Greenheck's make-up air FAQ says its units support VFDs. On direct-fired units, Greenheck requires a burner by-pass damper to keep the air velocity constant. Before you add or replace a drive, also confirm that the fan motor is rated for VFD duty and check its turndown limit. The FAQ does not cover drive sizing.
What Greenheck says about VFDs on make-up air units
Greenheck's make-up air FAQ makes two statements that matter here. The first is that its units support VFDs. The second is that direct-fired units require a burner by-pass damper to keep the air velocity constant.
The FAQ does not say how to size a drive, which motor to use, or how to retrofit a drive to a unit that did not ship with one. It also does not list models. If you are adding a drive to an existing unit, use your unit's model and serial number and confirm the arrangement with Greenheck before you buy anything. Our article on finding Greenheck parts by serial number covers the lookup.
Why the burner by-pass damper is part of the VFD question
A direct-fired make-up air unit heats the airstream with a gas burner in the air path. A VFD changes fan speed, and fan speed changes the airflow through the unit. Greenheck ties the by-pass damper to VFD use on direct-fired units and gives its purpose as keeping the air velocity constant.
In practice, this means a drive is not a stand-alone swap on a direct-fired unit. The drive, the by-pass damper and the burner controls work together, and the unit's IOM and wiring diagram show how. Treat any plan that adds a VFD but leaves out the by-pass damper as incomplete until Greenheck confirms otherwise for your model.
Maxitrol 14 and Maxitrol 44: which control does your unit use
Greenheck's FAQ describes two Maxitrol modulating controls for direct-fired units:
- Maxitrol 14 uses discharge-air sensors. Greenheck names kitchens and paint booths as typical applications.
- Maxitrol 44 uses remote thermostats for space control. Greenheck names factories and warehouses as typical applications.
The FAQ also says the burner can take an external 2 to 10 VDC or 4 to 20 mA signal. That matters if building controls will also command the unit. Find out which control your unit has before you order sensors or a drive, since the control type decides which sensors the unit needs.
Motor duty and turndown
A drive only helps if the motor can live on it. Two facts apply to fan motors on drives.
First, turndown. Greenheck's Airxchange IOM says inverter-duty motors should not exceed 20:1 turndown. That manual covers Greenheck's energy recovery product, not make-up air units, so check the IOM for your make-up air unit for its own motor limits. An article in Consulting-Specifying Engineer makes a related point: it says basic inverter-duty motors are generally rated for 20:1 turndown on variable-torque loads such as fans, and that constant-torque loads may be limited to about 2:1.
Second, motor rating. The same article notes that many manufacturers reduce a 1.15 service factor to 1.0 on a VFD, which removes the overload margin. That is a per-manufacturer practice, so read the motor nameplate and the manufacturer's instructions. Our article on inverter-duty and standard motors on a VFD covers the motor side in more detail.
If the unit already has a drive
Replacing a failed drive on a unit that already runs one is a different job from adding the first one. The by-pass damper, the burner control and the motor are already part of the installation, so the work is matching the replacement to what the old drive did. Read the old drive's nameplate for input voltage, phase and output rating. Note the enclosure and mounting. If the old drive still powers up, record its settings before you remove it, and photograph the control and power wiring so the replacement goes back the same way. If the drive is dead, the unit's IOM and wiring diagram are the starting point for the settings.
What to confirm before you add or replace a drive
- Read the unit nameplate. Record the model and serial number, and photograph the plate.
- Find out whether the unit is direct-fired. If it is, the by-pass damper requirement applies.
- Identify the existing burner control. Confirm whether the unit uses Maxitrol 14 or Maxitrol 44 and what signal it takes.
- Read the fan motor nameplate. Record horsepower, voltage, phase and any inverter-duty or VFD-rated marking.
- Check the motor against the drive's output. Voltage and phase on the drive must match the motor. The drive manufacturer's documentation governs sizing, and we do not size drives here.
- Confirm the turndown range you plan to use. Compare it with the limit in the IOM for your unit.
- Get Greenheck's confirmation for retrofits. The FAQ says its units support VFDs. It does not cover your specific unit.
Parts involved
The listings below show the kinds of items that come up on a drive project. They are listing details, not fitment statements for any unit.
| Component | Role in this issue | Where to find it |
|---|---|---|
| VFD | Varies fan speed; listed as preprogrammed with a voltage in the title | 885047 — VFD, Preprogrammed, PS20040306210, 208/230V |
| VFD | Pre-programmed Emerson drive; title carries voltage and kW | 386480 — Variable Frequency Drive, Pre-Programmed, Emerson EV2055M-K9-891, 380/480V, 5.5Kw |
| VFD control board | Control board for a listed drive; title carries the drive model | 485227 — Control Board, PCBA-VFD, EV2080M-L1-291 |
| Fan motor, not VFD rated | Shows why the VFD rating is something to check; this title says "Non VFD Rated" | 305656 — Motor, Non VFD Rated, M3543, 0.75 HP, 3 Phase |
| Baldor motors | Motor listings by enclosure, frame and voltage | Baldor Motors |
| AYK580 HVAC drives | York-branded drives, listed as compare to ABB ACH580; a different line from Greenheck, shown for the drive range only | York AYK580 HVAC Drives (Compare to ABB ACH580) |
Scope: The VFD support statement, the by-pass damper requirement and the Maxitrol descriptions come from Greenheck's make-up air FAQ. The 20:1 inverter-duty turndown figure comes from the Airxchange IOM, which covers Greenheck's energy recovery product, not make-up air units. The turndown and service factor comments on fan loads come from a Consulting-Specifying Engineer article. None of these gives drive sizing, and each make-up air unit needs its own IOM.
Before you order
Confirm your unit's exact model and serial number from its nameplate before ordering. Search the part number in our site search, browse everything listed in our ventilation and make-up air collection and the Valent rooftop unit and DOAS parts collection, and see the Baldor motors collection for motors. To confirm fit, contact us or email help@totalhvacparts.com with your model and serial number.
Frequently Asked Questions
Can I add a VFD to a Greenheck make-up air unit?
Greenheck's make-up air FAQ says its units support VFDs. It does not cover retrofitting a specific unit, so confirm with Greenheck using your model and serial number. Direct-fired units need a burner by-pass damper.
Does a direct-fired make-up air unit need a bypass damper with a VFD?
Yes. Greenheck's FAQ says direct-fired units require a burner by-pass damper to keep the air velocity constant. Check your unit's IOM for how the damper is arranged.
What is the difference between Maxitrol 14 and Maxitrol 44?
Greenheck says Maxitrol 14 uses discharge-air sensors and suits applications like kitchens and paint booths. Maxitrol 44 uses remote thermostats for space control, as in factories and warehouses.
How far can I turn down a fan motor on a VFD?
Greenheck's Airxchange IOM says inverter-duty motors should not exceed 20:1 turndown. That manual covers the energy recovery product, so check the IOM for your make-up air unit.
Sources
- Greenheck Make-Up Air FAQs
- Airxchange Installation, Operation and Maintenance Manual, Airxchange_iom.pdf
- Consulting-Specifying Engineer article on common pitfalls in specifying motors and VFDs (csemag.com), cited in text, not linked
Direct-fired make-up air units involve gas combustion, line-voltage electrical components and rotating equipment. Drive and motor work should be performed only by qualified personnel, following the manufacturer's installation, operation and maintenance manual for your unit, applicable fuel-gas codes and your facility's lockout/tagout procedures.