Why showering uses more energy than you might think, and how shower heat recovery can almost halve it.

When people think about saving energy at home, solar panels, insulation and heat pumps are often the first solutions that come to mind. But there is another significant source of energy consumption that is easy to overlook: hot water.

Every time you shower, warm water disappears down the drain while still containing a considerable amount of thermal energy.

A shower wastewater heat recovery system (WWHR) can capture part of that energy and immediately reuse it. Depending on the type of system and installation, this can reduce the energy required for showering by around 40–50%.

Vertical shower heat recovery systems are particularly interesting because they generally achieve higher energy savings than horizontal systems.

Why showering uses so much energy

Producing hot water requires a considerable amount of energy. And as homes become better insulated and require less energy for space heating, domestic hot water can represent an increasingly important part of household energy consumption.

During a shower, heated water is used for only a few seconds before disappearing down the drain.

That means a considerable amount of heat produced by your:

• gas boiler;
• heat pump;
• hot water cylinder;
• or other water-heating system

is normally lost through the wastewater.

Shower heat recovery provides a way to capture some of that energy before it leaves the building.

What is shower wastewater heat recovery?

A shower wastewater heat recovery system uses heat from warm shower wastewater to preheat incoming cold mains water.

Importantly, the clean drinking water and wastewater never mix. They remain separated inside the heat exchanger, with only heat passing from one water stream to the other.

Because the incoming cold water has already been partially heated, less hot water needs to be added to achieve the desired shower temperature.

As a result, the boiler, heat pump or hot water cylinder needs to supply less energy.

According to Milieu Centraal, shower heat recovery can reduce gas or electricity consumption for showering by around 40–50%.

Why vertical shower heat recovery systems are so effective

Several types of shower heat recovery system are available, including:

• vertical shower drainpipe heat exchangers;
• heat recovery shower drains;
• systems integrated into shower trays;
• wall-mounted or pumped systems;
• standalone systems installed on the shower floor.

Vertical systems generally achieve higher energy savings than horizontal systems such as heat recovery shower drains or shower trays.

Why does vertical installation help?

In a conventional vertical shower drainpipe heat exchanger, warm wastewater flows down the inside surface of the vertical pipe.

This creates a relatively large heat-transfer area between the warm wastewater and the incoming cold water flowing through the heat exchanger.

The heat from the wastewater can therefore be transferred efficiently to the incoming water before the wastewater leaves the building.

The exact heat recovery efficiency depends on the product, flow rate, installation configuration and test conditions. High-performance vertical systems can achieve very high heat recovery efficiencies under specified conditions, but these technical efficiency figures should not be confused with the actual percentage reduction in a household's total energy consumption.

For real-world household energy use, Milieu Centraal estimates that shower heat recovery can typically reduce the energy required for showering by around 40–50%.

Why systems such as Hamwells are interesting

There are also systems that make it possible to use vertical heat recovery technology where a conventional vertical drainpipe arrangement may be difficult to install.

Hamwells, for example, has developed shower systems that use vertical wastewater heat recovery pipes together with a pump, sensor and water reservoir.

The warm wastewater is collected and pumped through the heat recovery system, where its residual heat is transferred to the incoming cold water.

Systems of this type can be interesting because they combine:

• efficient heat recovery;
• relatively low maintenance requirements;
• immediate energy savings while showering;
• the possibility of installation in or near the bathroom;
• reduced demand for hot water.

Because less hot water needs to be produced, the required capacity of the hot water installation may also be reduced in some situations.

Particularly interesting with heat pumps

Shower heat recovery can be especially useful in homes with:

• a heat pump;
• a heat pump water heater;
• or an electric hot water cylinder.

With a heat pump, preheating the incoming water means that less energy needs to be supplied to produce water at the desired shower temperature.

This can help:

• reduce electricity consumption for showering;
• make the available stored hot water last longer;
• potentially reduce the required hot water cylinder capacity;
• reduce demand on the hot water system.

Milieu Centraal states that a shower heat recovery system can reduce electricity consumption for showering with a heat pump by up to around 50%.

Is shower heat recovery always a good choice?

Not necessarily. The best solution depends on the building and the location of the shower.

A conventional vertical shower drainpipe heat exchanger is particularly suitable when:

• there is enough vertical space beneath or near the shower;
• the required pipework can be installed;
• and the shower is used regularly.

In some renovations, particularly where there is insufficient vertical installation space beneath the shower, a conventional vertical heat recovery pipe may be difficult to install.

A heat recovery shower drain, shower tray or pumped system can then provide an alternative, although horizontal systems generally achieve somewhat lower energy savings than vertical systems.

The financial return also depends on factors such as:

• energy prices;
• the number of occupants;
• shower duration and frequency;
• shower flow rate;
• the type of hot water system;
• purchase and installation costs.

The basic principle, however, remains very effective: instead of allowing useful heat to disappear down the drain, part of it is immediately reused.

Conclusion

Producing hot water for showering requires a significant amount of energy, and much of that heat would normally disappear down the drain immediately after use.

Shower wastewater heat recovery addresses this loss by transferring heat from outgoing wastewater to incoming cold water.

According to Milieu Centraal, this can reduce the gas or electricity required for showering by around 40–50%. Vertical shower heat recovery systems generally provide the greatest energy savings, although the best solution depends on the layout and installation possibilities of the home.

The technology can be particularly useful in combination with a heat pump or hot water cylinder, because it reduces the amount of energy and stored hot water required for each shower.

Instead of producing more heat, shower heat recovery simply makes better use of energy that is already there.