Library » FAQs » Roof Mounted Fans
FAQs - Roof Mounted Fans
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: Can Centrifugal Inline fans be mounted outside?
A: If the Centrifugal Inline Fan is Greenheck's Model TCB or TCF the answer is Yes. These fans are constructed with welded housings, which make them weathertight. If the fan is Greenheck's Model SQ or BSQ, they are not recommended for outdoor mounting. The construction of these fans is such that they are not weathertight.
Q: Can we mount an S-CUBE or USGF on the sidewall for emergency smoke?
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. Recommend using an inline smoke rated model SWB, TBI, TDI, TCF, AX, QEI for this sidewall emergency smoke applications.
Q: Do spark resistant fans have any third party certification?
A: Spark resistant fans do not have any third party certification. In other words, you will not see a sticker on the fan which states that the fan is spark resistant. The guidelines for spark resistant construction are noted in AMCA Standard 99-0401-86. If spark resistant construction is ordered, the fan manufacturer will build the fan to meet these requirements.
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 factors prevent fans from not performing as specified?
A: There are numerous reasons why fans may 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 which 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 which fall below this range are typically the results 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 to 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: 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.
- What specific chemicals are involved?
- What are the concentration levels of these chemicals?
- What will the airstream temperature be?
- What is relative humidity of airstream?
- What amount of time will the fan see this environment? (continuous or intermittent)
- What part of the country will the fan be located? (seaboard, snow country or desert)
- What are the specifications or requirements for this coating?
- 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.
