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Recycling water with an inline heater

by Alison Empey
(London, Ontario, Canada)

I am researching water and energy conservation technology as I redesign our master bathroom. My husband loves long showers because he has a stiff back (yes, he goes to the chiropractor to help with this). However, for energy and water savings, I have trained our family to have "sailor showers" instead of long ones, but my husband misses the therapeutic benefits of his long showers.

I am considering putting a floor sink under the shower stall to allow us to collect water and recycle it. We would take our sailor shower first then after rinsing fill the floor sink with fresh water that would have a waterfall (pond) pump in it to recycle the water to a different set of shower heads. Since the water would cool I will need an inline water heater to reheat the recycled water. Because body jets are good for massaging my husband's back, I would want very good volume; somewhere in the neighborhood of 10 g/min.

Any suggestions on what I should be looking for in an in-line water heater? And does an in-line water heater make sense or should we just go back to regular showers?

I might add, our water costs are extremely high ... we are just outside a major city and our water costs are nearly double the city's prices. Even with our sailor showers we're spending over $2000 (CDN) on water each year ... very typical of households in our neighborhood.

Answer from Green Energy Efficient Homes

This is an interesting twist on the problem of long showers. For most people the major expense is the cost of heating the water, so one way to cut costs there is to use a Thermo Drain or Power Pipe in your drain stack, which captures the heat from the water outflow and transfers that heat to the hot water intake. But the water itself is still wasted, and you won't cut your high water costs by using one of these.

If you're trying to recycle the water as well, an inline water heater (also known as a tankless water heater) coupled with re-cycling the water, may make more sense. The trouble with tankless water heaters is that they have a limited flow rate that would potentially make it hard for you to achieve the 10 GPM flow you are after. Most are only rated for 2-4 gallons per minute of flow - enough to take a shower using a low-flow showerhead but probably not enough for a full blast with the massage heads.

The GPM ratings of tankless water heaters are based partly on the temperature differential between the incoming and outgoing water. For example, the Stiebel Eltron Tempra 24 electric tankless water heater provides a temperature gain of roughly 42F at 4 gallons per minute, but only 21F at 8 gallons per minute. You can in theory get a higher flow rate for a given temperature differential, by putting two inline water heaters in series, and if you are capturing the outflow from the shower and feeding that back through an inline water heater, the water going in will already be substantially warmer than a typical municipal water supply, which means the heater can sustain higher GPM flows and still provide the desired output heat. But ultimately there may be a maximum flow rate regardless of your heating requirements because of the heat exchanger and other heater plumbing the water has to go through, which could prevent you reaching a full 10 GPM.

The other factor to consider is the effect on the interior plumbing of the water heater of cycling 'gray' water through the heating element. Inline water heaters are not designed for water recycling, and any dirt, sediment, grit, skin, soap or other contaminants could shorten the life of the heater. I suspect that most tankless water heater manufacturers will not honor their warranty if you are cycling used water through it. For example the standard Stiebel Eltron warranty states that it does not apply "to installation methods which do not conform to relevant national, state or local codes and ordinances", so if it is not legal in your country, state/province or municipality to recycle gray water through a hot water heater, that could void the warranty even if the warranty doesn't explicitly preclude heating grey water.

A pond pump can probably handle the contaminants in the drain water better than the hot water heater. And there are pond pumps that can certainly handle the flow, such as the Danner Pond Mag 950GPH pump which can provide up to 16 gallons per minute of flow, and comes with a filter. But I'm not sure how well pond pumps deal with hot water; it may wear them out prematurely.

I guess you are charting new ground here, and given your husband's need for a high pressure hot shower and your need to keep your water use low, you will likely have to come up with a solution the manufacturers of water heaters and water pumps haven't figured out themselves. I hope the above observations are helpful but you probably already know more about this subject from your research than I do. If you are able to come up with a good solution feel free to post it on the Energy saving tips section of Green Energy Efficient Homes.

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