Your HVAC is the Wrong Size

Unfortunately, it’s true. Your HVAC system is the wrong size. Based on outdoor temperatures, people in the home, electronics, cooking in the kitchen, laundry, etc. the BTU Requirements of your home change minute by minute, day by day. If you currently have a single-stage furnace with an air conditioner or a single-stage heat pump, your HVAC system is the wrong size for your home 85% of the year. If you’re lucky enough to have a two-stage HVAC system, then it’s the wrong size, only 65% of the time. These are indisputable statistical facts. Why do we continue to accept this as OK? For this reason alone, you should seriously consider a variable-speed compressor with a variable speed blower for your home. It’s just like having your comfort system on cruise control. But there are many other reasons your system is the wrong size, and we will outline many of them in this article.

Let’s think about it in terms of your car that you just drove home. (Or, with this quarantine, you may not remember if you still have a car.) You’re at a stoplight, and the light turns green. Do you stomp on the gas pedal to the metal, with the engine at max RPM’s and race up to 55 miles per hour? Or do you slowly and efficiently press the gas pedal up to your desired speed? Then, suddenly, you notice the next light up ahead turns red. Do you slam on the brakes and skid to a stop or gently press the brakes and slowly come to a stop? Which do you think is better for the health and longevity of your car? And which has the lowest cost of operation? Which do you think is better for your HVAC system? If you have a single-stage HVAC system, it’s either 100% on or 100% off. There is no gradual increase in speed up to the total output. Then there is no slow reduction in speed down to off. It’s 100% On. Then 100% Off. No in-between. This is not good.

To make matters worse, Air Conditioning Contractors of America (ACCA) requires that we design your HVAC system to maintain a winter temperature inside your home of 70 degrees Fahrenheit with an outdoor temperature of 23 degrees Fahrenheit. In addition, they also want us to maintain a summer temperature inside your home of 75 degrees Fahrenheit at 50% relative humidity, with an outdoor temperature of 92 degrees Fahrenheit. The State of North Carolina strictly enforces these requirements through the Energy Code and the Mechanical Code. With these temperatures in mind, I’m sure you are already thinking, “It gets hotter than 92 degrees Fahrenheit and colder than 23 degrees Fahrenheit around here.” And you would be 100% correct. You would also be right in thinking that there are a lot of days that it doesn’t get up to 92 degrees Fahrenheit or down to 23 degrees Fahrenheit that your HVAC system is running. Again, you would be 100% correct. In fact, in Wake County, your HVAC system will operate 1,239 hours in cooling mode and 1,883 hours in heating mode. This translates to 3,238 heating degree days and 1,928 cooling degree days, using a 65 degrees Fahrenheit base temperature. These are a lot of critical data points to show that there is a lot of variability in the annual load demand for your home and that your HVAC system runs a lot of hours each year. With that said, why would anyone want an inefficient and improperly sized single-stage HVAC system? It’s a bad idea on so many levels.

Trane’s TruComfort Variable Speed system maintains a consistent temperature with maximum efficiency by automatically making minor and continuous adjustments in output all day long in up to 750 steps. The result is efficient, affordable, reliable comfort in your home. With this technology, the compressor, outdoor fan, and indoor fan will vary in as little as 1/10th of 1% increments to maintain the comfort in your home within 1/2 degree Fahrenheit of the thermostat setting. This is a comfort system that is always the correct size for your home regardless of the time of day, the time of year, or the internal load. Now, that’s control!

This is not a new technology for the HVAC industry. It has just taken a little while for homeowners to demand this product be readily available and affordable. Now, Trane brings this variable speed technology to the next level with a system that intuitively adjusts to changing heating and cooling needs, working only as hard as required. The majority of the time at lower and much more efficient speeds. Variable speed technology is finally available in residential equipment at very affordable prices. And, let’s remember, the price you pay today is not the complete cost of this system. You’re stuck with the operational expenses for this system as you live with it for the next 20 to 30 years. You will be paying for this system to run for a long time, so you definitely want it to be reliable and efficient.

Not only do you need to make sure the equipment itself is reliable and staged for varying loads conditions; you need to ensure that it is sized correctly for your home during the times that we do reach maximum capacity. We find a large majority of the existing HVAC systems that we encounter are incorrectly sized for the homes that they serve. We routinely find that the first floor systems are too large, and the second floor systems are too small for the homes. Too large sounds great, but it causes short cycles, doesn’t adequately control moisture in the home, leaves hot spots and cold spots throughout the home, and is terribly inefficient. In the state of North Carolina with our high humidity levels, it would almost be better to have a system that is slightly undersized so that it would have longer run times. The longer run times would be much better for moisture control and lead to fewer unbalanced temperatures throughout the home. But either way, too large or too small, it’s a bad situation.

The State of North Carolina requires that an ACCA Manual J Load Calculation be completed on every HVAC installation, new construction, or replacement. A load calculation is just a mathematical model of your home to determine the exact BTU requirements to heat and cool your home. It determines the heat loss and heat gain in the floors, doors, windows, walls, and ceilings. This load calculation also incorporates the number of occupants and the number of appliances in the home. The good HVAC contractors will do a very detailed room-by-room load calculation so that we can determine if your home has enough supply air to condition that space.

The State of North Carolina also requires that we complete an ACCA Manual D Residential Duct Design on the home. With the known amount of conditioned supply BTU’s for a specific space from the Manual J, we translate this into CFM’s, cubic feet per minute. Now knowing the CFM of air going into a space, we can correctly size the supply trunk and branches going into the various parts of the home. This allows us to get adequate airflow into the proper spaces with the highest efficiency. During this phase of the HVAC design, we can determine if the supply trunks, branches, and registers are properly sized for the efficient flow of conditioned air. At the same time, we can evaluate the grills, branches, and trunks of the return side of the system. In fact, the return side has more stringent requirements for the efficient flow of air back to the HVAC system.

On top of having the wrong size and number of stages of an HVAC system, now we find that your duct system is most likely not correctly sized for your HVAC system either. The overwhelming majority of the homes that we go into today do not have enough return and barely have enough conditioned supply. Most barely have 50% of the necessary return grill area. A simple test is to take every supply register in your home and lay them side by side over the face of your return grills. If you have more supply area than return area, then you most like do not have enough return. Or simply measure each of the supply registers face area and compare that to the measured face area of the returns if you can’t physically remove them.

With the list of potential pitfalls outlined above, you may already be thinking that you have an issue with your home’s comfort system. If so, you don’t have to throw the baby out with the bathwater. Call Mitchell Heating & Cooling if you need a consultation or evaluation of your current HVAC system. You may just need a simple maintenance visit to get your system back to its peak efficiency. As we mentioned above, your HVAC just operated 3,122 hours over the last year. You wouldn’t drive your car that many hours without rotating the tires and changing the oil.

There are too many options and solutions available these days to be uncomfortable and suffer. My last words: Regardless of outdoor temperature or humidity, don’t be uncomfortable in your own castle.

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