Home savvy magazine- tankless heaters

From the article in the current edition (a few typos in it):

**Tank Water Heater **vs. Demand WH

Average Annual Traditional Tank Heater Costs

  • Annual hot water demand: 87 gallons / day

  • 335 days / year = 29,145 gallons/ year (I guess we’re not home for 30 days/year)

Power to heat up 1 gallon of water from 57 to 110F according to the multi-housing laundry association is 0.2 kWh. The annual power requirement of heating the water from 57 to 110F for an American household is: 29,145 gallons/ year x 0.2 kWh/ gallon = 5,900 kWh/ year

Annual cost of 5,900 kWh: 5,900 kWh/ year x $0.11/ kWh = $649

The cost to keep one gallon of water hot is $0.003. So, the cost to keep 21,145 gallons of water hot (standby): 21,945 gallons/ year x $0.003/ gallon = $87

**So the total annual cost of heating water using a traditional tank system is $649 + $85 or **$736

Average annual Tankless Water Heater Costs

  • Average energy to heat water to 110F is 6.2 kW

  • Average daily usage is 1.5 hours

  • Daily power needed to heat the water to 110F is: 1.5 hours x 6.2 kW = 9.3 kWh

Annual costs to heat water using a tankless water heater is: 9.5 kWh/ day x 335 days/ year x $0.11/kWh = $343

The total cost of heating water using a tankless water heater is $343 + $17 or $360.



> 87 gal/day x 8.33 lb/gal** x** 1 btu/lb-deg x 57 deg temp rise = 41,308 btu’s/day
> divided by 3413 btu’s/kWh
= 12.1 kWh/day used

> 12.1 kWh/day x 335 days/year x $0.11/kWh= $445.89 + $87 (yearly standby loss)= $532.89…a bit different than $736 in the article


The tankless instantaneous unit will still have to heat the same 87 gal/day of 57 degree water to 110 degrees and will use the same $445.89 electricity as the tank**. The only difference should be the** $87/per year standby losses.

SOOOO…Let’s come at it another way:

  1. The tankless claims to use 9.3 kWh/day = 31,741 btu’s/day

  2. 8.33 lbs/gal X 1 btu/lb-deg X 57 deg temp rise = 474.8 btu’s/gal

  3. Divide 1 by 2 = 66.9 gallons per day heated!!! A MISTAKE or A MISREPRESENTATION???

And they assumed that the tankless was 100% efficient but figures by better authors/orgs claim energy factors (EF) of from 0.82 to 0.94 for them. so add another 10-15% onto the $445.89 comes to $490.48 to $512.77.

versus… $532.89 for a storage tank

Something’s amiss in the article!!!

Brain I agree that tankless heaters are not for every home owner. Especially families of 3 or more. They do not save, annually, the extra cost of the unit and retro fit installation for most homes.
Good post.
When was this article printed, and by whom?

This month’s edition of Home Savvy E-zine that some HI’s subscribe to and email to their clients. I don’t think they attributed it to anyone in particular.

The articles clearly state that the information is provided by the U.S. Department of Energy and GoWithGreen.com.

No, most people are not. I am home 322 day out of the year. The rest of the time is spent traveling.

The math in the mini-article is shown step-by-step. If you dispute it, I recommend contacting GoWithGreen.com directly or the Multi-Housing Laundry Association (where GoWithGreen.com got their numbers).

Additionally, it appears that the original authors of the GoWithGreen.com mini-article rounded-up numbers. This is a common practice when providing editorial examples. They aren’t “typos”.

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When the payback time for installing a tankless heater becomes 5-7 years IMHO people will switch.

That will only happen if energy prices escalate quite a bit.
But that probably is going to happen when the environmentalists get their way. :frowning:

There are certainly benefits (unlimited hot water) but not much in the way of cost savings for now.

Oh yeah, Brian…

Great post…from everyone. Good eyes and brilliant insight.

It was up last month. Oh well. As I’m sure you can understand, we aren’t responsible for other people’s Websites though that is a bummer that their site is down.

While I have not attempted to research and recalculate all of the numbers above, I do agree with some of the facts stated. In particular, the amount of electrical energy (kwh) that needs to be transferred to a given volume of waterfor the same temperature rise is about the same no matter which electric heater is used. So the basic difference would be in the standby losses, which the original calculation says is about $87/year. However, even if that is correct, it is misleading because it does not consider difference in heater locations. For example, if a heater tank is located inside the heated space of residence in a predominatly cold climate, the ‘losses’ will reduce the heating load. On the other hand in some place like Miami, the losses will add to the AC requirement. Using ‘average’ numbers is always risky.

Now, having said all that about electric heaters, IMHO tankless heaters can probably only be justified, in most cases, if they are gas fired because in $/MMbtu electricity is considerably more costly than natural gas even at today’s prices.