Heat Pump Water Heaters: Everything you need to know

A Heat Pump Water Heater (HPWH), also known as a Domestic Hot Water heat pump (DHW), provides sanitary hot water using heat pump technology. The heat pump water heater captures heat from the ambient air to heat the water, resulting in exceptional energy efficiency.

Heat pump water heaters are a very practical solution for anyone who doesn’t want to invest in a full heat pump system but wants to benefit from the most efficient water heating solution available.

How do Heat Pump Water Heaters Work?

Air-source heat pump water heaters work just like heat pumps and air conditioners. They pull heat from the surrounding air to heat up the water. The heat is extracted from the air adjacent to the HPWH unit, and through the same process used by heat pumps, it is transferred to the water in the tank.

The process uses significantly less energy to produce heat, which is why HPWHs are the ultimate alternative to a heat pump for the purpose of heating water.

The HPWHs components are all housed in a single unit, just like monobloc air source heat pumps. 

The intake fan pulls air from the surrounding air into the evaporator. The refrigerant that flows through the evaporator absorbs heat from this air and, in vapor form, reaches the compressor. There, it is compressed, raising its pressure and temperature. The refrigerant then passes through the condenser, where it releases the heat into the water tank, which acts as a heat sink. This is how the room’s ambient heat is used to produce hot water.

As the refrigerant cools, it starts to condense into liquid form. It then reaches the expansion valve, which helps it drop its pressure and temperature. In this cooled, liquid state, it is ready to absorb heat again through the evaporator.

The cycle is repeated, and the water keeps heating up while the HPWH absorbs the heat from the air in the room. As a byproduct of this process, the room’s temperature can drop slightly since the heat is absorbed by the HPWH. That is why placing an HPWH in a room that benefits from cooling, such as a wine cellar or a server room, is an even more energy-efficient design.

This process, known as the vapor compression cycle, is what makes heat pump water heaters draw less energy than conventional water heaters to produce the same amount of heat.

Energy efficiency benefits and cost savings of heat pump water heaters

So, how energy efficient are heat pump water heaters?

Your savings will depend on many factors such as your local cost per kWh, how much hot water usage you require, and what other systems are currently in place. Let’s take a look at an example.

Annual Savings Example

Let’s consider a conservative example residence with 5 inhabitants with an assumed local kWh average cost of €0.283 (the EU average in February 2023*). 

With an average power consumption using a standard electric hot water boiler, the annual consumption of 4,500kWhs would cost €1,273.5. With a TermoPlus domestic hot water pump, consumption would drop to 1100 kWhs annually costing €311.3. That would be an annual saving of €962.2 which over the 25-year life of the product would amount to €23.055 in total savings, and that isn’t even taking into account the rising cost of electricity. 

Payback Time Example

Using the data from the above example, a HPWH that costs 1,925 would reach the break-even point in exactly two years.

Return On Investment (ROI) Example

Having recouped an investment cost of €1,925 in the first two years of the estimated 25 year lifetime would result in a total return of €23,055 – €1,925 = €21,130. That would be a ROI of 1098% ((23,055/1925)*100). 

No other water heater system technology (other than a full heat pump system with DHW) can provide such a high return on investment so far. Let’s take a look at what other benefits are provided by HPWHs.

Key benefits of HPWHs

HPWHs are one of the highest-performing investments a home owner can make. This is generally the main reason people choose to buy HPWHs, except, of course, if they opt for a fully fledged heat pump system. Other than the incredible energy efficiency and savings offered by HPWHs, let’s take a look at what else they bring to the table:

  • They cost less than a full heat pump system
  • They have a very fast payback period and an exceptional return on investment over their very long lifetime (yes, this deserves another mention!)
  • They have a much longer lifespan than most conventional water heaters
  • They are very easy to install and the whole refrigeration circuit is hermetically sealed which means installers don’t need to be certified in handling refrigerants
  • They can be combined with other heat sources to offer even greater efficiency
  • They can be powered by a solar PV installation and act as a heat battery, reducing any energy waste or grid charges
  • They can provide 100% carbon-neutral hot water when powered by green energy
  • They can provide cooling for a small room such as a pantry, cellar or server cabinet
  • They are extremely reliable
  • They are easy and economical to service and maintain
  • They are safer than conventional water heaters (reduced combustion risks)
  • They can be moved to other properties without major works or sizing considerations
  • They are quiet
  • They take up less space compared to most other options (except for tankless water heaters)
  • They are odorless, unlike most combustion-based water heaters
  • They offer hot water on-demand while conserving water (compared to tankless water heaters which require flow to warm up)
  • In most cases they can be subsidized through government initiatives

Comparison of heat pump water heaters with traditional water heaters

Compared to conventional resistance water heaters or gas-powered water heaters heat pump water heaters are significantly more efficient. Most heat pump water heaters have a typical energy efficiency of 200-400%.

Let’s take a look at the other types of water heaters, and which water heater is right for each situation.

Hybrid water heaters

A hybrid water heater is essentially a heat pump water heater that also uses an electrical heating element (resistance) to heat the water further. They are essentially a combination of a conventional electric resistance water heater and a heat pump.

While the additional resistance can provide additional heating power in colder climates, as well as greater capacity when demand for sanitary hot water is very high, it does consume more electricity in the process. Ideally, the element would only be used when the heat pump needs a temporary boost.

While all our heat pump water heaters are designed as heaterless units, we do offer the optional upgrade to include an electric resistance heater. This upgrade is entirely unnecessary for most users and is mostly for users that need an electric heater to produce vast quantities of hot water for constant, intense use up to temperatures of 70°, or where a backup may be needed.

To offer some perspective, our heaterless design uses a rotary compressor (which ensures low operating costs and a long lifetime) to produce hot water up to 62 ˚C without the need for an electric heater. This also ensures sanitary hot water is safe from legionella without the need for further heating.

So, if you are looking for the most efficient heat pump water heater system possible that can provide you with plenty of hot water, you should opt for our heaterless units. If you really, really need that extra capacity and heat though, you can pick a hybrid version instead.

Solar water heaters

Solar water heaters are very efficient at warming up water but they are not consistently effective. Solar water heaters don’t use any power (unless minimal power is used to pump the water through them). Water flows through their collector and is warmed by the sun. They are excellent during hot summer days but are limited in capacity through the rest of the year in most climates. They also require rooftop space which could be used by solar panels instead.

Solar water heaters can be combined with a heat pump water heater though, so that the water enters the heat pump in a warmer state, requiring even less power to heat. They can also solve the consistency and capacity issues solar water heaters face by ensuring there is always ample hot water available.

If you are combining a solar water heater with a hybrid water heater then it is likely that the electric resistance heater will not need to operate as much, resulting in more energy-efficient heating.

Solar water heaters have a typical energy efficiency of 52-80%. Despite not using any power, this rating refers to the conversion of input energy into output energy.

Tankless Water Heaters

Tankless water heaters do not use a tank to warm up water, but do so on demand. These take up significantly less space than other systems and are an option for very small apartments or where there is no space for a tank. They are typically conventional electric water heaters or natural gas water heaters. As so, they are very inefficient at heating water and can easily increase your gas or electric bill.

Tankless water heaters have a typical energy efficiency of 80-98% if powered by natural gas, around 98%-100% for electric resistance units.

Natural Gas Water Heaters

Gas water heaters are typically tankless water heaters that heat water on-demand while burning natural gas. Natural gas is a fossil fuel that is currently very expensive to use for heating. If you are stuck with natural gas as a heating source for your home and can’t afford a full heat pump system, an electric heat pump water heater could reduce your gas bills considerably without breaking the bank.

Natural gas water heaters have a typical energy efficiency of 80-98%. During summer months, when they are used only for domestic hot water heating, they can be very inefficient. In summer their efficiency can drop below 50% because of the short operational time (the heater needs to operate every time water is used, while no space heating occurs).

Oil / Gas Fueled Boilers and Furnaces

Oil or gas-fueled boilers and furnaces are conventional heating systems that can also heat sanitary hot water through heat exchangers. They are expensive to own and operate as the fossil fuels they use are costly. They also require access to combustible fuels which is a risk that can be avoided with a heat pump water heater. Diesel-fueled systems especially are noisy, smelly and take up a lot of space. These systems need a separate water tank in addition to their heating apparatus.

Oil or gas-fueled boilers have a typical energy efficiency of 80-98%. As with natural gas heaters, in summer their efficiency can drop below 50%, because of their short operational time (the heater needs to operate every time water is used, while no space heating occurs).

The Environmental Impact of Heat Pump Water Heaters

Heat pump water heaters are exceptionally environmentally friendly for two reasons.

The first reason is that they are exceptionally efficient hot water heaters, especially compared to traditional systems.

The second reason is that they can be powered 100% by green electricity, such as a solar panel installation, or a green energy provider. In this case they do not produce any greenhouse gases. Our TermoPlus HPWH / DHW product range has a very long lifespan compared to conventional water heaters and can be fully recycled – this contributes to less waste.

Common misconceptions about heat pump water heaters

Heat pump water heaters are sometimes misunderstood because they use the same technology as heat pumps. Let’s take a look at some of the most common misconceptions and myths surrounding heat pump water heaters.

Heat pump water heaters are expensive

Heat pump water heaters are not as cheap as conventional water heaters, but they are also not nearly as expensive as fully fledged heat pump systems. Although quite a bit more expensive than conventional heaters, the cost of HPWHs is very reasonable considering the substantial savings they produce over their very long lifespan.

Heat pump water heaters don’t work well in the cold

This is an old myth that’s been busted. HPWH’s are even less prone to freezing as the units are placed indoors and use the warmth of the space they are in to heat the water. Some models of the TermoPlus Pro range, for instance, can operate efficiently even at temperatures as low as -5°C.

HPWHs are hard to install

While installing standard heat pumps can be complex and requires proper sizing to achieve optimum performance, Heat Pump Water Heaters are extremely easy to install. They can be installed by any conventional water heater installer, and don’t require major plumbing work.

HPWHs also don’t require refrigerant lines as split units do since they are essentially monobloc units. Since the refrigerant is hermetically charged, installers don’t have to be certified to handle refrigerants.

HPWHs can replace virtually any other water heater as a retrofit, and the installer doesn’t even have to have experience with heat pumps as the systems are built ready to plug and play. 

HPWHs are noisy

Older heat pumps had noisy external units. As the technology has matured, they have become much quieter. HPWHs however, are designed to be used entirely indoors and, for that reason, are quiet by design. Modern HPWHs can be quieter than most traditional water heaters. The most audible component in most units is the fan, and for that reason, TermoPlus HPWHs only use brushless fans that are almost silent.

HPWHs can’t supply enough hot water or won’t heat your water enough

HPWHs can provide very hot water. Our heaterless units heat water to 62°C, while our hybrid heat pump water heaters reach 70°C. To provide some context, water at 49°C is scalding, and many plumbing laws require the temperature to be limited to 50°C to avoid burns.

As for the sufficiency of HPWHs to provide ample capacity, our units can provide either 230L or 300L of capacity which are pretty standard sizes. A 300L unit can very easily satisfy the needs of a six person residence. If you anticipate a constant need for large quantities of hot water (gym showers, hotels etc.) then you may want to consider cascading systems, adding another tank or opting for the hybrid model instead.

HPWHs need maintenance

With the exception of electric resistance water heater tanks, most conventional systems require regular maintenance. HPWHs don’t have any special requirements but they do need an annual service maintenance, which is cleaner, faster and less costly than other conventional systems.

Real Cons of Heat Pump Water Heaters and Switching to a Heat Pump

There really aren’t many downsides to owning a HPWH unit. Let’s take a look at the real pros and cons of heat pump water heaters.

For one, if you consistently exceed the heat pump water heater’s tank capacity, you may need to consider your capacity requirements – as you would with a conventional water heater. In that case, you can consider choosing a larger tank, investing in an additional storage tank, or opting for our hybrid system. It is however highly unlikely that you will need to exceed this capacity unless you use the system for commercial purposes.

If you need a new water heater for your home or business, but you plan on switching over to a full heat pump system that includes domestic hot water functionality, investing in a heat pump water heater is redundant. Since HPWHs have a lifecycle of over 20 years, if you plan on switching to a heat pump during this period then you may as well invest in the heat pump now to cover all your needs. This isn’t a hard rule of course, as you could invest in a smaller split or monobloc heat pump system without DHW functionality later on, if you needed the HPWH to replace a conventional water heater immediately.

Government subsidies, incentives and schemes (rebates, grants, loans etc).

Heat pump water heaters are eligible for various government incentive schemes, depending on where you are located. Generally they are treated as an energy-saving investment such as heat pumps and can benefit owners through tax credits, grands, low-interest loans, utility incentives and rebates, but these all vary from country to country.

What to Look for When Shopping for a Heaterless or Hybrid Heat Pump Water Heater.

Energy Efficiency (COP)

Not all HPWHs are the same. Some are more efficient than others. TermoPlus HPWHs have a coefficient of performance (COP) rating of up to 4.55 (EN 255-3). Any COP rating above 3, (or even 2) is worth switching over to a HPWH, but the higher the rating, the greater the savings you will achieve. However, the most efficient unit is not always the right choice, since there are tradeoffs between features and efficiency.

Heaterless vs Hybrid HPWHs

A heaterless HPWH is more energy efficient and is most likely going to be enough to cover the needs of a typical residence. A hybrid HPWH consumes more energy due to the additional electrical resistance heater but can provide a larger, constant flow of hot water. If you need the efficiency of a HPWH combined with the redundancy of an electrical heater, our hybrid HPWHs have a high efficiency setting that will not use the heating element unless absolutely necessary.

Tank Water Volume

You probably need at least a 230L tank for a household of 3-4 residents, and a 300L tanks for 6 residents. You can also install two or more units in parallel if you have greater demands. This can be augmented with an additional storage tank for more residents, or you can choose a hybrid HPHW system for constant, uninterruptible hot water for more intensive applications such as gyms, restaurants and so on. 

Anti-legionella program, and 60°C heating at a minimum

Proper anti-legionella programs help maintain your water safe and clean from Legionella bacteria. The HPWH must heat to at least 60°C to be safe. These are pretty standard requirements and found in most models in the market.

Anti-corrosion protection for longevity

HPWHs can cost a pretty penny, so to get the most value out of your purchase, you need the unit to last as long as possible. Although HPWHs can last much longer than some conventional systems, their weak spot is the tank’s risk of corrosion. TermoPlus units are coated with a top-quality enamel layer and use a Magnesium anode to protect the tank from rust and prolong its lifespan. The minimum life expectancy of our HPWHs is 25 years with correct maintenance of the system and anode.

Low temperature operation option

If you are located in a colder climate and anticipate this to be a problem, you will need to select a model that can work in colder climates. Our Pro range is available with a Low Temperature upgrade that allows efficient operation down to -5°C.

PV input option

If you have solar PV panels installed, you may wish to take advantage of this nifty optional feature available for our Pro HPWH range. The photovoltaic input allows the HPWH unit to automatically store excess solar energy in the form of hot water. This reduces energy transfer losses, net metering surcharges, or even wasted energy if your off-grid storage is topped up.

Ducted cooling option

If you cannot place your HPWH in a space you want to cool (such as a cellar or server room), you may wish to connect the spaces through a small cooling duct. This will allow you to discharge cool air into a small server cabinet or cellar, providing the duct isn’t longer than 10m long.

Solar / double heat exchanger option

A double heat exchanger will allow you to recover heat from additional sources, making your HPWH even more energy-efficient. You can combine it with solar water heaters, woodstoves, conventional boilers, and so on. If you do have such sources available, this upgrade makes sense from an efficiency standpoint.


Ideally you want to make sure that at the very least, your HPWH has a warranty that is equal to your investment’s payback period, so that you are have some financial assurances. So, for instance, if your HPWH takes three years to break even, then it would be useful to have a warranty that covers these three years. HPWHs typically reach payback in two to five years. An investment in a TermoPlus HPWH is usually recouped within two years.

Due to the quality of our products and in order to guarantee peace of mind TermoPlus offers a 2+3 year warranty. This means that with a quick check-up every year you are guaranteed 5 years of absolute peace of mind. We also support our products for their entire lifetime and guarantee spare part availability. The minimum life expectancy of our HPWHs is 25 years with proper maintenance.

Cost and value

Some heat pump water heaters can be quite costly, while others can be extremely cheap. You need to make sure that the unit you buy will provide the value, features and benefits you need as well as a track record of a reliable, long lifecycle that will help you make the most out of your investment. Did we mention that the minimum life expectancy of our HPWHs is 25 years?

Are you planning on buying a heat pump soon?

If you are planning on investing in a heat pump in the next couple of years, then it may be worth considering a heat pump that produces sanitary hot water functionality instead of a HPWH. This will result in a lower cost overall for the entire system that will stack many energy bill savings over the coming decades.

Shop for TermoPlus Heat Pump Water Heaters and Hybrid HPWHs

If you’re interested in finding out more about our heaterless and hybrid electric heat pump water heaters we’ve got plenty of information for you. Find out more about our HPWH range, specifications or get a quote here.