The ACCA Manual J is the bible of the HVAC world when it comes to proper equipment sizing. This manual contains weather data from all over the united states and scientific information from construction materials which it brings together through algorithms to guide is in the direction of properly sized equipment for homes.
When we talk about equipment sizes (for cooling/air conditioning) we typically refer to it in terms of "tons". A ton of cooling is roughly equal to 12,000 British thermal units (BTU), the amount of energy per hour required to melt 2,000 lbs (1 ton) of ice. Another way to think about this is a burning match produces about 1 BTU or the fact that it takes 1 BTU to raise 1 lb of water 1 degree at sea level. Common equipment sizes are 1 ton, 1.5 ton, 2 ton, 2.5 ton, 3 ton, 3.5 ton, 4 ton, and 5 ton. That scale ranges from 12,000 btu/hour up to 60,000 btu/hour. This is a huge scale to chose from and a dangerous one to guess at. It sounds great in theory to say just put the biggest reasonable air condition in, however as with anything in life, for every up there's a down!
The first reason a proper load calculation and equipment sizing is important is life expectancy of the equipment. Oversized equipment will "short cycle" meaning it will start and before it can run an appropriate amount of time, it shuts down; then shortly after that it needs to start up again. This happens because oversized equipment will cool the air in a space too quickly and shut down because the thermostat has satisfied. Note that I said it cools the air too quickly and I didn't specify the home. Nate Adams was the first person to introduce me to the term "mean radiant temperature" (MRT). The MRT deals with the average temperature of the surfaces in home such as furniture, floor coverings, walls, windows, ceilings... As Bryan Orr likes to say, nature will always equalize and if we cool the air in a home to lets say 70 degrees too quickly but those surfaces are 76, the heat will move out of those surfaces into the air causing the thermostat to demand cooling again. This leads to short cycling and it can happen very quickly. You wouldn't turn your car on and off, on and off, on and off repeatedly cause you know it's not good for it. Same thing goes for your ac unit. There is oil in the refrigerant circuit that lubricates the compressor and this oil doesn't move properly until the system has reached it's full compression which is a minimum of 15 minutes after start up. If a system is only running 12 to 20 minutes at a time, that oil is not moving properly back to the compressor where it needs to be, ultimately leading to a failed compressor.
If the idea of replacing a very expensive compressor isn't scary enough, how about the propagation of fungi and bacteria in your home? This is an issue in my market as I live in coastal North Carolina where we deal with heavy latent (moisture) loads. Let's go back to Bryan Orr's statement about nature equalizing. If it's 80%RH outside we need to dehumidify our homes not only for comfort but safety as well. That moisture from outside is going to be constantly moving inside through a multitude of avenues because our homes are not air tight. The only time your HVAC system can dehumidify your home is while it's running, so an oversized system that isn't running enough won't be able to meet this need. The worse case of this I ever witnessed was about 7 years ago I had a service call to property in Richlands NC where the man said he had mold growing in his house. The contractor he had been using for 20 years put in a brand new 5 ton ac in his home just 2 years before this service call and they were claiming his mold issue wasn't due to the HVAC or an issue with it. When I arrived at the home I was shown that he literally had mold growing on the walls in large accumulations in every room of the home to include his kitchen cabinets where all his cookware and dishes were. I asked him how big the home is and he said it was roughly 2300 square feet. Anyone in the HVAC industry will tell you right away (or they should if they know what they are doing) that a 2300 square foot home needs at most a 4 ton unit and that would be on a very very leaky home here in our area. This particular home was built in the early 2000's and was actually of good (custom build) quality and after performing a load calculation I found that he only needed a 3 ton unit (which was still just a hair oversized). We ended up installing that 3 ton unit for him and wouldn't you know, his mold issue went away.
The last point I will make about the importance of proper equipment sizing will likely be the most appealing to consumers and that is the cost. There are two costs we have to consider when sizing equipment and that is the cost of the equipment itself and the cost of operation. It should go without saying but I'll say it anyway, smaller tonnage equipment is cheaper than bigger equipment. So the smallest tonnage unit that you need to be perfectly comfortable in your home will save you cost on install as opposed to an oversized unit. The longer more impactful savings come from the actual operation. Bigger tonnage units draw more power than smaller tonnage units because they are doing more work. Just like a 2500 series pickup truck needs more fuel to drive down the highway than a sedan. If you were going to run a 25 mile marathon, would you sprint it or would you pace it? This is the same for your HVAC in terms of cost of operation. We want to see longer, smoother, lower energy run times so that we can use as little energy as possible to make sure we achieve good MRT, proper dehumidification, as well as make sure the equipment is getting the proper oil return to the compressor.
It's in your best interest as a consumer to ensure when you get a new system installed that it's appropriate for the situation. I see all too often contractors quoting exactly the same size equipment you already have and sometimes that's the right size, however I will surprise you when I say that I find in my area 80% of the systems I see are oversized. 9 times out of 10 it's not a gross oversizing, but oversized none the less. You as a consumer have every right to request a copy of the load calculation from your contractor prior to having a new unit installed and I encourage you to do so. We (contractors) in North Carolina are required by law to have done this before we install a new unit so we should have that information readily available.
Kenneth Casebier
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