As many of us started in the industry, we learned quick rules to simplify complicated topics. One of these first rules is 12,000 Btus (British thermal units) per one ton of cooling capacity. It’s a rule that many in our industry accept, but is it always true? Could this rule get you in trouble if you don’t dig a little deeper? Let’s look at how equipment cooling capacity tables can answer these questions with the tons of information they contain.
12,000 Btus = One Ton?
A long time ago, we didn’t have mechanical refrigeration systems for cooling. In those days, ice was used for refrigeration. The ice industry needed a standard reference for refrigeration capacity. That reference became the weight of ice and how long it took to melt.
Think of it this way: It takes 144 Btus to melt one pound of ice at 32°F. One ton of ice weighs 2,000 pounds. If we multiply these together, we end up with 288,000 Btus (2,000 x 144 = 288,000). If you divide 288,000 by 24 hours in a day, you get 12,000 Btuh per hour (288,000 ÷ 24 = 12,000). So, if you’re dealing with a 2,000-pound block of ice, 12,000 Btu per ton is perfect.
Mechanical refrigeration systems are complex and have more to consider than melting ice. The amount of Btus per ton is a moving target. To account for this, equipment manufacturers publish expanded cooling performance tables. You can look at them and predict how many Btus a cooling system should remove at certain conditions. It’s surprising how often it isn’t 12,000 Btus per ton.
Equipment Cooling Capacity Btu Breakdown
As you look at equipment cooling capacity tables, there are three Btu types to consider. They are sensible, latent, and total Btus. In cooling mode, you must account for all three.
Sensible Btus are those you can feel and cause the temperature to change. When a cooling system removes sensible Btus from the air, the supply air temperature drops. The refrigeration cycle removes heat as air passes across the cold evaporator coil.
Cooling systems also remove moisture from the air. As warm, moist air contacts the colder evaporator coil surface, moisture in the air condenses. It then runs into the drain pan, where it flows down the condensate drain. Latent Btus are part of the moisture removal side.
Since a cooling system removes both sensible Btus (temperature change) and latent Btus (moisture change), you need to account for both on the manufacturer tables. Total Btus are a combination of the two and how manufacturers rate total cooling equipment capacity. It’s the value we often think of as Btus per ton.
Some manufacturers use the term MBtuh in their tables. The M stands for one thousand, so a unit removing 36,000 Btus would appear as 36.00 MBtuh on the table. Don’t let the differences confuse you. They are different ways of saying the same thing.
Four Airside Factors to Consider
Now that you’re familiar with the different Btu types you’ll see on the tables, there are also four airside factors to consider. They influence how many sensible, latent, and total Btus the equipment will remove. Once you identify these factors for your test or design conditions, you can see the different Btu capacities.
Airflow across the evaporator is the first factor to consider. It appears as cfm (cubic feet per minute) on the manufacturer tables. As airflow across the evaporator increases, total and sensible Btus increase while moisture removal (latent Btu) decreases. Although airflow rates of 400 cfm per ton are average, humid climates may need 350 cfm per ton, and dry/arid climates may need 450 cfm per ton or more. You can easily plot fan airflow using manufacturer fan tables.
Evaporator entering dry bulb temperature is the next factor to consider. It appears as EDB (entering dry bulb) on the manufacturer tables. As indoor entering dry bulb temperature increases, more cooling is used to reduce the air temperature (sensible) rather than remove humidity (latent). Take your EDB measurement at the evaporator coil, not from the thermostat display.
Evaporator entering wet bulb is the third factor. It is a temperature reading taken at the evaporator coil. It appears as EWB (entering wet bulb) on the manufacturer tables. As EWB temperatures increase, total cooling capacity also increases. More cooling is used to reduce humidity (latent) rather than lower the air temperature (sensible). With the right test instrument, you can measure dry bulb and wet bulb temperatures together at a single test location.
Condenser entering dry bulb temperature is the last factor to consider. It appears as OAT (outdoor ambient temperature) on the manufacturer tables. Cooling equipment capacity (total, sensible, and latent) decreases as the outdoor air temperature increases. When you measure OAT, take your readings in the shade near the condensing unit. Avoid any radiant heat sources like brick walls, dark roofs, or direct sunlight. It’s also wise not to use the weather app on your smartphone. Depending on your location, the temperatures could differ a lot.
AHRI and Real-World Conditions
To add to the changing Btu capacity, also consider the equipment laboratory-rating conditions versus real-world operating conditions. They can vary and change the expected Btus from the cooling equipment.
Laboratory-rating conditions are often referred to as AHRI (Air-Conditioning, Heating, & Refrigeration Institute) test conditions. Many manufacturers use 400 cfm per ton, 80° EDB, 67° EWB, and 95° OAT. Unfortunately, many of your customers aren’t comfortable when it’s 80°F with 50% relative humidity in their home. So, you need to adjust to more practical circumstances.
Real-world conditions are how you tweak capacity for a more comfortable living environment. Let’s say you live in Texas. Your real-world conditions are a little different from AHRI test conditions. You may use values like 400 cfm per ton, 75° EDB, 63° EWB, 105° OAT. How would this compare to AHRI test conditions?
Let’s say you have a three-ton equipment matchup rated under AHRI test conditions for 36,000 total Btus. Next, adapt the four factors to reflect the Texas numbers above, and you see the equipment rated capacity drops to 31,600 total Btus. At these conditions, the three-ton equipment functions closer to 2.5-ton equipment. The result is a 12% decrease and lower-than-expected Btu per ton values.
Now that you understand the differences in equipment capacity, I hope it puts the term “tons” in a different context. As you can see, equipment cooling Btus lower the space temperature and remove moisture. With the manufacturer tables, you can see how much equipment capacity goes to fulfilling each purpose.
Don’t assume equipment capacity or conditions. Instead, do some digging and find the tables for the equipment you install and service. You can locate many of them on the manufacturer’s websites, equipment specifications, or with a quick Google search. If you run into older equipment, the challenge is finding the information.
As you dig into the tables, you will see a story unfold about how equipment has changed over the years. You’ll also find answers to why a homeowner didn’t experience comfort problems with their old equipment. However, since they upgraded to newer equipment, they now have humidity issues, and their home cools down too fast.
For further information on this subject, I recommend ACCA Manual S, Residential Equipment Selection. It has details about how to select equipment appropriately. If you like this subject, you’ll enjoy the read.