Control Two Stages with Zoning

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When zoning was first introduced…

we learned that we had to control excess equipment capacity if we were going to close dampers and dramatically change the airflow or the air temperature across the coil or heat exchanger.  We used mainly single speed equipment with a bypass duct that would allow excess capacity to be dumped back into some unobjectionable space.  So, what part of your home would you dump 800 cfm of 140⁰F air with no control and call it unobjectionable? We finally abandoned the idea of a dump zone and began using a bypass duct that directed excess supply air back into the return air plenum.  Two or Three zones would work as long as the zoning manufacturer provided a temperature sensor to de-energize the unit when the supply air temperature was out of the correct operating range.  The indoor blower would continue to blow into the zones that were calling and the system would restart after a brief time off.  This was a simple way to control the excess capacity, but we saw some system short cycling.

Then we learned that a two-stage system…

would allow us to control capacity if we could correctly control the shift from low speed to high speed and back to low because the capacity requirements change so dramatically when zones open and close.  Some zoning manufacturers energize second stage after so many minutes of run time.  This method does not control excess capacity; it just arbitrarily turns on second stage based on running some length of time.  Other manufacturers energize second stage if any thermostat is calling for second stage.  This method does not control based on the excess system capacity, and can unnecessarily shift into high speed based on only a single thermostat’s request while all other zones are not calling.

At Comfort Products, Inc. we advocate…

a zone controller with a temperature sensor that not only controls high and low supply temperature cut-out, but also let’s use the supply air temperature to control staging.  This method has been applied to commercial vav systems for years with great success.  After 8 minutes of initial run time in low speed, if the supply air temperature is out of range, turn on second stage.  If the temperature in the supply plenum changes too far, we drop out of second stage and go back into first stage, and so on.  Also, with this method we no longer need two-stage thermostats since the stat is not responsible for staging.  This method allows the excess air that is being bypassed in low load conditions to affect the supply temperature and directly control staging.  No matter how many zones open or close the supply air temperature must be correct or staging (up or down) will occur automatically.

It is imperative that…

we have the ability to adjust the supply air temperature setpoint.  This allows us to fine tune the staging based on exactly how a particular machine operates in each application.  This just means that the contractor controls how cold or warm the supply air will be before staging occurs.  In high humidity regions, high speed can be used to maintain a cooler supply air temperature.  Especially in mild but humid conditions, we have more humidity removal during the limited run time.  When less than all zones are open, we can control to a specific supply air temperature and consequently the power bill can be lower.

Two boards can be…

connected together to give the client as many as 8 zones easily.  The ability to control stages using supply air temperature is required if you are using many zones and/or small zones.  It is very important to control the bypass damper with a static pressure sensor when using more than two or three zones.  This allows for adequate pressure sensing and consequently good pressure relief through the bypass damper.

The way we see it…

the clear plastic enclosure, the supply temperature display, screwless terminal blocks, the correctly colored LED’s beside each terminal, the power open/power close motor terminals with LED’s, one controller operates gas/electric, electric/electric, heat pump, dual fuel, or forced air with radiant zoned heat on a separate transformer, heavy-duty damper and equipment relays, and a removable fuse for low voltage are all just icing on a delicious cake.

Written by Greg Talley, President

Comfort Products, Inc.

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