The most recent misconception to be encountered when performing field training or when discussing system specifications includes the use of variable-speed indoor blower motors with a forced-air zoning system. For example, many contractors state, “I can’t install a zoning system with a residential variable-speed air handler or variable-speed furnace.”
The confusion behind this statement is that many do not realize that variable-speed motors are controlled internally via the air handler’s integrally controlled motor (ICM). These integrated logic controls can accept but do not require an external pulse width modulation (PWM) or 4-20 mA input signal to control the motor’s speed and ramp-up rate. The only signal required to operate these variable-speed motors is the standard (G) input from a conventional thermostat. Just as a standard 24 VAC thermostat connects to and operates a variable-speed furnace, so does a zone control panel. Both simply output a demand for the fan to operate. The variable fan speed operation is determined internally by the system mode and stage of operation.
This gradual ramp rate starts at 70 percent of full load and takes five minutes or more to reach 100 percent full load. This slow ramp rate increases the latent cooling effect of removing moisture from the air, and results in lower-than-normal supply air temperatures — just like a conventional zoned air system with a bypass damper has been doing for 40 years. Simply adjust the low temperature limit potentiometer setting on your zone control system to avoid a “nuisance” trip of the freeze protection setting.
The fact is that a variable-speed fan motor operates at full load (FLA) or near full load at different times, depending on the mode or stage of operation. A variable-speed air handler or furnace is not aware of how many zones are demanding conditioned air, or how many dampers are open or closed. In the same way, it has no idea if the connected ductwork has been properly sized and installed. The use of a barometric, or preferably an electronic, static pressure-operated bypass damper prevents overpressurizing of the ductwork, subsequent noise, and the negative effects from excessive static pressure. When installed in a variable-speed system, the bypass damper simply may not open as often as it does in a constant-volume system. However, it still proves to be an indispensable component in any zoned forced-air system. As to the never-ending dilemma of using a dump zone instead of a bypass damper, we respond again with this question: Why spend money to condition the air, and then dump it where nobody benefits from it?
The more advanced and costly zone control systems do provide an external pulse width modulation (PWM) or 4-20 ma output signal to the air handler’s integrated motor control. These zone systems can control and modulate the PWM output, in direct response to the number of zones that are demanding conditioned air, and the preprogrammed cfm requirements of each zone. These systems may not require a bypass damper, but they are very unforgiving with poor duct design and layout. These systems could probably benefit from a bypass damper, just in case.
The diagram in Figure 1 reflects a typical motorized bypass arrangement based on static pressure. The static pressure control is field adjustable from 0.01 inches water column (wc) to 4.0 inches wc.
The control is multipositional. That is, it can be mounted in any position and will still function properly. Note that the supply air sensor is installed upstream of the bypass takeoff and wires back to the zone control panel. This is to ensure that the zone control microprocessor can accurately determine the supply air temperature, regardless of the position of the bypass damper.
The ability of the zone control system to properly monitor the supply air temperature is critical to any successful installation. Some zoning manufacturers provide the supply air sensor as an option only. Other manufacturers include the sensor in every zone control package. The ability to adjust cooling and heating limit setpoints is built right into the zone control panel, which makes it very easy to fine-tune the zoning installation. Older-style zone control systems will not accept a supply air sensor at all. External field-installed temperature limit controls must be used.
The field-installed temperature controls (external or nonintegrated) can be expensive, bulky, and difficult to install and wire. A better solution is to utilize the supply air sensor provided with these state-of-the-art zone control systems. They are accurate, easy to install, and provide superior air temperature monitoring directly to the processor.
A properly designed and installed forced-air zone system will provide exceptional comfort levels as well as energy savings. Specifying variable-speed equipment adds an even higher degree of control and comfort. The two systems complement each other and function very well together. They are definitely not as complicated to install and set up as some would have you believe.
Brown is chief engineer with EWC Controls Inc., Englishtown, N.J. He can be contacted by e-mail at email@example.com.
Publication date: 05/26/2003