Over the past few years, I've specified and installed Hitachi VFD drives on everything from chiller pumps to large blowers in industrial HVAC. I've also made some expensive mistakes. This FAQ covers the questions I get asked most—and the ones I wish I'd asked myself before a $3,200 redo in early 2023.
If you're short on time, jump to Q3 and Q5. Those are the ones that cost me.
1. Why choose a Hitachi VFD drive over a standard starter for my HVAC fan or pump?
I used to think a VFD was overkill unless you needed variable speed. Turns out, for any motor over 5 HP on a fan or centrifugal pump, it's often the smarter choice. A standard starter just turns it on. A Hitachi VFD (like the WJ200 or SJ700 series) lets you ramp up slowly.
Why does that matter? Inrush current. Starting a motor direct-on-line can pull 6-8 times its full-load current. In a facility with multiple units, that adds up—both on your utility bill and on mechanical wear. A VFD brings it up gently. The difference is night and day on belt-driven blowers. I've seen belts last twice as long.
That said, if you have a fixed-speed load running 24/7, a VFD might not pay back. It's about the application.
2. Can I use a Hitachi VFD drive on my existing HVAC motor? Or do I need a special 'inverter-duty' motor?
This is where I made my first mistake. In my first year (2017), I slapped a VFD on an old 20 HP standard-efficiency motor. It worked. For about six months. Then the winding insulation failed.
Here's what no one told me: standard motors aren't designed for the voltage spikes a VFD can produce—especially with longer cable runs. The PWM waveform reflects back, and the voltage at the motor terminals can spike significantly. Hitachi VFDs have dV/dt filters available. I didn't use one. That was the mistake.
So, the real answer? You can use a standard motor, but you should:
- Keep the motor lead length short (under 50 feet is ideal)
- Use inverter-grade wire insulation
- Consider adding a dV/dt filter or load reactor from Hitachi's accessory line
- Upgrade if the motor is old or if you see pitting on the bearings (common electrical fluting issue)
If you're buying new, get an inverter-duty motor. It's cheaper than the replacement cost. Trust me.
3. How do Hitachi VFD drives compare to other brands (like a certain competitor's AC drive)?
I'll say this carefully, because I have opinions. I think Hitachi VFDs are excellent for industrial HVAC and pump applications because of their built-in PID control and simple auto-tuning. For a chiller condenser fan or a cooling tower, they're hard to beat out of the box.
The surprise wasn't the performance. It was the manual readability. Hitachi's documentation used to be terrible—very engineering-focused. The newer SJ700 series manuals are much better, but the initial setup still has a learning curve if you're coming from a simpler drive. Expect to spend an hour on the first one.
For basic fan control, most drives are similar. But if you need precise pressure control for a blower system, the Hitachi's PID tuning is relatively forgiving. The frustration I have: the default acceleration/deceleration times. They're set too fast for most HVAC loads. Change those first. Otherwise you'll trip a fault on startup.
4. Does a humidifier or dehumidifier help in a space conditioned by a VFD-driven heat pump?
This is a common question, and the answer is tricky. A VFD-driven heat pump (like a Hitachi air conditioning system) is excellent at maintaining temperature. But humidity control on a heat pump during part-load can be poor. Because the fan runs slower, the coil gets colder, which should dehumidify. But the compressor also modulates down, which means the coil might not stay cold enough long enough to condense moisture effectively.
In short: it depends.
If your space feels clammy (above 60% RH), despite the thermostat showing 72°F, a dehumidifier might be the better solution. Here's my rule of thumb from a 2022 warehouse project:
- Use a dehumidifier if the heat pump runs at low speed >70% of the time during humid months. The heat pump won't keep the coil cold enough.
- Use a humidifier only if the heat pump runs often but the space feels dry (below 30% RH). This happens in winter with good heat recovery systems.
Neither option is perfect alone. On that warehouse job, we added a small standalone dehumidifier for the storage area. Problem solved.
5. What about the Hitachi thermal transfer overprinter? Why is it in an HVAC discussion?
Honest question. The Hitachi thermal transfer overprinter (TTO) is a packaging marking solution—nothing to do with HVAC directly. But I include it because if you're managing an industrial site, you'll eventually deal with labeling. And the mistake I made was not thinking about component tags early enough.
I once ordered 50 replacement VFD drives for a plant expansion. We installed them, programmed them, and then realized we hadn't printed labels for the cabinets. We had to use a cheap handheld labeler. They started peeling within a month. The TTO is for high-quality, durable printing on cables and panels.
If you're doing a large installation, plan your labeling before commissioning. A good thermal transfer printer saves you hours of rework. I recommend it for any project over 5 drives. It's one of those details that makes you look professional.
6. Can a Hitachi VFD help my air compressor run more efficiently? And what about air filters (K&N style)?
Yes, on the VFD for an air compressor. A fixed-speed compressor unloads or cycles on/off, wasting energy. A VFD-driven rotary screw compressor matches demand. Hitachi makes dedicated compressor packages, but you can retrofit a standard Hitachi VFD onto a compressor motor. Pro tip: Make sure the VFD is rated for the high starting torque of a compressor. The Hitachi SJ700 is well-suited for this.
On air filters: I see people put high-flow filters (like a K&N air filter) on compressor intakes thinking it reduces restriction. Don't. Compressors need high-efficiency particulate filters, not high-flow. A K&N is designed for engines, where you accept lower filtration for horsepower. For a compressor, you want clean air. A standard paper filter or a Donaldson-type is better. You'll change it more often, but your compressor oil and vanes will last longer. A K&N on a compressor? I've seen it. The oil analysis was not pretty.
7. What about heater sizing (Mr. Heater type) in a VFD-controlled space?
This is a niche one. If you're using a VFD to control a fan in a space that also has a Mr. Heater or similar forced-air propane heater, be careful. The VFD doesn't care about the heat source, but the airflow does.
If the VFD slows the fan down too much, the heat exchanger in the heater doesn't get enough airflow. It can overheat and trip the limit switch—or worse. I saw it happen in a workshop in 2021. The heater kept cycling off, the space didn't warm up, and the fan was barely moving.
Solution: Minimum speed limit. Set the VFD so the fan never drops below the heater's minimum required airflow. This is usually stated in the heater's manual (often 200-300 CFM for a small unit). If you can't guarantee that, don't use a VFD on that fan—or bypass the heater safety with a separate interlock.
Simple fix. Cost me a weekend of troubleshooting to learn it.
