Catalogue

UV-PRC003-EN 15
Application Considerations
ASHRAE Cycle I
All standard unit ventilator cycles automatically close the outside air damper whenever maximum
heating capacity is required. As room temperature approaches the comfort setpoint, the outside
air damper opens fully, and the unit handles 100 percent outside air. Unit capacity is then controlled
by modulating the heating element capacity.
ASHRAE Cycle I is typically used in areas where a large quantity of outdoor air is required to offset
the air being exhausted to relieve the room of unpleasant odors and particles.
ASHRAE Cycle II
ASHRAE Cycle II is the most widely used ventilation control. Similar to Cycle I, the outside air
damper is closed during warm-up. But with Cycle II, the unit handles recirculated air through the
return air system. As temperature approaches the comfort setting, the outside air damper opens
to admit a predetermined minimum amount of outside air. This minimum has been established by
local code requirements and good engineering practices. Unit capacity is controlled by varying the
heating output. If room temperature rises above the comfort setting, the heating is turned off and
an increasing amount of outside air is admitted until only outside air is being delivered.
ASHRAE Cycle II is a very economical control sequence often referred to as integrated
economizing. This design supports optimum ventilation and provides the greatest energy savings.
This is further proof of why AHRI-840 certification is important in minimizing energy consumption
through economizer performance.
Freeze Protection
The most important advantage the Trane blow-through design provides is additional protection
against coil freeze-up. In contrast, draw-through configurations allow little mixing of the return and
outside air stream while locating the coil very close to the outside air inlet. This process creates
“cold spots” on the coil which could lead to coil freeze-up.
With a blow-through design, face and bypass with isolation valve control is not necessary to
provide proper freeze protection to the unit vent. The placement of the coil above the fan allows
enough space for the coil to avoid “cold spots” that could cause freezing.
Figure 10. ASHRAE Cycle graph
Ventilation
UV-PRC003-EN.book Page 15 Tuesday, June 4, 2013 8:59 PM