WATER HEATER: Return on Investment (ROI) on Tankless Water Heaters

Also, there are reasons other than ?energy efficiency? to buy a tankless water heater. For example I have commercial unit in my house as most of the time it?s only my wife and myself, but when we have house guests we can have a tub, a shower, the dishwasher, the laundry and the kitchen sink running more or less at the same time ? I?d have to keep 140+ gallons of water hot to have a fighting chance of keeping up with the demand with a conventional tank type heater, and if 6 or 7 people in a row all wanted hot showers, two at a time, we would still be out of luck. The tankless heater was expensive to install, but it provides what we paid for: hot water, as hot as we want it, for as long as we want it. Meanwhile, we don?t spend a dime to heat water if we are out of town.

Another example: in some of our rehab rental units the smaller utility area possible with a wall-mounted tankless unit providers a few critical additional square feet of floor space that can translate into a half bath or additional closet space ? which matters in a highly competitive rental market.

That said there ARE a lot of issues with tankless installations; I have a web page up where I discuss some of the ones we have encountered

http://paragoninspects.com/home-inspection-tankless-water-heater-installation-problems-faq.html

Hello,

A little problem with your water heater math. You ’round-up’ significantly! Too significantly, as a matter of fact. The cost in a tank is $416, not $736.

And, most years that I know have 365 days, not 335 days. But, using your 335 days for the year, please find the corrections.

According to wikipedia and my high school physics book, a BTU is defined as the amount of heat required to raise the temperature of one pound of water by one degree. Since water weighs 8.34 pounds per gallon, this would mean that it takes 8.34 BTUs to heat one gallon of water one degree. To raise that one gallon of water 53 degrees takes 442.02 BTUs (8,34 BTU/degree x 53 degrees = 442.02 BTUs).

Since 1 Btu = 0.00029307 kilowatt hour, 442.02 BTUs = 0.1295 Kilowatt hours. Not 0.2!

According to the calculation in the article, 87 gallons per day x 335 days/yr = 29,145 gallons/yr. So, the amount of kwhrs needed for one year is
29145 gal/yr x 0.1295 kwhr/gal = 3774.28 kwhr/yr

Based on the utility rate of $0.11/kwhr, the cost will then be:
3774.28 kwhr/yr x $0.11/kwhr = $415.17.

I’m not sure where the number of 6.2 kw came from. but based on the author’s math, I’d like to see the source!

Thanks!

Eric, thanks so much for your response. Per your request we’ve reached back out to the author to join your conversation. In addition, we’ve contacted another one of our experts to weigh in. Stay tuned, and thanks again for contributing!

David

Eric, thank you for taking the time to read our article and provide your critical thinking. To answer your questions, the data provided in this article was generated through actual, experimental data from water heaters working in houses. Regarding the days: yes a year still has 365 days however in actual engineering calculations you have to account for days that the household is “away” (holidays,weekend trips etc.) and based on Department of Labor numbers the average American takes a 35 day leave from their place of work and residence. We rounded it up to 30 (not everyone leaves all days).

Regarding BTU/kWhr calculations: Your high school physics and wikipedia are, of course, right but they both refer to laboratory and controlled environment conditions. In real life there are losses… a lot of them! In real life, water has impurities (metals, chemicals etc.) that impair its ability to conduct heat. As a results it takes more BTU’s to raise a pound of water. In addition, all man made appliances (including tankless water heaters) have losses of their own stemming from resistances, pipe losses, fouling etc.

Again, our numbers were obtained from real applications in different states and provinces and by default are going to be different from the literature values. We tried to assimilate the costing situation that an average user will most likely encounter so there will not be any surprises and false promises.

We hope this answers your great questions. We appreciate your time and consideration.

Another variable you could consider is that if a water heater is within a conditioned space, a water heater operation is increasing a structure’s cooing cost and decreasing it’s heating cost, with the relative sized of the effects depending on the distribution of heating and cooling demand durign the year - so a tank type heater would be somewhat less efficient in FL than MN.

First, the calculations are incomplete. Installation can be a significant cost, particularly upgrading the gas line feed. There is also little service for tank heaters, while tankless may require more and it probably won’t be within the ability of DIYers. Water quality also affects efficiency due to scaling.

Second, I don’t believe the life expectancy claims. The short life of tank heaters is due to consumers buying low cost units. You can buy expensive ones that are long lasting. Time will tell if the more sophisticated and complex functioning of tankless will be long lasting.

I still think the numbers are bogus. The energy required to heat the water should be essentially the same in both cases — electric heat is 100% efficient. The only significant difference should be in standby losses, and, for the tank, they should be purely a function of time and temperature differential, not the amount of water used.

And somewhere it should be noted that the equations will shift significantly for gas units vs electric, since standby losses will be considerably higher.

Some more questions for Eric to think about:

Does 1 gallon of water weigh 8.34 pounds when it is 57F and 8.34 pounds at 110F? I think 1 gallon of water will weigh less as the temperature increases. What happens to your formula then?

I cant see how a gallon of water would weigh less at different temperatures!

it s like the old trick Question, which weighs more, a pound of lead or a pound of feathers ?? its still a gallon of water either way unless you take it to the moon it won’t change weight!

No, Ben, you aren’t correct. Density of water changes by temperature (although by a small amount). Since water is measured in gallons(volume, not weight), and Density = Mass/Volume, if the Volume stays the same and the Density increases, the Mass increases.

It’s not like the old trick Question.