Homeowner’s Guide to Sizing an AC System (and who is responsible when you’re uncomfortable)

This tech tip was a collaborative effort between Matt Bruner, Adam Mufich, Ed Janowiak, and Genry Garcia. Thank you to everybody who gave their input!

The next time you come across a homeowner who wants a bigger HVAC system, or you simply want to set expectations for a customer, share this tech tip with them. I hope it will allow them to understand how complicated—and expensive—it can be to get an HVAC system that can handle heating and cooling 100% of the time.


When you, the homeowner, need to replace your HVAC system, you call an AC company, which will then send you somebody to install the new one. Mind you, this person may or may not be following the best practices for installation.

Not long after the new system is installed, you start having comfort complaints that you either never had before or worse versions of the same old ones. As time passes, you might start wondering:

“But why is it so humid?”

“Why is it so hot?”

“Why is it so cold?”

“This company has been out several times, why can't they figure it out?” 

At this point, you’ve had enough. You may have taken to Facebook groups, Google, and all sorts of websites to find whatever explanation you could get your hands on. Not long after that, you realize: 

“Crud! I’ve been misinformed! I trusted these people, and they never told me that they were using rules of thumb to determine the size of my new system! This is all their fault, and they’ll pay for what they did!” 

As frustrating as it is, the situation is usually more nuanced than that.

 

Whose fault is it? 

When you’re feeling uncomfortable in your house, you want to hold the responsible person accountable. Strong arguments can certainly be made on the contractor's failure to perform load calculations and duct inspections and look at the house as a system. 

But you, me, and the people next door are not exempt from responsibility. Consumers are well served to understand that the budget tends to match the quality of the work; if a price seems too good to be true, it probably is. Like most things that require design, skill, and knowledge, a good result is not a commodity—like comparing the price of gas between local stations. 

Regardless of the size of your budget, it is your responsibility to use it wisely—this means doing research and asking hard questions. This article is meant to be a starting point that will help you make an informed decision about your new HVAC system and take ownership of that choice.

After you’ve made it obvious you’re willing to challenge any little sales pitch that gets thrown at you, the good contractors will begin to reveal themselves. These contractors understand the complexity of HVAC systems and won’t just throw a bigger unit at the problem and walk away. 

 

What does a bigger A/C unit really do?

If your A/C unit has a hard time keeping you cool or warm in extreme weather, you may wonder if a bigger unit will make you feel more comfortable. But will it? It’s like when someone has a car with flat tires without realizing it; when they get up to speed, they think, “Man, this car is slow!” So, they get a new car with a bigger engine and think the problem is solved. That is certainly one solution, but it’s not the best solution. What if they had gone to a trustworthy mechanic, patched the holes, and had some air added to the tires? Was the best solution really to go bigger?

Believe it or not, bigger isn’t always better for HVAC systems. In fact, many oversized systems perform below expectations. Systems that are too big can’t run long enough to remove moisture from the air, which can leave houses feeling muggy. At worst, too much moisture inside a house can lead to microbial growth. Many systems are oversized; if we want to stick to the car metaphor, it’s like having a giant truck on tiny wheels. It has to fit to make sense, and bigger isn’t always better. 

This article will cover the proper system design process and lay out what you can expect from a properly sized and designed HVAC system with a low to moderate budget. It will also contain some tips that can help you manage the temperature in your home to manage heat gains and losses beyond your HVAC system.

 

Heat in, heat out

Heat can enter or leave a building in many different ways; it can pass through walls via direct contact with insulation, through air passing through openings and cracks, or through glass windows via radiant heat, mostly from the sun. Inside a home, pets, furniture, cooking appliances, electronics, and our bodies also generate heat.

An HVAC professional needs to know how much heat enters or leaves a structure so that they’ll know how much heat the HVAC system needs to compensate for. We quantify that heat in BTUs, also known as British thermal units, which is a measurement of heat content. In an HVAC system, we can imagine the BTUs riding on several boxes of air per minute.

Heat is not the same thing as temperature; BTUs represent a quantity of heat rather than temperature (which we measure in degrees). Something can have more heat but a lower temperature. Let’s say we have a swimming pool of water at 80°F and a cup of coffee at 125°F. The cup clearly has the higher temperature, but it contains far less overall heat than the pool (8 ounces vs. 10,000+ gallons). That is because BTUs measure the actual quantity of heat, not the average intensity of the heat in a given object.

Heat moves from higher temperature to lower temperature, but the size of the air conditioner or heater needed is all about the number of BTUs we need to move.

 

Two types of BTUs

There are two types of BTUs we want to add or remove: sensible and latent. Sensible BTUs deal with the heat we can feel on our skin. Latent BTUs deal with the energy of water vapor in the air, which we can’t sense by temperature; even though latent heat is “hidden,” it’s also part of the heat load. 

HVAC units are designed to move sensible heat; the sensible heat affects the thermostat temperature and is what makes the system turn on and off. However, a system should run long enough that the indoor coil gets cold enough to hit the dew point. The dew point is the temperature at which the air can’t hold any more water vapor; the relative humidity is 100%. At the dew point, any excess water vapor becomes liquid and condenses on cooler surfaces (like dew). In HVAC systems, the indoor coil will get cold if it runs for a long time. Some of the water vapor in the air passing over the coil becomes liquid water on the coil. From there, that water drains out of the home; this allows the HVAC to remove humidity from your home—but only when it runs.

HVAC contractors need to size the equipment properly to match the heat in/heat out in terms of BTUs. However, BTUs are the tip of the iceberg; sizing may also affect things like energy costs and system longevity.

 

HVAC unit sizing as it relates to BTUs

HVAC units are sized by “tons,” which refers to how many BTUs they can move per hour. Generally, one ton of heating or cooling is equal to 12,000 BTUs per hour. So, a three-ton unit can move 36,000 BTUs per hour. 

In the sizing process, some HVAC contractors use rules of thumb related to the square footage of a residence. While it is true that the square footage of a house matters in the sizing process, it doesn’t tell us anything about heat gains and losses in the structure, which is what the HVAC is really trying to compensate for. These rules of thumb also don’t account for the temperature the residents want to maintain or how their lifestyles affect indoor heat loads. The contractor has to know a lot more about the structure, the climate, and the way people live to design and size a system appropriately.

 

Sizing considerations

Systems that are too small or too large may make your home uncomfortable, but their effects go further than that.

Systems that are too small won’t be able to keep up with the sensible heat load, leaving the home a little warm or cold during the hottest and coldest days. 

On the other hand, systems that are too large will have shorter runtimes and may cause high duct pressure. Shorter runtimes reduce the lifespan of your equipment; constant starting and stopping will take a toll on your unit’s health over time and make it more prone to failure. High duct pressure may also lead to shorter lifespans, but it also increases electrical energy usage and can hike up your power bills. In homes with duct leakage, more pressure in the ducts will worsen that leakage and cause a pressure imbalance, which negatively affects comfort.

Oversizing typically presents more problems in humid climates, as those shorter runtimes can lead to poor moisture control. Larger units will be able to move more sensible BTUs in a shorter time, but the runtimes will be too short for the coil to get cold and pull much moisture out of the air. When this happens, homes can become muggy, which can make you feel uncomfortable even when the temperature seems fine.

 

What factors do HVAC contractors consider when sizing equipment?

When HVAC professionals calculate those BTUs, they have to think about the test conditions. These may vary wildly by climate, weather, and occupant behavior.

Location

One of the broadest design considerations is the location of the HVAC system installation. North America has a wide range of climates, and it doesn’t make sense to use the same design considerations in Phoenix (arid) for HVAC systems in Seattle (cool and humid) or Miami (hot and humid). 

Average ambient conditions (ACCA design days)

So, contractors partially base their design conditions on the location and use specific data to make their calculations; hourly temperatures are tracked throughout the country and recorded as “bin data,” which represents the blocks of hours in which an area falls into specific temperature bins. It just doesn’t make sense to size an HVAC system based on the hottest or coldest possible days; those are only going to happen a few times a year. 

Let’s say, for example, that an HVAC system was sized based on an outdoor temperature of over 100 degrees for three days of the year. That unit won’t run long enough to remove enough latent BTUs from the air on most days when the temperature is significantly lower. (Remember, the lower sensible heat load will make it so that the thermostat shuts the unit off when it meets the desired temperature.) When homes get too humid (typically above 60% relative humidity), microbial growth may start in your home. When bacteria and fungi thrive in your home, they can make your family sick.

Household activities

Each household is a bit different, and some will produce more heat and humidity than others. So, the contractor won’t be able to predict occupant behavior perfectly, which can be a challenge for sizing and comfort.

One person living alone in a small house will generate a lot less heat and humidity than a family; an HVAC system that fits a single person’s lifestyle may not perform very well for a family that may move into a house during the equipment’s lifespan. When there are more people in a house, there will usually be more heat—especially latent heat—from bathing, cooking, and doing laundry. When cooking or bathing, the use of a range hood or bath fan can also help control humidity resulting from those activities, but those have to be correctly sized for them to be most effective. Not to mention, one person gives off much less body heat than three or more people. 

There are several other factors, too. For example, pets also give off body heat and moisture. Homes with pools may also have high humidity if people constantly go in and out of the house while using the pool or if they leave the door open. Those will almost surely affect comfort and HVAC system performance, but the contractor can’t control those or factor them into the design precisely. 

Keeping doors and windows open also allows heat to enter or leave a building via convection, which heavily influences the heat load. Blinds and shade can also affect the amount of heat that enters a building via radiation; if a structure is underneath a tree, the leaves will likely absorb most of the solar radiation before it reaches a house. The same applies to blinds, which block the sun’s electromagnetic waves from passing through windows.  

 

Sizing considerations and options

When designing and sizing a system, we have to consider that the process won’t be perfect; on some days, the HVAC system may not be able to heat or cool a home as well as you would like in the absolute hottest and coldest hours of the year. However, I think almost everyone would rather be very comfortable 99% of the time instead of 1% of the time.

So, it’s better to design an HVAC system to handle the most common conditions for the location and, to the best of the contractor’s ability, the occupants’ habits and routine; it should be able to manage your comfort on all but a handful of hours on a few outlier days of the year. You won’t freeze or get heatstroke if you stay indoors on those days, but you may be a bit less comfortable than usual. 

The other option is to oversize the unit and maintain temperature on those outlier days—with the downside of dealing with high humidity on many other days and excess wear and tear on your unit. This second option will be more costly, uncomfortable, and possibly unhealthy in the long run. 

In some cases, HVAC contractors may be able to install a secondary dehumidifier to keep up with the high humidity on most days, but this option does come at a cost. 

 

Design standards for sizing equipment

HVAC professionals use a set of formulas to find out how many BTUs enter or leave a space. They consult a manual published by the Air Conditioning Contractors of America (ACCA), Manual J. In many cases, they use software to help them determine how many BTUs the HVAC unit will need to compensate for. 

These load calculations account for the heat loads generated by the people in the home, their activities, and the structural features (including building or duct leakage) to come up with an estimate of BTUs that need to be added or removed per hour. On the heating side, Manual J also helps determine how many BTUs the HVAC unit needs to add to counter the BTUs lost via the walls, cracks, and through windows.

Manual J isn't perfect

Contractors rely on several data points to determine how many BTUs a system needs to move; however, some of these markers will be estimates at best.

Manual J’s worst enemy, however, can be its interpreter and practitioner. The overwhelming majority of contractors who won’t bother with Manual J have never actually read it—maybe because they don’t have the time or will. 

We also can’t rule out that a few contractors may deliberately act in bad faith. Think of Manual J as a given set of laws; when applied by corrupt practitioners, these laws will wrongfully cause harm. But when applied without any biases or agendas, it can be the biggest ally to honest practitioners. Manual J is the same. If an HVAC professional were to shrink or expand the boundaries set by Manual J deliberately, it could do more harm than good. But at the end of the day, that dishonest practitioner could still use the “I did a Manual J” argument as a shield.

Manual J formulas also won’t account for the heat loads on all days, as that will change depending on the weather and seasonal conditions. Contractors need to keep these things in mind and explain them to their customers to establish realistic expectations. Consumers need to hold contractors accountable to follow a process but understand that there is no “perfect” process and, no matter what, compromises must be made.  

Once a contractor knows how many BTUs the equipment needs to move most of the time, they will consult another manual, ACCA Manual S. This manual is the ultimate guide to equipment selection based on the BTU capacity required. One ton of capacity is equivalent to 12,000 BTUs per hour, and contractors use that figure along with their Manual J calculation to determine the capacity.

 

Other ACCA Manuals

If all the design standards are followed to a T, Manuals D and T will likely also come into play. These deal with the transport of air throughout a residential building. 

Manual D helps contractors size and run ductwork that fits the structure and equipment. Turns and fittings in the ductwork add extra “length” because they add friction or turbulence to the ductwork. Manual D contains formulas and guidelines that help contractors deal with those when designing ducts. Flex ducts behave similarly when they are compressed, as compression increases friction; Manual D also helps with that.

Generally, people don’t like having air blasted directly on them, so that’s where Manual T comes in; it ensures that the air mixes properly and that the duct system can help the house maintain the desired temperature while going mostly unnoticed. Manual T helps contractors select, size, and place vents. 

Many multi-family (and some single-family) homes may also be designed to have zoning capabilities. Zoning allows the HVAC system to adjust the airflow distribution between zones of the structure based on where it is and isn’t needed, resulting in energy savings. There is an ACCA manual that also helps with the zoning of HVAC systems: Manual Zr.  

 

How much does the ductwork matter?

The quality of your ductwork will have a pretty big impact on the comfort of your home. The ducts carry the air from the HVAC unit to different rooms of your house, so improper sizing, leaks, and poorly selected filters can all negatively affect your comfort. 

A study by the Department of Energy concluded that the average duct system leaks about 30–40% of the air moving through it, meaning duct sealing or repair can have a huge impact on your system’s ability to keep up on hot or cold days. These leaky ducts can also pull in contaminants from nasty spaces like walls, attics, and crawl spaces, so duct sealing can have a beneficial health impact as well.

Generally, larger ducts are preferable to smaller ducts, and large filters are preferable to small filters. Air has weight because it contains many different elements and creates resistance. That’s why cars are aerodynamically designed and parachutes slow people down when falling from the sky. Duct systems need to be designed carefully with these principles of physics in mind. A well-designed duct system will be quiet and leak-free and keep you comfortable from room to room. Depending on your needs, you can also design your filtration system to capture very small particles in the air, helping your family stay healthy. 

 

Retrofit or Replacement Systems

The ACCA Manuals come in handy for designing new systems, but what about retrofit options for existing systems?

A contractor will typically approach a retrofit by doing a Manual J block load calculation. A block load calculation determines the total BTUs that need to be moved into or out of the entire home; it is not a room-by-room calculation.

In some cases, contractors may also use Manual S to select equipment based on the Manual J block load calculation. Furnace oversizing is relatively common and isn’t quite as big of a deal as heat pump oversizing. In furnace-to-heat pump retrofits, consulting Manual S can allow contractors to correct previous oversizing (or undersizing).

 

How can we control heat loads without the HVAC?

Even when designed perfectly, most HVAC systems designed on a low to moderate budget simply can’t handle extreme temperatures or highly variable indoor heat loads without some compromise. However, in many cases, we can control comfort better by adjusting the home conditions to fit the HVAC instead of making the HVAC fit the home. You can target some heat loads at the source by focusing on ways to manage heat transfer in your own home.

If we’re trying to minimize heat gains and losses via conduction (touch), we’ll want to focus on improving the insulation. 

To manage heat gains and losses via cracks or openings in the structure, we would benefit most from focusing on air sealing. Duct sealing may also be worth considering if the ductwork is leaky and contributes to comfort problems in the home. Of course, habits like keeping doors open also affect those gains significantly, so you can keep windows and doors closed to control a good chunk of the heat load.

We have to get creative at managing radiant gains. Sometimes, you can minimize the rate at which the sun’s rays shine through windows by using curtains or blinds; a fancier and more expensive solution is to use Low-E window film, which reflects incoming electromagnetic waves. When it comes to the attic, some homeowners opt to use reflective materials, also known as radiant barriers. These look like large space blankets and reflect the incoming heat back toward the roof instead of letting it pass into the attic. However, the cooler conditions on hot, humid days may lead to condensation in the attic—and eventually bacterial and fungal growth, a pretty big drawback.

 

Conclusion

All of that is to say that a properly sized HVAC unit can manage a good chunk of the temperature and humidity demands of a building under typical conditions for the location and climate. It’s the contractor’s responsibility to make sure that the BTUs gained or lost through heat transfer are factored into the design process.

However, occupants also affect heat loads in ways that the contractor can’t predict. HVAC contractors can’t design for heat loads from leaving doors or windows open, doing laundry, cooking, or showering. When possible, it’s usually better to adjust home conditions and practices to fit the HVAC rather than the other way around on those extra hot or cold days.

The HVAC contractor’s obligation is to design the system so that it will work properly in most cases, but we must understand that the home occupants and their behaviors may change during the life of the system. 

If homeowners don’t want to experience discomfort on the hottest and coldest days of the year or have trouble managing indoor heat loads, then the only safe solution may be to install a higher-end HVAC setup as well as a supplemental dehumidifier in many climates.

In the end, only a good design, implementation from a competent contractor, and an appropriate budget will get a result that covers all the bases. 

Comments

Alice Carroll
Alice Carroll
3/24/23 at 02:11 AM

Wow, I didn’t know that there are also some calculations that should be done to ensure that an air conditioner would be the right size. I’d like to find a good air conditioning contractor soon because I might need to have one installed in my workshop. It gets way too hot in there sometimes.

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