August 2013

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And Another Collection of Short Stories

An ongoing series of tales told and lessons learned

Steven R Calabrese

Steven R. Calabrese
Control Engineering Corp.

Contributing Editor

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Continuing from June’s installment of this column, I present the next in an ongoing series of shorts, fueled by email suggestions and real-life experience.

Using CO2 to Detect Human Life

Simply put, we humans off-gas the stuff! In (not-so) scientific terms, the concentration of carbon dioxide (CO2) in the great outdoors is relatively constant at around 400 parts per million (ppm). When an occupied indoor space is ventilated with the generally mandated 15 cubic feet per minute (CFM) of fresh air per occupant, the indoor CO2 level stabilizes at approximately 700 ppm above the outdoor air level, or around 1,100 ppm. So there is a direct correlation between the level of CO2 in an occupied space, and the occupancy level. Now, we don’t use the CO2 level measured to do a “hard count” of the number of people in the space. However, if we were to theorize that these values are “firm”, and given that we’re modulating the outside air damper to maintain a 1,100 ppm CO2 level “setpoint”, then theoretically we can measure the amount of outside air being brought in via the outside air damper, and correlate that to the number of people in the space served. For example, if we were to use an airflow measuring station at the outside air intake, and this device measured a flow rate of, say, 90 CFM, then we conclude that there are six people in the space served (15 CFM x 6 = 90). If three occupants were to vacate, the CO2 level in the space would tend to drop, and thus the outside air damper would be modulated to bring in less fresh air, until it stabilized at 45 CFM.

The above discussion is simply “food for thought”, but it does demonstrate the theory behind using CO2 level to detect occupancy and thus to properly and adequately ventilate indoor spaces. When utilized in this manner, the process is termed Demand Controlled Ventilation, and although it does have its limitations, it’s a step up from just opening up the outside air damper to satisfy the maximum occupancy level, with no regard as to the actual “real-time” occupancy level.

Determining the Number of Stages for Electric Duct Heaters

Determining the number of stages of electric heat requires looking at the total delta T of the particular heating coil, as well as considering the degree of control required. For precise temperature control, the delta T per stage should be no more than five degrees. For mediocre control, the delta T per stage can be in the range of 6-12 degrees, and for coarse control, 13 degrees and up. A good rule of thumb is, 10 degrees per stage, for mediocre control.

It’s a common misconception that the larger the electric duct heater, the more stages of control are needed. Yet from the above discussion, we see that the number of stages is more or less a function of the total delta T and the desired delta T per stage. A 20 KW electric duct heater handling 3,150 CFM of air yields the same delta T as a 3 KW heater handling 475 CFM of air: 20 degrees. Thus, given the same application, both of these heaters can be supplied with simple two-stage control.

The whole origin of the “mediocre control” rule of thumb stems from the notion that the larger the delta T per stage, the more noticeable the difference in supply air temperatures is when staging takes place. With a 10 degree delta T per stage, the occupant may be more likely to feel the difference when a stage of control is added or dropped out. The larger the difference is, the more likely he/she is to feel the difference. This is normally an undesirable and often an unacceptable condition for HVAC comfort control. For more critical applications, the delta T per stage should be lessened, by increasing the number of stages.

What’s Your Comfort Range?

Everyone has one…a range of temperature that they feel comfortable with. Some individuals’ ranges are larger than others. As well, ranges differ from one individual to another. I can tell you that I can be comfortable through a relatively large range of indoor temperatures, meaning that it takes a real departure from the standard “room temperature” of 72 degrees for me to complain. I fare better on the cold side than on the warm side, for if it gets too warm, my head starts to sweat, hot blooded as I am! Must get that from my mother’s side…she likes it cold. Thinking that her comfort range is more like 60-65 degrees!

Of course on the other end of that spectrum are those folks who like it on the warm side, like 74 degrees on up. You can begin to understand the reason why service departments have their hands full with so many hot and cold calls. It’s not so much that the equipment is malfunctioning. Oftentimes it’s more so that the occupants of a more or less common space have different comfort ranges. It’s just human nature. Some like it hot, and some like it cold…so what’s your comfort range?

Seasonal Setpoints

So we just got done discussing individual comfort range, and how that relates to the accepted “room temperature setpoint”. However thermostats don’t typically operate to maintain a precise setpoint. This is especially true for staged thermostats, those of which operate rooftop units and residential heating and air conditioning systems.

What you typically get, when you set your home thermostat, are two setpoints separated by a deadband. For instance, in the summer you might set the cooling setpoint to 72 degrees, which may automatically establish the heating setpoint as 70 degrees (2-degree deadband). When the space temperature is within deadband, all is well and neither the heating nor the air conditioning is engaged. On a rise in temperature to the cooling setpoint, the air conditioning comes on, and conversely on a fall to the heating setpoint, the heat comes on.

So what’s the point? Well, to finally explain the title of this short story, setpoints such as these are often “seasonal”. In the summer, a cooling setpoint of 72 degrees may be suitable, however in the winter, a heating setpoint of 70 may be considered “too low”. So you go to the thermostat and kick the overall setpoints up two degrees. The moral of the story is, your thermostat may need to be adjusted on a seasonal basis, in order to satisfy your personal preference.

The Truth Behind Economizers

One of the most clever features of a typical, standard rooftop unit, and perhaps also one of the most misunderstood, is the economizer section. The economizer section of a rooftop unit, or for any air handler for that matter, is designed to utilize outside air as a source of “free cooling” if outside air conditions permit. This is done by modulating the outside and return air dampers to bring in the proper amount of cool, dry outside air. The colder (and less humid) it is outside, the more of an opportunity there is to economize.

Reliable Controls The decision on whether to economize or not is based on the “condition” of the outside air. If it’s 80 degrees outside, there’s no opportunity for economizer operation. Humidity is another factor to consider in making the decision to economize. If it’s cool yet humid outside, we don’t want to use the outside air for free cooling, if at the same time we’d be contributing to an indoor humidity problem. On the other hand, if it’s warm and dry outside, the use of outside air may be at least partially beneficial to the cooling needs of the space served. An enthalpy controller is utilized to determine the value of the outside air, in terms of its cooling ability.

Once the decision is made to economize, and a call for cooling has been requested by the space thermostat, then economizer operation is implemented. It is to be noted that the rooftop unit’s refrigeration cycle, i.e., DX cooling, will not necessarily be entirely disabled, for there are instances where the outside air, although beneficial to the cooling process, may not be sufficient to completely satisfy the cooling needs of the space. In other words, these two processes (economizer operation and DX cooling) are not mutually exclusive. Sometimes those new to the concept tend to mistakenly interpret it that way. The fact is, economizer and DX may both be implemented at the same time, if that’s what’s called for to satisfy setpoint. For example, if the outside air is 60 degrees and dry, the enthalpy controller may permit the use of the economizer, and the 60-degree air definitely helps the cause. However 60 degrees into the space may not be adequate to satisfy the thermostat, and so the stat calls for more, and what do you get? Yep, DX cooling to the rescue!

Tip of the Month: Challenge yourself to find your comfort range. Find your optimal temperature, then tweak it down, one degree at a time, until you find yourself shivering off the chill. Then tweak it upward, again one degree at a time, until you start to sweat. You may be surprised, either by how wide it is, or more likely, by how narrow!


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