will end up quoting duct improvement solutions that will drop the TESP, maybe even throwing in some return duct upgrades.
Let’s say the customer agrees, and once the work is done, you perform a test and balance. You ultimately confirm that the TESP is now within acceptable levels.
You may think yourself a hero! But if you did improve the duct system to move more air than before, then the problem just got worse.
Next time you realize you are in one of these situations, ask yourself:
More, colder air. Do I really want to increase the sensible capacity of this already oversized system?
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 that report to the thermostat on the warmest areas of the house, therefore tricking the system into running longer. The thermostat may also 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 potential for 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, especially reducing the airflow. The lower the supply air temperature inside the duct is below the surrounding air dew point (i.e., unconditioned spaces like attics), the more condensation will occur. 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 may be able to 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 may be moisture that has been forming on building materials for a while. It was never a problem until now that the nasty “M-word” is growing out of the cracks in the walls and baseboards. A “moisture” remediator gets called next, and what follows is an unfortunate string 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.
—Genry Garcia & Kaleb Saleeby
Comfort Dynamics, Inc.