This page is about using thermostatic mixing valves as a shower energy saving measure. This page will continue to evolve for months after the earthbag village and Duplicable City Center are complete, sharing our experience, feedback, and savings data.
This page contains the following sections:
A thermostatic mixing valve (TMV) is a valve that blends hot water with cold water to ensure constant and safe shower and bath outlet temperatures. The use of a thermostat, rather than a static mixing valve, provides:
This graphic shows the inner workings of these valves and how the thermostatic element (regulated in many by a wax thermostat) moves to change the amount of hot and cold water that mixes. In the case of a water pressure loss and/or hot or cold supply failure, water is shut off rapidly (less than 2 seconds) to prevent scalding or thermal shock.
In short, if output temperature is important, a thermostatic mixing valve can give you the exact temperature you desire. Click here for more TMV details on Wikipedia.
Open source sharing thermostatic mixing valve use in a teacher/demonstration village scenario like One Community has the potential to change the way people look at hot water use and its related energy costs. At One Community, the hydronic and showering subsystems of the Duplicable City Center and Earthbag Village communal showers are prime examples of conditions where this kind of valve can make the most difference in energy savings, user comfort, and safety.
Open source sharing data we gather on these areas, along with water saving shower head data and user satisfaction, and other modifiable components of the bathrooms will help:
Ron Payne: Mechanical Engineer and HVAC / Thermal Designer
A thermostatic mixing valve works much like a thermostat in your house. You set a desired temperature and the device opens or closes the hot water valve in order to maintain the proper temperature setting. In the case of our showers, the valve would only use the point-source water heater when necessary to attain the correct temperature thus decreasing the probability of wasted hot water and the resultant wasted energy. The ease of use and safety of these valves make them perfect for domestic use. Because the temperature can be set before the water is turned on, the user can depend on, and monitor the temperature of the water that they are using. This is helpful to prevent scalding injury and to limit only the necessary hot water usage to the user of the shower.
The constant temperature output make them well-suited for in floor thermal heating such as the system utilized in the Duplicable City Center. An input from a BACNet (building automation and control networks) to a digital thermostatic valve (or regulator) would ensure that the temperature of the water running through the radiant heating system would be the correct (and safe) temperature to heat the space safely.
Though the units themselves are hardly more-or less expensive than a “normal” shower valve, the main benefit to One Community and others is the ability to control the exact temperature of the shower.
Let’s do a thought experiment to see what a reduction of 1°C in everybody’s shower would mean in terms of energy, water, and money. For this experiment we’ll assume that the average shower flow rate is 1.5 GPM and the average shower is 8 minutes. Energy is usually measured as the amount of energy it takes to raise a amount of water by a certain temperature. So finding energy from a water temperature increase, or decrease, is relatively simple.
Raising 1.5 gallons of water per minute by 1°C would be the same as 52.20 watt-hours (WH) worth of energy for every shower. When you consider that 150 people will be taking showers every day, that adds up to nearly 7.83 kWh every day.
Standard energy cost @ 40°C
Increased by 1°C / 1.8°F
At the standard rate of 10¢ per kWh this is only 78¢ per day difference. However, One Community, being off-grid, pays dearly for it’s power and the infrastructure needed to provide that extra 7.83 kWh/day would cost nearly $11,700 of extra infrastructure. That power-generating infrastructure would need to be purchased before the buildings were built.
What does that 7.83 kWh/day of energy really amount to? It’s 6,740 Calories for you gym people. It’s 5.5 standard 60W incandescent bulbs left on all day. It’s the amount of energy in a quarter gallon of gasoline (about 6 miles driving in the average car). Or, It’s 23 miles driving an electric car.
All that, every day, in just one degree difference for 150 people.
The biggest water savings, however, would come from the immediacy of the correct temperature upon starting the shower. Three factors affect the timing of the hot water’s arrival to an individual about to take a shower:
In a ½” pipe there are 0.002236 gallons per inch of pipe. At a standard shower flow rate of 1.5 GPM we can find that it takes 0.04403 seconds per inch for water to move through it. When the TMV is opened the valve will be sensing cold and therefore the hot water will be completely open. The water heater would heat the water and it would start to rise up to the thermostatic mixing valve. The valve would not begin to react to the hot water until the water reached that point.
If the heater is 36” from the valve it would take 1.59 seconds before the valve would react. The NHS D08 TMV Model Engineering Specification makes reference to this, and permits a 7°C temperature ‘spike’ to last for 1.2 seconds, with 50°C permitted for a maximum of 0.5 seconds. Assuming incoming hot water as a “spike” the valve would react to the correct temperature in about 1.7 seconds. Assuming 36” again for the mixed water pipe from the valve to the showerhead, the correct temperature would come out of the shower head 1.59 seconds after the TMV has found the correct temperature.
Total estimated time from start of shower to correct temperature:
1.59s + 1.7s + 1.59s = 4.88s
Now this may not seem like much time savings, I’m sure in the showers in your own home you have a pretty good idea of where the shower valve needs to be to give you the best temperature. Where this valve shines is its ability to work consistently no matter which shower you use.
When entering a strange shower people usually take time to adjust the temperature and find the right position for their comfort. In a large group of people using several different showers, the ability to get an ideal temp in a shower in less than 5 seconds, no matter what shower, adds up to consistent and significant savings in water and power.
Let’s assume a 5 second savings:
Here are some additional water and energy saving measures worth testing.
It is our hope that the addition of heat lamps in the bathing and dressing area will help reduce water and energy consumption as well. The assumption is that the addition of a heat lamp to the showering area either causes the showerer to decrease the time in the water or decrease the temperature to which the shower is set. Heat lamp effectiveness, however, is hard to analyze because it depends on a person’s comfort level at different distances.
Here are the calculations we can do though: If we were to use two 250W Heat lamps like the ones shown at left, the benefit would need to either make the user shorten their shower time by more than 47 seconds on average, or decrease their temperature by 1.3°C (2.34°F) or more to be a net savings of energy and/or water.
Using timer-switches to reduce negligent energy use is not a new idea. It is common to see these switches on large energy consuming devices that serve guests (i.e. hot tubs, saunas, and heat lamps.) The idea is simple, the person selects how much time they will use the unit (i.e. 5 minutes) and the unit will only operate while that countdown is going. If the person wants more time, they select more time. Nothing prevents the person from using the device, but it requires that the person be present to reset the timer. This saves the facility from having to power the device while no one is using it.
There are many options on the market for timers for standard devices, but what time selection would be the best for energy saving? We shall put timers on the Heat Lamps, which is a semi-standard practice, and separate timers on the showers.
The heat lamp, along with allowing a person to take cooler showers, would also serve as a time-metering device while in the water. Since while in the shower most of us are without our time devices, the heat lamp and shower timers would also act as timekeepers, turning off after an allotted time.
But what length of time would yield the best results? It figures that a long timer would not alert the showerer and the shower would run long. However, a very short timer would cause the person to adjust it over and over perhaps interrupting the showering and creating a longer shower time as well.
Since this is a question of human behavior we will have to see what effect different timers have on the energy and water use in the shower areas. With several showering facilities, we can (and will) test the different timers against each other in an experiment. Here is the initial plan:
1/4 of the showers would have:
1/4 of the showers would have:
1/4 of the showers would have:
1/4 of the showers would have:
More analysis is needed to design the experiment thoroughly, however, the idea remains the same:
Clearly identify how to use maximum comfort of the shower and changing environment, combined with convenient ways to self-monitor and adjust water temperature and time showering, to maximize water and energy savings.
Thermostatic mixing valves, heat lamps, and timers can be used to save water and energy. They do this by providing increased control and awareness. Through creative experimentation, savings can be maximized while also educating people about their own usage patterns and ways that they can be more conservative.
Q: Can the heating system influence the thermostatic mixing valve’s performance?
Yes, if the TMV is installed incorrectly it will not operate optimally. The solution to this is to carefully follow the manufacturer’s installation suggestion.
Q: How quickly will a thermostatic mixing valve adapt when a user changes the shower water temperature?
Assuming 36” for the mixed water pipe from the valve to the shower head, a TMVs temperature will changes in about 1.7 seconds, significantly faster than adjusting a traditional shower. This almost instant closure in the event of a hot or cold supply failure is what makes them so effective at preventing scalding or thermal shock.