Automatic Duct Temperature Modulating Control
Enables the end user to maintain a pre-set minimum inside temperature.
In the 100% outside air mode, the outside air intake damper is in the open position and the inside air recirculation damper is in the closed position.
When the outside temperature (and duct temperature) falls below a predetermined set point, the modulating motor opens the recirculation damper and closes the outside air damper until the temperature of the mix of outside and inside air reaches the set point.
Proportional Temperature Controller:
A Honeywell #T991A proportional temperature controller mounts on the exterior of the MUA housing with a capillary tube inside the air straightener parallel to the air flow. The temperature controller has a range of temperatures from which to select a specific set point.
Sensor Control Set Point:
The maximum volume of outside make-up air and the greatest energy savings will be achieved at the lowest possible set point. The point can be best established on a cold day.
Temperature - Turn knob on the front of the case until the pointer indicates the desired set point temperature. This is the center point of the proportional range.
Range Adjustment - The Honeywell T991A may be adjusted to vary the temperature range within which the proportional action is desired. With the cover off, turn the adjustment wheel until the pointer indicates the desired range.
If the temperature of the controlled medium is to be maintained at 80°F and the proportional action from 75°F to 85°F is desired (a range of 10 degrees), turn the temperature set point indicator to 80°F and the proportional range adjustment wheel to 10.
Checkout - After mounting and wiring have been completed, let the controlled equipment operate until the system temperature stabilizes. If the motor shaft constantly moves back and forth, widen the Honeywell Model T991A proportional range (about five degrees at a time) until the system is stable.
On a cold day, select a set point at the bottom of one of the ranges. If the setting tends to cool the plant in the area below 70°F, increase the set point until no cooling is taking place. This setting (winter mode set point) should not need to be adjusted during the winter season.
In the spring and early fall, plant heating systems may not be operating and outside temperatures may go as low as 40°F to 50°F. Operating MUA units under these circumstances may further cool an already cool plant. To remedy this situation, the set point should be increased and then re-set in late fall to the winter mode set point.