This is probably one of my most favorite subjects to talk about and it deals with comfort and psychrometrics (the study of air). We will talk about design conditions vs reality and what we are feeling that makes us want to keep turning the air conditioner lower and lower.
Lets first discuss the standard design conditions for a home in the United States so we can get the shock factor out of the way early. Design for a home is almost universally 75 degrees at a 50% relative humidity. I know many of you read that and said "75 degrees is way too high", but if that were true; then why does that standard exist? It's not to save energy although that would be a good guess. If you could truly experience an environment maintained at these conditions, you would see that it's rather comfortable. Some might want it a degree lower, but none the less you would see that it's perfectly acceptable. The issue we run into is that very rarely do I ever find a home that actually maintains these conditions.
The moisture has a greater effect on our "feel" of the air around us than the actual temperature. Our bodies are cooled by moisture evaporating off the surface of our skin and when the rate of evaporation is higher, we feel colder. I tell you that so we can understand that if the air around our bodies is dry, then moisture evaporates at a higher rate and we feel cooler or more comfortable. As professionals in the HVAC industry, we typically use Dew Point as opposed to relative humidity to properly asses the comfort of a home. That's because relative humidity is "relative" to a temperature, it doesn't directly tell us anything about the actual moisture content in the air... directly. If we have any two of the 4 major data points of air properties (temperature, dew point, relative humidity, wet bulb), we can calculate the others. I tell you all that to state that a better way to look at indoor comfort targets is keeping the dew point below 60 degrees. Dew point is that temperature at which air can no longer hold water. If you take a cold drink outside in an early summer morning, condensation will form on the outside of the container. This is because the surface temperature of the container is below the dew point and when this happens, water vapor condenses out of the air. Think about that process in reverse which is what's happening on the surface of your skin in a dry environment.
Nature is always trying to balance, it's just a rule. Like Bryan Orr likes to say "high energy goes to low energy". The energy contained in the moisture that our bodies generate is higher than the energy of dry air (75 degrees at 50% RH), so our energy in the form of heat is absorbed by the air around us. This gives us a greater satisfaction of comfort than a particular temperature. You still may not believe me but think about commercial buildings that you've been in like hotels, convention centers, government office buildings... 99% percent of the time these types of facilities adhere to a very strict standard where they are more concerned about moisture control than set temp for both economical reasons as well as comfort. When you go into most government office buildings you might have never thought that they were cold or hot, just neutral. That's because they are controlling the properties of the air to a very specific standard, that same standard I mentioned earlier.
Outdoor moisture content is higher in the morning and evening than it is during the heat of the day. When the air warms up, the relative humidity goes down, this is because warmer air can hold more moisture than cooler air; hence the name RELATIVE humidity. The solution to the problem is proper equipment sizing first and second is load matching. When we perform a load calculation we are using historical weather data and selecting equipment that will satisfy the highest temperature on average that our areas have. For Jacksonville NC the design parameters are targeted to maintain a home at 75 degrees when the outdoor temp hits 91 degrees, but what about when it’s only 75 degrees outside with 80% RH? Single stage equipment will be oversized in these milder conditions and thus won’t be able to properly MATCH the actual load of the structure and likewise won’t be able to control the humidity to our liking. Multi stage equipment can typically accommodate the necessity for proper load matching and humidity control, however it may also be necessary to have a dehumidifier.
Dehumidifiers offer two great aspects with one drawback. A dehumidifier will obviously help with moisture control, as well as rejecting the heat produced by the refrigeration circuit back into the home. You may think that’s counterproductive, but in fact it can be a good thing. By adding heat to the home, it can force the central air conditioner to run and when your central air is running, it too is removing moisture. The heat rejected back into the conditioned space also raises the dew point of the air, making your central air more efficient in terms of moisture removal. The drawback of dehumidifiers is they are another appliance running and increasing energy consumption… so the electric bill goes up. The trade off is comfort and you can look at this way, as opposed to driving the temp in the home down (which also consumes energy), you will be more comfortable at a higher temperature which may offset the increased energy consumption of a dehumidifier.
I often advise my clients to get a substantial portable dehumidifier and give it a try for a bit and see if they feel a difference in the comfort of their home. If comfort improves then they may be a candidate for 1 of 3 options… Keep utilizing the portable unit, have a permanent ducted dehumidifier installed, or consider a proper load calculation with a properly sized and installed central air system. I myself like the sensible approach of a portable dehumidifier as they rarely cost much and it’s a good way to truly find your comfort zone.
Kenneth Casebier
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