Please share your experience with SDHW and On-Demand hot water. Thank you,

My Solar Hot Water Experience

Coincidentally, I installed a 2 panel solar (West Sand Lake, NY) hot water system back in the early 1980’s when NYS and the FEDS were offering tax incentives to do so.  It was a pair of copper plate collectors insulated on the back with flat black paint on the front in an insulated box.  The collector front was a tempered high solar gain glass.  It used an anti-freeze solution (in case of pump failure in winter) circulated two stories down to my basement into a heavily insulated 80 gal storage tank with an exchange loop.  Controls consisted of a temperature sensor inside the collector attached to the collector plate and one attached to the storage tank.  Comparative sensing turned on/off a simple 1/10 HP Grundfos circulator pump.  Grundfos was somewhat new at the time with its low power consumption and considered high tech.

Anyway, no matter how I plumbed the panels, attached valves, routed pipes I always had a problem of thermal siphoning causing an unwanted circulation within the loop caused by the differential temperatures that were always present. Today that would be solved by installing solenoid valves to shut off the flow unless the pump was active but at the time, things weren’t that advanced. I didn’t spend a great deal of time harvesting data, but I never noticed any significant change in my electric utility bill (back up electric element in storage tank).

Having the finishing heating element in the same tank was also a negative factor as thermal looping within the tank caused the electric element to activate prematurely.  In the McDaniel/Borton article, they had a much better setup with the preheat tank separate from the on demand finishing heater.  Even that didn’t allow for much of an operational savings though in their installation.
I considered my installation a failure and decommissioned it years later.  Fortunately, the tax incentives softened the financial blow.  The roof top presence of the collectors made putting on a new roof more difficult as well.  My conclusion was simply that our area lacks sufficient high intensity solar gain to make direct harvesting economical.  Even in the hot summer months, the circulator did not run all day as you might expect because the storage tank temperature rose to its target fairly quickly.  The collectors could not get to a high enough temperature in the deep winter months for the system to activate even in full sun.  The ambient temperatures were simply too cold to allow a high enough temperature rise within the collector box.  The entire system ultimately provided only preheating BTU’s to the incoming well water and the electric element always was used to finish off raising the water to use temperature.

I would agree completely with the statement that direct solar domestic hot water heating does not make sense economically or environmentally.  Some of this discussion relates to why I also feel active solar heat systems in the northern tier make little sense.  It all comes back to not having enough sun at the time of the year when we need it.

Smorton is a fastidious Do-it-Yourselfer. He has built his own highly insulated home that now requires the proverbial candle to heat it – well actually baseboard electric that consumes an estimated $1800 a year for a 3000SF two story house with a passive solar gain design.  Link to house design with pictures: http://www.sunplans.com/select/plan/__details/Adirondack_Atrium

I’m surprised to read of the ineffectiveness of your solar hot water system, smorton - I would have thought that it couldn’t really go wrong, but of course it makes sense what you are saying about not having the sun when we need it most.

I have a thought for a solar hot water system to heat a tiny house that I’ve built that looks something like this:
- a heat (hot-water) reservoir (a well-insulated hot water tank - perhaps a double-walled tank with closed-cell foam insulation)
- a solar-heating loop, circulating through solar collectors much like you describe (though I was thinking of a drain-down system, which heats the water directly, and then drains out to prevent freezing)
- a secondary heating loop, circulating through a heating jacket built into a wood-burning rocket stove
- a circulation loop which travels through a radiant floor in pex tubing, providing heat for the space
- an outlet, which can be used for bathing water

This is a conceptual design, and I haven’t figured out all of the technical ins-and-outs of it. I wouldn’t imagine that the solar loop would be the primary heat source, but sort of like a booster which supplement the wood heating system.

This design is based on the thoughts that water is a great heat-transferrence and heat-storage agent, and that it makes sense to capture the sun’s energy directly and store it in water for use as heat energy. Any input based on direct experience would be welcome!

In response to your proposed drain down solar heat system design I can only say what you describe was in vogue in the 1970’s and has been kicked about in many magazines for DIY inclined people since that time.  Sadly, it offers no more promise today than it did 40 years ago.  Examine the thermodynamics for a moment.  You propose capturing solar energy to heat a house…a much more demanding situation than heating DHW.  In the northern tier, the sun has to be up about 2 hours before much energy is captured due to the extremely low angle in the sky and the attenuation of the sun’s energy through long atmospheric distances before reaching your collector.  The same is true at the end of the day leaving you with a short period to capture enough BTU’s to heat an entire house for 24 hours(or more depending on our weather).  It just doesn’t work.  Instead of building large storage spaces and complex plumbing that requires maintenance, spend your money on foam insulation, air seal all infiltration points, and have properly positioned south facing windows.  This web site helps in window placement:http://susdesign.com/windowheatgain/  My background includes a graduate engineering degree from Rensselaer Polytechnic Institute if that helps lend any credence to my comments.

markintheworld,

I generally agree with smorton - you are better off financially highly insulating and air sealing your house, and then you can heat with expensive electric since you will need so little. However, if you are driven to extremes as I am, then highly insulating and air sealing your house may very well allow you to heat it with the sun and a little wood. Certainly the sun part will not be the economical choice, but it will provide some flexibility that the wood stove will not. I am building a 100% Solar House and plan to use a combination of wood and solar to heat it. Since this house has insulation and air sealing that rivals Passive House, the heating requirements may make solar laughable, but until I finish it and can capture the results I won’t know. You can see my what I have done so far at www.HomeEnergyAdvisors.com / 100% Solar House icon. I will be putting that information in this website eventually… Thank you for thinking of the sun. And thank you smorton for your practical advice! Have a Sunny Day!

I do have foam insulation (5.5” of EPS foam in walls, ceiling and floor), I have worked on sealing all air infiltration points, and I have south-facing windows.

I mentioned that I am heating a tiny house, but I didn’t mention how tiny: the space that I am heating is 84 square feet. I don’t know how many BTUs it would require to heat it, but I’m sure it’s not much. It sounds like you feel as if solar is the wrong tree to bark up, especially in this climate, but I do need a source of heat besides the sun.

I chose to investigate solar heating because I am loathe to turn such a high-grade energy like electricity (you can run a computer with it) to such a low-grade energy as heat (you can burn a stick to get it). Not to mention the fact that it is only about 30% efficient by the time it is delivered.

The other option that I mentioned is burning a stick (in a rocket stove): I would like to heat my heat reservoir with a heat exchanger installed in the wall of a rocket stove. I hope to store this heat in a large mass of water to be circulated throughout the radiant floor.

I guess I would focus on this wood-burning approach rather than solar hot-water, which does not seem to be suited to this climate. I wonder if there are any successful solar hot-water systems in our area?

Thanks for your input!

Hi Markintheworld,
You are doing a great job! The biggest factor in home energy is size. 5.5” of EPS is great, just pressurize your house with a fan and look where smoke goes to find any remaining leaks. You will need ventilation in a space this size as tight as it will be. Panasonic offers some very quiet and energy efficient fans that can be set on a timer – automatically runs X minutes per hour will probably be best. We’ll need to think more about what X should be. Still be careful about any indoor pollutants in your space. Regarding heating, all traditional rules of thumb are off when you build an 84 square foot house. The human body gives off 300 BTUs per hour - this is darn near enough heat! Take a look at: http://www.sundogsolar.net/blog.html (top and bottom of page) and talk to Betsy and Aldo. I expect you will have a lot in common Aldo, and Betsy is really smart about what you can do with active solar heat. Please let me know how your home turns out and I’d love it if you would write a blog with some pictures of the project to share with our members! Thank you for asking, Dan

Hey Everyone,

I just wanted to share the energy savings that I experienced from installing a SDHW system.  I built my system from used collectors.  It is a closed-loop glycol system.  I’ll provide details on the system in a future article. I have experienced a minimum of 2/3 lower fuel costs for domestic hot water heating based on propane gallons delivered.  I get my last delivery for the heating season generally in March and the first delivery for the next heating season in October or November.  So, I can calculate the gallons per day over this period. Even recognizing that there may be some furnace use on either end this summer period, my post versus pre-SDHW is at least 2/3 less.  There is no question in my mind that SDHW works here and saves money. Plus, it is not entirely about how short the economic payback is.  We must reduce our fossil fuel use.  That reduction in fossil fuel use and satisfaction of using the sun to heat your water is instant with SDHW.  The sun is throwing free fuel at us. All we have to do is start collecting it.

McDaniel/Burton/Woodruff Debate

McDaniel/Burton have done a great job defending their study, yet miss one key point. Woodruff did have “two major reasons” for his analysis, which all parties agreed were technically valid. Yet he also made it very clear that to state that “the fundamental conclusion of the paper still stands-evacuated tube hot water systems are not cost effective for domestic hot water heating in Oberlin-like climates” with the results
and cost analysis of just ONE example, no matter what the technical circumstances are, is against the rules of making a scientific conclusion, and is academically irresponsible. Just because this ONE example matches the conclusion, does not mean you can make this conclusion for ALL domestic hot water heating systems.

Claiming that the “physics” involved proves the case? This is not rocket science. I hold a degree in physics as well. Physics is used to properly describe, design and manufacture solar heating systems, not evaluate cost effectiveness. A responsible contractor always provides the client with a cost savings analysis, BEFORE an installation. If the numbers don’t look great, then debate the option of implementing the presented system. Whether it be a tankless water heater, or a solar heating system, or ANY type of efficiency upgrade investment.

To: McDaniel/Burton, there is an economic scale of cost effectiveness for solar heating systems. You even admit in your rebuttal that ET systems ARE cost effective for larger consumers i.e. laundry matts… Larger consumers can also be for domestic hot water, residential or commercial uses. Which makes your conclusion, and title of your study =
FALSE. No matter how you want to explain the specifics of your study.

Michael Paradis of Green Earth Energy. 
.(JavaScript must be enabled to view this email address)
www.greenearthenergy.info

smorton, sorry to hear of your experiences, and even more so that you were unable to find professional help.

I worked extensively is solar heating “back in the day” (even before your system!) and recall the initial puzzlement about the thermosiphon issue. We finally realized it must be that *within* an individual pipe a flow would develop, one stream up one edge of the pipe, and vice versa. That would then kick on the pump - which would happily pump hot water up to the cold collectors!

The check valve installed to prevent the siphon within the tank-collector system was not designed for this.

I solved it by installing the swing check valve so that gravity would swing it closed, even when the pump was not running. Or, used a spring check valve.

On another note, as I’m sure you are aware, the effectiveness of a type of system can never be judged by a single instance - which is vulnerable to all the possible “early on the learning curve” errors, and other individual factors..

I plumbed the panels, attached valves, routed pipes I always had a problem of thermal siphoning causing an unwanted circulation within the loop caused by the differential temperatures that were always present.

Mr Fulton,

I’m unclear about the reason for your post. Are you asking for advice? Thermosiphon issues are always engineering issues; I’d be glad to help out if you’d care to describe your system.

Christian