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Thread: How to Construct a Solar Water Heater

  1. #16

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    I have 2, 3 by 8 collectors, one soft tank, and one differential controller that have been in operation since 1991. Its a drain back system with a 007 Taco bronze pump. I have had a homemade heat exchanger leak in the tank from using copper wire to tie it up. Other than that, the system has had 0 maintenance. I live in Maine, is that cold enough for ya?

  2. #17
    In the trades Dana's Avatar
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    Quote Originally Posted by wallyworld View Post
    I have 2, 3 by 8 collectors, one soft tank, and one differential controller that have been in operation since 1991. Its a drain back system with a 007 Taco bronze pump. I have had a homemade heat exchanger leak in the tank from using copper wire to tie it up. Other than that, the system has had 0 maintenance. I live in Maine, is that cold enough for ya?
    You might be close to breaking even by now then, eh? :-) (Or not, depending on how much hot water you actually use, what fuels you would have otherwise paid for and how much you had to pay for the repair when the heat exchanger problem. If it's was a DIY repair to a DIY HX, this doesn't qualify as "full retail" solar, not even close.) And if you're near the coastal hurricane zones, collector-longevity might be much better some decades than others...

  3. #18
    Extreme DIY Homeowner Scuba_Dave's Avatar
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    Solat water heaters are cost effective, even in colder climates
    You can deny the truth all you want
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  4. #19
    In the trades Dana's Avatar
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    Quote Originally Posted by Scuba_Dave View Post
    Solat water heaters are cost effective, even in colder climates
    You can deny the truth all you want
    There's no denial- it's calculable. Sometimes 'tis cost effective, other times, well... not so much.

    At full-retail with fat-margins for the manufacturers & installer, $14KUSD up front for a net thermal input of ~10-15MBTU/year, maybe not... (depends on the discount rate and energy price inflation and mainenance cost assumptions in your net-present-value analyis.)

    At $5KUSD- maybe- depends on the cost of energy from other sources, and the maintenance cost over the anticipatied lifetime of the equipment. It's still not a no-brainer.

    But I DO want to know the truth, so show me the math. (Self-maintained DIY systems for under $2.5KUSD, no argument whatsovever, eh?) The math will set you free...

  5. #20
    Extreme DIY Homeowner Scuba_Dave's Avatar
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    Why buy an overpriced retail unit?
    But payback has been proven even on those when the price is OK

    A complete freeze proof system is $5k or less
    No need to buy a cadillac
    Warmer areas you can buy one for a LOT less, under $3k
    Last edited by Scuba_Dave; 01-29-2010 at 04:45 PM.
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  6. #21
    In the trades Dana's Avatar
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    Quote Originally Posted by Scuba_Dave View Post
    Why buy an overpriced retail unit?
    But payback has been proven even on those when the price is OK

    A complete freeze proof system is $5k or less
    No need to buy a cadillac
    Warmer areas you can buy one for a LOT less, under $3k
    Payback is only "proven" at some price point, which isn't necessarily the current price for what a non-DIY installation costs. At current energy prices the economic argument is pretty easy at 3k, but just fer yuks, show me where the NPV turns postive on a (non-existent in my neighborhood without subisidy), of a $5KUSD unit that deliivers 12MBTU/year output vs. heating hot water with an indirect & mod-con boiler @ $1 or even 1.50/therm. (Assume a very modest 80% average efficiency while heating water if you like, but it's probably better than that on an annualized basis.)

    Seriously- SHOW ME THE MATH! In simple-payback terms it takes decades to get to zero, but ever turning positive in net-present-value terms, about never without making some dubious/difficult to support assumptions about maintenance and future costs of fuel &/or money.

    Simply asserting multiple times that it's cost effective doesn't make it true. SHOW me how it's cost-effective at $5K up front, cuz' it's not so obvious to me when I run the numbers.

    But recommisioning systems or panels from the '80s obtained via craigslist, DIY homebuilts for $2-3K, inexpensive commercial batch heaters, yes, that's an easy argument to make.

  7. #22
    Extreme DIY Homeowner Scuba_Dave's Avatar
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    It's been proven again & again that they pay for themselves
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  8. #23
    In the trades Dana's Avatar
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    Quote Originally Posted by Scuba_Dave View Post
    It's been proven again & again that they pay for themselves
    It must be true then, since you've repeated yet again (with nothing but the bare allegation for support.)

    The proof goes someting like:

    1>If Dave sez so, it must be so.

    2>Dave sez so.

    ...ergo..

    3> It is so.

    Q.E.D., eh? ;-)

    So if it's been proven again & again, what's the problem with showing the proof (yet again)?

    This doesn't take hard math, but on any investment with a term over 5 years I'd at least want to see it as a Net Present Value with reasonable discount & energy inflation rates to be able to assess the assumptions. Simple-payback calculation methods don't work- it has to be compared to investing the up-front money in something conservative and using the proceeds to offset your fuel costs with the alternative scenario.

    The math matters. Heating water with diesel generated island-utility electricity at 47cents/kwh in a 0.90EF tank is very different from $1/therm NG burned in a 0.82EF tankless, and a cost-effiectiveness analysis for solar under those two scenarios is substantially different, but on different ends of the scale.

    Pick something middle of the road for you analysi if you like, but use something and DO THE ANALYSIS.

    Too much to ask?

  9. #24
    Extreme DIY Homeowner Scuba_Dave's Avatar
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    Yer a bright boy
    Do a search
    People buy these & install them because they work & save $$
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  10. #25
    In the trades Dana's Avatar
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    Quote Originally Posted by Scuba_Dave View Post
    Yer a bright boy
    Do a search
    People buy these & install them because they work & save $$

    I've been searching and analyzing the issue for quite some time, which is why I've concluded that it's not a slam-dunk for cost-effectivness in many (or even most) situations. You're the one making the assertion that this isn't correct, so I need to see your analysis to figure out where I've gone rong, eh?

    The simple math on a $5K solution that delivers an 80% solar fraction with electric element backup @ 12 cents/kwh (considerably less than I pay) for the remainder, compared to a 0.58EF gas fired tank at $1/therm (what I paid, full-retail, on my last gas bill) goes something like this:

    Tank capiliziation: $500 up front, $500/decade for replacement & maintenance.

    Tank fuel use: 250therms/year or $2500/decade, (delivering ~15MBTU to the water/year)

    10 year cost: $3000.

    20 year cost $6000

    Solar system capitilalizaon: $5000, assume zero mainenance for 20+ years, 80% solar fraction.

    Solar backup power: 20% of 15MBTU, or 3mbtu/year, which is 3x 293kwh/mbtu= 879kwh/year, so x $0.12/kwh= $105/year, $1050/decade

    10 year cost: $6050 (more than the 20 year cost of the cheapo tank)

    20 year cost: $7100

    But simple analysis doesn't reflect the real world in two important respects:

    1> The $4500 up-front different in cost between a cheapo tank and a cheapo solar could be invested very conservatively to return 5% after taxes, the proceeds of which could be applied to the higher utility costs of the tank. The tank uses $250 (in gas), the solar system uses $105 in electricity, for a difference of $145 in operational cost. With the returns on the cash applied to the utilites that's now reduced to a ~$100/year cost difference.

    2> The price of natural gas (and electricity) are not static, and some assumptions have to be made about the future costs of each if we're looking out 5+ years. With the ramping up of production in the Allegheny shale formations predictions of natural gas prices INCREASING dramaticaly over the next 20 years are very dubious indeed, and fraught with many many impossible-to-know factors. Will the US start taxing carbon emissions heavily, driving electricity production to shift from coal to gas quicker than the gas production can increase? If yes prices will indeed rise on average, but how much requires a crystal ball. If not, we could be looking at decades of lower NG pricing relative to the upramp of the 1900s and early part of this century. At the same time, if electricity is ever increasingly supplied by the same natural gas burners, the price of electricity will also rise.

    This is not simple to model, and by no means a no-brainer at $5KUSD. In a net-present-value analysis the underlying assumptions for discount & fuel/electricity inflation rates are likely to have large errors, and it could fall far to either side of the cost effectiveness in a 20 year term, but there's no way it can be demonstrably cost effective in a shorter years than that without some fairly wild assumptions.

    It's dead-obvious 10 years at $3K up front though.

    But if you have a better analysis (or can point me to one online) let's have it. The math will tell you "the truth".

  11. #26
    DIY Member Hardt's Avatar
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    Hmm...I haven't read anything about all the tax credits that come with installing a solar hot water system! My first solar system was installed in 1980, 120 gal 4 panels for $4200. After federal and state tax credits, the net cost was $2100. The cost of electricity then was about $.25/kwh. now it averages $.42/kwh (yeah, it's expensive but you gotta pay the price when you live in Hawaii). After 29 yrs of reliable service ( during which I maintained it my self, I replaced the recirculating pump, the heating element, a couple of ptr & air release valves and a spring loaded check valve with a more reliable swing check valve), I had a new system installed last May, 120 gal 2 panels for $6100. I reroofed the entire house and it would not have been cost effective to remove/reinstall the old panels/piping, etc. The electric co. rebate was $1000, then federal and state tax credits kicked in of 30% and 35% and a real property tax credit of $300 and the net cost was down to $1485. If I did not have this system, my guess is that I would spending about $60/mo on electricity for my hot water needs. I should have pay back period of about 2 yrs. which is not bad economics and besides that as a hard-core DIY person it is a fun system to maintain.

  12. #27
    Extreme DIY Homeowner Scuba_Dave's Avatar
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    Tank cost ?
    I have a HW tank, I don't need another one
    So that cost is out, you need to buy a tank with or without solar
    Early Spring to Late fall full HW provided
    Solar works as long as it is set up correctly
    And 30% back of the cost
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  13. #28
    Plumbing Contractor for 49 years johnjh2o1's Avatar
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    I find it hard to believe that the 120 gal tank lasted 29 yrs. You had to replace it once if not twice in that time period.

    John

  14. #29
    In the trades Dana's Avatar
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    Quote Originally Posted by Hardt View Post
    Hmm...I haven't read anything about all the tax credits that come with installing a solar hot water system! My first solar system was installed in 1980, 120 gal 4 panels for $4200. After federal and state tax credits, the net cost was $2100. The cost of electricity then was about $.25/kwh. now it averages $.42/kwh (yeah, it's expensive but you gotta pay the price when you live in Hawaii). After 29 yrs of reliable service ( during which I maintained it my self, I replaced the recirculating pump, the heating element, a couple of ptr & air release valves and a spring loaded check valve with a more reliable swing check valve), I had a new system installed last May, 120 gal 2 panels for $6100. I reroofed the entire house and it would not have been cost effective to remove/reinstall the old panels/piping, etc. The electric co. rebate was $1000, then federal and state tax credits kicked in of 30% and 35% and a real property tax credit of $300 and the net cost was down to $1485. If I did not have this system, my guess is that I would spending about $60/mo on electricity for my hot water needs. I should have pay back period of about 2 yrs. which is not bad economics and besides that as a hard-core DIY person it is a fun system to maintain.
    It's very easy to make the math work for you when your utility rates are high, the year-round insolation levels are high, outdoor temps are moderate to high, and the capitalization is heavily subsidies. In your case you have all four, making it a no-brainer. (It's an easy argument in your case even without the subsidy!)

    Cost effectiveness at the national average utility rates isn't plausible without subsidy on the hardware. This is analyzed in great detail within governing bodies & utilities when trying to determine the appropriate level of subsidy to achieve a policy objective. At least until recently in CA the size of the total subsidy by the natural gas ratepayers is required to be at parity with how much that reduction in load tp the natural gas grid lowers the retail price of the gas delivered, as a matter of fairness to the ratepayer. It took a very complex economic analysis to come up a credible defensible number. This may soon be superceded by newer state policies with broader goals, with an entirely different analysis.

    BTW: I'm assuming that in HI the systems can be simpler since they don't require freeze protection. Are you running potable water in the collectors, or is it an isolated loop for corrosion/scaling control on the panels?

  15. #30
    DIY Member Hardt's Avatar
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    Dana, yes I use potable water in the collectors. I'm surprised that a solar hot water system would not be cost effective in Ca. I would have thought that even with marginal subsidies from the utilities, the federal and state(?) tax credits would make it viable. But I do remember when we lived in Sacramento 30+ yrs ago that SMUD & PG&E rates were extremely low! John, yes the tank was made by American Appliance Mfg. Solar Stream model, glass lining, etc... In my previous post I left out the most problematic area that I had with the tank. After about 10 yrs of use the dip tubes/anode rods gave out. I tried various plastic tubes ( I gave up trying to find replacement anode rods ) but they didn't last long. I finally put in 1/2" copper tubes and that did the trick. When the new system was installed and the installers were hauling my old tank to the landfill, they commented on how heavy the tank was and one of them said "they don't make 'em like this anymore"!! So that set my expectations that this new tank will last 10 yrs at most! Did I mention that I still use a Snapper 22" rotary mower that I bought in February 1972! I was thinking "green" before it became fashionable...

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