February 2011
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Controls Devices (Part 1 of 3)
Rules of thumb to follow for TC installation & design

Steven R. Calabrese


Steven R. Calabrese

Control Engineering Corp.

Contributing Editor

 
This month we get back to basics with a column about modern temperature controls devices. Note that the term “temperature controls” is a catchall for all controls devices, not just thermostats and temperature sensors, but pressure and humidity sensors, current-sensing switches, safety devices, and end devices. In the DDC domain, all of these devices, in one way or another, wire back to a digital controller, which in turn monitors inputs and affects outputs in conformance with the intended Sequence of Operation. We’ll not talk about the controller this time around, just about the stuff that connects to the controller, specifically the topics in the order that follows:
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This installment is part one of a three-part series. When completed in April, the series can serve as a good “quick reference” guide to temperature controls devices, covering not all but a good amount of what makes up a typical HVAC temperature control system in this day and age.

Sensors & Transmitters

OAT Sensors

Outside air temperature sensors need to be located outdoors (yeah!), preferably on a northern exposure, and out of the direct sunlight. Keep away from areas where rainwater may accumulate. And while you’re at it, go the extra mile and get the temperature + humidity variety. Yeah, it’s more money, but monitoring outside air humidity is probably specified anyway, and if not, you really do get bang for your buck on spending the extra dough. Labor-wise it’s an extra pair of wires, so no real additional cost there. And controller-wise, you eat up an input, so unless you are down to your last input and would need to add another controller to accommodate the humidity sensor, you really should “default” on providing it.

Duct & Pipe Temperature Sensors

Probes are your “general purpose” duct temperature sensors. They come in a multitude of lengths, and there are various rules of thumb for properly selecting them. One states that the probe should reach halfway into the duct, which probably makes good sense, as far as getting a good, consistent temperature reading, as that is the point at which the velocity of the air is at its greatest, with the velocity dropping off as you reach the sides of the duct. Given this rule of thumb, it follows that, for terminal units (VAV and fan-powered boxes), a duct probe of 8” is a good all-around standard size.

For larger ducts (over 4’ wide), use an averaging sensor, which is not a probe but a continuous pliable metal tube with temperature sensors embedded within, along the length of the tube. A good rule of thumb dictates that, for every square foot of cross-sectional area of duct, provide one linear foot of averaging sensor. Also use an averaging sensor where temperature stratification is likely to occur, as in the mixed air chamber of an air handling unit.

Pipe temperature sensors generally come in two varieties: immersion and strap-on. Immersion sensors are for new construction, to be installed in the pipeline prior to completion and system fill. Standard sizes suitable for most HVAC-level applications are 2” and 4”. Installation consists of welding the sensor “well” into the pipe, and then screwing the sensor into the well. If you have a pipe size of less than 2”, use a strap-on sensor, along with some thermal conductivity grease. Strap-on sensors are also good for existing operational systems, and other “specialty” situations (like in new construction when the sensor is overlooked or all but forgotten about until the very end of the project after the system has been filled!).

Space Temperature Sensors

Space sensors come in a variety of styles. It takes some up-front thought before deciding on a particular style and specific options. Does the application call for a decorative enclosure or low-profile one? Is visual indication of room temperature and setpoint important? What about occupant adjustability? An override pushbutton, to allow override to the occupied mode during “off” hours? Answers to these questions require some insight as to how the end-user will use their system. Sometimes visual indication of the temperature and setpoint does more harm than good. And occupant adjustability may be deemed to be undesirable, although with the typical DDC system you can limit the adjustability of the temperature range of the dial or slidebar, pretty much down to 0 degrees.

For public areas, such as corridors, and for areas of mass congregation, such as classrooms and meeting halls, a tamper-proof plastic guard may be required, even if the sensor has no amenities. Metal cage guards are necessary for gymnasiums, where a stray basketball could otherwise eradicate a wall-mounted temperature sensor.

Reliable Controls Although nowadays, “vertical” mounting is the norm for the kinds of space temperature sensors that most BAS accommodate, in retrofit applications, you may run across a situation in which you need to replace an old horizontal-mount thermostat with a new vertical-mount temperature sensor. Wall plates specifically manufactured for these applications (sometimes called “goofplates”, presumably named so after an installer cut in an electrical wall box in the wrong orientation and thus had to use one of these) are available which cover the existing hole in the wall and provide mounting holes for the new sensor.

Humidity & CO2 Transmitters

As discussed above, an outside air humidity sensor makes good sense (pun intended!), and is necessary in order to make outside air enthalpy calculations, which are required for economizer changeover decisions based on both outside air temperature and humidity. For instance, when it’s warm yet dry outside, there may be an opportunity to economize. On the other hand, if it’s cool and rainy, you may not want to bring in outside air if it’s going to contribute to an indoor humidity problem. An enthalpy calculation is required to allow the BAS to make these types of decisions. As far as return air goes, RA humidity is a good indicator of the average space relative humidity levels, and is a typical spec item for larger air handling systems.

Carbon dioxide (CO2) sensing has its place in HVAC control, nowadays more than ever! As CO2 is a good indicator of human occupancy, CO2 transmitters are used in indoor environmental control strategies, in an effort to maintain a suitable “fresh-air” environment within the occupied building spaces. An outside air CO2 transmitter can serve as a reference for Demand Controlled Ventilation (DCV) strategies. The outdoor air CO2 level is measurable and while generally consistent, it can vary due to certain environmental conditions. By comparing indoor levels with the outdoor level, an effective DCV strategy can be implemented. Return air CO2 level is a good indicator of the average levels of CO2 within the occupied spaces, however for “true” DCV, individual space CO2 transmitters may be required, especially when there’s occupant diversity within the building, as is true with schools, theatre houses, and even commercial office buildings.

Tip of the Month: Tech-talk…what’s the difference between a sensor and a transmitter? From a functionality standpoint, not much. Which is why these terms are often used interchangeably. Technically speaking, a sensor is a passive device requiring no separate power, as with a common thermistor type temperature sensor. Hook a pair of wires up to it and read the resistance across it, which is a function of the temperature surrounding it, and you have your temperature value. For sensing humidity and pressure, active electronics are typically required, and so these sensing devices are called transmitters. In our everyday vernacular, most of us tend to refer to any device that senses something as a sensor, even when it’s technically a transmitter. In the end, as long as we understand the difference, we should be “good to go”.

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