Air Conditioning Can Be Counterintuitive

Facility Managers are responsible for the comfort of the building occupants. If it is too hot or too cold, Facilities needs to find a solution to the problem. Much of this has to to with H...

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Facility Managers are responsible for the comfort of the building occupants. If it is too hot or too cold, Facilities needs to find a solution to the problem. Much of this has to to with HVAC (Heating, Ventilation Another problem related to comfort is the humidity level. If the air is too dry, it can cause discomfort. If the air is too humid, there is also discomfort. The goal of most facilities is to have 74 degrees Fahrenheit and 50 percent humidity. Those two goals will satisfy the most occupants. Most buildings will lock in the settings at these setpoints. If you leave a thermostat unattended, all manner of problems can occur, some of which is counterintuitive.

Why It's Always Too Hot

If the problem is that the room is too hot, it's quite possible it is too hot. That's the first thing to check, certainly. If, however, the temperature is at 74, then the next thing to check is the humidity. Once you start getting near 60% humidity, it starts to get uncomfortable. It feels stuffy. Here is the tricky part and why thermostats need to stay locked. Most people will turn down the air conditioner to 68 or lower if you let them. Most systems are designed to drop the temperature about 20 degrees. More than that, you'll be running your unit constantly and not reach the setpoint. This is a problem because most HVAC units are designed to draw in fresh outdoor air. So, as the temperature drops, the humidity climbs. This results in a cold room that still feels stuffy. Naturally, people will want to crank down the temperature even more.

How to fix it

How do you fix this problem? Unfortunately, your best bet is to just leave the setpoint at 74 and apologize. Some units will run two or three degrees cooler than the setpoint as they dehumidify the air. Once they reach 71 or so, they'll shut off and wait for the room to warm to resume dehumidifying. There is no rushing this, unfortunately. If your HVAC does not have a built-in dehumidifyer, you may need to invest in a separate dehumidifying unit. It will save you money in the long-run.

Some HVAC units have a built in dehumidifyer that will preheat the air right before it reaches the cooling coils. This sounds counterintuitive. You are heating the air before you cool it. It's wasting energy, right? Not necessarily. By preheating the air, the HVAC unit can maintain the room temperature at a constant 74 degrees while it is dehumidifying the air. It is actually more efficient to dehumidify warm air than to dehumidify cool air. Therefore, the unit will continue to run until you reach the preset humidity level of 50 to 55 percent and shut off. Whereas the previous example would have the HVAC unit cooling and cooling, never reaching the setpoint nor feeling comfortable.

If you have the choice between buying an HVAC system with dehumidification or without, spend the extra money on dehumidification. It will save you from getting frequent calls that the room is too hot. It will also save you on electricity cost. And, one thing we haven't mentioned, lower humidity prevents mold growth. By cranking down the temperature with high humidity, you will promote condensation which will breed mold and mildew. If this happens, you can expect complaints about the smell and complaints about allergies and air quality.

If you live in a climate that needs air conditioning, but has dry air. I'm sorry, I don't have experience with those conditions. There certainly are HVAC units that will humidify the air. Dryer air is more comfortable than humid air up to a point. If it's too dry you start zapping yourself with static electricity everywhere. For some people, dry sinuses are uncomfortable. And for some people it dries out and irritates their skin. We really are most comfortable at about 50% humidity.