Three System-Sizing Superstitions The HVAC Industry Suffers From
“When you believe in things you don’t understand, then you suffer — superstition ain’t the way.” – Stevie Wonder
The HVAC industry has suffered from system-sizing superstitions for generations. These superstitions are passed down with the honest assumption that they work. After all, if it did the job for grandad, it must be right. Unfortunately, these beliefs often have hidden consequences that lead to long-standing issues and a lot of aggravation.
Many customers suffer as a result because they fail to get the system they expected and often must deal with long-term issues that lead to discomfort and shortened equipment life. You (HVAC professionals) suffer from these beliefs because you’re left trying to correct the issues they create.
It doesn’t end there — the cycle expands beyond you and your customers. For example, distributors suffer because they are stuck in the middle, trying to find a solution that helps the consumer without throwing their customer (the HVAC professional) under the bus. Manufacturers suffer because they bear the brunt of warranty claims and the associated costs that go with them. They spend millions of dollars per year because so many believe and act on these superstitions.
Long-accepted superstitions make the professional who does things right look like the crazy guy talking to himself alone in the park. It’s tough being the professional who proposes a 2-ton cooling system when everyone else proclaims 6 tons is the way to go. Let’s look at three sizing superstitions our industry suffers from and how we can overcome them.
BIGGER IS BETTER
One common superstition says bigger equipment is better. If it works for car engines, it must work for HVAC systems. All of us dread callbacks on extreme temperature days when a system cannot maintain temperature. This fear often leads to oversizing equipment.
Oversized equipment provides a large cushion of extra heating or cooling to account for inferior installations, thermostat setback, and corner cutting that’s beyond an HVAC professional’s scope. “If 3 tons will work, let’s install 5 tons and cover ourselves!”
Another contributing fact is that some contractors use simplified pricing models, based on how many tons of cooling they install. Fewer tons equals lower prices, so less money is brought in when equipment capacity drops.
Instead of looking at HVAC equipment sizing like a car engine, perhaps we should look at it like shoe size. Your feet have a corresponding shoe size that’s just right, and HVAC equipment is the same. It either fits or it doesn’t. When a shoe is too big for your foot, you have no control over it, and it can’t perform its intended function. Temperature and humidity will respond the same way with oversized equipment.
If you believe in the bigger-is-better superstition, the consequences may be continuous equipment problems and comfort complaints from customers. If you’ve been told the living space quickly gets cold but still feels muggy, check the equipment sizing.
To overcome the bigger-is-better belief, do your best to prevent it before it happens. Follow industry best practices for equipment sizing and selection based on accurate load calculations. If the installed equipment is already oversized, recommend replacement. While this can be a tough pill to swallow, it’s ultimately what’s best for you and your customer.
500 SQUARE FEET PER TON
Another equipment sizing superstition used by many of our grandfathers is the 500 square feet/ton rule. There was a time, long ago, when one ton of cooling for every 500 square feet of living space worked in many parts of the country. But buildings have changed quite a bit since that time.
You have to go back to early 1970s construction practices to find conditions that needed one ton of cooling for every 500 square feet of living space.
The building components in a house of that era typically included:
- R-11 insulated walls
- R-19 rock wool/fiberglass batts in the ceiling
- Uninsulated crawlspace
- Single-pane aluminum frame windows
- Unsealed and poorly insulated ducts outside the conditioned space
- No air sealing
- Single story structure with eight-foot-high ceilings
A typical house built today requires one ton of cooling for anywhere between 750 to 1,500 square feet of living space. That’s a massive change from sizing methods used in the early 1970s that can be hard to accept.
Buildings are better insulated today. Insulation levels in attics, walls, and floors can be double what they were in the 1970s. The knowledge of how to properly install these products has increased, so insulation is more effective in the field. Windows don’t transfer as much heat as they used to — both the glass and the frames.
Building connections are also tighter and sealed to prevent uncontrolled leakage to unconditioned areas like attics and the outdoors. Infiltration is reduced to minimal levels compared to what they were almost 50 years ago. As you can see, building practices have improved, but our sizing methods haven’t.
If you still rely on 500 square feet/ton, it’s time to look closely at the environment you’re conditioning. Buildings have changed. The building is what connects air from the supply registers back to the return grilles — it is essentially an extension of the duct system. Let’s make sure we understand its influence.
Another superstition that has been brought along from the early 1970s is the 12,000 Btu/ton rule — also known as nominal cooling capacity. Cooling capacity is measured in Btu/hr (British thermal units per hour). It is a measurement of how much total heat the cooling equipment removes.
You can break down the total amount of heat removed into two forms — sensible and latent heat. Sensible heat creates a change in temperature, while latent heat creates a change in moisture.
It was common for cooling equipment in the early 1970s to have indoor coil temperatures designed around 40°F. This results in 70 percent of the equipment’s capacity doing sensible heat removal, while the other 30 percent removed latent heat. Under these conditions, equipment typically removed 12,000 Btus/ton, 8,400 Btus of sensible heat (lowering room temperature), and 3,600 Btus of latent heat (lowering room humidity).
Newer equipment is designed with warmer indoor coil temperatures and different sensible and latent heat ratios. Don’t be surprised if you see 85 percent of the equipment’s capacity lowering air temperature and only 15 percent for moisture removal. The equipment delivers roughly 11,200 Btus of total cooling — 7.5 percent less cooling per ton. When equipment capacity is adjusted for real world operation, additional capacity is lost.
Continuing to assume 12,000 Btu/ton has resulted in many contractors wondering why systems that worked great prior to equipment replacement now can’t keep up and dehumidify properly.
To keep this from happening to you, research manufacturer specifications for the equipment you install and make sure it can do the job. Understand the factors that impact equipment capacity and learn how to adjust for them. If there isn’t enough latent heat capacity, you may need to consider other equipment options or supplemental dehumidification.
DEBUNK THE SUPERSTITIONS
When superstitions are replaced with understanding and measurement, our industry wins. Customers get what they invest in, and HVAC professionals are paid like the specialists we truly are. Distributors and manufacturers can have peace of mind knowing they aren’t sweeping a problem under the rug and paying for it with their reputation. They can be assured their brand is protected when sizing and installations are done right.
Fixing these superstitions is easier than we think if we correctly put the puzzle pieces together. We can overcome these superstitions one customer at a time. It starts with understanding the environments we condition each day. This means looking beyond the equipment and even the duct system. There is a huge difference in conditioning an old farmhouse and an insulated concrete foam building.
The next step is to use proven design practices instead of guesses and assumptions to lay the foundation for your systems. Once the plan is in place, install the system according to industry best practices — don’t cut any corners, or the design won’t work. When the installation is complete, test it to assure your design truly works.
Isn’t it time our industry got paid handsomely for systems that address the unique needs of each customer and really work? Let’s stop believing in superstitions and start understanding what truly succeeds and move our industry forward.
If you’re an HVAC contractor or technician interested in learning more about debunking superstitions, contact me at firstname.lastname@example.org or call 800-633-7058. NCI’s website, www.nationalcomfortinstitute.com, is full of free technical articles and downloads to help you improve your professionalism and strengthen your company.
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