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FAQs - Gravity Ventilators

• Can a solid state speed control be used on a belt drive fan?
• For a two-speed fan, how is the low speed performance determined?
• What information needs to be known in order to properly select a special coating?
• What is the maximum roof pitch that a fan can be mounted to without correcting the curb for the pitch?

• Q: Can a solid state speed control be used on a belt drive fan?

  A: Greenheck does not offer solid state speed controls on belt drive fans because in order to use a solid state speed control, the motors have to be either a PSC (permanent split capacitor) or shaded pole type. These types of motors are not used on belt drive fans because they have very low starting and running torque. Belt drive fans have added torque requirements due to bearings, belts, and pulleys, all which hinder the ability of the motor to start the fan or even keep it running if the voltage is dialed down on the speed control.

• Q: For a two-speed fan, how is the low speed performance determined?

  A: The fan laws should be used to determine how a fan would perform on low speed. CFM (low) = CFM (high) x [RPM (low) / RPM (high)] Ps (low) = Ps (high) x [RPM (low) / RPM (high)]2 BHP (low) = BHP (high) x [RPM (low) / RPM (high)]3

• Q: What information needs to be known in order to properly select a special coating?

  A:

  To properly select a special coating for a fan, there are eight questions that should be answered.

  1. What specific chemicals are involved?
  2. What are the concentration levels of these chemicals?
  3. What will the airstream temperature be?
  4. What is relative humidity of airstream?
  5. What amount of time will the fan see this environment? (continuous or intermittent)
  6. What part of the country will the fan be located? (seaboard, snow country or desert)
  7. What are the specifications or requirements for this coating?
  8. What part of the fan is to be coated? (interior, exterior or entire)

• Q: What is the maximum roof pitch that a fan can be mounted to without correcting the curb for the pitch?

  A: As a general rule, the recommended maximum roof pitch is 2:12 or 10 degrees from horizontal. Steeper pitches will increase the possibility of moisture infiltration into the building as a result of water splashing off the roof and entering the fan. For roof pitches greater than 2:12, it is recommended to have a pitched curb, which will allow the fan to sit level and not at an angle.