The biggest question mark is what the future will bring on supply/demand relationships for energy and taxation. I remember hearing a virtually unanimous chorus of doomsayers during the energy crisis of the late 70's proclaiming that the price of oil could ONLY go up, since the world's supply was finite and demand was skyrocketing. Despite that, 30 years later, the inflation-adjusted price of oil is LOWER than it was then. As my econ professor used to point out, you begin to realize the capacity of the marketplace to increase supply when you discover that the number of Rembrandt paintings in museums around the world is substantially higher than the number of paintings that the artist actually painted in his lifetime!
On another subject, Consumer Reports magazine's latest issue has an interesting article on high-efficiency water heaters and suggests that hybrid heaters are a good solution. At eight years old, my existing heater probably has no more than two more years of life. Based on how I described my low usage, would replacing it with a hybrid be worth considering when the time comes?
Thanks again for your insights.
Bob
In 1980 the US essentially WAS the oil market, consuming some ridiculous fraction of the world output, and US energy policy essentially drove the world price. That will not be the case in 2020. With the deregulation of oil prices and the unprecedented & massive shift toward higher-fuel economy cars in the US in the early 80s the prices crashed and stayed low until the fleet of econo-boxes got replaced by SUVs & pickups. The US consumer was the tail wagging the oil dog. In 2000 the US was still consuming about 25% of world production, China & India combined were consuming barely 10%. But by 2020 we'll be less than 20% of the total oil market, and the emerging Asian tigers will be matching or exceeding our consumption- US energy policy will still have an effect on world oil pricing, but not nearly as much as it did in 1980. (Past performance is not a guarantee of future returns.)
While clever methods have been developed to squeeze the last bits out of aging oil fields, exentending their useful life (and expanding the known reserves), the days of just poking a hole in the ground and pumping are long gone. Maybe we should start burning Rembrandts instead...
(The deep water stuff has yet to be exploited but it's not cheap to develop.) Experts will differ on "by how much", but nobody I know of in the industry is expecting oil production to keep pace with increased world demand (predominantly from increases in Asian consumption) at even $150/bbl of a coupla summers ago, let alone at a sufficient pace to keep it trading in the $70-80/bbl range. Even if the US & Europe embark on a massive electrification of the transport sector it can probably only change the slope, not the average direction of the oil price curve over the next decade. Inflation-adjusted oil prices have never fallen back to 1970 levels- being at-parity or slighly lower than 1980 is nothing to crow about. The near doubling of oil-price between 1979 & 1981 was about OPEC price-fixing. The price spike of 2008 was driven by anxiety about pumping capacity vs. world consumption rate. World consumption rate has (temporarily) fallen, storage tanks everywere are near capacity, keeping a lid on prices. But the pumping capacity hasn't changed appreciably, nor will it in the forseeable future. As the world economy picks up, so will world demand- the problem hasn't exactly gone away.
The warranty on your heater may be up in a coupla years, but if you replace the sacrificial anode and don't have ridiculously hard or acidic water there's plenty of reason to believe it'll last another decade or more.
Whether a hybrid HW heater makes sense to you depends a lot on what you pay for electricity, gas, and how you heat your home in winter. They all totally suck for recovery time and first-hour gallon ratings (compared to gas fired tanks.) Since they pull the heat from the surrounding room as well as from the compressor's motor, they represent a net heating load during the heating season, but reduce the cooling load slightly during the cooling season (albeit at lower efficiency than your central AC.) Odds of it ever being cheaper to operate than a standard efficiency gas-fired tank are low unless you have very cheap electricity. They have COPs of ~2.0 -2.2-ish, but if you're paying many times the price per source BTU with electricity as you do for gas, it may not exactly be cheaper to run. Standby losses will be lower though. eg: 15 cents/kwh electricity is the same as $4.40/therm natural gas. (29.3kwh ==1 therm). If you can truly get a 2x efficiency out of the hybrid at your low usage (maybe, you can, but it's probably lower) the hybrid would then be like $2.20/therm source fuel, and if a pretty-good gas-fired tank delivers 50% efficiency in your app you'll about break even on operating cost with $1.10/therm gas. If you have 10cent electricity it might be a good deal, if yours (like mine) hits 20 cents during economic good-times, maybe not.
If you heat your home hydronically (pumped hot water, whether radiant floor, fin-tube baseboard, or radiators) you'll get better average overall efficiency with an "indirect" tank running as a zone off the boiler. With a low-mass boiler the summertime efficiency will be somewhat better than your existing tank (lower standby losses, since there isn't a center-flue convecting heat out of the tank, or a pilot light.) During the heating season it improves the overall AFUE of the system by giving the boiler a higher, more efficient duty-cycle. But if you're heating with heat pumps/force-hot air, steam, etc. fuggedaboudit.
The CU isn't exactly the best source on this sort of stuff (the hack-job they did on the tankless-on-demands a few years back depended a lot on high-volume use, and exaggerated some of the quirks of tankless units.) The high standby loss of gas-fired tanks makes them far less efficient for low-volume users such as yourself. With low-volume use, atmospheric-drafted tanks with pilot ignition only pull ~ 40-45% efficiency. See:
http://old.aceee.org/conf/08whforum/presentations/1a_davis.pdf
Mind you, the test setup is less than ideal- they didn't fix the standby loss of the uninsulated plumbing, but perhaps that was intended to mimic the typical "as installed" situation. You may find the effect of R2 insulation on solar fraction in the system tested here an interesting datapoint (see the discussion on page 3)
http://www.fsec.ucf.edu/en/publications/pdf/FSEC-CR-1856-10.pdf
The effect of pipe insulation is significant in any stored-heat situation, but still counts for tankless units.
If you can tolerate the quirks of tankless on-demands, they'll have the lowest operating cost, but the bigger-deal units cost quite a bit, and still have a not-insubstantial electricity standby cost, but their efficiency doesn't fall straight off a cliff at low volume use the way tank heaters do. The unit I keep recommending for off-gridders is the
Bosch 1600H, since it has no standby loss, no standing pilot and decent (but not rebate-worthy) 0.80EF efficiency (below the 0.82EF cutoff for subsidy), doesn't require expensive venting (cheap galvanized B-vent will do), and you don't need to run electrical circuit to run it. It'll come in at half the installed cost of it's microprocessor-controlled cousins, if still 50% more than a 50 gallon tank, but by going from 40-45% to 70% (real-world efficiency) it'll use dramatically less fuel. If you have very hard water you'll need to de-scale the thing on a schedule (probably annually- more often for high volume users.)
The quirky bits about on-demands in general is that there's a hesitation on ignition, so a slug of cold water gets through between the warm-ish water from the previous draw before the newly heated hot water shows up (the "coldwater sandwich".) Much is made of this, but in relatively warm-water areas (like Sacramento, as opposed to Winnipeg) it's not very jarring. The other issue to watch out for with a unit like the 1600H is that it's lowest-modulated fire isn't super-low, so in summer when the incoming water from the street is warm and the flow out the tap is low the heater itself may have a hard time regulating the temp or even flame-out to keep from overheating (you have to run the flow stronger than you might normally do for hand washing or warm-rinsing.) When used with very short-draw appliances like many new front-loading washers that pull a pint at a time, the ignition delay can even be long enough that very little hot water actually makes it to the appliance, etc. But for things like showers, filling large tubs, you'll never run out (as long as you pay your gas bill.) Many of the more expensive tankless heaters modulate down low enough that some of these issues won't be a big deal, but some might. While the high-fire output of something like the 1600H isn't going to support 2 showers and a clotheswashing draw simultaneously, it'll be plenty for your application, at least until you install a 4-sidespray 12gpm shower (at which point the raw BTU-boost of drainwater heat recovery becomes more important than NPV.)
If you replace it with another gas-fired tank, going with a forced-draft & electronic ignition unit that scores over 0.65 on an EF test is still worth it. The convection losses at the flue are lower, and you don't have a pilot burning 24/365, and you may go from 40-45% as-used efficiency to ~50%. Still low efficiency, but a 15-25% fuel savings.