Context

While renovating our bathroom with my SO, we stumbled upon the concept of grey water heat recovery devices.

Basically, the idea is that grey water (i.e. the waste water from a shower) is sometimes quite warm. These warm calories could be used to heat the incoming water to a shower.

There are many different devices. Some are quite complicated (heat pumps) whereas others are quite simple (low-tech). To make it short, they consist of a tube within a tube. The internal tube houses the greywater, and the outer englobing external one contains the cold water inlet. That way they form a heat exchanger. Connected to a shower, it will reclaim the calories from the shower’s grey water and use them to preheat the cold water of the shower.

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Cost analysis

Before buying or making such a device, it is always interesting to know what would be the theoretical cost savings by using it.

Let’s suppose the following variables:

• Cold water temperature: 10°C
• Shower temperature: 40°C
• Shower water usage: 50l

Then the theoretical maximum (assuming 100% efficiency) energy reclamation using such device would be: $$ (40K-10K) * 50\% * 50kg/\text{shower} * \text{water specific heat} = 0.9 kWh/\text{shower} $$

Please note that in this case, 100% conversion efficiency translates into a 50% multiplier as the reclaimed temperature cannot be higher that the grey water output temperature.

Assuming a family of 2, it would thus save $ 0.9kWh/\text{day}/\text{person} * 2 \text{person} * 1 \text{year} = 660 kWh $

Such device would thus save $660 kWh$ per year. We then have to convert this into money, to be able to calculate an ROI:

• We assume an electricity cost of $ 0.16EUR/kWh $
• Using a normal electric boiler, it would save us $ 660 kWh/\text{year} * 0.16 EUR/kWh = 105 EUR/\text{year}$
• Using a heat pump with a COP of 2: $ 660 kWh/\text{year} * 0.16 EUR/kWh * 50\% = 53 EUR/\text{year} $

Of course, saving 100 euros per year is not much in the grand scheme of things, but this figure can go higher once the family grows. Also, it is important to take into account the cost of the heat reclamation device in order to decide if it is worth it or not.

Commercial products

Sadly, there are very few products on the EU market as of 2021.

Gaïa Green products

ReQup-Plus

TODO

ReQup-Floor

TODO

joulia Inline

Joulia is a Swiss-based company that makes a shower channel with the heat reclamation device embedded in the channel.

As of november 2021, the characteristics are as follow:

• Cost: 1'200 CHF (1'145 €)
• Efficiency: 80%

That gives us a gain of:

• $105 €/year * 80\% = 84€/year $ for an electric water heater
• $53 €/year * 80\% = 42€/year $ for a heat pump water heater

The ROI can then be calculated:

• Electric water heater: $ 1145 EUR / (84 €/year) = 13 year$
• Heat pump water heater: $ 1145 EUR / (42 €/year) = 27 year$

Conclusion

Having water heated via a heat pumps increases the ROI as less energy is needed to heat water.

In my opinion, 20 years is a bit long but still in the reasonable range.