Outside Air Temperature Sensor Location

Typical OSA Temperature or RH Transmitters Mounting with Weather Shields:

Outside Air (OSA) sensor placement is critical to good HVAC performance. The OSA sensor must be mounted in the shade and never above building windows, doors, vents or dampers. Sensors should not be placed in direct sunlight as temperature readings can be altered by as much as +30%. The ideal shaded location in the Northern Hemisphere is on the north side of the building. Note that in the Southern Hemisphere the south side of the building is ideal. Never mount over a sun-drenched wall, roof or parking lot because the heat rises and gives a false temperature reading.

The sensor weather shield and probe should always point down or horizontal per the manufacturer’s recommendation. The probe should not point upward as water and debris could collect resulting in long-term issues. This shield is not a sun shield because when hit by the sun it will heat up and through heat radiation will affect the temperature probe.  The OSA sensor should be mounted between four feet above the ground and one-foot minimum below the eave. (Note: Four feet keeps the sensor above ground radiation and one foot under the eave prevents measurement of trapped heat from under the eave.)

Summary of Do’s & Don’ts:

Do’s

  • Always mount in the shade on the north side of the building, minimizing sun exposure.
  • Mount a minimum of four feet above the ground surface to prevent thermal radiation from rising up and impacting performance.
  • Mount at least one foot below the eave, preventing trapped heat at the top of the eave from affecting performance.
  • Point the weather shield down or horizontal to prevent debris from altering the performance.

Don’ts   

  • Never mount in direct sunlight.
  • Never mount above building windows, doors, vents or dampers.
  • Never mount above a sun-drenched parking lot.
  • Never mount within 1 foot of the eave.

Sun Exposed Mounting: Not recommended if possible. 

There are some shields made for mounting in the sun.  The most popular is a “Gill-style” that uses stacked plates (like a pagoda) tilted to take the wind and direct the air up in a sucking effect to bring fresh air in from the bottom of the assembly to the sensor within and expel the air out the top.  If there is no wind, the plates heat up and provide a lifting effect of the hot air thus bringing in fresh air from the bottom of the structure.  In full sun, this works OK in high winds >5.6 MPH (2.5 m/s) with a temperature gain of about +5.4⁰F (+3⁰C).  In higher winds, this drops to +3.6⁰F (+2⁰ C).  Lower winds experience an unacceptable rise of +9⁰F (+5⁰C) or more due to simple heat radiation from the plates impacting the sensor within.

Economizer controls require a very good reading of the true OSA temperature/humidity to function properly.  A temperature rise of 2⁰F is not acceptable. It is not recommended to use a sensor located in the direct sun using a non-aspirated enclosure for any HVAC control operation.

Some aspirated shields perform very well in any wind or sunlight condition. If an OSA temperature or humidity transmitter must be mounted in direct sunlight ensure an aspirated-style enclosure is used.  These will require power to operate a fan.  An aspirated enclosure mounted in the direct sun has a temperature gain of between 0⁰-.9⁰F (0⁰-.5⁰C).  The higher the fan speed the less solar effect is realized.

Conclusion:

The optimal location to mount an OSA sensor is on the north side of the building regardless of the OSA enclosure used.

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