When sizing generators, they should be of sufficient capacity to provide the
required voltage and amperage listed on the HVAC unit's nameplate, plus any
additional capacity requirement for add-ons and other appliances.Generators
may be used to test or operate HVAC units. Their effectiveness requires an
understanding of the operation of both devices. Improper use of generators on
HVAC units may affect the HVAC unit warranty.
HVAC units are tested and listed at 230v for compressor cooling/heating, and
240v for heat strip heating. When generators are used, they must supply a
minimum of 230v to the HVAC units at all times, including startup and shut
down. Supplying anything less than 230v increases the wattage/amperage
requirement of the generator. To prevent damage from generator power
fluctuations, a voltage monitor (brown out control) is required for every HVAC
unit powered by a generator, whether the use is permanent, emergency, or
temporary.
Therefore, when sizing generators, they should be of sufficient capacity to
provide the required voltage and amperage listed on the HVAC unit's nameplate,
plus any additional capacity requirement for add-ons and other appliances. I
suggest sizing generators at 125% of the HVAC unit's minimum circuit ampacity.
Use the following formula to calculate the generator size needed to power only
the HVAC unit. Then increase the generator size to include power for lights and
any other appliance loads. Understand that heating and cooling units may have
separate MCA requirements on the heating and cooling sides, but the generator
must be sized for the largest load. The formula for 3 Phase equipment is
slightly different.
 For example, when an HVAC unit is in
heating mode, a heat strip usually has a higher running wattage draw than a
compressor. Heat strips are rated at 240v, while the HVAC units are rated at
230v for the cooling side. Supplying anything less than 240v to an HVAC unit
reduces the listed btuh output of the heat strip.
A compressor is used in the cooling side of an air conditioner, and also on the
heating/cooling side of a heat pump. Sometimes compressors create a
larger-than-usual load. Compressors have a starting load that creates a large,
instantaneous wattage draw. High outdoor temperatures also increase the wattage
draw of the compressor. These large loads must be taken into account when
sizing the generator.
A heat pump usually has an auxiliary heat strip. If the compressor circuit
cannot be separated from the auxiliary heat strip circuit, then both circuits
must be added together to size the generator. Be aware that separating the
auxiliary heat strip from the generator to simply reduce the generator size
will also remove the heating capacity produced by the heat strip.
To prevent damage to the HVAC system, the generator voltage should not be
supplied to the HVAC unit until it has reached a steady 230 volts. If an HVAC
unit's thermostat is "calling" for compressor heating or cooling while a
generator is starting up or shut down, the chance of compressor and/or control
damage is very likely. A voltage monitor (brown out control) protects the HVAC
unit from low or high voltage damage during these periods.
Involve your electrical engineer, mechanical engineer, or HVAC supplier in
selecting/preparing your HVAC unit for generator use.
Keep your voltage steady until the next time we meet in . . . The Comfort Zone.
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