How to Construct a Solar Water Heater

Discussion in 'Solar and Geothermal Water Heating Forum' started by nehalive, Oct 9, 2009.

  1. Dana

    Dana In the trades

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    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.
  2. Scuba_Dave

    Scuba_Dave Extreme DIY Homeowner

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    It's been proven again & again that they pay for themselves
  3. Dana

    Dana In the trades

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    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?
  4. Scuba_Dave

    Scuba_Dave Extreme DIY Homeowner

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    Yer a bright boy
    Do a search
    People buy these & install them because they work & save $$
  5. Dana

    Dana In the trades

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    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".
  6. Hardt

    Hardt Member

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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.
  7. Scuba_Dave

    Scuba_Dave Extreme DIY Homeowner

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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
  8. johnjh2o1

    johnjh2o1 Plumbing Contractor for 49 years

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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
  9. Dana

    Dana In the trades

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    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?
  10. Hardt

    Hardt Member

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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... ;)
  11. Plumbermurrieta

    Plumbermurrieta Plumber

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    SOLAR HEATING Solar heating is feasible today. The average American household consumes between 1000 and 2000 gallons of number two fuel oil per year. Efficient use of the sun’s energy could easily cut this consumption in half or eliminate it entirely. The heating of water is perhaps the easiest, most cost effective solar project a person can get involved with.
  12. Tasha Faith

    Tasha Faith Guest

    Be careful working with the water heater since the water and the heater parts can become extremely hot. Wear gloves and safety goggles during this project. Make sure that you read, understand, and follow all instructions that come with your equipment. Do not let children or animals near this device. The sharp edges of the metal can be coated with tool dip after using a Dremel tool on them if their sharpness is a potential issue.
  13. Mikey

    Mikey Aspiring Old Fart, EE, computer & networking geek

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    We maybe should have a thread dedicated to cost/benefit analyses, but since the discussion is here, here's my $.02. I've had a professionally-installed (about $2800 after rebates, etc.), open-loop, PV-driven circulation pump, 80-gal tank, 4x10 collector system installed for about 4 1/2 years. It was installed in December, and by accident we left the backup electric power off. We didn't notice anything for about 3 weeks, during a cloudy few days. When running normally, the hot water in the tank is about 160°F. Occasionally the overtemp ciruit breaker in the WH opens (set point is 180°), so after a run of cloudy weather we have to reset it. So, we believe it's working. Here's the rub: I keep detailed records of my electrical power consumption, and can detect no significant reduction in consumption following the installation of the solar system. Nor for the installation of triple-glazed windows, high-efficiency AC and R-30 insulation (augmenting R-6 or so). I'm puzzled, needless to say, but mention all this to caution that, as emphasised above, you can't guesstimate savings. Get the numbers, before and after, and you can see what really happened.
  14. Dana

    Dana In the trades

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    In FL the incoming water temps are pretty high (70-75F on an annualized average), so water heating energy per gallon is typically 2/3 or less what it is in MN or the upper peninsula of MI where the average temps are 25F lower, and it'll be "in the noise" on electrical power use relative to cooling bills. If you track power use with cooling degree-day(CDD) data you'll usually find some monthly baseline number with a roughly linear CDD power use function on top. If you have older billing data with exact dates, you may be able to look up the CDD history to correlate to those exact dates. (Uncorrelated monthy numbers won't work with any accuracy but kwh/CDD for whole cooling seasons might.)

    If your window & insulation upgrades haven't been measurable in the bill I'd be looking at duct leakage & air infiltration issues. While FL does have significant sensible cooling loads, the latent loads are still typically higher. If you're dragging 1-2 air-exchanges/hr into your home with poorly implemented duct design or other air leakage issues, keeping up with the latent loads becomes the dominant air conditioning issue, not much affected by the insulation levels or the rated efficiency of the AC unit. A tight house and tigher/better-balanced ducts acn make a huge difference. See: http://www.buildingscience.com/docu...residential-ventilation-and-latent-loads/view

    Blower door testing on the building envelope, and pressurized duct testing may tell you where the bigger problems are. If the air-handlers & ducts are in an uninsulated attic the "fix" can be as simple as foam-insulating/sealing the ducts, or foam insulating the roof deck (sealing the soffits), making the attic into semi-conditioned space, etc., putting the ducts & air handler inside the pressure boundary of the structure. (If you do the latter, don't let the foam installers try to convince you that you need to remove insulation at the attic floor, or than some number of inches of foam "...is all you need...", which seems to be a common misconception in the trades. ) See:

    http://www.buildingscience.com/docu...-climates/view?searchterm=latent load florida

    http://www.buildingscience.com/docu...n-conditioned-space/?searchterm=Sealed Attics

    http://www.buildingscience.com/docu...in-hot-climates/view?searchterm=Sealed Attics

    In general, window replacement is almost never cost effective in an NPV analysis on energy use, but can be good from a comfort point of view. In cooling dominated climates exterior shading of the windows with awnings &/or operable exterior screens/shades are usually a better bet, but every house is different. Truly leaky or horribly placed single-pane uncoated windows from a solar-gain point of view can sometimes be cost-effective to replace, but they're the exception, not the rule.
  15. Mikey

    Mikey Aspiring Old Fart, EE, computer & networking geek

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    Thanks, Dana. That's good advice and consistent with I've found and guessed. I'd love to do a blower-door test, but haven't found anybody around yet that does them. I did the best I could to seal everything in the attic before putting in the insulation, but never did any quantitative testing. The utility company used to, but no longer. My weak link, though, I'm pretty sure, is the walls. They're 6" "jumbo brick" with 3/4" foam on the inside. I don't know offhand what that works out to in R value, but it ain't much. Hard to fix.
  16. Dana

    Dana In the trades

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    If it's 3/4" bead-board EPS ("Styrofoam"), it's ~R3. If it's foil-faced polyisocyanurate it's ~ R5. If it's pink or blue XPS you're looking at ~ R4, none of which adds up to much, but it's way better than nothing, and the thermal mass of the brick helps even out the peak loads (but not the average.) If it's bare foam without a foil or poly facers it has very modest vapor retardency too, but vapor permeance is likely to be only a secondary factor (well behind infiltration, in most homes) for your latent loads. Add another R0.5-0.75 for the brick, and that's pretty much your clear-wall R-value.

    Hopefully it's a cavity wall with, at least a 1/2" gap between the brick and the interior foam, which would allow a good portion of the high vapor drive that occurs when sun hit's a rain or dew-wetted brick to vent rather cook into the interior. But if the brick tight up against unfaced foam, with no studwall on the interior, just plaster or drywall it can be a signficant moisture path. In either case, some amount of that type of vapor drive can be mitigated using masonry sealers on the exterior, which keeps dew or rain from wicking into the brick only to be vaporized and transmitted into the interior as the brick heats up. In some instances it might be better to use a vapor-retadent sealer, but I wouldn't recommend that without a lot of site investigation. Vapor permeable or semi-permeable sealers reject liquid water, yet still allow the brick to release moisture as vapor to the exterior (it's more forgiving, since it can't create moisture traps.)

    Plantings or shades to keep direct sun off the E,S, & W sides of the house can also make a difference. Brick is (unfortunately, in this case) a pretty good solar absorber, and surface temps can easily hit 20-40F above the exterior air temp in direct sun. When it's 90F out at 11AM the surface of sun-baked brick might already be 110F, and may hit an egg-frying uncomfortable-to-touch 120F+ in the afternoon, even if the air temps only hit 95F. A sunbaked section of 120F wall is letting in about twice the heat per square foot as a shaded section of 90-95F wall. Shrubs, trees, trellisis, fences, awnings, roof overhangs, etc all help reduce the amount of direct solar gain from both windows and low-R walls. Air-tight & shaded walls, with R30+ rafter-mounted radiant barrier (or cool-roof materials) on the roof/attic are about as much as you can do to get the cooling load down, but it's substantial.

    The bigger air-infiltration holes in most homes are generally easy to fix: Dryer vents, fireplace & furnace flues, bathroom vents without backdraft control, etc.. Then there's the PITA leaks to fix, like recessed lights, flue chases, plumbing, & electrical penetrations into the attic, etc, which can be time consuming to fix (or even find, in some cases.) Rare is the home where window & door weather tightness is the primary air leak. Crawlspaces & full basements can also have large air leaks, but I'm assuming you're slab on grade?
  17. Mikey

    Mikey Aspiring Old Fart, EE, computer & networking geek

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    Yup, slab on grade, mostly. I discovered the slab was sinking a while back, and a company came in and pump some kind of magic foam underneath through holes drilled in the slab all over the house. It was magic -- the slab rose up to nearly where it belonged. Long story. Anyway, some of the slab lies on an insulating layer of foam of unknown and varying thickness.

    The foam in the walls was sprayed on, some kind of closed-cell urethane, it looks like, ca. 1973. It's brick-foam-gypsum board only. With large shading trees, large overhangs, and general orientation, I don't think the solar-absorption aspect of the brick is a major factor, but I will look into sealing it -- I know interior humidity is higher than I'd like it, but the sprayed-on foam (I thought) should be impervious. There may be leakage around the 1x2 nailers, though.

    The garage used to have a no-brainer problem: a west-facing uninsulated steel 18' door. In the late afternoon, the door was a 120° radiator. I replaced that door with a the latest & greatest hurricane-proof insulated model, and the difference is as dramatic as you'd expect. Its 4 walls are the door to the W, uninsulated brick to the N, and the house (3 1/2" fiberglass in the studwalls) to the E and S. It's bearable year-round in the garage.

    Laundry room is outside the AC'd envelope, between the house and garage. It has its own source of makeup air for the dryer, but that source exits right next to the dryer intake, so it doesn't affect the room much, which stays moderately comfortable just from traffic to and from the house exchanging conditioned air.

    Fireplace flue is an interesting issue. Some days when the AC is on, I can smell the air coming into the house via the chimney. That tells me there's negative pressure in the house, which could mean too many registers are restricted, or duct leakage in the attic (which I doubt). I haven't looked into this any further, but intend to go around and check & open up all the registers for starters.

    Leakage into the attic may be a big deal. As I said, the ductwork should be pretty tight, and I sealed all the electrical penetrations that were easy to reach (maybe 30% of the total), but there are several can lights that are IC but not AT. Even so-called AT lights were a long way from airtight, I found, so I sealed them up with 3M fireblock caulk. Those suckers are AT now. Plumbing is overhead, and all penetrations are sealed.

    The bottom line, though, is that we keep ourselves cool in the summer and warm in the winter, and it apparently costs us. But for now, it's worth it. Our bills relative to our neighbors' are better, in some cases significantly, but I still want to do the best I can on general principles. It's definitely our fault, though -- when we were both away from home in February, our bill was cut in half. Maybe we should just stay away.

    Thanks again; sounds like you do this sort of thing for a living, and are good at it. I appreciate the comments.

    Mike
  18. Dana

    Dana In the trades

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    It sounds like you've taken on quite a bit of the PITA air-sealing work, but if you have detectable air flows coming down the flue, there are still more leaks to plug, possibly some big ones. Top closing fireplace dampers are much tighter than old-school steel flappers at the firebox, and may be worthwhile here- but that's only a start. (There are inflatable "chimney balloons" too, but they're a pain to swap in/out seasonally when you might want to use that fireplace.) But the flue is the return path- there has to be a big supply to get much flow, but it may be 20 smaller supplies. Pressuringizing/depressurizing the house with a big window-fan and running around with a smoke pencil, can of foam, and labels for marking the bigger leaks can be as effective at this "rat-killing" of leaks as a calibrated blower door test that actually measures the volume.

    Closed cell polyurethane foam from the '70s is about R6/inch, so you're at better than R4. But while it's waterproof to liquid water, it's semi-permeable to water vapor. At 1" thickness it's permeance is about 2 perms, and higher at 3/4" compared to 6-mill polyethylene sheet's ~0.05 perms, or kraft paper facer on fiberglass batts ~0.4 perms. It is passing in on the order of 40-50x the diffusion that poly sheeting would(!).

    In CA under Title 24 building efficiency laws ducts have to meet both design-balance and pressurized leakage tests. To pass the leakage test usually requires mastic or FSK tape on every seam & joint, and sometimes even the air handler panels need to be taped. Very few ducted AC or heating systems elsewhere in the US would actualy pass, but the Title 24 provisions all went through cost-effectiveness vetting as part of the process (efficiency measures that were a net economic burden to the homeowner were cut from the code.) There are many companies there that now offer duct leakage test & remediation services, but I don't know of any operating to that spec in FL.

    Evening out room-to-room air pressure via transom grills or jump ducts can often have huge positive effect on reducing air infiltration related to duct imbalances, now ensrhined in FL code for new construction See: http://www.baihp.org/casestud/return_air/index.htm Partition-wall stud cavities can often be used as jump ducts with less light sound transfer by putting a grill on the bottom of the cavity on one side of the wall, and at the top on the other. In new constrution or as retrofit jump ducts are often a U of flex duct at the ceiling. But lowering the overall resistance of the supply & return paths internal to the house reduces the volume of air-handler induced infiltration.

    Ducts & air handler in the attic are far more suscetptible to radiated heat from a superheated roof deck than the insulated attic floor, and need to be insulated for best efficiency. Radiant barrier (the aluminized fabric type) stapled to the rafters can cut the heat gain at the ducts roughly in half, even if they're already insulated. Duct insulation must be air-tight, or you can end up with substantial condensation on the duct it self in a ventilated attic. (An inch or two of closed cell foam or foil-faced rigid-board with mastic/FSK-tape sealed seams is good. Unfaced fiberglass wrap, not so good.)

    Don't feel guilty about using the AC- it's necessary for both health & comfort when living in a hot humid climate- it's what you have it for, eh?

    I'm currently working as an electrical engineer, I'm not a building pro. (My degrees are in math & physics.) But building practices & energy efficiency stuff has been of personal interest for decades now. My father was in the constuction biz (general contractor- mostly commercial & industrial stuff) in the Pacific Northwest during '60, '70s & '80s. I've seen a lot of construction practices come & go, and a lot of half-understood vapor retarder & insulation stuff mis-applied, even by the pros. A lot of stuff previously enshrined in code based on best-guesses or best practices in one climate zone mis-applied to another has slowly been tossed out since the mid-'70s, but there's still more work to be done. There's still a wealth of ignorance in the biz about moisture control regarding proper use of vapor retarders (or even what is/isn't waterproof vs. vapor retardent vs. air barrier.)
  19. hj

    hj Moderator & Master Plumber Staff Member

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    I am not about to try to read everything that has been posted, but a solar system depends on a ration "radiation area" to water surface to absorb the energy efficiently. A large tank will have a very low ration since the only water in the tank which is absorbing the solar energy is that which is in contact with the tank's surface. The rest of the water is draining that temperature away. The best collectors have large radiant "fins" attached to small copper tubes, which results in a very high efficiency. DIY panels can use "finned baseboard radiators" painted black and installed in parallel manifolds to absorb the heat and transfer it to the storage tank. A serial manifold could result in overheating the water before it arrives in the storage tank.
  20. A system in which the sun’s heat is gathered by a solar collector and used to increase the temperature of a heat-transfer fluid which flows through the pipes in the collector
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