By Randy Peoples, M/E/P Coordinator
Most people have no idea how critical humidity is to our comfort level. There’s a large list of variables that make humidity either not a factor or not acceptable. For years I have used the following phrase to describe humidity, ‘You can be in a 90* room with 5% humidity and a slight breeze and feel fairly comfortable, but you can also be in a 65* room with 85-90% humidity and stick to everything you touch’. You may be cool, but are still miserable.
I will begin with an explanation of the 2 most commonly used terms:
Humidity is the amount of water vapor in the air. Higher humidity reduces the effectivness of sweat in cooling the body by reducing the rate of evaporation of moisture from the skin. This effect is calculated in the heat index we hear about in the weather forecast.
Dew Point Temperature is the temperature at which the air can no longer hold all of the water vapor and some of the vapor must condense into liquid water. This temperature is always at the current temperature or below. Sometimes your iced tea glass sweats and sometimes it doesn’t.
That said, the worst combination for human comfort is a high dewpoint (65 or above) combined with a high RH. If the dewpoint is above 65, generally it will feel uncomfortable. Comparitively, the optimum combination for human comfort is a dewpoint of about 60 and a RH between 50-70% (this would put the temperature at about 75*).
Our bodies begin to sweat in response to a rise in our core body temperature as opposed to a rise in our skin temperature. Our brain regulates our body temperature by evaporative cooling, which is sweating. Humans have an average of 2-4 million sweat glands. The amount we sweat can be 27-47 ounces per hour during exercise, or daily 2.3 ounces up to 270 ounces average. When the humidity is high, our sweat doesn’t evaporate into the air as well making it harder for the body to cool. Air movement does help but only because it is giving just a tiny bit more evaporative cooling as opposed to no cooling whatsoever.
Comparitivly, the only sweat glands dogs have are around their feet. They cool by panting which evaporates the saliva cooling the tounge, and the deep breathing evaporates moisture on the surface of their lungs cooling from the inside. Yep, that is good to know!
The 2 most commonly used mechanicisms of cooling used in the US are evaporative cooling and mechanical cooling. Evaporative cooling has been around in one form or another for thousands of years. Mechanical air conditioning was patented by Willis Carrier in 1902 and has evolved ever since. Carrier found that his cooling machine was also able to remove moisture from the air, solving a problem he had with printing operations at his job in Buffalo, New York.
Evaporative cooling is still heavily used primarily in the desert regions of the country and works very well but only when the humidity is low. Phoenix has a typical summer humidity of 3-7% which is ideal for swamp coolers. I would say roughly half of the homes there still use swamp coolers, until the humidity gets too high, then they switch to the mechanical cooling until the humidity drops back down later in the summer.
The information below is accurate as of the first of November this year showing what you already know; it is the most humid in the mornings, without rainfall. With rainfall, the entire game changes.
The humidity is lowest during the later part of the day, typically in the afternoon due to the temperature raising which makes the air expand. You have the same amount of water vapor in the air, but with more air that moisture spreads out more resulting in lower humidity readings.
This past summer, from what I’ve been told, we saw record high humidity in most of the state. CMSWillowbrook had numerous calls from current and previous customers asking if their systems were working properly because their comfort level was unacceptable. I personally worked for months with the EOR on Pioneer Library to determine the cause of their excessive humidity and to find resolution. Below are findings and changes that we made to help reduce the problems. It was not a single point issue but a group of minor adjustments which helped to ease the complaints.
Some of their interior humidity (Relative Humidity) readings were in the mid 50% range all the way up to mid 80% range at most any time during the day, but highest in the mornings. Paper throughout the building was curling with copier paper miss-feeding and their large format plotter paper being ruined. Book page corners would swell and magazines looked weeks old the first day on the shelf.
The owner bought 7-8 hygrometers (humidity meters) and took several readings throughout the day compiling lists that they would send to me every couple of days. This gave us a clear precise picture of the moisture in the building and finally gave us indication when the humidity finally started to drop.
Heath McGee with Allen Consulting (EOR), Yarbrough and Sons and I first found that the approved thermostats from Trane were insufficient and did nothing except to turn on/off the heating and cooling. They had no fan control and the outside air was controlled only from a setpoint on the controller in the rooftop unit. We replaced those limited thermostats with digital setback thermostats which did have control of the outside air damper. We found that at night the units would run the blower non-stop while still bringing in the outside air. With a lower cooling load (compressors running less)the fans would load up the building with humidity overnight then not be able to remove in the following day causing the cycle to repeat. We programmed the fans to be off when not cooling and the outside air closed. At night the fans would turn on only to maintain the setpoint temperature, but brought in no outside air. This alone cut the humidity in the building down to the low 60% range.
We as well found that the occupants would turn down the temperature thinking this would provide more comfort. As I said above, this gave them the cool/clammy feeling. Cold air doesn’t give up moisture as well as warm air. Turning the temp down reduced the ability to remove moisture as easily.
We also found that none of the filters in the entire building had been changed in nearly the entire year since C of O. I arranged a quote from Yarbrough for annual maintenance which they accepted. The EOR found that one return transfer opening above ceiling had not been provided. Once the opening was cut the airflow increased. All of the individually simple and small changes combined made all the difference. If the building controls not had been cut out we could have done this virtually for no cost with exception of the filters.
In summation, the best possible situation is to have the lowest humidity possible for cooling and add (if necessary) humidity during heating. Moderate your temperature settings to make the units run as much as possible while not allowing the temperature to drop too low.
For our projects, be aware of the sequence of operations in the mechanical design documents. This tells you what happens during which time of the day. If you see that the blowers never shut off and outside air is introduced 24/7, ask why. This will eventually become a problem. Never-please never, VE out building controls, because you will have a very limited ability to modify the HVAC system globally afterwards. Yes, it adds money to the bids but will nearly always be less than trying to fix problems after.
CMS has determined that we will try as best possible to not VE out the capacity to remove excessive humidity in the mechanical equipment. Once this capability is removed from the equipment you have severely limited your options to control high humidity if and when complaints start coming in. On the Pioneer Library project the cost to add in the high humidity package back in (which was omitted by a single sentence in the specifications) was equal to the cost of new units+ approx. $2,500.00 per unit. Had this capability not been removed the upfront add would have been roughly $10k. After CMS and the A/E spent time and resources not to mention the added thermostats, the $10k would have been a wash.
As a reminder, I am nearly always available to help by phone with any MEP problems or questions and can be on site with a little notice. As my schedule permits I will be visiting most of the projects routinely to provide any input and QA/QC walk through, along with punch lists of items I see that should be resolved prior to A/E/Owner punch lists.