How an A/C System got to be Oversized (Maybe)

But My Old Unit Worked Fine?!

Most of us have heard this at some point. This complaint typically comes from a particularly unhappy customer after the installation of a brand new A/C system. Throughout this article, we’re going to explore the possible root causes of this situation, but first, some ground rules:

• We are going to navigate this issue from the perspective and comfort needs of climate zone 1. In other words, it’s hot and humid!
• Also, the intention is to describe the impact that progressive accumulation of a set of factors can have on the comfort (or lack thereof) of people within a building envelope over time, which means that, while a hypothetical story, it is based on actual events plus some reasonable assumptions.

So, going back to our complaint.

The complaint comes from whom we’re going to call homeowners C. The Cs just bought their first house about seven years ago. The home was ready to move in, but they did have to replace the old dark shingle roof shortly after. The new roof is much more cosmetically appealing with the light tile they used instead. It is a 1980 production home, and with the converted garage, it adds up to about 1,450 square feet. It's not much, but it’s home, and they’re proud of it. So much so that when the old A/C system started having some problems a few months back, they decided it was time for a new one.

Through the reference of friends/family, they got your number and requested a free estimate. You showed up on time, wore your booties, petted their dog, and had the right answers for all their questions. “You had me at 0% interest!” said the husband.

The wife asks, “What are all these returns you said we need?” She only asked because you’ve pointed out how all three bedrooms and the converted garage have only the undercut on the doors as return air paths.

The existing unit is a 14-year-old, 4-ton split system. “Do you guys feel this system keeps the house comfortable?” you asked.

“Sure,” they said, “but we can probably use a bigger one. It gets warm in here every time we have a party.” But you know better than that; you convince them to stay with the 4 ton, but as an act of good faith, you’ll run a new refrigeration line set. It’s only 50 feet, but the existing suction line is 3/4″, and you know that the manufacturer recommends a 7/8” for better system capacity and performance. The liquid line will, of course, be 3/8”.

So, the system gets replaced, including the new lines, and the returns are added for all four spaces. That’s when the fun starts! For months, you’ve had to go back every 2 or 3 weeks, all to address the same chief complaint. They feel uncomfortable no matter how low they set the temperature. You’ve tried replacing and relocating the thermostat. You’ve also been in touch with the manufacturer a few times and confirmed that the subcooling and superheat are within range. The TESP is high, but that’s “OK” because this new air handler has a constant CFM motor (2.3 or 3.0), and according to the fan performance table, the unit is indeed moving enough air.

“What did I do wrong??”  you say to yourself. Nothing, but the Cs are very upset! Right now, that 0% interest doesn’t feel as sexy as it did back when you were petting their dog. What’s worse—they are blaming you for it! They are convinced that all their predicaments started when the new system was installed, but little do they know that it was many years in the making.

From the top

It’s 1980, and the house is brand new. It’s a slab-on-grade, 1,200 square feet initially with 9’ ceilings, 3/2 with an adjacent garage. It has a dark shingle roof housing the ductwork in a ventilated attic, and the back of the house faces west. The backyard can be admired from inside the house since there are some seriously large glass sliding doors. A load calc gets done on the house, and an engineer estimates that a 3-ton system will work (perhaps a smaller one would have worked too). The indoor unit is installed in a closet with a louvered door inside the conditioned space.

Enter homeowners A. The As are a mild-mannered semi-retired couple, and some peace is all they’re after at this stage in their lives. They like the new house and add a back porch where they can hang out and BBQ occasionally. But the now porch-covered back of the house that faces west gets too hot in the afternoon. “Let’s plant a couple of trees, dear,” she says while the husband changes the subject with the hope that she’ll forget about it. The trees get planted, and the perspective of enjoying some shade in the afternoon looks promising.

Fast forward 15 years, and among other things, it looks like it’s time for a new A/C system. Mrs. A has been thinking about converting the garage into a room so her husband can watch sports and drink beer undisturbed. She hasn’t, however, decided on the wallpaper yet, but it’s happening. In addition, as the system has aged, it has decreased performance and can’t seem to keep up on the hot summer days, which can only mean one thing: they need a bigger unit!

A 3.5-ton system gets installed. It's the same ductwork, but a new 8” supply duct gets added to the garage. “That’s 200 CFM for you, Mr. A,” said the installing contractor as he got into his truck and drove off. The garage finally gets converted into a room. The garage door gets removed, and a block wall goes up instead. The room has a brand new TV, minibar, and a nice comfortable chair, but it has no dedicated return air path to go with that new supply vent—just the door undercut. The conditioned space is now roughly 1450 square feet (I don’t know the aspect ratio of the garage; let’s use round numbers).

As the turn of the century approaches, traffic has been getting hectic, and there were a couple of near-hits last hurricane season. The As have had it and are ready to sell the house and move on! By now, those young trees that Mr. A reluctantly planted are towering over the back of the house, providing cool shade.

The cosmetic inclined home remodel

Cue homeowners B. Now, the Bs are in it to win it; they have some capital to burn and show up with a sledgehammer in hand. It’s time for some serious remodeling.

They want the house to look modern but also want it to be safer. As such, they rip out all the windows and replace them with low SHGC, low E, impact glass windows. As for the sliding glass doors, they replace them with smaller, wood-framed double doors and a similar type of glass as the windows. Now, they don’t have to board up the house three times every hurricane season, they thought.

For the interior, among other things, they get a new kitchen and add a 100 CFM exhaust hood. To give the house the ultimate modern look, they replace all the existing lighting with ceiling recessed lights. About two dozen of those evil things throughout the house! No LED bulbs; they are the vented cans type, so they can dissipate the heat up into the attic by convection.

When all the construction work is done, they move in and crank down the air conditioner. But something’s wrong. “We made all these improvements to the house: new, more efficient windows and doors. But the thermostat won’t go below 76?! It must be time for a new A/C unit.”

As they’re walking through the local big chain store of home goods one day, they see a kiosk where they are promised a better life by way of installing a new “Comfort System” for their home. “Just what we need!” they said. The Bs secure a conveniently prompt, free visit from the ‘Comfort Advisor/Specialist/Commission-Based Former Car Salesman.’ This so-called specialist shows up at their house, and after listening to their problem, he concludes that their unit must be too small.

“You need a 4 ton for this house, I am sure. Not only that, because today is Wednesday, we are going to give you a complimentary UV Light kit that is going to clean the air in your house!” LOL!

The system gets replaced with a 4-ton. The owners still have some comfort issues, but it does feel more refreshing in the house, and so, they move on with their lives.

Fast forward a decade or so, and now they no longer like it there; one of them got a job offer out of town, they don’t like their local Sedano’s supermarket, traffic got worse—I don’t know, you pick the reason.

The Cs buy the house from the Bs, and here you are. We have a brand new, perfectly operational 4-ton AC system and a very unhappy customer.

Where did it go wrong?

By now, you probably know where I’m going with this, but let’s try to break it down anyway:

1 – How the block load decreased:

Beware of the shade. The west and south-facing sides are the two with the most heat gain incidence. By building a porch over the west side and adding trees, a good chunk of the block load got chopped off.

In addition to that, the fenestration factor of the load was profoundly affected by upgrading the windows and doors. The more energy-efficient windows and doors installed have also decreased the amount of heat that the A/C system was supposed to handle.

Lastly, there is a new roof. As compared to shingles (especially dark-colored ones), roof tiles will reflect some heat as opposed to absorbing it. This keeps the roof surface cooler, and therefore, also the attic. A cooler attic will result in a cooler ceiling, and of course, less heat gain. I highly recommend this excellent research study by the FSEC on the subject.

2 – How the cooling load increased:

As the A/C system kept getting bigger, the lack of return consistently worked to offset the decrease of the block load.

The table below can be found on page 28 of ACCA’s Manual D.

You can also read this article by Allison Bailes quickly before you keep going. That way, it’ll make more sense.

Remember when, in anticipation of the garage being converted into a conditioned space, how the system was oversized by the half-ton? How come that didn’t cause any trouble?

Because we lost about 100 CFM of return air from the former garage and replaced it with unconditioned infiltration air coming into the envelope through every crack and crevice, if a supply vent brings in 160-180 CFM of cold air and all you have for return is a door undercut of 1” on a 36” door, on an isolated room, you’ll only get 80 CFM back. Because this is not a tight envelope, the balance will likely come from the attic through unsealed access doors and ancillary spaces. Therefore, this tonnage increased was pretty much a wash and an extra expense on the utility bill.

But remember how the car salesman/comfort specialist sold homeowners B a 4-ton? And they upgraded all those windows and doors—how come that didn’t cause any trouble?

Let’s start with the consequences of connecting a 4-ton air handler to the existing ductwork designed for a 3-ton.

The old 4-ton unit had a PSC evaporator motor. Connecting the larger air handler to the smaller air distribution system will cause the TESP to go up, but it’s also going to move more air than the previous, smaller unit. It will likely not move enough air for the system to deliver its 4-ton nominal capacity, but it will nonetheless move more air.

If the duct configuration remains unaltered, this, in turn, will proportionally increase the airflow out of each vent. That means that if you had 100 CFM coming out of a given supply vent and the total system airflow is increased by 20%, then there’d be 120 CFM approximately being delivered by that same vent.

But what does that mean? Remember how we can only get about 80 CFM worth of return air? Now, there is more airflow going into each room with a door closed, but we are getting a lower percentage of it back. If, for example, there were four bedrooms with the doors closed, and the total combined airflow for these rooms was 700 CFM, then we are getting 320 CFM of it back, or 46%. If the total airflow went up by 20% with the bigger unit, then there are 840 CFM going in. Still, only 320 CFM is coming back, which is now 38%. Consequently, the pressure in the central open area is even lower, and there is a higher pressure differential driver bringing unconditioned air into the building envelope. If there isn’t a low-resistance path for the return air to travel from the isolated rooms to the core/main return area, then upsizing the system will result in more infiltration, not necessarily more cooling of the space.

I get that these doors may not always be closed, but when they are, this is going to happen. These rooms not being isolated all the time will intermittently mask this condition and make it much harder to diagnose.

To add insult to injury, the reverse stack effect has kicked in. Remember what else the modern homeowners B added? High hat can lights! Those aren’t the LED sealed can ones available nowadays. No, these are the evil ones, full of little openings. Not only is the pressure differential between the attic and the conditioned space higher, but now there is a highway for the reverse stack effect of bringing nasty, humid attic air into the space.

Lastly, there is the occasional use of the kitchen exhaust hood at 100 CFM without makeup air and the more than likely small percentage of duct leakage to factor in. At this point, it wouldn’t be unthinkable to have at least 5-10% duct leakage. Duct leakage has the same space depressurization effect as the lack of return mentioned above (for different reasons). If you want to look at some hard numbers on its consequences, check this article by Neil Comparetto.

But wait for a second! Isn’t this an air conditioner?! If the air conditioner is bigger, then it should be able to handle more significant loads.

Correct, but those are higher loads of design conditions at 80 °F DB and 67 °F WB, not of infiltration air at 75 °F dew point! Imagine that you were trying to thread the needle, and every time you miss the hole, somebody kept handing you a thicker thread.

The capacity of the system went up

So, now it's the present day, and it’s your turn to screw up. Of course, you meant well and wanted to do the best possible job but, you were doomed right out of the gate. If you see a 4-ton system, and the consumers tell you that it has been keeping the house comfortable, then what do you replace it with? A newer, shinier, and more efficient 4-ton system—with a constant CFM ECM (not the same as a continuous torque) evaporator motor, a larger-diameter suction line, and added low resistance return air paths to all the isolated rooms.

The new ECM motor will deliver the design airflow against up to 1” WC for most manufacturers. This means that, though at an energy penalty, now the system is moving as much air as it was designed to do, so the system capacity improved over the old unit.

With the larger-diameter suction line, you got about 2.5% of total capacity back over the old system with the 3/4 line. It's about 1,000 BTU/H, which isn't much, but the system capacity did go up.

Adding returns to all the infamous isolated rooms put it over the top. Now the pressure differential between these rooms and the core return area is nearly if not zero. Because of this, the system is no longer pulling in as much infiltration air as it was before. All the volume that is being delivered by the supply vents is now going smoothly, making it back to the air handler in the form of cool, crisp return air. And this is where it gets hairy.

Because the block load of the house was decreased by the new roof, new windows and doors, and the shading of the trees, and now, because there is less infiltration of unconditioned air, there is more air moving across the evaporator, and the total system capacity went up, there is less of a load for the A/C system to handle which makes it…(drum roll)… oversized!

Sure, the ceiling recessed lights are still there, and the reverse stack effect is still doing its thing, but in light of the new circumstances, the occupants will have to double down on those to have a chance at the system, not short-cycling—as much.

By now, you, the installing contractor, have figured it out. You take a big gulp, brace yourself for impact, and walk up to poor homeowners C. After an elaborate but oversimplified explanation of the situation, you gather the courage and confess to them: “Ma'am, this unit is oversized…”

Pause for effect…

“But My Old Unit Worked Fine?!”

—Genry Garcia