ROOF MOUNTED FANS FAQs
Answers to your most Frequently Asked Questions about Greenheck roof mounted fans.
Q:
Q:Can a solid-state speed control be used on a belt drive fan?
A:
A:Greenheck does not offer solid-state speed controls on belt drive fans because the motors must be a PSC (permanent split capacitor) or shaded pole type. These motor types are not used on belt drive fans because of very low starting and running torque. Belt drive fans have added torque requirements due to bearings, belts, and pulleys, 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:
Q:Can centrifugal inline fans be mounted outside?
A:
A:If the centrifugal inline fan is Greenheck Model TCB or TCF the answer is yes. These fans are constructed with welded housings, which make them weathertight. Greenheck Model SQ or BSQ fans are not recommended for outdoor mounting. The construction of these fans is such that they are not weathertight.
Q:
Q:Can we mount a USGF on the sidewall for emergency smoke?
A:
A:No, the fan body will not support the steel wheel and motor when mounting on a sidewall. The isolators will be put in shear, the fan will not be weatherproof, and will not maintain UL for smoke. We recommend using an inline smoke rated model QEID, TBI-FS, AX, QEI for sidewall emergency smoke applications.
Q:
Q:For a two-speed fan, how is the low speed performance determined?
A:
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:
Q:What factors prevent fans from not performing as specified?
A:
A:

There are numerous reasons why fans fail to perform as specified. But first it is important to understand what defines acceptable performance. According to AMCA Publication 200, a fan installed in a ventilation system should expect a tolerance of +/- 7.5% for flow (cfm). In other words, a fan that produces 1,000 cfm in a lab environment should provide a volume flow rate of 925 to 1,075 cfm when installed in a properly designed ventilation system. Volume flow rates falling below this range are typically the result of variances in system static pressure or mechanical problems with the fan.

Common symptoms include:

  • Obstructions in the duct system - closed dampers, closed registers, dirty filters, clogged coils
  • Obstructions in the fan inlet - elbows too close to the inlet, walls too close to the inlet
  • Duct design - improperly designed turning vanes, leaks in supply or exhaust ducts
  • Fan related - impeller running backwards, fan speed too low, impeller dirty or clogged, clearances between inlet cone and wheel cone are incorrect
Q:
Q:What information do I need to select a special coating?
A:
A:

There are eight questions to have answered to select a special coating for a fan.

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