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"How Does Low Site
Voltage Affect
a HVAC System?"
The Comfort Zone
September 1995
by Maury Tiernan |
Well . . . a good analogy is to say . . . it's like trying to pull a freight
train with a Volkswagen! The Mobile Modular Industry is almost exclusively 230
volt, single phase power. The operating range of this type HVAC unit is from
197-253 volts. Although most low site voltage at the HVAC unit is because the
power to the building is low, it also is caused by the length and size of an
entrance cable. Whatever the reason, the result is the same: decreased output
(a 3 ton is a 2.5 ton at 200 volt), chattering 24V controls, tripped circuit
breakers, tripped high pressure switch, unit won't start, burnt compressors and
transformers. All these turn into costly customer complaints and service calls.
If you own the building, do you really want your HVAC compressors burning out?
Low site voltage causes high amperage and the insulation to deteriorate on the
compressor and transformer windings. A service call on a burnt compressor will
be a very expensive repair since it takes extra material/labor to clean up the
system. HVAC units do not recover or heal themselves after a low voltage
condition has damaged the windings. It can only continue to deteriorate.
So now that I have you "shaking in your boots," what can be done about low site
voltage?
Check the voltage at the unit's circuit breaker/disconnect at 2:00 p.m. (the
worse time/temperature of the day). If it is below 200 volts, consider the
following:
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Check/insure the temporary power supply entrance cable is sized properly for
the load and length of run.
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Add a buck/boost transformer to the HVAC unit if the installation is permanent.
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Most HVAC units have a 24V transformer with a 230 and 208 volt "stake on"
terminal. Simply move as needed to protect the 24V circuit from burning out.
Remember the voltage will need to be rechecked each new lease site.
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Ask the power company to check/boost the site voltage.
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Consider adding a "brown-out" control which protects the unit from low site
voltage. It takes about 10 minutes to install and costs about .
Each project has it's own specific problems and solutions, but the addition of a
brown-out control is probably the best overall choice since it will check the
voltage for you . . . every job, . . . every day, . . . every hour.
I probably answered a few questions, but raised some others. So fax any
questions or comments you have into the MBI c/o The Comfort Zone.
Keep those cards and letters coming. Until the next time we enter The Comfort
Zone . . . Be Cool.
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