This prompted some more research on tank v. tankless. The punch line is: hmmm.... There are lots of things to balance here: Efficiency of energy to heat, overall efficiency (including standby), cost efficiency, space efficiency, maintenance efficiency, life of product, carbon footprint, water efficiency, house design, numbers of occupants, local climate. Unfortunately, not much pulls in the same direction to where there is a clear winner. Again: hmmm....
Electric is the most efficient at converting energy to heat. Most report the efficiency at close to 100 percent if not 100 percent. Can't beat 100 percent (unless...well, hold on, we'll get to that). Standard gas-burning water heaters have efficiencies from 60 to 80 percent. Condensing gas-burning water heaters (gas heaters that harvest heat from the exhaust fumes) can approach 95 percent. Not bad, but not 100 percent. And then there are heat-pump electric water heaters that achieve (wait for it...) 150 to 250 percent efficiency! Even a standard electric can achieve a greater-than-100-percent efficiency with photovoltaics, so imagine what a heat pump water heater coupled with PV can do!
I'm vaguely head-over-heels in love with heat-pump water heaters. These heat-pump heaters harvest heat from the air around them to heat water and, as a by product, cool (and dehumidify) the air around them. That sounds good for hot ole Texas. This has prompted dreams of putting one in the pantry, opening the wall behind the refrigerator (which is also a heat pump, but is pumping heat out of it's interior and dumping it out the back), and having the water heater absorb heat (in part) from the fridge. This system heats the water and cools the room (good for the wine!).
However, for reasons that are not entirely clear to me, pretty much all the experts (including the Green Building Advisor) recommend using gas if you have access to inexpensive gas. And gas these days is pretty darn inexpensive and appears to be so for the next 50 to 100 years (there's a whole lotta, whole lotta fracking going on...). So I think the recommendation for gas comes primarily from financial concerns. Heat-pump water heaters are great, but if you use up your hot water, standard electric heaters fire up and suck up the juice. Unfortunately, I haven't been able to find a heat-pump water heater that uses gas as a back-up (a dream device?).
The folks at the American Council for an Energy-Efficient Economy produced this table that says electric heat pumps are the most cost-effective approach over the life of the technology:
but the table doesn't include a condensing tankless gas unit. Electricity in Austin goes for 6.02 cents per kWh over 500 kWh in the winter and 7.82 cents per kWh over 500 kWh in the summer. Gas goes for ??? Couldn't find on the interwebs...
Gary Klein is partial to tanks and loops. He's also partial to small diameter distribution lines. His point is that the more water you have stored in your lines, the longer you wait for hot water and the more water (and energy) you waste. First and foremost, when designing a house, you want your plumbing to be as close as possible to each other to centralize the hot water demand. Although this wasn't a driver for us in designing the house (and wasn't mentioned), the architects did this very thing (those sneaky green devils...). I'm assuming that the water heater, in whatever form it takes, will be in the pantry outer-wall area, so perhaps this is how the water gets run:
So.... as near as I can figure out:
If you have a choice between geothermal (i.e., you already have or are putting in a geothermal system), electric, and gas, the order of priority is (1) geothermal, (2) gas, and (3) electricity.
If you have a geothermal system, use a desuperheater.
If you have access to gas, use gas. If your primary hot water use areas are relatively compact, consider a condensing tankless system. If your water is hard (thus impacting the life of a tanked system), consider a condensing tankless system. If your hot water areas are more distant, consider a tanked system with a circulating pump.
And that brings us to [trumpets]: A tankless system! So after all the (appropriate) handwringing, here's the original (unpublished) blog post, still relevant after all:
We are currently spec'd for a tankless water heater, which seems to be what all the cool kids are getting (unless you're really cool; in that case, you have a desuperheater!). It shows up as an allowance in the builder's budget for $950 (the architects have it at $1,200). Given that it's an allowance, which one should we get?
I'm thinking a Rinnai, specifically a RC80e (KA2530WD-US) condensing tankless water heater:
Kinda looks like something Iron Man would have at his house. It has a 97 percent thermal efficiency (as compared to the 80 percent of some of their other models). Amazon has the sucker for $1,066.91!
Rinnai recommends a cold climate kit for those of us in the cold climates. The kit auto drains the outdoor bit of the system when the power shuts off. Although we don't live in the colder climes, it does freeze down here from time to time in the winter and, with the growing unreliability of our electric grid, methinks we should get one (the bride wants a back-up generator, but those cost more...).
More stuff on water heating to confuse you::
- Water heating.
- Heat pump water heaters come of age.
- Solar thermal is dead.
- Get rid of your gas water heater!