Looking for a Comfortable Low-Flow Shower Experience

May 06, 2009
May/June 2009
A version of this article appears in the May/June 2009 issue of Home Energy Magazine.
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 Q. John Koeller notes how the plumbing industry has taken advantage of the lack of any prohibition against multiple head and body spray luxury shower installations resulting in many very high flow installations (“Danger in the Shower: 2008 Forum Looks at Hot Water,” HE Jan/Feb ’09, p. 6). My own theory is that consumers want these systems primarily to get and stay warm in their oversized and unheated showers. If we add radiant heat to the floor, three walls, and even the ceiling, they will be very comfortable right away and be happy with less water over a shorter period of time.

GARY KLEIN has been involved in energy efficiency and renewable energy since 1973. After 19 years with the California Energy Commission, his new firm, Affiliated International Management LLC, provides consulting on sustainability through their international team of affiliates. He has a passion for hot water: getting into it, getting out of it and efficiently delivering it to meet customer’s needs.
For electric radiant panels in the shower to be effective, they would have to be 15 watts per square foot times, say, 36 square feet of panel. So 540 watts for 30 minutes would be around 270 watt-hours or 5¢–6¢ of electricity. How much water can you heat for that? A truly high-volume shower uses 6 to 8 gallons per minute (gpm) more than needed over, say, 15 minutes, or 90 to 120 gallons.

I think that both the energy costs and the water usage are higher in the unheated shower.

I would never advocate a bigger shower requiring more radiant panel area. It must be minimized and should certainly accommodate two persons, even with only one showerhead. I keep getting clients that want them bigger and have continuing difficulty in persuading them that bigger leads to less comfort. A 3 foot x 3 foot shower works; even better is a 3 foot 4 inch x 3 foot 4 inch shower with a neo angle door, which is less than a square foot bigger.

My point is that heating the shower really adds to its comfort and utility, thus enabling the user to use less water (and perhaps even less energy) and thus justifying the heating installation.

I think that electric systems are best for this, as they are very responsive beneath thin tile, but currently NEC 424.12(A) and 424.93(a)(2) prohibit these installations except in the floor. I ask all readers who agree to join me in appealing to NFPA and UL to get this situation changed. The joy of a warm shower, even with low-flow equipment, has to be experienced to be appreciated. The way the heated shower dries out quickly is another side benefit!

Ned Baldwin
Baldwin & Franklin Architects
Hastings on Hudson, New York

Author John Koeller replies:
A. This would be a good topic to bring to one of our favorite water-energy gurus, Gary Klein. Are we saving water at the expense of energy consumption? Or might we be doing the opposite by trying to heat the shower stall with flowing hot water atomized by a shower system? Interesting stuff!

Here are my thoughts
  1. Energy needed to heat water for a shower
    1. At 1 gpm, a 70°F temperature rise (50°F to 120°F) takes, say, 600 Btu. More energy than this is needed, depending on the efficiency of the water heater system, but we will ignore that for now. The temperature of the water drops between the water heater and the shower, the amount depending on the distance, the temperature of the environment surrounding the pipes, the amount of pipe insulation, the flow rate, and the diameter of the pipes. We can safely assume a 5°F temperature drop with no pipe insulation (2.5°F if the pipes are insulated); the actual temperature drop could be much more. This 115°F water is then mixed with cold water to get the desired temperature.
    2. The maximum legal flow rate for a single showerhead is 2.5 gpm. Given the conditions described above, approximately 1.9 gpm of the mix is hot and 0.6 is cold. This means that we need 1,140 Btu per minute to heat the hot portion of the mix.
    3. In a ten-minute shower, this would be 11,400 Btu, or 3.3 kWh. (Eight minutes is the national average, with a large standard deviation.)
  2. Heated shower stall
    1. The benefit of the heated shower stall is that in principle it will allow for the use of a lower flow rate or a lower mixed water temperature, enough so that the savings from either or both of these methods will be greater than the cost of heating the shower stall.
    2. In Ned’s example, I assumed that heating the shower stall could be done for about 270 watt-hours, or 0.27 kWh. Any reduction in flow rate or mixed water temperature must be greater than this amount in order for there to be any energy savings. The radiant heater must be able to reduce the temperature difference between the mixed temperature hot water (115°F in this case) and the incoming cold water (50°F in this case) by at least 8% to make it worthwhile. This works out to be a reduction of 3.6 degrees. If we take all of the reduction on the hot side, this means reducing the mixed temperature water down to 111.4°F, just to get to the balance point between the two methods. In any event, I would want to see bigger savings, due to the added construction costs. At higher flow rates, say 5 gpm, the same energy input from the radiant panels only has to offset 1.8°F, but then much more energy is being used to heat the water. I would point out that these higher flow rate shower systems are clearly not energy saving to begin with.
    3. I like the idea of heating the shower surround, particularly for the drying feature, which should keep down the buildup of mold and mildew, but I do not know how the energy balance will turn out. If someone is willing to monitor the temperature changes and the energy costs of radiant panel operation, please let me know.
  3. More-effective showers
    1. In his letter, Ned states that “ consumers want these [high flow rate shower] systems primarily to get and stay warm in their oversized and unheated showers.” While that may certainly be part of the motivation, I would observe that the first step is to build more- effective shower stalls and put them in generally more-comfortable houses.
    2. I would be a bit concerned about the longevity of the radiant heat solution. If we build an intentionally large shower system, which can really be comfortable only if the radiant heating is used, what will happen when the system fails (which it may very well do in a number of years)? Won’t the likely result be that the people will adjust the temperature or the flow rate up, thereby increasing energy and water consumption? Oops!
    3. A more-effective shower stall would be high enough to prevent drafts, located away from air vents for the same reason, and not too large, even if it had two showerheads (each serving one person), so the shower experience could be shared.
    4. I would also think that it would be good idea to have the showerhead on an adjustable bar so that it could always be the right distance from the person taking the shower. I suspect that much of our discomfort in our current showers has to do with this distance.
    5. Shower satisfaction also has to do with the droplet size and the area of the spray pattern. Smaller droplets cool off more quickly as they move away from the showerhead. The area determines the coverage on the body. Both matter.
    6. It also seems reasonable to think about putting a ceiling on the shower so that the steam will not escape, and it is possible for the vapor pressure in the shower stall to increase. If this works out, then it will also be possible to lower the flow rate or the water temperature.
I hope this helps.  
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