Wednesday, February 17, 2016

Do Aerator Shower Heads Use More Energy?

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Our house had an old “high flow” shower head. As California is in the midst of an epic drought (despite all the recent rain), I was planning on installing a low flow “aerator” shower head. These shower heads mix in air with the outgoing water to lower the flow rate while theoretically not sacrificing comfort. Compared to a traditional shower head the water pressure felt is higher but the flow rate is lower, so they end up saving water.
But right as I was about to install one I heard a theory that these aerator shower heads actually end up using more energy because the mist causes the shower to feel colder, which means the user turns the hot water dial up, thereby using more hot water, which requires the hot water heater to burn more natural gas (or use more electricity, which burns more coal and natural gas) to reheat more water.
At first I thought this was a calculus problem, (think high school calculus – water goes in to a tank at one rate and leaves at another rate, how long until the tank is empty?) but then I realized it’s actually a simple algebra problem. Since the user turns up the amount of hot water in the mix, we can just assume a temperature change differential and calculate the change in hot water usage.
I calculated the existing flow rate of the shower using a stopwatch and a bucket at 3.5 gallons per minute (it took 45 seconds to fill a 10 liter bucket). Other older high flow shower heads can be up to 5.5 GPM. According to the EPA, the average shower length is 8 minutes. This means my existing shower head uses 28 gallons per shower.
According to Bosch, the average shower temperature is 106 degrees Fahrenheit and the average groundwater temperature is 58°F. The Department of Energy recommends setting your hot water heater at 120°F. Our hot water heater was set to a scalding 140 degrees Fahrenheit, so I turned it down to 120. This means for each gallon of hot water you pull out of the hot water heater it needs to heat another gallon of water by 62°F (Delta T).
It takes 1 British Thermal Unit to heat 1 pound of water 1°F. At 58°F, 1 gallon of water weighs8.34 pounds. At 120°F, 1 gallon of water weighs 8.25 pounds. Using the average of the two, and assuming a 90% thermal efficiency in converting natural gas to hot water, a hot water heater uses about 570 BTUs for each gallon of hot water consumed.
By the time the 120°F hot water makes it from the hot water heater through the copper pipes to your shower it has lost about 5°F of heat. If you prefer your shower at 106°F then you will need to set your shower dial at a mix of 84% hot water (.84*115+(1-.84)*58=106). This checks out with the shower dial position of the old shower head being about 85% of the way towards full hot water. So the “high flow” shower head used about 13,406 BTUs per shower. (570 BTUs per gallon of hot water * 3.5 gallons per minute * 84% hot water mix * 8 minute shower = 13,406.4 BTUs).
After installing the new low-flow shower head, I noticed that I needed to move the shower dial slightly closer to the “full hot” position to achieve the same comfort level. I’d estimate it is about 95% of the way to hot. This means that the shower head lowers the temperature of the shower by about 6°F (106-(.95*115+(1-.95)*58)). The new low-flow shower head uses 2 gallons per minute. So at this new hot-cold mix, it uses about 8,664 BTUs per shower. (570 BTUs per gallon of hot water * 2.0 gallons per minute * 95% hot water mix * 8 minute shower = 8,664 BTUs).
So to answer the question of this blog: YES, aerator shower heads do save energy (and lots of water).
But how long does it take to pay for itself? According to the EIA, natural gas contains about 1,028 BTUs per cubic foot. At a cost of $10 per thousand cubic feet of natural gas (about the residential average in California for the past few years), each shower saves about 6.3 cents of energy. ((570 BTUs per gallon of hot water * 1.5 gallons per minute difference between the high flow and low flow shower heads * .95 percent hot water mix * 8 minutes) / 1028 BTUs per cubic foot of natural gas) * $0.01 per cubic foot of natural gas). If we reach “peak gas” in the near future, this cost could increase dramatically. Since the shower head cost $14, it will pay for itself after about 222 showers. Some water districts (including our own EBMUD) give away these shower heads for free, making the return on investment infinite!
While I was installing the new shower head I also installed a “ladybug” water saving temperature-controlled shutoff valve. This ingenious device shuts off the flow of water once it reaches a certain temperature. To restart the flow you simply pull a cord. So when you want to take a shower, you simply run it as you normally would; once the ladybug detects the shower is hot, it slows the flow to a trickle; then you just hop in and pull the cord to restart the flow.
The Ladybug Temperature-Controlled Shutoff Valve
The Ladybug Temperature-Controlled Shutoff Valve
The alternative way to avoid wasting water while you wait for the shower to heat up is to install a recirculating pump. This pump sits under your bathroom sink and is connected to the hot and cold water lines. When you’re ready to take a shower you push a button and the pump sucks water from the hot water line and forces it down the cold water line until the hot water line reaches the desired temperature. Besides being expensive (they cost about $200 without installation), a recirculating pump also causes your cold water line to have some warm water in it, so when you go to the sink to get cold water after a shower, it will be warm for a bit (which bothers some people).
Waiting for a shower to heat up wastes water and energy because most people don’t want to sit around with their hand in the shower stream waiting for it to heat up. This means that they might let it run for a minute or two longer than they need to. At 2 gallons per minute, an extra 2 minutes of run time amounts to a savings of about 2.1 cents per shower ((570 BTUs per gallon of hot water * 2 gallons per minute * .95 percent hot water mix * 2 minutes) / 1028 BTUs per cubic foot of natural gas) * $0.01 per cubic foot of natural gas). At a cost of $29, this will pay for itself after about 1,377 showers. While this may seem like a while, for a family of four, this is a payback period of less than a year.