What Is a Thermostat?
We all have one on a wall in our home, but what exactly is it? In its elemental form, a room thermostat contains (at least) a sensor that measures temperature and a (simple or complex) controller that operates HVAC
equipment to maintain room temperature at the desired setpoint. Additional bells and whistles might include humidity, motion, and CO2 sensors, as well as programmable schedules, networking, and other
advanced features that
maximize occupant comfort while reducing energy and environmental cost.
To meet your specific control needs, KMC offers a variety of thermostats that include different digital, analog electronic, and even pneumatic solutions.
This article describes different types of thermostat technology and representative models of each as well as upgrade options. (For details on these
thermostats, see their data sheets as well as the KMC product catalogs.)
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KMC Model Series
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Type
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Integrated Sensors
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Temperature
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Humidity
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Motion
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CO2
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AppStat
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Digital B-ASC
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Standard
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Optional
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Optional
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|
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FlexStat
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Digital B-AAC
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Standard
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Optional
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Optional
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Optional
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CTE-5202
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Analog Electronic
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Standard
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|
|
|
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CTC-16xx
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Pneumatic
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Standard
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Digital Power
The BAC-4000 series AppStat combines a
controller, multiple sensor options, and BACnet networking into a single, integrated space-mounted device. Appstat offers a unique, cost-effective
combination of networking, application, and sensor options along with easy, intuitive installation (with a two-piece design), configuration (with
contextual menus and no obscure numeric codes), and operation (with an intuitive interface).
The user interface is a color LCD display (with LED back lighting) and five push buttons. Through the menu-driven display, an operator can change
setpoints, configure available options, and commission the installation. Separate passwords can be set-up for users (normal operation) and administrators
(installation and configuration).
AppStat is an integrated native BACnet Application Specific Controller (B-ASC)
for connection with a BACnet network. (No external communication or occupancy modules are required.) All models include an integrated schedule and hardware
real-time clock with 72-hour capacitor backup for stand-alone operation or network time synchronization. If desired, the AppStat can be installed simply as
a stand-alone thermostat (with a built-in clock and schedule). An MS/TP network connection to a building automation system can easily be added
at a later time. Installation is scalable from a single room to a network of multiple rooms.
AppStat is easily configurable according to the model’s factory-programmed application (FCU, HPU, or RTU). For a more powerful multi-application digital
thermostat that is also field-programmable with custom Control Basic programming, see the KMC FlexStat.
The award-winning FlexStat series of intelligent
temperature/humidity/motion/CO2-sensing, wall-mounted, thermostat/controllers are native BACnet Advanced Application Controllers (B-AAC). The
FlexStat simplifies networked zone control for common AHU, FCU, HPU, and RTU HVAC equipment, which can be controlled via the on-board libraries of programs
built into the FlexStat.
FlexStats can provide flexible, set-and-forget control over the indoor environmental quality (IEQ) in your space without the expense of
a large building automation system. FlexStat IEQ control for your space optimizes temperature, humidity, and ventilation for your health and
comfort...while saving energy at the same time with advanced energy management control.
For communication with other FlexStats and/or a building automation system, BACnet over MS/TP communication is standard on all FlexStats. “E” versions,
with an RJ-45 jack, add BACnet over Ethernet, BACnet over IP, and BACnet over IP as Foreign Device (for communication across the Internet).
Economical Analog Electronic
If you don’t need networking, the new CTE-5202 has a
user-friendly LCD display and push buttons (instead of hard-to-see wire indicators and inexact sliders or dials common on many older electronic
thermostats). It has two adjustable-span 0-12 VDC outputs with independent control loops and one external input for morning warmup, changeover, or setback
options.
The thermostat is typically used with KMC CEP/CSP-4000 and CSP-5000 series of electronic pressure-independent VAV controllers, MEP-4002 proportional electronic actuators in pressure-dependent VAV
applications, and VEB-43/46 series proportional control valves in baseboard, chilled
beam, and other heating/cooling applications. Single or dual setpoint, single or dual ducts, and with or without reheat are available configurations.
Venerable Pneumatic Legacy
They may not be considered sexy and cool, but pneumatic thermostats
are still reliable workhorses that control temperature in many older buildings across the country. While digital and newer electronic thermostats measure
the resistance of a thermistor as it reacts to temperature, CTC-1600 series pneumatic thermostats have a bimetal element. The
element deflects as it reacts to temperature, opening or closing a nozzle, which in turn controls air pressure leading to an actuator on a damper or valve.
A more high-tech approach to pneumatics can be found in KMC’s OEM relationship with Cypress Environsystems. KMC builds the pneumatic thermostat heart of
their award-winning Wireless Pneumatic Thermostat. It still uses
air pressure to control an actuator, but it also communicates wirelessly with a digital building automation system.
Thermostat Tips
Whatever kind of thermostat is purchased, it must be properly installed and configured to ensure long life and optimal operation. Accurate operation is
hindered by poor mounting. A thermostat must NOT be:
-
Mounted on an exterior wall.
- Mounted on or near a large thermal mass (e.g., concrete block wall).
- Blocked from normal air circulation by obstructions.
- Exposed to heat sources (e.g., lights, computers, copiers, or coffee makers) or to sunlight (at any time of the day).
- Exposed to drafts from windows, diffusers, or returns.
- Exposed to air flow through the conduit (from leaks in plenum ducts)—put plumber’s putty or similar material inside the conduit to block air flow.
If the thermostat includes a motion sensor, it must have an unobstructed view of the most typical traffic area. For more information about motion and CO2 sensors in thermostats, see the FlexStat Application Guide. For wiring and
configuration information, see the thermostat’s installation guide.
Thermostat Upgrades
But what if you already have a thermostat on the wall that’s been working fine for years? A building’s brick and mortar can last over a century, but is the
original HVAC system going to meet the occupants’ needs that long? Would you want a furnace in your home that was installed a century ago? Even if you
could still get coal for it, it would be horribly dirty and inefficient by today’s standards. Since today’s standards exceed those of the past, modern
thermostats are generally superior to the older models they replace.
However, even if improved energy-saving replacements exist, some people follow a “if it ain’t broke, don’t fix it” policy. Thermostats are generally quite
sturdy, but years of wear from people sliding levers or rotating dials (not to mention deliberate vandalism) result in thermostats breaking eventually. By
the time most thermostats are replaced, identical models often aren’t made anymore. But even if an identical model is available, most people want an
upgrade that is more energy efficient, more intuitive, and more attractive.
For upgrading analog electronic thermostats, the highly configurable CTE-5202 easily replaces many older KMC and other manufacturer models. It replaces
older KMC CTE-1x0x, CTE-50xx, and CTE-51xx thermostats, as well as Barber Colman (Schneider Electric, TAC, Invensys) TP-8101, TP-8102, TP-8103, TP-8124,
and TP-8125 thermostats. It also replaces a large number of other models from Anemostat, Carnes, Metal Industries, Metalaire, Price Industries, ASC/Titus,
Honeywell, Johnson Controls, Kele, PEKO, Siemens, and Viconics. (See the CTE-5202 Application Guide
for more details.)
For even more advanced capabilities, the networked, digital FlexStat and AppStat thermostats are excellent choices for working with building automation
systems. Feature for feature, they are cost competitive and can functionally replace many competitors’ products.
Bringing an old pneumatic system closer to current standards is more challenging. One of the CTC-1600 series may be an easy “drop-in” replacement. It
replaces most of the older KMC CTC-1xxx/15xx and CTC-5xxx models as well as many competitor products. For a substantial system upgrade, hybrid pneumatic/digital systems keep the parts of the old pneumatic
system that are most costly to replace and add digital monitoring and control. Although this approach does not achieve the energy efficiency and control
capability of a complete digital system retrofit, it is a far less expensive and time-consuming alternative.
Additional Resources
Small Building Automation (with the KMC Controls FlexStat)
KMC Sensors: Sensing Your Needs
Benefits of Controls in Green Buildings
Why Green Buildings Are Good