Hello all.
My new plumber says that the existing 10 yr. old 100gal.water heater
is as efficient or more efficient (meaning uses less gas) than a new
flash gas heater. The house had two BIG roman tubs. We are
remodeling and got rid of one. Now there is a shower and one big tub.
My plumber says his supplier told him that the customer would not be
happy with his gas bill if we install a new flash heater. What are
your experiences? I have installed a couple of flash heaters on other
jobs and have never heard a complaint about energy usage, just
happiness that two people could use the shower at the same time. I
never asked about energy consumption though. Thank you for sharing
your experiences with me. Rob Z
Replies
We install the Rinnai brand of propane demand water heater and they are 86% eff compared to 70% and less for the best tank heaters that have a flue up through the middle of the tank. You won't save enough to pay for the switch but it is the more efficient option.
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"You cannot work hard enough to make up for a sloppy estimate."
Thanks for the reply. That is about what I thought. I don't know where my plumber is getting his info.
I'm pretty sure that tankless and gas tank heaters are roughly equivalent in terms of efficiency. The flue in a tankless is directly above the heat exchanger, just like the flue in a tank unit. You can get both in the ~85% range.
Where you save is in standby loss. If you have few users and there are long periods without hot water use, there is a savings with a tankless. If you have four teenagers and little standby time then there would be little savings.
Where you really save is in terms of space. If you have nowhere good to put a tank, a tankless can be a great thing.
With gas heaters it's more of a horserace than with electric. An electric tank can be 100% efficient with virtually zero standby losses, whereas a gas unit (tank or tankless) struggles to get over about 85% and the gas tank is harder to insulate well.Also, with an electric the input BTUs is fairly limited, limiting max flow rate for tankless and making the tank much more attractive. With gas you can have fairly high input BTUs, so achieving a good flow rate is relatively easy.So for gas it mostly comes down to initial/maintenance cost vs space saved. In some cases the tankless will save a little in gas, but probably not enough to pay for itself. The tankless will also have a little harder time maintaining a constant temp, if that's a big issue for you.I'm guessing that a 90+% gas tank could be made (probably is somewhere), but the initial cost is not justified by the savings in operating efficiency. A gas water heater just doesn't consume that much gas in most cases, compared to a gas furnace.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
I agree with you that the savings is in the standby loss but that is figured into the published efficiency standards. The highest efficiency rating I remember seeing is the State 50 gallon direct vent which was 70 percent. When the burners turn off the hot water in the tank creates a thermo-siphon effect that draws cold outdoor air up through the heat exchanger and cools the tank. The same thing happens in a tank-less but it just cools a small amount of water that is left in the heat exchanger from the last use. This creates a cold water sandwich effect where the water in the pipe leading from the heater tot he shower cools much more slowly than the water in the heat exchanger and when some one else calls for hot water shortly after another person took a shower that little bit of cold water flows down the pipes and causes a brief moment of cooler water on the back of the person taking a shower. This is a real problem with tankless water heaters that you should know about before installing one. If you have copper pipes that copper may hold enough heat to mitigate this problem, its more of an issue with well insulated pex piping.------------------"You cannot work hard enough to make up for a sloppy estimate."
Edited 8/8/2007 9:31 am ET by ShelterNerd
Standby loss is figured into the EF of the water heater for one particular scenario. The more you use the water heater, the smaller the standby loss as a percentage of energy usage. The EF factor cannot take into account all possible scenarios and the standby loss is a significant percentage of energy when you use a small amount of energy, and it's not signficant as a percentage when you use more. So technically speaking, as your usage increases beyond whatever the EF was calculated at, your EF would increase.
However as an absolute number, standby loss would cost the same amount of money in either case.
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-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
I have also considered installing a flash heater. I also cannot find more than anecdotal testimony as to the overall efficiency. A major factor has to be keeping the water hot all the time and the number of showers per day and length of showers still there has to be some solid facts to start with.
In the realm that includes 100 gallon (i.e. commercial) water heaters, gas fired condensing units with efficiencies in the milddle to high 90's (% combustion efficiency, not AFUE) are readily available. They are expensive. A modulating, condensing boiler with an indirect fired water heater, also will produce copious amounts of 140 degF water while operating on the over 90% efficiency range.
Its unfortunate that the American consumer has not demanded better in the residential markets and that the energy efficiency codes, where they exist and are enforced, have not mandated condensing technology (i.e. 90+ efficiency) on new gas fired heating appliances.
Most water heaters are not rated in terms of AFUE, but simply "energy factor". This is done purposefully, to be somewhat decieving and to mask the gross inefficiencies of the basic residential unit.
OTOH, the tankless water heater, while thermally inefficient in all but the most recent incarnations (my Bosch AquaStar operates at 65% combustion efficiency, now used to provided hot water to tubes in my garage slab), will provide how ater as long as fuel is available, but the do not react to transients quickley enough. Two people can take individual showers simultaneously, but when one stops before the other, someone will get a slug of hotter than desired, then colder than desired water. I hated the tankless in the house. The savings, (due to no standby losses) for me were real, but minimal. I would never recommend a tankless except dedicated to a single, large fixture.
Tim, that's a little unfair.AFUE is meant to measure performance in a heating mode. Partially oversized heat source, no inherent water capacity to note, variable firing characteristics (part loads, changing load sizes, etc).A water heater fires under much more limited scenarios. AFUE testing wouldn't even apply, just like EF determination would not apply to a boiler used in a heating mode either.They are two different types of usage, two orders of magnitude different in their volume of usage, and could never both be addressed by one set of testing.Having AFUE testing for water heaters intended for use in a heating system would be cool, though, definitely. The flip side would be good too.. what's the EF of a cast iron boiler vs a mod con heating an indirect? I bet they are much more different than you might expect.-------------------------------------
-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
What's unfair? The combustion efficiency is based on (my) performance measurements. It simply is.
Regardless of how the heat of combustion is ultimately utilized, the ability of the device to capture and transfer a usable amount of heat from a fuel is the most basic efficency comparison parameter. The net heat output of a system, divided by the gross heating value of the fuel going into the system is a simple ratio of "what you get/what you pay for". It is simple and fair.
As a domestic hot water system goes, there is no good standard by which to objectively compare products that differ in such a basic operating approach as do the storage tank unit vs the tankless unit. I did say, that while I did not prefer the general performance and operating limitations of my tankless, it saved me money. W
"AFUE testing for water heaters intended for use in a heating system would be cool.." This is available from the manufacturers that have units intended and specifically listed/rated for comfort heat applications. At least it was available from Bradford-White when I was associated with that company years ago.
Yes, but AFUE is not combustion efficiency either, and combustion efficiency would not be the ultimate arbiter of heating system efficiency, because combustion efficiency varies under different firing situations, so when would you measure it?on a water heater, it might be more fair, since it has more limited firing patterns. But for a boiler? combustion efficiency would be grossly misleading if it were the only thing used to determine the efficiency of the boiler. It might have 100% combustion efficiency, but if most of that heat goes to heating up the boiler, and is wasted after it shuts down, for example, it doesn't tell you much.-------------------------------------
-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
It would be worthwhile if some "authority" were to establish 3-4 realistic "profiles of use" for water heaters and then the heaters were tested to those. Of course that would be TMI, and the mfgrs wouldn't like it.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
We're not talking about mod con boilers.
The gross inefficiency to which I refer, is simply a matter of cheap products. The basic tank water heater has a burner under a tank, with a hole running up the middle of it, and little else, short of some very simplistic controls. Regardless of how you dress it up or cover it up, this is an inefficient piece of junk.
In an era inwhich we have to have condensing units that are 12 SEER or better on residensces, the EF's of less than 0.8 are pitiful. For that matter, AFUEs less than 90 ought to be phased out, prohibited in all new construction and no longer manufactured.
Then you're saying oil shouldn't be allowed as a fuel anymore... not exactly a reasonable stance. Though I might still agree with you.However, if you have a small load, a true efficiency of 95, 85, 80 or even 75 may not really make much of a difference. I run into this regularly. The difference in those numbers could be $100/year on some systems... less, sometimes. So the owner should be forced to spend thousands more to gain a 30 year payback? Hell, on DHW it's *certainly* not worthwhile to spend thousands to gain 10% for an average family. The loads simply are not high enough to justify it.On full systems I'll stand by your side and praise the virtues of modulating technology. But water heaters have their place, and they perform better than their EF's indicate in heating systems, typically. Not so good that a mod/con doesn't make a lot more sense on most full size systems, but they do have a place in small load situations, which can mean in mild climates, superinsulated homes, small areas, or some combination of those factors. All assuming "typical" DHW loads.-------------------------------------
-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
Not being from or in the northeast, I cetainly have no preference for oil. The attachement to ancient technologies and fuels is the reason I said Phased Out, not criminalized.
IF higher efficiencies were mandated (for new construction and replacements), the cost of the more efficient units would tumble.
Why keep stooping to the lowest common denominator? Besides, if we saved 10% on domestic HW across the board, that would be a staggering reduction in fuel usage for the nation. The additional cost for 90% furnaces over 80%'ers is hundreds not thousands. A 50 gal GWH with an EF of 0.8 does not cost thousands of dollars either, maybe $1000 vs $500.