My short answer.
Have fun everyone.
Theory only works perfect in a vacuum.
Anything that has a fast large change in temperature is under a lot of stress.
Using the wrong material is a disaster.
You should look into what the heater is made of, and figure the cost of maintaining it.
There is more to the formula than fuel savings.
Last edited by DonL; 09-27-2013 at 07:11 PM.
Theory only works perfect in a vacuum.
If you can't perform the annual demineralization on the tankless, the cost savings (if there are any) over a traditional tank will be negative, especially when you consider the rework of the utilities...that service will take at least an hour of labor. If you wish to do it yourself, you'd have the first-year equipment costs, and learning curve.
Important note - I'm not a pro
Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013
Apparently there are lots of people who don't like these things. Everyone is entitled to their opinion, but opinion is not a substitute for proven performance. After two years I don't see anything but great performance, economy and convenience from my heater. I guess we'll see how it's working in 10 years.
Because I live in an area with very hard water I installed a water softener. This has been great for ALL the plumbing in the house, especially the water heater. So I expect to do little to no maintenance on the heater, although I will flush it with vinegar every few years despite it not being strictly necessary.
Last edited by lifespeed; 09-29-2013 at 05:37 PM.
Lifespeed: I definitely don't hate 'em (I even heat my house with one!) but I don't have rose colored glasses about their performance or their economics either. Properly maintained you can get 20+ years out of most of them, and you never run out of hot water. Limitations of max flow rates is their primary down side for tub-filling apps, but since you never run out of hot water you CAN fill a tub of arbitrarily large size, if you have the patience. For most of the past 25 years I've used gas fired tankless HW heaters for my hot water (both in the US and in Europe)- my current HW heating situation is an exception (unless you argued that because the heat source for my indirect happens to be a tankless HW heater it should somehow count, even though I've configured it as a boiler.)
I don't know what hot water heaters come for free, and would not insert a zero-cost into a lifecycle cost comparison even if it were subsidized to the point where it were indeed free. Taking into account competing subsidies may be appropriate at the point of an particular buying decision though. I assume unrealistic long lifespans or zero maintenance costs tanks either. Apples-to-apples the economics aren't there at buck-a-therm gas, even if you add on a (reasonable) carbon cost to cover some of the externalities.
Finned water tube heat exchangers have high tolerance for big delta-Ts and rapid slewing of temperature & heat rates- that's the least of my concerns regarding tankless water heaters. There is over a century of history on this type of heat exchanger- they can definitely take it.
But the toll on efficiency of low volume draw short-cycling is real, as is the electrical power use (even in standby mode) that often doesn't reach the surface many forum type discussions of tankless HW technologies (not that it concerns me very much for this application either.)
PG & E, the Davis Energy Group, Lawrence Berkeley National Laboratories, et al as well as the US D.O.E. have tons of actual monitoring data on comparative systems, both in lab bench testing, and in-situ measurements in occupied dwellings. This was done quite rigorously in CA in order to determine how much subsidy would be appropriately carried by the retail rate-payers, and how much the utilities could earn on subsidy-investment in customer efficiency. In the CA case the analysis even went as far as to determine how much the efficiency saved the utility & ratepayers in lower market price for fuel, and for the diminished or increased infrastructure capital that comes along with tankless water heaters. This was not a one-off simple process, and it took well over a decade to get there, but the alternatives are now well understood, way beyond marketing hype or armchair opining of either advocates or detractors.
If you're going to count your personal perceptions as "...proven performance..." of "...great economy..." we'll need to see the test data & financial analysis on both your current & prior systems, and not just take your word for it, eh? ;-)
For now I'll take the word of the engineers and economists who spent a chunk of their careers on it. If you want to PM me your email address I can send you the PDF of a presentation one of PG & E's engineers (Robert Davis, PE)
delivered at the June 2008 ACEEE Water Heating Forum entitled, "Testing Water Heaters with Different Draw Profiles". It's not the slickest presenation ever, but it does a pretty good job of describing the real operational efficiency differences of at least a handful of realistic options (including a condensing tankless, as well as condensing tanks), under different scenarios. Yes, they're more efficient, than bottom-of-the-line tanks, but not by nearly as much as we all wished they were.
Just a quick update:
1) You guys have convinced me to keep the tank heater instead of replacing it with a tankless.
2) Any suggestions on a good 75 gallon tank heater?