When you work in the heat pump dryer supply business, every question that comes through from customers—whether it’s a small bakery owner testing a unit for her countertop production line or a large pharmaceutical plant manager upgrading their drying process—boils down to one core priority: reliability. I’ve spent the last six years talking to these kinds of decision-makers, and the question I get asked most often, after “how much is this going to save me on my utility bill?” is always: “What kind of ventilation does a heat pump dryer actually need?” Heat Pump Dryer
It’s a fair question. If you’ve used a traditional vented dryer before, you know it dumps hot, humid air straight outside through a long duct, and you’ve had to make sure that duct doesn’t get kinked or blocked by dust. Heat pump dryers work differently, of course—they reuse most of the heat that would be lost in a traditional unit, pulling moisture out of the air, cooling it to condense water, and recycling the warm, dry air back into the drying chamber. That efficiency is why so many businesses switch to them, but that doesn’t mean ventilation doesn’t matter. Get the ventilation wrong, and you’ll end up with longer dry times, higher energy bills, or even damage to the unit’s components.
Let’s start with the basics so we’re all on the same page. Heat pump dryers are closed-loop systems by design, which means they don’t need the same kind of continuous, large-volume exhaust that vented dryers do. But that doesn’t mean they are airtight. The system relies on a small, controlled amount of airflow outside the closed loop to balance internal pressure, remove any trace non-condensable gases that can build up over time, and prevent excess humidity from lingering if the load is particularly moist. Too little of this airflow, and you’ll see the drying efficiency drop; too much, and you waste the very heat that makes the heat pump dryer worth investing in.
So what’s the recommended ventilation for a commercial or industrial heat pump dryer? I’ve tested this with dozens of clients, and the general guideline I share comes from years of on-site adjustments, combined with the data we’ve gathered from our own unit testing. For most standard heat pump dryers (10 to 500kg load capacity), the recommended ventilation rate is 0.5 to 2 air changes per hour (ACH) of the total volume of the drying chamber. That number is not arbitrary—it’s calibrated to let just enough fresh air in and conditioned air out to keep non-condensables from building up, while not siphoning off the heat that the compressor works so hard to create.
Wait, let me break that down with an example. If you have a heat pump dryer with a chamber volume of 10 cubic meters (roughly 350 cubic feet), 0.5 ACH means you need to ventilate 5 cubic meters of air per hour, and 2 ACH means 20 cubic meters per hour. That might sound small compared to a vented clothes dryer, which can move 200 cubic feet per minute, but remember: heat pump dryers don’t need to move all that air. Their efficiency is in recirculation, so only a small, steady purge is required.
But ventilation isn’t just about volume. The design of the ventilation path matters too, and that’s where a lot of people go wrong. The vent connections on our heat pump dryers are sized specifically to match the required airflow, usually 100mm to 150mm for standard units. You don’t need a huge, wide duct here—actually, a duct that’s too large can create negative pressure inside the drying chamber, pulling in unconditioned outdoor air that the heat pump has to work to heat up. That defeats the purpose of the closed loop. Instead, you want a straight, short duct, as much as possible. If you have to run the duct to the outside, keep it under 3 meters long, with no more than two 90-degree turns. Each turn creates airflow resistance, and extra length means more heat loss. We always recommend smooth, rigid ducting for this, no flexible foil duct if you can avoid it—flex duct traps dust and creates more friction, which can throw off the airflow rate you need.
Location of the vent is another key point. The exhaust vent should be placed high on the drying unit, at the opposite end of the fresh air intake. That way, when the conditioned air enters the chamber, it flows through the load, picks up moisture, and exits through the exhaust vent without short-circuiting—meaning it doesn’t just flow straight through from intake to exhaust without interacting with the material you’re drying. If you put the intake and exhaust next to each other, you’ll get that short-circuiting, and the ventilation won’t do anything to improve drying efficiency. For outdoor exhaust, you want to point the vent down slightly, or add a bird screen and rain cap, so rain and debris don’t get into the unit.
Now, there are exceptions to this guideline, and that’s where working with someone who knows heat pump dryers (like our team) makes all the difference. For example, if you’re drying very high-moisture materials—think fresh fruit, wet laundry, or newly mixed pharmaceuticals—you might need to bump the ventilation rate up to 2 to 3 ACH. Why? Because those loads release more moisture faster, and even the closed loop can only process so much water before you need to purge some of the humid air out. If you don’t increase the ventilation in that case, the system’s humidity sensor will think the load is still wet and run longer, wasting energy.
On the flip side, if you’re drying low-moisture materials—like dried herbs, finished grains, or cured wood—you can drop the ventilation rate down to 0.25 ACH. These loads don’t release much moisture, so you don’t need to purge as much air. Too much ventilation here would just pull out the heat the unit has already generated, making the compressor work harder than it needs to.
Another exception is if you’re installing a heat pump dryer in a space with poor air circulation, like a small workshop or a basement. If the room around the dryer is already humid, you might need to add a small dedicated exhaust fan to vent the air directly outside, instead of relying on the room’s general airflow. A lot of people make the mistake of just opening a window to vent the dryer, but if the window is small or the room is humid, that doesn’t move enough air. We recommend a inline duct fan for installations like this, sized to match the recommended ventilation rate. You don’t need a powerful industrial fan—something that moves 5 to 20 cubic meters per hour will work, depending on your unit size.
Wait, let’s talk about common mistakes I see all the time, because even with the right guidelines, installation errors happen. The biggest one I see is people using the same ventilation rate as a vented dryer. They install a 200mm duct and a powerful exhaust fan, and wonder why their energy bill is 30% higher than they expected. That’s because they’re losing all the recycled heat. Another mistake is blocking the vent with dust or lint. If you dry textiles or sawdust, lint and dust can build up in the vent duct over time, reducing airflow and making the ventilation rate too low. We recommend checking the vent and cleaning it every three months, or once a month if you’re running the dryer 24/7.
Another mistake is not accounting for altitude or temperature. If you’re installing the heat pump dryer at a high altitude—above 1,000 meters—or in a very cold climate, the ventilation rate might need to be adjusted slightly. At high altitude, air is less dense, so you need a bit more airflow to remove non-condensables. In very cold climates, the outdoor air is much colder, so if you exhaust too much conditioned air, the heat pump has to work harder to warm the incoming air. Our technical team can help you adjust the rate for your specific location, just ask when you reach out.
Let’s also touch on indoor vs outdoor installation, because that affects ventilation too. If you’re installing the dryer outdoors, you don’t need much ventilation at all—just the small duct to balance pressure, because the humid air can disperse into the outdoor atmosphere. If you’re installing it indoors, like in a workshop or production facility, you have to make sure the exhaust air is vented outside, not just into the room. If you vent it into the room, you’ll raise the humidity level of the entire space, which can make drying other materials harder and even cause mold or corrosion.
I’ve seen a customer who learned this the hard way. A small woodworking shop installed a heat pump dryer indoors, vented the exhaust into the shop, and within a month, their tools started rusting and the new lumber they were storing next to the dryer absorbed extra moisture, ruining a batch of custom furniture. It was an easy fix—we added a short duct to vent the exhaust outside, and their problems went away. That’s why I always emphasize: indoor installation means venting the exhaust outside, period. No exceptions.
So what does all this mean for you, if you’re in the market for a heat pump dryer? The short answer is: follow the 0.5 to 2 ACH guideline for most loads, adjust up for high-moisture materials and down for low-moisture ones, use a properly sized short duct, and vent the exhaust outside whether the dryer is installed indoors or out. But the long answer is that there’s no one-size-fits-all, and that’s why we’re here.
If you’re planning to install a heat pump dryer, the best thing you can do is reach out to our team. We can help you calculate the exact ventilation rate you need, based on the size of your drying chamber, the type of material you’ll be processing, and where you’ll be installing the unit. We also provide pre-sized ducting and fans that are designed to work with our heat pump dryers, so you don’t have to guess at what will work.
I’ve been in this business long enough to know that the biggest reason customers switch to heat pump dryers is the energy savings—up to 60% compared to traditional vented dryers. But those savings only happen if the unit is installed and operated correctly. Bad ventilation erases those savings, leads to longer dry times, and even shortens the lifespan of the unit. That’s why we don’t just sell heat pump dryers—we partner with our customers to make sure every step, from installation to daily operation, is done right.
If you’re ready to upgrade to a heat pump dryer, or have questions about ventilation for your specific setup, we’d love to hear from you. Our team has worked with bakeries, pharmaceutical plants, woodworking shops, and countless other businesses to get their ventilation just right, and we’re ready to help you too. Don’t take chances with your drying process—get the ventilation right, and you’ll reap the energy savings and reliability of a heat pump dryer for years to come.
Potato Chips Production Line References
ASHRAE. (2020). HVAC Applications Handbook. American Society of Heating, Refrigerating and Air-Conditioning Engineers.
International Electrotechnical Commission. (2018). Standard for Heat Pump Dryers (IEC 62301).
U.S. Department of Energy. (2022). Energy Efficiency Guide for Commercial Drying Equipment.
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