Domestic hot water (DHW) via heat pump: advantages and disadvantages

The heat pump (PAC) with integrated domestic hot water (DHW) production is an interesting solution to satisfy two needs with a clean, renewable and free energy.

By combining two essential functionalities in one unit, this system offers many advantages in terms of energy efficiency, cost savings and space optimization.

However, like any device, it also has limits and drawbacks... which can be neutralized with another option, the thermodynamic water heater. Explanations...

Heat pump with domestic hot water production

The heat pump with DHW system consists of an air-to-water heat pump and a storage tank and therefore provides both heating and DHW for the house. 

Heat pump with integrated DHW: how does it work?

The operation of the heat pump can be summarized as follows: 

• The refrigerant, which is very cold in its liquid state, captures the heat from the outside air even at low temperatures. When heated, it becomes a gas in the evaporator of the heat pump's outdoor unit;
• To further increase its temperature, this gas will pass through the compressor, where it will be heated by compression;
• Condenser: this heat will then be transmitted to the water of the heating circuit. By transmitting its heat to the heating network, the refrigerant will cool and return to the liquid state;
• Expansion valve: the temperature of the fluid is further lowered by reducing its pressure to be able to recover calories from the outside air again. And the loop starts again.

In the case of a heat pump with integrated DHW, the indoor unit also includes a hot water tank with a capacity of generally between 170 and 300 liters, depending on the size of the household. It complements the other heat emitters, such as radiators and underfloor heating. It is also important to note the presence of a regulation system to manage the different needs.

This hot water tank is crossed by a serpentine tube containing the heated gas which transfers its heat, the tank playing the role of energy stock. After delivering its heat, the gas returns to the liquid state, passes through the pressure reducer which reduces its pressure and then collects the heat again outside the dwelling.

Heat pump with integrated DHW: what are the advantages?

Using a heat pump to produce domestic hot water has several advantages: 

• Optimization of energy consumptionThe heat pump can be used to produce hot water, but it can also take advantage of the "Inverter" technology, which adapts the power of the heat pump to the needs of the household (if you opt for a model equipped with this technology). Depending on the COP of the heat pump, the energy savings can reach 70 %;
• Space savingThis is because the domestic hot water tank and the heat pump indoor unit are located in the same column. This is an advantage, especially when you consider that the price per square meter is constantly rising, especially in urban areas. Please note: The indoor unit is larger with a DHW cylinder;
• Lower initial investmentThis is especially true if you plan to replace a dual service boiler that provides both heating and DHW. Indeed, the alternative to the heat pump with integrated DHW would be, for example, the installation of two new separate systems, one for heating and one for DHW (conventional air-to-water heat pump and electric water heater, for example) ;
• The integration or not of the DHW is not really what impacts the most on the purchase price of the heat pump;
• Reduced ecological footprint housing ;
• The device is eligible for aid of the state.

Heat pump with integrated DHW: what are the drawbacks?

The advantages of the heat pump system with integrated DHW are interesting, but some disadvantages may affect your comfort.

In regions with harsh winters, the efficiency of the heat pump system with integrated DHW is reduced. Indeed, the heat pump will prioritize the production of domestic hot water, which directly impacts its capacity to heat the home. A supplementary heating system is required, with all the associated disadvantages (additional cost and reduced thermal comfort).

Secondly, the production of domestic hot water can lead to excessive electricity consumption with a low or medium temperature heat pump. The production of domestic hot water (DHW) by a low-temperature heat pump (HP) requires a higher temperature than that required for heating.

This means that the heat pump will have to work harder and more often to reach this higher temperature, which can lead to faster equipment wear.

To avoid the proliferation of bacteria, in particular legionella, the hot water must be brought to a temperature of at least 60°C regularly. This is called the "anti-legionella cycle".

Low temperature heat pumps have to make an extra effort to reach this temperature, which accelerates their wear. This is why the high temperature heat pump (70°C) is strongly recommended.

Finally, and even if there is a space saving insofar as a single column is sufficient for the PAC + DHW tank system (as opposed to two columns with a remote system), the whole system can be relatively cumbersome, with a height of 2 meters in the case of a large tank.

This can be a problem for homes that do not have a large enough room. An alternative exists: a heat pump with remote DHW production. Here, the hot water tank and the heat pump are physically separated, but the operating principle remains the same.

Independent thermodynamic water heater

This option is just as interesting, insofar as we remain on the same principle of operation of the heat pump air - water.

How does the thermodynamic water heater work?

The thermodynamic water heater is actually a specific form of heat pump. While a conventional air-to-water heat pump provides heat to a heating system, the thermodynamic water heater is specifically designed to heat domestic water. Here is how a thermodynamic water heater works:

1. Heat extraction Like the heat pump, the thermodynamic water heater takes thermal energy from the ambient air, for example the outside air, the air of an unheated room or even the air extracted from a ventilation system. Most of the time, it is located in a technical room, in a cellar or in a laundry room;
2. Water heating The thermal energy extracted by the evaporation of a refrigerant in an evaporator. The refrigerant becomes gaseous and then circulates through a compressor which increases its temperature. This hot fluid then passes into a condenser, where it gives up its heat by condensation to the water in the storage tank;
3. Return to the liquid state After giving up its heat, the refrigerant returns to the liquid state and the cycle can start again.

The main difference between a conventional heat pump and a thermodynamic water heater lies in their use. The heat pump is generally used for heating rooms in a building and sometimes for the production of domestic hot water, while the thermodynamic water heater is used exclusively for the production of DHW.

Therefore, this equipment is more compact, as it does not need to be connected to a central heating system.

What are the advantages of the thermodynamic water heater?

The thermodynamic water heater has several advantages, including its energy efficiency and low environmental impact:

1. Energy efficiency The thermodynamic water heater is very energy efficient, since it can reduce the energy consumption for the production of domestic hot water up to three times compared to a conventional electric water heater;
2. Environmental impact The thermodynamic water heater has a lower environmental impact, as it emits ten times less CO2 than a device using fossil energy;
3. Eligibility for grants This device is eligible for state aid because it uses renewable energy; 
4. Storage capacity The thermodynamic water heater generally has a large tank to store hot water (up to 300 liters);
5. System independence By separating heating and hot water production, you can avoid premature wear and tear on your heating system in summer when you only need hot water;
6. Booster heater The thermodynamic water heater has an electric heating element that can be used when needed, for example during the coldest days of the winter.

What are the disadvantages of the thermodynamic water heater?

Despite the energy savings, the thermodynamic water heater is a relatively expensive equipment.

Then, the fact that it embeds a small heat pump to heat the water will inevitably produce additional noise (compressor), which eliminates the possibility of installing it in a living room.

You must therefore install it in a technical room. Finally, in the most common case where the thermodynamic balloon pumps heat into the air of the room in which it is located, then it results in a drop in ambient temperature of this room of about one degree (depending on the volume of the room).

What to choose: heat pump with integrated DHW or thermodynamic water heater?

Both options offer great energy savings while reducing the environmental footprint of your home... but each has its own advantages and disadvantages. The following table will help you make your choice.

Heat pump with integrated DHW (or heat pump with remote tank)	Thermodynamic water heater
Replacement of an old oil-fired boilerReplacement of an old gas boilerNeed to optimize space by using a single columnRegion with relatively temperate wintersPreference for a single maintenance contract	Need for DHW only, no need for heatingHouse without central heating systemHouse located in a region with a harsh climateFamily house (6 persons), with a significant need for DHW

Heat pump with integrated DHW: what you need to know

The heat pump system with integrated domestic hot water (DHW) production is an economical and ecological solution for heating and producing the DHW you need.

The result: optimization of space, reduced initial investment (in the case of replacing a dual-purpose boiler), reduction of the ecological footprint of the home, eligibility for government assistance and savings on energy bills.

However, there are some disadvantages to this option, especially in regions with harsh winters, where heating performance may be affected. In addition, DHW production can lead to excessive electricity consumption and faster wear and tear on the equipment, especially with a low or medium temperature heat pump.

The independent thermodynamic water heater is an interesting alternative for producing DHW. It is very energy efficient, has a low environmental impact, is eligible for state aid, offers a large hot water storage capacity and allows system independence. However, it requires a dedicated space for its installation, can produce noise and is relatively expensive.