Dedicated Outdoor Air Systems
Corpus sync: neutrality remakes, note hygiene, datasheet relocation, transformer-split cross-refs
Showing changes from Rev 5
to Rev 6
in Dedicated Outdoor Air Systems.
---
title: Dedicated Outdoor Air Systems
…168 unchanged lines
### Each unit shall receive a factory functional run test before shipment, operating the refrigeration or heat-pump circuit (where integral), the fans, the energy recovery drive, and the dehumidification and reheat sequence, and confirming that the unit produces the specified leaving dewpoint at the test condition.
−### A factory test report shall be submitted at closeout.
−
## NRTL Listing {toc}
…34 unchanged lines
### The controlling leaving condition is the supply air dewpoint, because the DOAS is selected to absorb the building latent load. {note}
−```datasheet
−label: Supply Air Leaving Dewpoint Setpoint
−type: range
−unit: °F
−drawing_ref: true
−options:
− min: 40
− max: 58
− setpoints: [40, 45, 48, 50, 52, 55, 58]
−default: 52
−```
+### The supply air dry-bulb strategy shall be specified, coordinated with the leaving dewpoint setpoint and the parallel sensible system serving the space.
```datasheet
…9 unchanged lines
### The leaving dewpoint setpoint shall be low enough that the DOAS supply air, after mixing with space air and absorbing the space moisture gains, holds the space relative humidity within the design range under ASHRAE 55.
+```datasheet
+label: Supply Air Leaving Dewpoint Setpoint
+type: range
+unit: °F
+drawing_ref: true
+options:
+ min: 40
+ max: 58
+ setpoints: [40, 45, 48, 50, 52, 55, 58]
+default: 52
+```
+
### A leaving dewpoint of about 50°F to 55°F is typical for comfort applications; lower dewpoints are specified for spaces with strict humidity limits. {note}
…174 unchanged lines
### AHRI 1060 quantifies cross-transfer from the exhaust stream into the supply stream as the exhaust air transfer ratio (EATR) and the outdoor air correction factor (OACF). {note}
+### The exhaust air stream classification shall be specified, establishing the permissible EATR limit and energy-recovery device type for the exhaust's contamination class.
+
```datasheet
label: Exhaust Air Stream Classification (ASHRAE 62.1)
…28 unchanged lines
### In cold climates the warm, moist exhaust gives up its moisture as it is cooled by the incoming air, and that moisture can freeze on the recovery surface, blocking the exhaust path and destroying recovery. {note}
+### Every recovery device in a freezing climate shall have a defined frost-control strategy.
+
```datasheet
label: Energy Recovery Frost Control Method
…8 unchanged lines
```
−### Every recovery device in a freezing climate shall have a defined frost-control strategy.
−
### Wheels shall be frost-controlled by modulating wheel speed or by a timed defrost; fixed plates and heat pipes shall be frost-controlled by preheating the incoming air, by an exhaust-side bypass, or by recirculation.
…19 unchanged lines
## The cooling section of a DOAS is selected to drive the air to a low leaving dewpoint, which usually means cooling the air below the temperature the space actually wants, then reheating it to the desired supply temperature while the moisture stays removed. {note}
−## To reach a 50°F to 55°F leaving dewpoint at a humid design condition, the coil leaving dry-bulb must be at or below that dewpoint (the air leaves the coil at or near saturation), which is colder than a comfortable supply temperature. {note}
+## To reach a 50°F to 55°F leaving dewpoint at a humid design condition, the coil leaving dry-bulb must be at or below that dewpoint (the air leaves the coil at or near saturation), which is colder than a comfortable supply temperature.
## AHRI 920 rates this behavior directly through its integrated seasonal moisture removal efficiency (ISMRE2, in pounds of moisture removed per kilowatt-hour), and AHRI 920 testing is run to a 55°F leaving dewpoint reference. {note}
…54 unchanged lines
### The decision is where the reheat energy comes from. {note}
+### Recovered reheat shall be used wherever the unit type allows it.
+
```datasheet
label: Reheat Method (detail)
…10 unchanged lines
```
+### Purchased reheat (a hot water or electric coil) shall be limited to applications where no recovered source is available or where the energy code permits it.
+
+### Recovered reheat — hot-gas (refrigerant) reheat that takes heat rejected by the unit's own condenser, a wraparound heat pipe or runaround coil that precools the entering air and reuses that captured heat to rewarm the leaving air, or a passive dehumidification wheel — spends no purchased energy on reheat, whereas purchased reheat spends energy heating air that was just cooled, which the energy code restricts. {note}
+
+### A wraparound device (heat pipe or runaround coil) improves dehumidification while it reheats: by precooling the entering air before the cooling coil it lets the coil reach a lower dewpoint for the same compressor capacity, and it then uses that captured heat to reheat the leaving air — typically lifting the supply from roughly 48°F to roughly 55°F — with no purchased energy and no controls. {note}
+
+### A passive wraparound is largely self-regulating and cannot independently set the supply temperature; where precise leaving-temperature control is needed, modulating hot-gas reheat or a controlled wraparound is required.
+
```datasheet
label: Reheat Leaving Air Temperature Setpoint
…7 unchanged lines
```
−### Recovered reheat shall be used wherever the unit type allows it.
−
−### Purchased reheat (a hot water or electric coil) shall be limited to applications where no recovered source is available or where the energy code permits it.
−
−### Recovered reheat — hot-gas (refrigerant) reheat that takes heat rejected by the unit's own condenser, a wraparound heat pipe or runaround coil that precools the entering air and reuses that captured heat to rewarm the leaving air, or a passive dehumidification wheel — spends no purchased energy on reheat, whereas purchased reheat spends energy heating air that was just cooled, which the energy code restricts. {note}
−
−### A wraparound device (heat pipe or runaround coil) improves dehumidification while it reheats: by precooling the entering air before the cooling coil it lets the coil reach a lower dewpoint for the same compressor capacity, and it then uses that captured heat to reheat the leaving air — typically lifting the supply from roughly 48°F to roughly 55°F — with no purchased energy and no controls. {note}
−
−### A passive wraparound is largely self-regulating and cannot independently set the supply temperature; where precise leaving-temperature control is needed, modulating hot-gas reheat or a controlled wraparound is required.
−
# Heating {toc}
…419 unchanged lines
## The O&M manual shall include the filter change schedule, the energy recovery maintenance schedule (wheel seal and belt inspection, or fixed-plate core cleaning), and the condensate trap cleaning procedure.