Questions You Should Know about Inverter Hot Water Heater， Hot Water Heat Pump

This is Part II of an expert best practices web guide to Heat Pump Water Heaters published by the ENERGY STAR Residential New Construction program. Read Part I of the guide.

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Design for Efficient Operation

To ensure efficient operation, a Heat Pump Water Heater should be installed in a sufficiently large room, or be properly vented. Manufacturers typically require access to a minimum of 450 or 700 cubic feet of free air space where the water heater is installed, along with ample space to allow installation and service. An 8-ft by 12-ft room with an 8-ft ceiling, for example, provides sufficient volume. The installation location relative to other parts of the home (hot water uses and living areas) will affect efficiency and may impact residents’ comfort.

Be sure to consult the manufacturer’s installation guide for recommendations specific to your model. 

Sound

Heat Pump Water Heaters have a fan and compressor, both of which can make a modest amount of noise when the Heat Pump Water Heater is heating water. Heat Pump Water Heaters that meet ENERGY STAR Version 5.0 product specifications emit sound levels less than 55 dBA—about the level of a background conversation. (NOTE: 55 dBA is also the maximum sound rating level allowed by the ENERGY STAR NextGen program for HPWHs installed in occupiable space.)

Sound levels vary among products. Some Heat Pump Water Heaters have sound pressure ratings of 45 dBA—about the level of a quiet dishwasher—and even softer-sounding products are in development.

Avoid locating a Heat Pump Water Heater next to rooms where sound levels are more important to comfort, such as bedrooms and living areas. To further reduce the impact of noise, consider additional sound insulation for the installation room.

Cool Exhaust Air

Heat Pump Water Heaters exhaust air that has been cooled and dried. Avoid locating an Heat Pump Water Heater near areas where residents will be sensitive to cooler air temperatures. Infrequently occupied spaces where temperature variations are less likely to be bothersome—like hallways, garages, and utility rooms—are good choices. Locations with waste heat available also serve as excellent locations. In some areas, the cooler and drier air can be a benefit, for example, in a laundry room or near a home gym.

Typical Installation Locations

The local climate will often determine best locations to install a Heat Pump Water Heater. Be sure to consult the manufacturer’s installation guide for recommendations specific to your model and follow applicable local codes.

• Basements, including unconditioned ones, are often the best locations for Heat Pump Water Heaters in any climate.
• Garages are also a great option because they can provide ample volume. In warmer climates, where outdoor temperatures are usually above 50° F, uninsulated garages are acceptable; however, if the building is in a cooler climate, consider possible freezing conditions and follow standard location practices in your area.
• Interior rooms (e.g., utility, laundry, and IT rooms) are other common choices that work in any climate. A HPWH can benefit from waste heat produced by other equipment in the space.  
• Rooms outside the thermal envelope—such as attached sheds and utility rooms—also work in warm climates. In hot climates, they can increase energy efficiency compared to interior spaces.

Venting in Closets

In some homes or dwelling units, it is necessary to plan for the Heat Pump Water Heater to be placed in a confined space, such as a small mechanical closet. If the Heat Pump Water Heater must be installed in closets adjacent to rooms that are frequently occupied, vent the cooler exhaust air through a duct or transfer grille into an area where temperature is not an issue.

Refer to manufacturer requirements for minimum requirements for the amount of air needed, and choose one of the following options to achieve proper airflow.

Passive venting options:

Best practice is to provide a total minimum net-free area of 240 square inches or greater, with both high and low openings to allow air to circulate. This can be done through a fully louvered door, using both high and low transfer grilles, or a high transfer grille and a ¾”door undercut.

Active venting options (ducted):

• Duct Heat Pump Water Heater  intake air directly into the HPWH. To allow cool exhaust air to leave the space, install a large louver or transfer grille that provides at least 130 square inches of net free area, placed in a location near the Heat Pump Water Heater exhaust.
• Duct Heat Pump Water Heater exhaust out of the space. To allow warm air to enter the space, install a large louver or transfer grille that provides at least 130 square inches of net free area or at least a ¾” door undercut to allow air to enter the space. 
• Duct both intake air and exhaust air with balanced airflow.

For active venting, ducts must be short, unrestricted, and as straight as possible. Design ducting to vent exhaust air into a location where a cool air stream will have minimal impact on occupant comfort. Only duct to the outside if located in a warm climate and ducting both the inlet and outlet. Refer to manufacturer guidance for duct sizing and maximum distance requirements.

DO NOT:

• Do not duct only the Heat Pump Water Heater intake air or exhaust air to the outside. Doing so will create a pressure imbalance that will lead to air infiltration or exfiltration, increasing the load on the space heating and cooling systems. 
• Do not run any ducts between the garage and the Heat Pump Water Heater. This may bring exhaust fumes or other contaminants into the living space. 
• Do not vent the water heater exhaust air near a thermostat. The cooler exhaust air will provide a false reading to heating and cooling systems. 
• Do not duct both the Heat Pump Water Heater intake and exhaust air to the outside in cold-climate regions or locate the Heat Pump Water Heater outside in cold-climate regions. Intake air temperatures below approximately 40°F will trigger electric resistance elements and significantly reduce Heat Pump Water Heater efficiency.

Tank Sizing

Follow the local plumbing code’s minimum first-hour rating (FHR) requirements and manufacturer recommendations for tank sizing. For maximum efficiency, upsize the tank over the standard practice used for electric resistance or fossil fuel-fired water heaters. Heat Pump Water Heaters typically come with auxiliary resistance elements that can run to meet periods of high water demand when the unit is set to Hybrid Mode. Resistance operation is less efficient than running the heat pump alone. Upsizing the tank will minimize inefficient resistance heating, allowing the heat pump to do the majority of the water-heating work. The ENERGY STAR NextGen program requires minimum rated tank volumes based on the number of bedrooms, as shown in the table below.

• Tank Size Tip #1: Upsizing tank size increases the potential for thermal energy storage, which will allow a user to take maximum advantage of utility load-management programs or time-of-use electric rates.

• Take Size Tip #2: Upsize the tank if the occupants are likely to have high hot water draw periods, such as a household with teenage children or occupants with a preference for baths.

Tank Size Guidelines (Gallon Capacity)Number of Bathrooms

1 to 1.5

2 to 2.5

3 to 3.5

Number of Bedrooms

1

2

3

2

3

4

5

3

4

5

6

FHR per Uniform Plumbing Code

38

49

49

49

62

62

74

62

74

74

74

ENERGY STAR NextGen Minimum Tank Size

36

45

59

45

59

72

72

59

72

72

72

Design Considerations for Multifamily Buildings

There is no one-size-fits all application option for heat pump water heaters in multifamily buildings. There are several design features that must be considered when planning to apply Heat Pump Water Heaters in multifamily buildings. The architect and mechanical designer must engage early in the design process to account for these needs. 

The best system for a specific building will depend on a variety of factors including climate, number of units, type and size of units, number of floors, presence of a parking garage, roof access, balcony presence/absence, and interior or exterior corridors. Heat Pump Water Heaters are most effectively integrated into multifamily buildings if considered from day one of the design process.

There are two primary domestic hot water system types for multifamily buildings: in-unit serving one dwelling unit per water heater or a central system serving all or part of the building. In-unit Heat Pump Water Heaters are typically located in a closet within the apartment or just outside it. A variation on this approach is using a single 80-gallon Heat Pump Water Heater to service two to four dwelling units.

Central systems heat and store hot water in a central location, such as a mechanical room, and use a recirculation loop to distribute the hot water. Unlike in-unit Heat Pump Water Heaters, with central systems there is no need to figure out how to keep the cool exhaust air from impacting occupant comfort. A downside to central systems is that they must pump water long distances, resulting in energy losses of 30 to 50%.

Venting Heat Pump Water Heaters into Corridors

Due to the smaller size of most apartments, for in-unit Heat Pump Water Heaters, exhausting cool air into the living space can potentially cause comfort problems, especially in heating dominated climates. One solution to prevent comfort issues is to locate the Heat Pump Water Heater in a (properly ventilated) closet off the corridor where heat can be harvested and cool air rejected. If the Heat Pump Water Heater is accessed from the corridor, code will typically require fire-rated assemblies between the Heat Pump Water Heater closet and the dwelling unit. These fire-rated assemblies will have the benefit of providing of additional sound proofing between the unit and the water heater.

When a heat pump water heater is actively operating (in heat pump mode), it generally is providing 2,500 to 5,000 Btus/hr of cooling. This will nominally increase the amount of heating needed for the space, and decrease the required cooling. Designers should account for this additional cool air when sizing heating and cooling systems when multiple Heat Pump Water Heaters reject cold air to the same space (e.g., a corridor). 

Anyone who owns a home that uses hot water should at least consider getting a heat pump water heater.

Even if your current water-heating system is working fine, you should still start thinking about it. After all, regular access to heated water is a luxury we often take for granted, so it’s all the more frustrating when your system fails on you.

It might take 10, 15, even 20 years, but eventually, your hot water heater is bound to break. So you need to have a plan in place before that happens. Otherwise, you’ll be left without hot water for who knows how long. And no one wants that.

Heat pump water heaters are the future. In fact, in July , the US Department of Energy proposed a new set of standards that would essentially phase out most non–heat pump models by . But the question of whether it’s the best choice for you right now is a bit more complicated.

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If you want the most energy-efficient option available, get a heat pump water heater. Unlike electric and gas heaters, which must create heat to warm the water, heat pump water heaters simply take the heat particles that already exist in the air and move them into the water. As a result, heat pump water heaters usually have an efficiency rating between 3.0 and 4.0 UEF (Uniform Energy Factor, the standard US measurement); that’s about three to five times higher than you get with even the best gas and standard electric water heaters out there.

You don’t have to worry about losing energy in the conversion process, either—every watt of energy that goes into a heat pump model results in three to four times the equivalent heat energy.

“A gas system is a great energy producer. But a heat pump water heater is a great energy transfer,” as Leo Pesegoginski, a project manager at Boston Standard Plumbing, succinctly explained it to us.

A heat pump water heater could be a great choice if you want to reduce your utility bills. The amount of savings will depend on your current setup, as well as where you live. If you already have an electric water heater, for example, a heat pump will use only about a quarter of the energy. If you’re switching from a gas-powered system, however, the savings might not be as impressive—though most people are still likely to save some money by making the switch.

It’s also great if you’re looking to electrify your home. In fact, there are some models—like the A. O. Smith model we recommend—that can plug directly into a standard wall outlet without any additional electrical upgrades, making it easier than ever to move away from fossil-fuel dependence.

Most heat pump water heaters also come with smart-home-integration options, including Demand Response (which can help you sync up to the local electrical grid), and they further optimize energy use not just in your home but across your entire community. This sort of electrification is a major step toward broader decarbonization and sustainability goals.

A heat pump is also the best way to take advantage of financial energy incentives from the Inflation Reduction Act. While heat pump water heaters tend to cost more upfront than other home water-heating systems, installing one can earn you a tax credit of up to $2,000, which can help to offset that price difference. And some states offer additional incentives on top of that. By contrast, even the most efficient natural gas water heaters will still only qualify for a $600 credit at best.

A heat pump water heater is a particularly good choice if you live in a humid area or you have a dank basement. Heat pumps tend to pull in moisture from the air around them (not unlike an air conditioner). This is a normal part of the process—but it can also be a nice bonus feature because it makes the water heater work like a dehumidifier at the same time.

That excess water still has to go somewhere, though. If you already have a sump pump or other wet-basement solution in place, then you should be fine; if not, you may need to add extra piping to make sure that water drains out with the rest of the plumbing.

But it isn’t a great choice if you expect heating on demand. “You can’t have hot water on demand because heat pumps only slowly accumulate the heat,” explained Joel Rosenberg, senior program manager for special projects at Rewiring America. As a result, heat pump water heaters tend to have a longer recovery time—that is, the time it takes to reheat a tank of water after you’ve used it all up.

Tank-based water-heating systems also inevitably deal with some level of standby heat loss, but heat pump units are so efficient that this should be minimal.

“Once the heat pump water heater gets to the right temperature, it maintains it with very little energy input,” Rosenberg said. (If you don’t want to deal with water storage or standby heat loss, you might want to look into a tankless system.)

A heat pump water heater is also not great if you value silence. Unlike other water heaters, heat pump water heaters use fans to help draw in the air. And fans tend to make a little noise (though it’s still quieter than what’s considered normal conversation level). If your water heater is tucked away in the basement or a utility closet, you might not even notice it; if it’s closer to your living space, however, it could be a source of frustration for some people.

And it might not be the best choice if you have limited space. Most heat pump water heaters need a minimum of at least 700 cubic feet of space around them to have enough air from which to extract that ambient heat. Though some models, like the 120-volt A.O. Smith, need only about 450 cubic feet. (The US Department of Energy recommends 1,000 cubic feet for optimal performance.)

If you’re planning to install your water heater in an open part of the basement, that’s great! If you’re trying to replace a tankless unit or squeeze your water heater into a tight utility closet, you might need to add some sort of venting or duct system, which could add to the cost.

Another thing to note is that you may need to increase the capacity when replacing an electric or gas model with a heat pump. Most contractors and manufacturers recommend upsizing the tank by roughly 30%, to compensate for the extra-long recovery time. A home that ran on a 50-gallon natural gas water heater, for example, should upsize to a 65-gallon heat pump water heater. This helps to ensure that there’s always some extra hot water, in case you’re about to run out—but it also means you need a little more clearance space.

Also, be warned: A heat pump water heater can have a higher upfront cost. Most heat pump models cost at least twice as much as an equivalent-size electric or gas water heater; any additional installation complications can add to the price as well.

There are plenty of financial incentives available to help reduce the costs, and the increased efficiency will likely help to make up the difference over time. But if you’re tight on money at the moment—and the rebates and tax breaks don’t work for you—then a standard gas or electric replacement might be the better choice right now.

Top pick

The most efficient water heater we’ve seen has a hybrid backup system and plenty of smart-home features. But it may have a slower recovery time than you’re used to.

Rheem has been manufacturing hot water heat pumps since the mid-s. The company’s fifth-generation model, the Rheem ProTerra XE65T10HS45U0, offers the most well-rounded combination of high-efficiency performance along with other convenient features, including smart-home connectivity and a hybrid electric backup system, which kicks on when the tank is running low.

Like most heat pump water heaters, this model also comes with a 10-year warranty.

The upfront cost is an investment, but it should pay off in the long run.

The Rheem ProTerra XE65T10HS45UO is extremely energy-efficient. Depending on what size tank you get, the ProTerra models clock in between 3.88 and 4.07 UEF; that makes them at least four times better than any other non–heat pump water heater out there. Nearly every heat pump model we’ve seen has been at least a 3.0, but very few make it all the way to 4.0 or beyond.

If you run out of hot water, this model has a hybrid backup heating system. If you have a hot-water emergency—like when kids and guests take way-too-long showers—the Rheem model also has a regular (non–heat pump) electric resistance coil that kicks on to refill the water tank faster.

“It’s basically a standard electric water heater on the bottom—just like millions of people already have—with a heat pump on the top,” explained Scott Cohen, director of marketing at Rheem.

Although the heat pump component can recover about 27 gallons of hot water per hour on its own, the electric backup can handle up to 75 gallons.

“It sacrifices some of the efficiency for the comfort of the homeowner,” Cohen said. “But that allows people to adopt this new technology without having to change their behavior.”

And with the unit sized correctly for the home’s typical demands, the backup system would rarely operate.

The Rheem model comes with a variety of handy smart-home features. That includes Demand Response and leak protection.

Whereas most heat pump hot waters have basic smart-home connectivity (app/voice control, scheduling, energy monitoring, and so forth), the Rheem also comes with built-in leak protection, including an automatic water-shutoff system in the case of an emergency. Water damage is one of the most common (and expensive) forms of home damage, and many insurance companies offer discounts to homeowners who install some kind of smart leak detector—which can save you even more money in the long run.

Like most hot water heat pumps, this one comes with a 10-year warranty. The energy savings you get from this model over the course of those 10 years can help make up for the higher upfront cost, too.

But it may require a little more-routine maintenance. Heat pump water heaters like the Rheem use fans to pull the air in, which means they also have a built-in air filter that will occasionally need to be cleaned. This isn’t much different from the filter you’d find on an air conditioner or dehumidifier (or other large appliances with fans), but it still marks a noticeable change from the set-it-and-leave-it water heaters that most people are used to.

You may also need to have some additional electrical work done. If you already have an electric water heater, then don’t worry about it; you can fit your new heat pump model right into the existing infrastructure.

But if you’re looking to upgrade from a natural gas system, you’re going to need to run the wires for a 240-volt connection—which could also mean upgrading your electrical panel, if you don’t already have a configuration that can support the additional breaker.

Also great

The Signature 900 offers all of the benefits of a heat pump water heater without the need for an electrical upgrade. It’s far more efficient than a traditional electric heater, but other heat pump models can outperform it.

The A. O. Smith Signature 900 HPV10-66H01DV offers a lot of the same perks as other heat pump water heaters, including a high efficiency rating, smart-home features, and a 10-year warranty.

But it stands out because it’s one of the few models that require only a 15-amp (120-volt) circuit. So if you have a standard outlet available where the heater’s located, you could install it without bringing in an electrician or having to upgrade your electric panel.

Even at that lower voltage, it still has room for a backup hybrid heating component, too.

If you’re going through a contractor, the A. O. Smith Voltex HPTV-66 is the wholesale version that’s otherwise essentially identical.

The A. O. Smith Signature 900 can be plugged into any standard electrical outlet. Yes, really; you can run an entire 80-gallon water heater system with a regular ol’ wall outlet, without having any additional electrical wiring (though you could still hire a plumber to do the water-supply fittings and related work).

“It’s really aimed at the emergency replacement of a gas water heater,” explained Arthur Smith, a product manager (and member of the namesake family) at A. O. Smith. “It simplifies the installation process but still provides the same efficiency benefit without the upfront cost.”

It doesn’t need as much space as other heat pump water heaters. While most hot water heat pumps require around 700 to 1,000 cubic feet of space to have enough hot air to pull from, the A. O. Smith model can work in spaces as small as 450 cubic feet. (It still works more efficiently in a larger space.)

The water in the tank gets even hotter, too, which means you use less of it. Heat pump water heaters can be slow to recover when they run out of water. But the Signature 900 has a clever solution: It overheats the water by default and then mixes it with cold water on the way out of the tank, so it meets your target temperature before it hits the pipes.

This ultimately uses less hot water to achieve the same results. “That means we can store more thermal energy and ultimately deliver more hot water,” Smith said.

It’s one of the quietest heat pump water heaters we’ve seen. With a decibel rating of just 45 dB-A, the Signature 900 falls well below the standard threshold for “normal conversation level.” For most people, a fan at that volume will just disappear into the white noise of the background (though, again, it will depend on your specific situation).

It has a backup heating system, but it’s not the most powerful. This is a trade-off for using a heat pump water heater with a standard wall plug—it just can’t pull the same amount of energy as a standard 240-volt model. Still, the Signature 900 can refill its tank in about an hour using just 900 watts, while our other picks all have capacity for up to 4,500 watts.

It’s not quite as efficient as a 240-volt system, either. The 120-volt Signature 900 has an efficiency rating between 3.0 and 3.46 UEF, depending on the size of the tank. While that’s not as high as it could be if you went for the full electrical-panel upgrade, it still beats any standard electric or natural gas alternative you can find.

It’s slightly more expensive, too. 120-volt heat pump water heaters tend to cost a little more than a 240-volt model of the same size. But, hey, at least you don’t have to pay for an electrician!

Also great

This is the quietest heat pump water heater we’ve seen, and the LG’s smart-home features can integrate with the company’s other appliances. But this is LG’s first heat pump water heater. So compared with our other picks, with this one you may find there’s less availability or fewer qualified installers.

The LG APHWC501M Inverter Heat Pump Water Heater is one of the best and quietest choices out there.

It also works with the LG app, so if you already have other LG appliances in your room, you can integrate them all together into a more-robust smart experience.

Plus, it looks cool—which is not a thing you often hear about a water heater.

The LG model is the absolute quietest and one of the most efficient. This model uses LG’s dual inverter compressor technology, which we’ve tested on other products, including a long-standing portable air conditioner pick. That compressor works in conjunction with the heat pump itself to result in an impressive efficiency rating of 3.9 UEF or higher (depending on the size of the tank).

The variable speed also means it has a gentler sound, even on the fan, with the LG typically measuring around 42 decibels—quieter than any heat pump water heater out there. That’s quieter than even the quietest dishwasher available right now.

It has more-robust smart-home integration. If you have other LG appliances in your home, you can run them off of the same app as your heat pump water heater, so you can consolidate your scheduling, energy monitoring, and more. You can even use Demand Response to sync your entire home to the grid.

The LG is the nicest-looking water heater we’ve ever seen. While researching this guide, we visited a large regional plumbing wholesale warehouse, where the clerk joked that the LG looked more like a “fancy refrigerator” than a water heater. Aesthetics might not be your top priority, but this is a nice perk.

But LG is new in the water heater space. While LG has plenty of experience with heat pump technology and heating/cooling products in general, the APHWC501M is the company’s first foray into water heating (though LG does sell an air-to-water heat pump system as well).

By contrast, A. O. Smith has been making heat pump water heaters since and other water heaters for nearly a century. Rheem is similarly on its fifth-generation model, with another 70 years of general water heater experience on top of that.

Although LG makes plenty of other great products, the company doesn’t have quite the same pedigree with water heaters.

You’ll also have to go through a contractor to get it. Even if you don’t need to hire a licensed contractor for a full electrical upgrade, the LG is currently available through wholesale channels. You could still ask your plumber for this specific model, but you won’t be able to buy one at a store and then just hire someone to switch out the connections.

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