I have both a 50 gallon water heater and an 80 gallon water heater.. Both High efficency ones. The annual energy usage cost for an 80 gallon one is $411 while the cost for the 50 gallon one is $420
Both brand new!
same manufacture.
It costs less to heat 80 gallons than 50 gallons?
Replies
Maybe what they're saying is that it costs less to heat 50 gallons in the 80-gallon heater than it does in the 50-gallon heater. Are you sure the efficiency levels are exactly the same?
Just out of curiosity, gas or electric?
rich1
Electric
Think about the ratio of internal volume (thermal mass) to exterior surface area (heat transfer boundry).
All else being equal, I bet the 50 gallon has relatively more surface area per gallon to lose heat across than the 80 gallon does.
Forrest
I thought that too, but then I wonder what they are doing for a formula.If they are calculating standby loss per gallon used for some standard amount amount of usage, that would work, if they assume the same usage for both tanks. Use X gallons/year, standby losses equal X times loss factor Y.However that makes NO sense from a real world perspective and would be very shoddy figuring indeed. Not that I expect much more from energy ratings these days... grumble grumble.-------------------------------------
-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
If anything the standard usage figures for water should be the same or more for the larger tank.But frenchy did not give any model numbers so we really don't know what the two units are.Also we only have $ cost figures. Not the gas usage figures. It is possilbe that they where done at different times and thus use different "average price" for the gas.
BillHartmann,
They are both GE high efficency electric models with 98% efficency ratings. I could run down to the basement and find the model numbers, I suppose if it really means anything I will but since they have the same logo and come from the same company and etc.. It just would seem obvious to me that it should cost more to heat 80 gallons than 50 gallons, the fact that they say it doesn't drives me crazy!
"They are both GE high efficency electric models with 98% efficency ratings"I don't believe it.First GE does not make any water heaters. Rheem makes them. And they are only sold by HD. Now I can't find any place that I can get detialed information from HD.But looking at Rheem's sit they have 3 lines of electric WH's. A professional line, a residential line, and Marathon which is a fiberglass/plastic tank.But NONE with a 98% energy factor.Looking at their Fury series of WH's there are a number of models in that line.http://www.rheem.com/dealers/catalogRes_detail.asp?id=24They have tall and med High Energy models and they have tall, medium, and short standard energy models.Based on operating energy cost here is what you have.A tall, HE model, 80 gal83VR80-2, EF 92%, operating cost $411A medium, standard model 50 gal.82MV52-2, EF 90%, operating cost $420.Now in the HE models they have a tall 50 gal, 94%, $402
and a medium 50 gal, 93%, $406.And in the standard eff models the 80 gal tall is .86% $438But what they sell to HD may or may not be different, but just based on the size and opearting cost I think that those are the ones.
Bill Hartmann
Ok it's confusing enough for me to go down and check. One is model 249046 (50 gallon model) and the other is model 249813 (80 gal.) .. Both given as "high efficency models" In identical boxes which were differant than their regular electric water heaters. They are standing side by side in my basement and I am reading the data off each one.. I bought both of them at the same time from HD after checking many other places for both prices and efficency.
Rheem has several models with lower efficency numbers as do most plumbing supply houses. Thus I went with the highest efficency numbers I could find.. I suppose that you could go down to HD and read in their manual where they show the efficency numbers. I guess as "proof" I could go down with my camera phone & take a picture of their manual, go to the radio shack and buy a patch cord to get my computer to accept the link from my phone and then somehow figure out how to post that on this site.. But since I haven't been able to figure that out and I really don't feel an overwheming need to "prove" anything Please accept what I'm saying..
I could be wrong.. I honestly don't know and that is exactly why I posted this.
I did a large amount of research looking for efficency and now I find that either I've been fooled or something else is going on here..
For what it's worth the cost per KWH rate was given as .09 which is considerably higher than I pay. I intend when finished with the full house to put water heaters on standby energy which can lower the cost down to around 3.7 cents per KWH. that's really a red herring issue and I hope won't be confusing.
The energy consumption for both is given as 4881 KWH/yr. (50 gal) and 4775 for the 80 gal. Nope! I didn't transpose them. Lower usage for 80 gallon than 50 gallon..
The ironic thing is I'd intended to buy two 80 gallon water heaters and use one for domestic hot water and the other for in floor radiant heat.. use the second one as standby for the radiant heat back up. The rep at the store convinced me that 80 gallons was overkill.. save your money he said. It wasn't untill both were home and wired in place that I happened to read the data sheet attached to their side.
Prior to that I read all of my information from data sheets given to me at plumbing supply houses.. I guess my questions are as follows,
First,
How can a larger model use less energy than a smaller model given that both are "high efficency" models?
Second
If the data I read is wrong then how can Home Depot publish such data and get away with it?
Third,
If data sheets I read at Home Depot are wrong wouldn't data sheets from plumbing supply houses also be wrong?
Frenchy, I believe Hartmann gave you the correct answer a while ago:
A tall, HE model, 80 gal
83VR80-2, EF 92%, operating cost $411
A medium, standard model 50 gal.
82MV52-2, EF 90%, operating cost $420.
It appears those figures match yours. And yes, it wouldn't be misleading to say that 90% and 92% are both High Efficiency. Especially at HD, where you need a large grain of salt.
Clearly the slightly higher EF and whatever other minor differences more than compensate for the higher surface area of the 80 gal.
We're beating a dead horse.
DG/Builder
"First, How can a larger model use less energy than a smaller model given that both are "high efficency" models?"The term High Efficent is relative. Just means that they have a higher EF than some other model.And as the PDF file from Rheem show each unit has different EF numbers and they vary by the size of the tank, the shape factor (tall, medium, or short) and if they are the regular or "high" EF models. And then they have the "professional series" that are only aviable through plumbers don't have any designation of "high eff", but some of them equal the Fury HE models. Other are equal to the standard line."Third, If data sheets I read at Home Depot are wrong wouldn't data sheets from plumbing supply houses also be wrong? "I don't know that HD data is wrong.First you started out that the water heaters had a 98% EF. AFAIK there are none that have that high of rating. The highest that I have found is AOSmith/State/Sears which has some 95% models.So I don't know what HD is claiming.Look on the labels, on the nameplates, and in the instructions for Energy Efficency (EF) numbers.Now I finally traced down a list of all water heaters. Took me awhile to find it as it kept by the GAS appliance manufuacters assoc.http://tinyurl.com/lr4ctAnd I this is probably the models that you have. The numbers that you posted earlier are HD's stock numbers. These are the actual model numbers.For the 80 gal
SE80T12For the 50 gal either
GE50M06
PE50M09AAH00
PE50M12AAH00
Bill,
None of those numbers appear anyplace on my water heaters.. one of the instruction booklets that comes with the water heater has one of those numbers on it {50 gallon model} ( along with about a zillion others) but the instructions are extremly genaric, in fact the the instructions seem to be written not to inform but rather to confuse..
I think I can agree with the fact that high efficency means nothing really. I mean every single water heater I got information on claimed to be high efficency...
According to the chart they print out, my 50 gallon water heater is below the lowest numbers in total KWH while the 80 gallon is above the highest numbers they print out chart wise.
The chart I'm refering to is the little yellow energy calulation chart they glue onto the side of the water heater... where they have the high and low printed out with whatever the water heater you purchased identified..
The 80 gallon is above the highest while the 50 gallon was below the lowest..
If any of the plumbing supply houses had a higher efficency model they didn't trot out the numbers, because every single one I looked at was lower (which somewhat goes along with the high rating on the charts) as for being talked into taking a 50 gallon instead of the 80 gallon at the last minute when I had done all the research on 80 gallon models , well , shame on me..
I should know better.. I wonder if HD will take back a water heater once installed.
Mc Design,
80 gallons has more surface area than 50 gallons, more potential for heat loss.
No.
Imagine. . .
A cube two feet on a side - 24 square feet exterior; 8 cubic feet volume. Three square feet of "radiator" area per cubic foot of internal volume.
Now -
A cube three feet on a side - 45 square feet exterior; 27 cubic feet volume. Just over two square feet of "radiator" area per cubic foot. Assuming equal temperatures and insulation, less heat will flow out of the big cube per unit volume of water.
Forrest
"...less heat will flow out of the big cube per unit volume of water."
Forrest, you're stuck in a rut. What these guys are trying to tell you is that "per unit volume" is irrelevant. You are paying for TOTAL heat loss.
A bigger tank has more skin area, so more TOTAL heat loss.
Per unit volume is no more relevant than per person, per acre or per number of cars in your garage.
DG/Builder
Wait - now you're going to tell me that the whole concept of "economies of scale" is worthless? ;-)
Let's think of it this way:
This imaginary household uses 4000 gallons of hot water per month. That's 80 "fills" of the small HWH and 50 "fills" of the the large HWH. As the hot water for each "fill" is being heated each time, the HWH is also losing heat from the boundary area.
It will cost more to heat the same 4000 gallons in the 80 "fills" of the small HWH because the heat loss per gallon is relatively greater - just like the Energy Cost tag says.
Forrest
"This imaginary household uses 4000 gallons of hot water per month. That's 80 "fills" of the small HWH and 50 "fills" of the the large HWH. As the hot water for each "fill" is being heated each time, the HWH is also losing heat from the boundary area."No, the tanks fill as needed. HW will be used in 3 gal, 5, gal, 20 gal gulps depending on what the HW is used for.Basic laws of physics is that it will take the same energy to heat the water now matter what rate it is being used at as long as the total of amount is the same.What is different is losses during the other 95% of the time.And that is determined by the amount of insulation and the surface area.And as I showed the two units most like are of different series with different amounts of insulation. Also the tanks are available in different heights (for the same gals) and thus would have different surface areas.
Read Hartmann's and Hoover's explanations above. Both correct. If you can't understand those, it ain't gonna happen.
DG/Builder
My inner child says you're a poopy head.
;-)
Forrest
dang but that is so funny
We can imagine something that only exists in our heads, in a form that has no measureable, tangible reality, and make it actually occur in the real world. Where there was nothing, now there is something.Forrest - makin' magic every day
I like your sign-off tagline - I'm honored!
Forrest
just gotta love magic is allView Image
We can imagine something that only exists in our heads, in a form that has no measureable, tangible reality, and make it actually occur in the real world. Where there was nothing, now there is something.Forrest - makin' magic every day
My inner child says you're a poopy head.
My outer child (8 years old) has no problem comprehending that a larger surface area will leak out more heat :)
Your inner child needs to grow up and take some 7th grade Physics...
DG/Builder
Mc Design,
Hmmm,
Why then is the 80 gallon tank bigger than the 50 gallon one.. doesn't that mean it has more surface area?
Yes, but it has relatively less surface area for the 4000 gallons in the example. You can imagine a set of (50) 80 gallon tanks, and a set of (80) 50-gallon tanks, each set holding 4000 gallons. The set of (80) 50-gallon tanks have more surface area.
Forrest
True -- Another way to look at it is that surface area varies as the square of linear dimension, and volume as the cube. That's also why bacteria don't need complicated lungs and intestines like us big animals. ;-)
-- J.S.
I question how much of the heat loss is actually calculated into the annual usage cost. Or perhaps with a high efficiency unit that number is small or at least, the difference in heat losses between the two containers is small.
My guess is that they are assuming the average annual hot water usage for the average family of 2.6 people is so many gallons. The cost to heat that much water with that particular hot water heater is about the $400 plus dollars. Makes no difference to them that someone buying a 80-gallon water heater may use more hot water in a year than someone who bought a 50-gallon water heater.
That seems a bad assumption but that all depends on why one chose to install the 80-gallon unit.
Frenchy,
You have simply discovered the uselesness of such "estimates". They are inconsistent, based on no standard criteria, as far as I know, and are nothing that can be verified by actual use.
Both 'high effiiency"
WHat makes yo think they are "high efficiency?" Have a draft inducer? That doen't make a water heater high efficiency - it maks it side wall ventable.
I spent asome time las winter comparing specs on different water heaters and there isn't much difference
Bob Walker (from another laptop)
paradigm
Both electric
Only nine dollars difference is real close to a wash. There are a lot of variables we don't know, like how much water you use from each, and where they are. Especially what are the ambient temps in the spaces where they are?
-- J.S.
John Sprung,
Both side by side! I'd assume the calculation is done in a lab under similar conditions..
John Sprung,
I would assume that the calculations are based on similar usage.
Frenchy,
The Estimated Annual Cost of Operation is based on the Energy Factor (EF) number and not directly on the efficiency (which refers only to how efficently the water is heated and does not account for standby losses as does the EF). My guess is that the larger unit has a slightly higher EF due to better insulation.
Bill
"The Estimated Annual Cost of Operation is based on the Energy Factor (EF) number and not directly on the efficiency (which refers only to how efficently the water is heated and does not account for standby losses as does the EF). "Could you reword this. I think that the there are two many "ands" and not sure which phrase refers to which part of the sentence.BTW look at;http://forums.taunton.com/tp-breaktime/messages?msg=77125.18I think that I have IDed the models and you are right they have different EF's and neither is the 98% that frenchy claims.