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WARNING
Hazardous Service
Procedures!
Modulating Discharge Temperature (MDT) Controls
MUA-SVX01A-EN • Direct-Fired Make-up Air
Trouble Shooting
The maintenance and troubleshooting procedures recommended in this section
of the manual could result in exposure to electrical, mechanical or other
potential safety hazards. Always refer to the safety warnings provided
throughout the manual concerning these procedures. When possible, disconnect
all electrical power including remote disconnects before servicing. Follow proper
lockout/tagout procedures to ensure the power can not be inadvertently
energized. When necessary to work with live electrical components, have a
qualified licensed electrician or other individual who has been trained in
handling live electrical components perform these tasks. Failure to follow all of
the recommended safety warnings provided, could result in death or serious
injury.
For ease in trouble shooting, it is advisable to wire the system as follows (this
differs from the normal connection). The discharge air sensor is disconnected
and replaced with a 10,700 ohm for DFC01 RTC Solutions™ or 10,000 ohm for
A1014L Maxitrol™, 1/2 watt test resistor (terminals 3 and 4). If inlet air sensor is
being used, disconnect and replace with a jumper. On units where the remote
temperature selector is located a considerable distance from the heater, it may be
advantageous to connect the selector at the heater location.
Connect the DC volt meter (capable of reading 0-24 VDC) on the modulator or
modulator-regulator valve terminals (Figure 13). Set the temperature to the
minimum dial setting. The DC voltage should read 0 Volts. The DC voltage should
gradually increase to at least 20 volts as the dial is slowly rotated to the
maximum dial setting. If these voltages are obtained, the valve function can now
be checked out.
The operation of the valve with regard to voltage is as follows:
•
From 0 volts to approximately 5 volts, the modulating valve should be on
bypass flow with the heater operating on low or minimum fire.
•
From approximately 5 volts to 15 volts DC the valve should be performing its
modulating function, and the heater should fire at a modulated flow rate
between low and high fire, depending upon the voltage (Figure 11b).
•
Above approximately 15 volts DC, the valve should be delivering full flow to
the heater and the unit should be on full fire.
Note: If the DC voltage is obtained on the valve terminals, but the heater does
not respond as described, the problem can be isolated to the valve itself or
to the gas control manifold of the heater.
In the event proper voltages are obtained, and the valve responds correctly to the
DC voltages, the wiring leading to the discharge air sensor or the discharge air
sensor should be reviewed in the check list.
If proper voltages are not obtained when wired as instructed, the problem can be
isolated to the electronics and this should be reviewed on the check list.
63
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