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

By: Evan, US

Evan, US — Wed, 11 Jun 2008 18:08:38 +0000

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.

By: ben, uk

ben, uk — Mon, 09 Jun 2008 14:50:34 +0000

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!

By: Eric What about this?

Eric What about this? — Wed, 30 Apr 2008 20:59:21 +0000

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?

By: Dan, Byron MN

Dan, Byron MN — Sat, 15 Mar 2008 21:28:33 +0000

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.

By: WATER HEATERS: Does you wait longer for hot water when going tankless? | GetWithGreen.com

Wed, 20 Feb 2008 13:26:17 +0000

[...] The space savings will be a big draw for those of you living in smaller homes, and for some it will be the energy consumption of the tankless vs. a conventional model, or which one pays back faster? [...]

By: Pete in Milwaukee

Pete in Milwaukee — Thu, 27 Dec 2007 22:13:14 +0000

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.

By: Michael Thomas

Michael Thomas — Sun, 23 Dec 2007 16:05:57 +0000

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.

By: Dimitri Svolos PEng,Panama City

Dimitri Svolos PEng,Panama City — Fri, 21 Dec 2007 12:48:22 +0000

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.