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"Two or
More HVAC Units, One Thermostat"
The Comfort Zone
September / October 2000
by Maury Tiernan, Geary Pacific Corp. |
Situations arise where the occupants of a multi-wide mobile/modular building
request one thermostat to control two or more HVAC units. It may be because a
conference room is open across all the module lines, the building is one large
room, or the end user has simply requested it.
When planning how to control more than one HVAC unit with one thermostat, start
by discussing the following with your customer/end user:
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Should this building have a "zoned" HVAC system, or the standard one HVAC unit
per one floor/module system?
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How will all of the other rooms be affected if the thermostat is located in the
main room?
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Is the main heat load located in the room with the one thermostat?
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What is the reason for the request? (Sometimes installing one thermostat isn't
the best solution to the perceived problem.)
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Will your building code require smoke detectors or economizers in the units
since the total combined capacity of the new "system" probably now exceeds the
code's maximum tonnage or cfm quantity for standard units?
With the additional input from your customer/end user, we can proceed to select
the most appropriate thermostat control method. As we do so, we must keep in
mind an important point.
Each HVAC unit/system has its own low voltage transformer. Wiring two or more
HVAC units together has the potential of burning out the low voltage wiring
throughout all of the units. To prevent this from happening, make sure the
units are properly phased. However, phasing the low voltage system once
initially, may not create a permanent condition. In the future, if anyone
switches a wire around, poof, the wiring in all the units will fry! This
caution applies to the first control method below and usually is not
encountered in method number two.
The first of the two popular methods to control two or more HVAC units with one
thermostat is to eliminate all of the low voltage transformers in all of the
HVAC units, and install one large low voltage transformer in one HVAC unit.
The second method involves using a master control panel. Looking at the first
method, total up the volt-amps (watts) rating of all the transformers, remove
them all, and put one large low voltage transformer in one unit. This first
method will require an HVAC service contractor. Next, interconnect all the
units together with a "common" low voltage leg. Next, bring all the wires from
the units to one single gang box for the stat. Wire nut all the appropriate
leads together prior to the stat. Once that is done, the contractor can measure
the setting for the heat anticipator requirement of the thermostat. (Caution:
some thermostat heat anticipators cannot handle the load of more than one HVAC
unit). Finally connect to the thermostat and turn on the power to the units.
The second method involves a master controller where each HVAC unit's low
voltage wiring is run to separate terminal strips of that device. The master
controller requires 110 volts and has it's own transformer for control power to
the relays in the device. One thermostat can be surface-mounted on the
controller box or it may be mounted remotely. This master controller can easily
be installed in the factory with very little chance of wiring errors.
Air conditioners with heat strips and gas heat HVAC units would require less
wiring, fewer relays, and less space in a master controller panel box than
would a heat pump. The sequence of operation of this multi-unit master
controller would be different than that of most master controllers offered on
the market today. Take care not to confuse one type for the other. The other
popular controller on the market today, a Lead/Lag master controller, does not
let both units run at the same time, and involves a maximum of two units.
A review of the cost associated with these two methods would be a minimum of
approximately $ 250 (for each unit of a two-HVAC-unit system) to $ 500 each
HVAC unit (for a three- to four-unit-system). Two or three units controlled by
one thermostat in a proper installation shouldn't pose too many complications,
but four or more should be reviewed for potential problems by your mechanical
engineer or consultant.
As expected, controlling two or more HVAC units with one thermostat is a job
for a professional. Don't let your kids try this at home, not even if they have
been bad! Contact a mechanical engineer, HVAC controls company, or your HVAC
supplier for assistance on this approach.
Summer is coming to an end, but it is still 110 deg F in Phoenix, and Dallas
has record heat and no rain. So, stay in the shade and keep cool until the next
time we meet in . . .
The Comfort Zone.
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