How useful are shower heat exchangers?

Contents

Media reports about shower heat exchangers that save a lot of money and help the environment by reducing CO₂ emissions have recently been making the rounds again. However, their benefits depend very much on your behavior and the heating system you use.

A few simple sample calculations show the amount of energy and the resulting costs that are actually involved. Such heat exchange systems must then significantly reduce these costs so that they pay for themselves within a reasonable period of time.

The basics

The physical principles and facts can be summarized simply. It takes 4182 joules to heat one liter of water by one Kelvin. Since one watt-hour corresponds to 3600 joules, that is 1.162 watt-hours per Kelvin of temperature increase per liter of water.

A standard showerhead without flow reduction lets around nine liters of water through per minute. And depending on showering behavior, there are those that finish in five minutes and those that let the water run for ten minutes. This results in a rough range of 45 to 90 liters per shower. Depending on the source, the water from the water supplier reaches your home at an average temperature of 10 to 15 degrees Celsius. Shower water at body temperature is around 37°C; hot showers use around 40 °C.

This results in a temperature range of between 22 and 30 Kelvin – for the extremes in the shower profiles under consideration, without considering any extreme "overheating," for example, with fossil heating systems that heat water storage tanks to more than 45°C. This results in an energy requirement per showering process of
Short shower cool: 45 l × 1.162 Wh × 22 K = 1150 Wh
Long shower hot: 90 l × 1.162 Wh × 30 K = 3137 Wh

To arrive at a realistic annual consumption, the frequency of showering must also be considered. Dermatologists recommend showering two to three times a week to avoid damaging the natural skin layer. The recommendation per person is to shower between 104 and 156 times a year for 52 weeks. This results in a range from 119.6 kWh for hot water generation for short showers to 489.4 kWh for more frequent long showers.

Heat generators are also relevant

The costs for this vary depending on the heating system. One liter of heating oil contains roughly 10 kWh of energy, as does 1 m³ of gas. Low-temperature boilers for oil burners should achieve an efficiency of almost 90 percent, modern condensing boilers even 98 percent; gas boilers end up in similar ranges. For the sake of simplicity, we assume an efficiency of 1, but in practice this is (significantly) lower. Heat pumps achieve lower coefficients of performance with the large temperature range up to around 45°C in the storage system; on average, around 3 – 1 kWh of electricity therefore produces 3 kWh of hot water.

This results in annual consumption:

Medium	Minimum	Maximum
Oil (l)	11.96	48.94
Gas (m^3)	11.96	48.94
Heatpump Electricity (kWh)	39.87	163.1

At the beginning of September, a liter of heating oil can be had for about 89.5 cents a cubic meter of gas for about 95 cents. A kWh of electricity, on the other hand, costs an average of 27 cents for new customers at this time. For the sake of simplicity, we assume cost parity for costs such as delivery or meter charges and therefore omit them.

Concrete costs – current

Depending on the shower profile, heat pump owners therefore pay between 10 euros and 44 euros per person per year. In the case of oil heating, the annual cost is 10 to 44 euros, and for gas boilers, 11 to 46 euros. At current prices, modern heating systems are effectively on a par. However, older heating systems using fossil fuels are already significantly pricier than these example calculations. Electric instantaneous water heaters are also much pricier. However, CO₂ pricing and the European CO₂ certificate trading that will start in the foreseeable future will significantly increase the cost of fossil fuels compared to the price of electricity. Although the latter will also rise as a result, only a fraction of the increases will be reflected in the heat pump costs – especially since the energy mix is getting greener and the additional CO₂ costs are lower.

For some heat exchanger shower systems, the co2online portal provides manufacturer information on how much energy can be saved when showering. The information ranges from 30 to an incredible 60 percent – according to the manufacturers' marketing, so the lower ranges can be assumed for average cases. Practical reports on the internet also tend to point to the lower range of these figures.

It is therefore optimistic to assume a savings potential of one-third of the energy recovered from wastewater during showering. This then ranges from 3 to 15 euros per person per year. The costs for such a recovery system and its installation are therefore more likely to pay off if many people under one roof take long and frequent showers – four people taking long showers will save around 300 euros in five years. The cheapest systems for self-connection in the form of a shower mat aren't available for that price and cost more. With fewer people and more efficient showering, however, the amortization period extends into the decades.

Using the example figures, it is easy to calculate your costs and potential savings. Anyone still using old heating systems should consider switching to modern systems, ideally a heat pump. This will certainly do more for the environment and your wallet in the long term than a shower heat exchanger.

(dmk)