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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.


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.

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.


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.

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