Storing hot water at 120F doesnt save energy

TURN YOUR WATER heater thermostat to 120F for safe delivery of hot water and to conserve energy. Sounds simple, right? On the surface, this statement makes perfect sense. A 120F delivery temperature is certainly safer than 140F, and it sounds perfectly logical that lower storage temperatures will save energy. Well, we explored the myth that 120F potable water delivery is adequate to ensure safety

TURN YOUR WATER heater thermostat to 120°F for safe delivery of hot water and to conserve energy. Sounds simple, right?

On the surface, this statement makes perfect sense. A 120°F delivery temperature is certainly safer than 140°F, and it sounds perfectly logical that lower storage temperatures will save energy. Well, we explored the myth that 120°F potable water delivery is adequate to ensure safety from scalding and we detailed the facts, proving safety remains elusive (November 2002, pg. 32 and December 2002, pg. 20). We’ve also explored the potentially devastating consequences of low-temperature water storage and delivery in potable hot water systems (April 2001, pg. 29).

Let’s explore the idea that 120°F water storage temperature saves energy. If all you ever did with potable hot water were store to it in some vessel, the heat loss to the surrounding space would invariably increase as you increase the difference between the storage temperature (Delta-T) and that of the surrounding space. Insulation values and stack losses (for flue-style heaters) would be the determining factors alone — provided there was no thermo-siphoning or use of hot water. And that’s why this myth is so believable.

The general public’s perception is that these storage tanks are simply sitting idly by, wasting valuable resources with higher thermostat settings. Link that to the misconception that lower water temperatures ensure safety for their kids and you’ve got one powerful message. If only it were the truth!

Knowing that 106°F water represents the threshold for pain, let’s use a finished mix temperature of 102°F for our example. We’ll need to know the incoming water delivery temperature from the source, be it a municipal or well water source. Let’s assume an average annual cold-water delivery temperature of 55°F. For this test, we’ll use a 50-gal. storage vessel set at 120°F and then 140°F to compare the supposed energy savings.

You’re already familiar with the Btu formula as it relates to 1 lb. of water (adding 1 Btu increases the temperature of 1 lb. of water by 1°F). For a 50-gal. storage vessel, we can convert that water to weight by multiplying 50 x 8.3, which equals 415 lb. Subtracting the initial inlet temperature of 55 from 120 or 140 equals 65 and 85 respectively. That means we’ll need 26,975 Btu (415 x 65) to achieve 120°F or 35,275 Btu (415 x 85) for 140°F finished temperatures.

Now, let’s imagine we’re dealing with a family of three or four people and that their combined water usage for showering comes to 60 gal. per day for a total of 21,900 gal. of annual mixed water volume (GPY). They’ll need to mix the potentially scalding hot water to achieve the desired delivery temperature and that’s where this gets interesting.

In order to achieve the desired mix of 102°F in the bathing environment, we’ll need to determine the ratio (R) of hot to cold water. You can do this by subtracting the finished bathing temperature (BT) by the cold water inlet temperature (IT) and then dividing that number by the storage temperature (ST) after subtracting the cold water inlet temperature (IT). R = (BT - IT) ÷ (ST - IT).

If we’re using a storage temperature of 120°F, that looks like this: (102 - 55) ÷ (120 - 55) or 47/65, which gives us a multiplier of 0.723. If you do the math for the 140°F storage system, you’ll get a multiplier of 0.553.

Bear in mind that the 21,900 GPY figure is not changing. All we’re doing is altering the hot to cold water ratio to achieve the desired 102°F bather’s comfort.

Multiplying the 21,900 GPY number by our ratio of 0.723 for 120°F storage yields a need to deliver 15,833.7 gal. of heated water during the course of one year. Doing the math for 140°F storage temperatures reveals a need for 12,110.7 GPY, a reduction of 3,723-gal. of heated water.

Comparing the two shows that 140°F requires just 1,817.7 additional Btu per year. A pittance when compared to potential health issues and the increased size of equipment needed to keep our customers in adequate volumes of hot water.

The facts are, where we once installed 40-gal. water heaters, we now install 50-gal. models to ensure equal volumes of mixed water delivery. Our customers often raise the thermostat settings, which negates any perceived safety, offsets any lower water temperature savings and increases our liability exposure. In the final analysis, the increased material/labor/energy costs for larger equipment or customer-induced changes in thermostat settings more than offset the minimal Btu savings — by a wide margin.

A bit simplistic? Anyone can argue that point because we’ve eliminated a multitude of variables that can affect the computations and muddy the waters. Things like standby heat losses, which are affected by tank dimensions, insulation, surrounding air temperatures and stack loss.

Then there are the wide disparities between fuel costs, efficiencies and thermal siphoning to consider.

But no matter how complicated you make the variables and calculations, it still boils down to this: Lower storage temperatures will not save energy for production and use of potable hot water in the final analysis.

(Editor’s note: Dave Yates’ two-part column on scalding can be found online at http://www.contractormag.com/articles/ column.cfm?columnid=162 and http:// www.contractormag.com/articles/column.cfm?columnid=168. His Legionnaires’ disease column may be found at http://www.contractormag.com/articles/ column.cfm?columnid=98.)

Dave Yates owns F.W. Behler Inc., a contracting company in York, Pa. He can be reached by phone at 717/843-4920 or by e-mail at behler@blazenet.net.