Guest
On Feb 13, 6:14 am, "Only Just" <ifixit2@hotmail(dot)com> wrote:
No, the tests are done per EPA rules and specs by an independent
testing lab.
Now, on the other hand, those tests are performed under a certain set
of criteria based on an estimate of how the water heater would
actually be used. I would say the difference in how someone will
actually uses it in practice as compared to how it was tested, could
easily outweigh small differences in the recorded test data under the
controlled conditions. I would bet that a heater rated at an eff of .
61 vs one at .58 could easily be a wash or even upside down in actual
use. Same thing for the first hour rating.
In other words, over analyzing this whole thing is likely a waste of
time. Even the above payback analysis is flawed, because it ignores
the time value of money. Laying out $135 today and getting it back
over the next 6 years doesn't account for the fact that the money
could be earning a return. Or if you put the heater on a credit card
and pay interest, even for a short time, and the perceived savings are
gone. Take any of that into account and the difference between these
heaters shrinks.
"Donna Ohl, Grady Volunteer Coordinator" <donna....@sbcglobal.net> wrote in
messagenews6xsj.587$fX7.417@nlpi061.nbdc.sbc.com...
On Wed, 13 Feb 2008 06:16:38 GMT, Donna Ohl, Grady Volunteer Coordinator
wrote:
The math is all spelled out for us in the PDF at:
http://tinyurl.com/38eh4d
http://www.gamanet.org/gama/inforesources.nsf/vAttachmentLaunch/C2AAF...
Since almost all of us were dead wrong on this one, it would be nice if
someone can read that reference document and check my calculations
PLEASE DOUBLECHECK THESE HOT-WATER-HEATER CALCULATIONS!
Here are the necessary calculations I believe we need to make in order to
compare two gas-fired water heaters. All calculations are courtesy of the
referenced PDF and charts atwww.gamanet.orgexcept the energy costs which
are courtesy of PG&E athttp://www.pge.com/tariffs/pdf/G-1.pdf
In reality, these calculations would need to be done for at least a
half-dozen different water heaters (two each from Lowes, Sears, and Home
Depot, for example) in order to properly choose an available water heater
intelligently.
For simplicity, I'll use the values I plugged into the calculations for my
home; yours may differ but the mathematical approach is exactly the same..
0. Calculate minimum legal efficiency (EF)
1. Calculate maximum peak-hour requirements (FHR)
2. Calculate total installed price ($/installation)
3. Calculate yearly operating costs ($/year)
4. Calculate payback period (months/installation)
Here are my calculations.
PLEASE CHECK FOR ERRORS OR OMISSIONS AS WE ALL ARE LEARNING FROM THIS!
0. Calculate the minimum energy factor required by federal & local law:
(based on October 1990 Dept. of Energy Test Procedure for Water Heaters as
published in the May 11, 1998 Federal Register.)
For gas-fired residential water heaters, the minimum energy factor is:
Minimum EF for 40-gallon water heaters = 0.67 - 0.0019 * 40 = 0.58
Minimum EF for 50-gallon water heaters = 0.67 - 0.0019 * 50 = 0.59
Note: Southern California uses different legal minimum numbers than
northern California so be advised to modify the calculation for your area!
1. Determine your peak requirements:
20 gallons per shower x 2 showers in one hour = 40 gallons per hour
20 gallons per bath x 0 baths in one hour = 0 gallons per hour
2 gallons per shave x 0 shaves in that hour = 0 additional gallons
4 gallons per personal wash x 0 = 0 additional gallons
4 gallons per shampoo x 0 = 0 additional gallons (do it in the shower)
4 gallons per hand dishwashing x 0 dishes = 0 gallons
14 gallons per dishwasher load x 1 load = 14 additional peak gallons
5 gallons per food preparation x 1 meal = 5 additional gallons
26 gallons per wringer wash x 1 load = 26 additional peak gallons
32 gallons per automatic wash x 0 loads = 0 additional peak gallons
-----
TOTAL PEAK GALLONS = 40 + 14 + 5 + 26 = 85 gallons First Hour Rating
2. Determine price installed (inclusive):
Sears 33154 is $882 ($449 for the heater + $433 for full installation)
HD 183-717 is $747 ($349 for the heater + $398 for full installation)
3. Determine yearly operating costs given your basal energy unit, FHR, EF,
and average-use assumption.
For simplification, I'll only compare two heaters but the calculator we
create needs to cover at least a few at a time. I did this in Excel for
current northern California baseline (Schedule G-1 Residential Service)
energy rates.
CHOICE A:
Sears 33154 (marketed as Kenmore but made by AO Smith) FHR=97 EF=0.63
(41,045 btu/0.63)($1.21106/therm * 1 therm/100,000 btu) x 365 = $288/year
CHOICE B:
Home Depot 183-717 (marketed as GE but made by Rheem) FHR=80 EF=0.58
(41,045 btu/0.58)($1.21106/therm * 1 therm/100,000 btu) x 365 = $313/year
Note: Irrelevant specs would have been tank size, burner BTUs, gallons to
recovery to 90 degrees in one hour, etc. as the only figures that matter
for the calculations are the EF and the FHR since they take into account
all other design-size specifications such as those you quoted.)
4. Determine payback period:
a. Additional cost of more efficient model = $882 - $747 = $135
b. Annual savings of more efficient model = $313 - $288 = $25 per year
c. Payback period = $135 / $25 * 365 / 30 = 65 months (5.4 years)
5. Determine overall savings:
The two water heaters compared in this simplified calcuation would be
equivalent in overall costs at approximately 5 and a half years.
Considering the average water heater lasts 13 years, total savings for the
more expensive yet more economical heater would be:
(13 years - 5.4 years) * $25/year = $190 savings overall
6. Choose the correct water heater:
Based on the math everyone should perform when selecting the proper water
heater, I would buy the Sears 33154, which will save me almost 200 dollars
over its lifetime over the Home Depot 183-717 assuming current energy
prices and average usage.
Note: In reality, one needs to compare at least a half-dozen water
heaters;
here I only compared two for simplicity. I knew none of these calculations
just two days ago, so, PLEASE CHECK MY NUMBERS after reading the reference
document I refer to.
If the numbers hold water, then this should go into the alt.home.repair
FAQ
for everyone to benefit from all our efforts to understand how to properly
size & select a home water heater replacement (yes, I know there are
physical size issues also but this tutorial is already too long to go into
those details).
Donna
This is all very good but can you trust the figures given by the
manufacturers as these are often derived by engineer calculations and
marketing guru's and are manipulated to suit their agenda.
an example of this is how many people ever get the rated fuel consumption as
stated by any vehicle manufacturers, also appliance manufactures have
devised ways of confusing the figures to get a better rating that it
actually uses. The best way to purchase the water system is to get one that
suits your requirements as in size and water capacity.
Justy.- Hide quoted text -
- Show quoted text -
No, the tests are done per EPA rules and specs by an independent
testing lab.
Now, on the other hand, those tests are performed under a certain set
of criteria based on an estimate of how the water heater would
actually be used. I would say the difference in how someone will
actually uses it in practice as compared to how it was tested, could
easily outweigh small differences in the recorded test data under the
controlled conditions. I would bet that a heater rated at an eff of .
61 vs one at .58 could easily be a wash or even upside down in actual
use. Same thing for the first hour rating.
In other words, over analyzing this whole thing is likely a waste of
time. Even the above payback analysis is flawed, because it ignores
the time value of money. Laying out $135 today and getting it back
over the next 6 years doesn't account for the fact that the money
could be earning a return. Or if you put the heater on a credit card
and pay interest, even for a short time, and the perceived savings are
gone. Take any of that into account and the difference between these
heaters shrinks.