Getting into Hot Water part 1

I've been a bit lax in posting but not because I haven't been working on the house! I've just about finished a new woodshed designed to hold two cords of wood, with the idea that we use one cord annually and the other is getting well-dried.

Once we said bye-bye to the Buderus boiler, we needed a solution to making our domestic hot water (DHW). I tossed in a 10 gallon electric water heater I had as a temporary solution. Jill and I needed to time our showers with other uses of DHW or we'd get chilly. We went along for a few weeks like this.

Once we eliminated fossil fuels as the fuel source for making DHW, our choices narrowed to using electricity or solar. In some cases, such as our former home in NH, biomass can be an option. We used a heat exchanger in our old pre-EPA VT Castings woodstove and a thermosyphon solar water heater with a back-up electric resistance element in that system. More info here. But that solution today either requires using a dirty old stove, or possibly finding and importing a European woodstove that includes a DHW coil while retaining a clean burn. As I wrote in The Right Target part 2 I didn't anticipate that we would be burning wood the whole heating season, but rather use wood where it makes the most sense - during the coldest portion of the year - and use the heat pump in the swing seasons, where it is more efficient and keeping a stove going is challenging. The restricted time of use makes wood-fired DHW less compelling.

I decided that whatever technology we would use to heat water would require a sizable, very well insulated tank, so I ordered an 85 gallon Marathon electric water heater. The Marathon has a polybutylene inner tank liner, which is durable enough for the manufacturer to offer a lifetime warranty. The insulation is continuous closed cell foam, and the outer jacket is plastic, so no rust. The Marathon heater doesn't come with an internal heat exchanger, unlike tanks designed to be heated with an external heat source such as a boiler, solar thermal collectors, or stand-alone heat pump, so I knew that whatever I did I would have to implement a heat exchanger as part of the solution (if I chose to do something other than use the Marathon as an electric resistance water heater.)

One aspect of the decision about what technology to choose to make DHW is how much hot water is needed. I lived in two successive houses I had built, for thirty years, with passive solar hot water systems that were built into the design of the homes and used no pumps, power, controls, or antifreeze. These systems can't be easily retrofitted into a conventional home. This means that a solar thermal system will typically have some of the above-mentioned cost-and-complexity adding components, which raises the cost. Professionally installed solar thermal DHW systems in New England seem to end up costing $6,000 to over $12,000, depending on size and difficulty of installation. Some of that cost is relatively fixed, regardless of system size, and therefore the economics of solar DHW is dependent on the DHW usage (and of course, the cost of energy - natural gas is much less costly per energy delivered than the fuels available on MV, which are fuel oil, propane, and electricity). Jill and I use an average of 13.3 gallons of DHW at 120F per day. With an average annual incoming water temperature of 60F (an estimate, may be somewhat lower over the year, I just have summer and early fall data) this is just about 2 kWh/day of DHW load. Add to that perhaps 1-1/2 kWh/day of heat loss off of the tank and piping near the tank (it's insulated - we'll get to this), for a total of 3-1/2 kWh/day. This is close - we were using about 3 kWh/day into the much smaller and much worse insulated 10 gallon electric water heater we used for a few weeks. So 3-1/2 kWh/day is just under 1,300 kWh/year, at a cost on MV of just under $250. This 1,300 kWh/year is the annual output of roughly 26 sf of good collector, and if we did have this small collector it wouldn't make all the DHW we need because it would make more than needed in the summer and less in the winter. But a single 26 sf collector is a very small system that still needs mounting racks, piping from the collector from the roof to the basement, pumps, controller, heat exchanger, pipe insulation, and installation labor - plumbing, wiring, solar installing - which don't scale with system size. If we were a larger family and, instead of using a somewhat European amount of 6.6 gallons/person/day, we used 15-20 gallons/person/day, we'd be figuring on an 80 sf system that might cost less than twice as much as the 26 sf system, and would deliver more than three times the usable energy - very different economics.

I looked at some interesting and innovative solar thermal systems in this process, and I'll write about them in the next post. I didn't initially choose solar DHW, in any case, so there's a bunch more to say.

Comments

[David Fay]

I'm impressed with how little hot water your household uses. Our household of two uses much more: 5.7 kWh/day and we spend much of the summer away in Maine. How do you do it?

And how do you measure your hot water consumption? I can track mine only by how much electricity it takes to heat it (we have a separate electric meter for DHW). But it would be nice to know how many gallons we use.

David

[Marc Rosenbaum]

Short showers? I'll speak more about measuring soon, but for now, I installed a DLJ water meter on the inlet water line to the water heater.
http://www.jerman.com/dljmeter.html

[Ben Southworth]

Oyster on a string here Marc. Very interesting!

[Marc Rosenbaum]

Marc is posting this for Bob Lemaire, who evidently had some trouble with posting:

We had a solar HW system for 20 years at our last house, installed with the Jimmy Carter incentives in 1981. Three Sunworks panels of about 60 sq ft oriented due south with clear exposure served our family of five well, but I never accumulated any data. All I know is that I didn't use any appreciable gas in the summer when it would be noticed.

So now we have a different house in NH and have converted the old oil fired indirect hw system to solar with electric backup. This time I've instrumented it. First I installed a 50 gallon marathon in June and stopped burning oil. We average 27 gallons/day and 5.4Kwh/day.

In August I brought online 56 sq ft of panels hooked to the 60 gallon Super Store tank that used to be the indirect fired primary HW tank. That tank pre-heats and feeds the marathon. Since installation, out water usage is the same, but the energy has averaged 1.83 Kwh, most of which is the standby loss of the Marathon as is characteristic of a pre-heat setup.

Now I know what you're thinking, with 56 sq ft we should be swimming in HW. The problem is that our solar exposure is only 9:30am-12:30pm facing ESE. There is a stand of huge (110') Eastern white pine almost due south, and a row of large oaks in the 8am-9:30am slot. Hopefully they will drop their leaves before December. So for the past eight years I've been thinking that solar was impossible, then I actually did the numbers.

As you say, there are a lot of fixed costs not associated with the capacity of the system. So with a small window of sun, I was able to double the collector area for an incremental cost of about $900 to reasonably compensate for the afternoon shade. I expect that we will be dumping a lot of water May-July when the sun comes over the pines.

Another feature is that the panels are mounted on a tilting frame that is flush to the 30 deg roof in the summer, but is propped up to 60 deg now to be more efficient in winter and to shed the snow that sits on our roof from Dec to Apr, sometimes as much as 24" deep.

The whole system cost about $3,000 with the recycled tank and pump, marathon 50, and self-installed. I used a Minomess 130 water meter that is fairly inexpensive and comes standard with a pulse output that can feed a Hobo energy logger, and an EKM electric meter that is also connected to the Hobo logger. With tax credits the system easily fits my 10 year return criteria.

I'll be interested to hear the details of your system performance Marc, and see how you went about weeding through the different approaches. I'm guessing that you have the solar 85 gallon tank with extra port to allow heating the bottom with an external heat exchanger. With standby heat loss being a larger part of your usage, that would probably be a better solution than my pre-heat tank system.

Regards,

Bob Lemaire

[David Foley]

Marc, have you considered a simple, seasonal, solar water heater/ Perhaps a simple "batch" type heater could provide significant hot water in the non-freezing time of year? Seems like much of the complexity and expense of solar DHW is freeze-proofing for that time of year when the solar resource is scantiest.

[Marc Rosenbaum]

David, I hadn't gotten there as yet, still want to see how the heat pump water heater performs over the year. I had a high performance batch heater in my first house - selective surface, nighttime insulating shutters - and they definitely work well.

[Heating and Plumbing]

Marathon are great to use on, they are unique and very simple electric heating devices with the three great features, that are, lifetime guarantee on the water tank, high efficient operation and easy installation.