Do you need to upgrade your ducts to get a heat pump? (2024)

The typical state of ductwork in American homes has what we can politely describe as “room for improvement.” For most people, air duct maintenance and replacement isn’t top of mind when it comes to home improvement projects, and it often gets overlooked in favor of other upgrades. But if you’re considering installing a heat pump, the condition of your ductwork plays an important role because a heat pump is only as good as the ductwork that it’s connected to. 

For homes built with central, forced-air heating and cooling, a ducted heat pump can be a simple, reasonably priced way to modernize your HVAC system. Once you hook up the heat pump to your ductwork, your home has super-efficient climate control, running on electricity. Your heat pump will help you reduce or eliminate your fossil fuel consumption, and can shrink your utility bills — or even eliminate them completely, if you pair it with a well-designed solar system.

While many homes have duct systems that will work fine with a modern heat pump, some are in better shape for a heat pump installation than others. If you already have central AC, for example, there’s a better chance your ducts are ready, whereas older homes in cold climates are more likely to need duct upgrades, extra insulation, or some kind of hybrid or backup heating system. Modern heat pumps are also more sensitive to issues such as leaky ductwork than traditional HVAC equipment – if your HVAC already doesn’t work well in certain rooms, a ducted heat pump won’t magically make it work better.

There are telltale signs to look out for when thinking about whether or not it makes sense to upgrade your ductwork. If you’re experiencing any of the following issues, it’s likely that upgrading is a smart move.

• Some of your vents barely blow any air when the heat or AC is running

• Your ductwork is already not in good shape: Leaky, uninsulated (especially in attics or basements), saggy or disconnected at points.

• You already have comfort problems with your existing HVAC, and it isn’t solved by improving insulation / air sealing

• If you’re still using 1-inch filters; thicker 4-inch filters work much better and aren’t very expensive to install

• You currently only have a heating system. Ductwork designed for old fossil-fuel heating is pretty small, and may not be able to handle the amount of air that a heat pump will need to move.

• You have no return vents in your bedrooms. This is probably a comfort issue already even if it’s something you’ve gotten used to, but it will be more noticeable with a modern heat pump.

If you’re working with a contractor who’s suggesting you need new ducts, but there aren’t obvious signs it’s time to replace them, there are a few factors to consider. Overall, aside from the cost, there’s generally no downside to making ductwork bigger and better. Replacing your ductwork can cost anywhere from a few hundred dollars to upwards of $10,000.

Even if you’re pretty comfortable with your current system, modifying your ducts could improve things further. The upgrades can increase energy efficiency and equipment longevity, and it gives your contractor the opportunity to fix issues like leaks, add dampers for zoning, improve airflow, or add a better filter cabinet if you want (it’s worth it).

• To move more heat, you need to move more air, and bigger ducts are the best way to do that. (Faster air speed can work, but it’s much louder and less energy efficient.)

• If you’re going by the book, you design the ductwork to handle the heat pump’s maximum output. A modern heat pump doesn’t run at its maximum output very often, it runs at a fraction of that power, so the extra duct capacity isn’t necessary very often either.

• It’s also possible, even likely, that you will never need as much heating capacity as the calculations suggest. Manual J is almost always a significant overestimate of how hard your heat pump will need to work, so the Manual D duct sizing recommendations are, too. 

• If your home already has an AC, it’s probably much bigger than you actually need, and the existing ductwork could be sized pretty generously, too. A heat pump that’s the right size for your actual heating needs could fit on those oversized AC ducts just fine.

One real downside to keep in mind when it comes to undersized ductwork is that your HVAC might run loud on the coldest or hottest days of the year. Plus, it could put some extra strain on the equipment.

If you choose to install a hybrid or backup system, your heat pump still does most of the work, but you’ll keep some other type of heating system to kick on when the temperature drops below a certain threshold. This could be a gas furnace or an electric heat strip kit, for example. Among other potential benefits, this could help you avoid a duct upgrade: Traditional heaters run hotter than heat pumps, so they don’t need to move as much air to deliver the same amount of heat.

But for what it’s worth, we’re seeing fewer and fewer quotes on the EnergySage Marketplace with backup systems built in. Cold-climate heat pumps have been around long enough now that contractors are getting a good sense of what works, and (among other disincentives for backups) they’re learning that you can get away without a backup (unless you want it from an alternative fuel source for peace of mind).

If your ductwork isn’t in the best shape, you can use your heat pump installation as the time to repair or replace it as needed. Leaky ducts, for example, can contribute to poor air quality or even mold growth. The heat pump itself is also easily affected by leaks. That’s because they use variable-speed fans that often rely on pressure sensors to calibrate their power use, so they run a lot less efficiently than they could, making them more sensitive to leaky ducts than traditional HVAC. If you leave leaks unrepaired, you won’t get the energy savings you expect and the relative waste is higher vs. traditional HVAC.

Another common issue you might encounter is that your existing ductwork was simply not assembled correctly — it could be disconnected at the joints, which means it’s likely there are some leaks at transfer points, where pieces of duct are fastened together. If you’re up for a DIY job, you can seal joints with mastic or some other sealant. Duct tape is not a recommended solution, as it doesn’t have the right characteristics, and will dry out and fall apart over time. If you can’t get at all the transfer points — behind drywall or insulation or a concrete slab, for example — some experts recommend AeroSeal. Developed with Department of Energy research, it blows a gummy substance through your ducts that will fill up all the little gaps and cracks.

Sloppy duct design

The way your air ducts were designed are a factor to take into consideration for heat pump installation as well. If you have uninsulated ductwork, especially in an attic or basement, it’s never great. But the main issue here is unnecessary air resistance—or more technically, high static pressure. 

• The fan has to push harder to get the air to flow. It uses extra energy, and also wears out the fan’s motor faster. 

• If it still can’t get the air to flow correctly, you’ll have problems with the heat exchanger, which can also lead to problems with the compressor.

• Undersized ductwork contributes to this problem, but that’s sort of a separate issue, and one that we’ll cover below.

Your ducts are too small

Small ducts aren’t necessarily sloppy; a layout that doesn’t work for a modern heat pump might have worked just fine for an older HVAC system. What we’re talking about here is bad practice that isn’t OK with any HVAC system — and will likely be worse with a modern heat pump.

• Saggy flex duct is a common offender. Flex duct is fine when it’s fully stretched out and cut to length. Tighten it up! 

• Overly restrictive air filters can also be a problem. Dirty filters are a common cause. But some filters are just too tight from the jump

• 90-degree turns instead of a curve / elbow can also increase resistance quite a bit

Heat pumps don’t run as hot as other types of heaters (gas, oil, wood, etc.), so they need to blow more air to deliver the same amount of heat. The best way to do that is with large ducts. 

• High air speeds are louder and less efficient—not the ideal approach for residential HVAC, and popular household heat pumps just aren’t built for this approach anyway.

In certain homes—typically older buildings in cold climates, especially if they don’t have a central AC system—the small ductwork could mean that a heat pump could struggle to warm your home, at least in theory. In practice, there’s disagreement among experts about how much this actually matters.

One school of thought is that, according to the best practices laid out by the HVAC industry, it isn’t acceptable to stick a heat pump onto undersized ductwork because it can’t deliver proper comfort in extreme conditions, and will cause premature failures—both of which will leave customers dissatisfied. You can use a heat pump for some of the heating (and typically all of the cooling), but you’ll need heat strips or a furnace for proper results in cold weather. That is, unless you increase your ductwork’s capacity to match the demands of the HVAC system—or improve your home’s weatherization, so that you can install a smaller heat pump that fits the existing ductwork.

The other school of thought is that heat pumps that have already been installed on old ducts have happy customers, so don’t overthink it. On the coldest days of the year it might run a little louder and less efficiently than it could, and it’s probably not ideal for the health of the equipment. But super cold days aren’t very common, and the industry’s “best practice” for calculating heating needs is almost always a huge overestimate, so a heat pump almost never needs to run anywhere close to its maximum power. The old ducts are perfectly suitable at least 95% of the time, and fine the other 5%. So why bother with a pricey modification, or the complication of a hybrid heating system? Maybe we’ll add an extra return duct to a bedroom if it’s easy, but unless it’s a complete trainwreck, we don’t bother with big duct renovations. It’s just not necessary.

The first approach probably yields better comfort on the least comfortable days of the year, and might help the heat pump last a little longer. HVAC contractors that serve demanding (high-paying) clients, and put an emphasis on long-term service and maintenance, are more likely to follow this school of thought.

The second approach costs less up front, and generally works fine given all the cushion built into a “proper” system design. Contractors with a heavy pro-electrification identity tend to fall into this camp.