How to Construct a Solar Water Heater

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

  1. Scuba_Dave

    Scuba_Dave Extreme DIY Homeowner

    Messages:
    885
    Location:
    South of Boston, MA
    I've been heating 450g of water in my hot tub since May
    I changed my saltwater aquariums setup at the same time (less water to heat- one less pump), so not sure of exact savings
    But my electric bill dropped $100 1st month (May), $110 the 2nd month
    Hot tub will exceed 110 degrees with one 4x12 panel
    I've actually had to turn it off after 3 hours instead fo letting it run 7-8 hours
  2. Dave D

    Dave D New Member

    Messages:
    12
    Location:
    Maine
    Tell that to Gary who heats his house in a harsher climate than Maine or Mass with all kinds of collectors including Pex-al-pex collectors. The lates project linked below is amazing with all the charted graphed info you can fit in your brain.


    http://www.builditsolar.com/Projects/SpaceHeating/SolarShed/solarshed.htm

    http://www.builditsolar.com/Experimental/PEXColDHW/Overview.htm
  3. Dana

    Dana In the trades

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    That post is popping to the top again? (Dude, that post is almost a year old!)

    Don't be mean to Peter- he's actually a pretty nice (and very bright) guy. :)

    BTW: Gary's solar shed is an all-copper flat plate design, not PEX, but his DIY water heaters use PEX.

    Note that Gary Reysa is an engineer, and is capable of designing around the deficiencies of PEX where a commercial panel manufacturer or installer would have a hard time guaranteeing performance & longevity. To be as successful as Gary at it you DO need to pay attention to the details, measure stagnation temps in-situ, and adjust other factors accordingly, which a typical commercial installer just wouldn't have the time to do. Peter is totally correct that a 300F pressurized design using PEX would blow up in your face.

    But for the DIY crowd the atmospheric-pressure low-tech low-temp approach that Gary uses works just fine if you pay attention to the particulars.

    But for the amount of fuel it actually saves, in most homes investing the money elsewhere would deliver more substantial reductions in fuel use. eg. If your house scores higher than 3.0 on an ACH/50 in a blower door test you'd be better of spending the cash on air-sealing rather than a ball o' PEX and related hardware for DIY solar collectors, as kewl as all that stuff is. Thermal air heating panels are also A: less hardware and B: lower temp/higher efficiency than any solar hot water heater, delivering better return in heating-dominated climates. A DIY-installed $600 drainwater heat recovery system is less work, delivering as much or better return per dollar than a DIY solar hot water system if your shower runs more than 20 minutes/day. Any active-solar (even thermal air panels) is typically well behind air-sealing & insulation upgrades for most homes.
  4. Dave D

    Dave D New Member

    Messages:
    12
    Location:
    Maine
    Doesn't matter how old his Pex project is, it works, and it works well. Its also very easy to reproduce and up size or down size depending on ambition. Ive built a test panel like Gary's with Pex with the exception, I use a flat sheet of poly carbonite. I will be tesitng it thru the winter this year and if it works as I expect, similar to Gary's, I will be building a full size unit like his. I live near Portland MAINE. There is no down side to this and it certain works and works well form what I have discovered. As for the waste water recovery, I have done some tests on that as well. I wrapped an insulation blanket around my waste line and installed a temp data logger, the temps are minimal at best seeing the pipe only heat up to around 75 +/- degrees for very short periods of time, certainly not the 120-140 you get from a solar system. My house is as good as its gonna get for insulation and sealing, solar is the next step. Currently I burn 200 gallons of oil and two chord of wood a year for heat. The goal of solar is to lower my electric bill from 100 to around 70 a month on average. So if the solar costs me 1000 to build and I average $25 a month savings for electricity thats a 40 month payoff so three or so years.... Sounds like solar works and so does PEX.....

    Dave D AIA
  5. Mikey

    Mikey Aspiring Old Fart, EE, computer & networking geek

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    Hey, Dave, what kind of temp datalogger do you use?
  6. Dana

    Dana In the trades

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    It may well work great in low-insolation ME in low-temp apps without much tweaking, but the PEX could still melt if you're not paying attention. Have you tested the stagation temp of your collector? If it runs 200+ in stagnation tests you need to adjust the angle for less summertime gain.

    For space heating you'll get a lot more bang for buck out of thermal air panels (lots of examples of those on Reysa's DIY site too), than with solar hydronic. Solar air is inherently lower temp/lower loss (== higher efficiency) than domestic hot water systems, but if you had say, radiant slabs or above-the-subfloor tubing systems such as WarmBoard(tm) the temp & efficiency of solar hydronic will be similar to solar air (at 5x the cost.) Serioiusly- getting $25/month's worth of electricity at typical NE rates out of a $1000 hydronic system like Gary's at 120F+ operation isn't very likely, whereas for the same money you could build 80-100 square feet of thermal air panel that would deliver as much space heating as you'd get out of ~150-200 gallons of oil burned in an 85% combustion-efficient boiler.

    Unless your datalogger tests were done on a section of thin-walled copper or brass drainpipe your temp numbers drainwater heat recovery don't mean much, and you don't have corresponding flow rates to work from, etc. Natural Resources Canada has studied drainwater heat recovery extensively. See: http://www.builditsolar.com/Projects/WaterHeating/NRCanDrainWaterHtRecov.pdf I can point you to other web resources for their test methods & data if you're really interested. 50%+ average efficiency on a 4" x 48" 3" x 60" or longer is quite realisitic at typical Maine incoming water temps. If you're heating HW with electricity at NE prices it's fairly cost effective, if natural gas (at current prices) less so.

    Also, high temperature is not a measure of efficiency or effectiveness on any of this- running systems at the lowest possible temperature that will actually deliver the heat to where it's wanted it key to keeping losses low, and efficiency high. This is true of everything from solar hot water to space heating systems. Running 140F water in a solar collector to maintain the temp in a 70F room when it's 20F outside means your collector is running at about 30% collection efficiency, but if you can still deliver the BTUs to the room with 90F water (which you probably can, if you have a slab-radiant) your efficiency will be around 50%- nearly twice as much heat per square foot of glazed area. For comfort reasons thermal air panels with active fans sometimes need to be run at 100F+ or the exit air has too much wind-chill to the nearby humans, but they don't need to run anywhere near 120F. Simple snap-disc thermosats on the heat exchanger panel that turns the fan on at 100-110F & off at 80-90F are often the cheapest & easiest way to go, and quite efficient.
  7. Dave D

    Dave D New Member

    Messages:
    12
    Location:
    Maine
    My goal all along has been to reduce my electric bill for hot water as we have an electric 80 gallon tank. Its already runng about 125 degrees insulated and has a time clock. My electric bill has been tweeked to about as low as i can get it. If i can preheat water and not rely and cut down my electric needs that would be fantastic. Oil heat in our home is forced warm air and I dont want to add propane as a hot water source.

    My collector has been vented and sun shadded all summer and has not expereinced any deterioration. I would rather a copper colector than the pex but fear the hot cold cycle may fatigue the sweat joints and the panel may fail. Its also set at 70 degrees from horizontal for low summer gain and max winter gain. It takes no time at all to heat up a 20 gallon tank of water to 120 degrees, Im actually considering installing an old cast iron radiator in my basement to pump the test collector water thru for added heat benifit, again more experimentation.
  8. Dana

    Dana In the trades

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    Timers buy you practically nothing with electric tanks since the stanby losses are so low. Turning it off overnight may result in a 5F drop in temp, which doesn't make a huge difference in the average heat loss, and you spend 98-99% of the energy lost overnight bringing it back up to temp.

    Insulating the near-tank plumbing (including temp & pressure outflow and cold-water feed) within 6' of the tank with 3/4" walled closed cell foam pipe insulation would buy you much more than the timer. (It's not availale at box stores, try Grainger or your plumbing supply house, or online sources. Box-stores only carry 3/8" walled stuff.). It'll cut standby losses in half, for low-mid single-digit percentage savings.

    Insulating all of the distribution plumbing with 3/4" walled stuff will buy you even more. Surveys by PG & E in CA show that between 15-20% of all hot water heating energy gets abandoned in distribution plumbing, mostly on short-draws. Insulating the pipes reduces the mix of hot in the flow on successive draws (within the first 30-50 minutes, anyway), and the energy savings are typically in the high single digit percentages.

    Combined with near-tank insulation expect at least a 10% drop in water heating energy.

    Low flow shower heads (or throttling the flow back to a practical minimum with a tiny shower cutoff ball valve) can make a big difference sometimes. In showering households showers account for 40-50% of all hot water use, and a 20% reduction in flow can mean a 10% reduction in HW heating energy use.

    Similarly, in those housesholds drainwater heat recovery typically cuts total HW heating energy by 15-25%.

    Only after those measures does even DIY solar become the next-most-cost-effective thing. (Unless you can find some $100 Craigslist recycled flat panels from the '80s to refurb & recommission, which is well worth keeing an eye out for.)

    On the cast-iron radiator front, it's a worthy experiment, and if it's a big 'un you can get most of heat into that basement. You can't run it at atmospheric pressure Reysa-style or you'll have serious rust & scale issues clogging your pumps. It doesn't have to be high pressure though- 12psi (measured at the highest point in the system) is more than sufficient to limit oxygen infiltratino into the system (unless your PEX isn't an oxygen-barrier type, in which case all bets are off.)

    When the tank is ready for replacement a tank-top heat pump hybrid will make a serious dent in the HW heating costs for1/2- 3/4 of the year, but will add somewhat to the winter wood/oil consumption. It's roughtly half the electricity use, but since it draws heat from conditioned space that heat has to be made-up by the heating system during the heating season. Overall it's still a net win.
    Last edited by a moderator: Feb 12, 2014
  9. Mikey

    Mikey Aspiring Old Fart, EE, computer & networking geek

    Messages:
    2,714
    Location:
    Central Florida
    That was my goal as well. Following the installation of the solar system, in January 2006, we inadvertently left the water heater circuit breaker off. In early March, following a week of dreary weather, we noticed the water wasn't very hot, and quickly remedied the situation by turning the power back on. The effect of this experiment on our electric bill? Undetectable.

    Now, during the summer, the overtemp breaker in the water heater trips regularly, so we are often "off the grid" for extended periods, but still see no difference in the electricity consumption. I'm not sure what this means, other than the solar system will reach payback about when I'm 172 years old.
  10. Dana

    Dana In the trades

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    Yabbut, to be fair, in FL the incoming water is warm, relative to ME (you only need ~2/3 the energy that Dave does to raise incoming water to 120F) and the cooling bills kinda swap the signal when looking for a difference in the electric bill. The total annual bill for electric hot water heating for two average people is going to be under $500, under $250 for those who take short showers and have water sipping appliances. Even if you cut that in half it's not going to be the lions-share of most people's electric bills. Your refrigerator likely uses as much or more electricity as your hot water heater- it could easily be 2x your hot water heating bill if built before 2001 when the most-recent efficiency standards went into effect in the US.

    Still, unless he lives on an island with diesel-fired powerplants for electricity, if Dave is burning through 200 gallons of $2.50/gallon oil + a few cords of wood, thermal air panels would be a better place to put the solar resources. A grand's worth of material in solar-air panels WILL show up on the heating-fuel use in a meaningful way. Before that, doing a blower-door test & air-sealing & spot-insulating the place would probably be a better use of the first $1000 of energy investment.

    The only thing cheap about solar is the fuel cost. It's usually far cheaper to reduce the thermal load than to build or buy the active-solar resources to support that load. Once you've reduced the load by well over half you eventually get to where active-solar can pick up a sizeable fraction of the rest. If your appliances aren't all Energy Star variants, and your lights all high-efficiency, with occupancy sensor switches/daylight sensors/ timers, etc, you'll get better ROI spending more on the conservation & efficiency end. I pass a house on may daily commute with a ~ 3kw photovoltaic on the roof, with typically 500-1000 watts of incandescent yard lighting on during a bright sunny afternoon. What's with that? $30K was spent to WHAT end?
  11. Scuba_Dave

    Scuba_Dave Extreme DIY Homeowner

    Messages:
    885
    Location:
    South of Boston, MA
    I connected the 2nd 4x12' panel about a week ago with colder weather
    Now with 2 panels its still heating the hot tub to 110 f
    I have 4 panels approx 3x6' that I need to connect for hw

    With solar heat I had my ~20,000g inground pool up to 88 in July
  12. Dave D

    Dave D New Member

    Messages:
    12
    Location:
    Maine
    Ive done a lot of researcha and spoken to local Mech Engineers we consult with and the concensus is tehy just don't perform in our climate UNLESS you have a serious amount of byproduct heat you are trying to deal with and can use. There is a building here in Portland that uses the mechanical room heat generated to make heat pump hot water - or a portion of it and its been successful. My basemnt is in the 60's all winter because i just use residual heat from my oil burner as heat. I am convinced if I can preheat the incoming 50 degree water form my well to 80 or 90 thru the use of DIY solar and or gray water recovery it would cut my electric bill substantially.
  13. Scuba_Dave

    Scuba_Dave Extreme DIY Homeowner

    Messages:
    885
    Location:
    South of Boston, MA
    I've measured my incoming water temp as low as 35 f
    I've been thinking of connecting a pre-heat tank in the basement
    Just having another 40-50g of water at the basement temp would decrease the water heating bill
  14. Dana

    Dana In the trades

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    It's a matter of what your relative per-btu costs are for your differing space-heating vs. water heating fuels as to whether a tank-top electric heater buys you anything. In most colder places it's not buying you anything, may even cost you more if you're switching from cheap natural gas to expensive electricity. With oil it may even cost you more, but if you're cutting your own wood and thus sort of "free" it may make sense. To be sure it's not a no-brainer in your case- it's trading expensive electricity for expensive oil. I only made the recommendation due to your focus on reducing the electricity (not the oil or wood) bill. In general I don't recommend them either- the money is better spent on other efficiency measures in most cold-climate homes. That "residual heat" from the boiler isn't "byproduct", it is heat you've paid for (and dearly) in oil. And unless you foundation & rim joist are sealed & insulated, you're throwing away a substantial portion of that heat. (15-20% of the total would be typical, but it varies by quite a bit on house-by-house basis.) Odds are pretty good that the boiler is 3x oversized for your design-day load too, and some amount of boiler-tweaking (or boiler-control tweaks) can probably reduce your fuel use by double-digit fractions (TBD.)

    Unless you're filling a huge soaking tub every day or regularly showering until the water runs cold, or have 4+ people living there hot water isn't likely to be even as much as half your electricity bill. If it's a 1-2 person household who take 5 minute showers it could even be less than your refrigeration, if your refrigerator was build before 1991. If yours is new enough to have a yellow EnergyGuide label on it, what's the label on your HW tank say? Then consider whether you fit the 4-person household water-chugging appliance profile on which that power use was estimated and adjust the estimate accordingly. If you're a 2 person household with EnergyStar appliances an bath with 2.5gpm showerheads a ~4800kwh/year pig of a tank (per EnergyGuide) is actually using less than 3000kwh/year, maybe less than 2000kwh/year (166kwh/month, on average.) Drainwater heat recovery in a "typical" house would reduces that by about 20%, but if you have water-sipping appliances and take 10 minute showers with a 3gpm gusher it'll return a much larger fraction. (But a lower flow showerhead would be a better investment. ;-) )

    Scuba_Dave: 35F would be a mid-late winter incoming temp- in summer (when you'd be getting the most benefit out of a solar preheat) it's probably north of 60F already- measure it now- it's probably not much cooler than peak summer temp. The shallower your mains are dug into the street the bigger the seasonal swings you get. In Worcester it'll get down to the 35-40F range in winter but on average it's closer to 50F. A few weeks ago it even made the low-mid 60s, after a much warmer than usual summer. I assume roughly 40F as my winter average for estimation purposes, even if I occasionally get slugs of cooler water.

    But a tempering tank in the basement isn't going to buy you much, since it represents a heat load on the house. Those BTUs have to be made up by the heating system. Drainwater heat recovery would raise your 35F incoming water to well over 60F during showers though, and that's heat that was literally going down the drain.
  15. Scuba_Dave

    Scuba_Dave Extreme DIY Homeowner

    Messages:
    885
    Location:
    South of Boston, MA
    Actually peak summer we have close to 70 degree water if not higher
    A tempering tank would buy me a lot in recovery time/longer showers
    I can easily run our 50g tank cold in the winter

    With a child in the house & older faucets/showe valves I do not keep the tank at 140+
    But even when I did we I could still run it cold
  16. Dana

    Dana In the trades

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    I s'pose I should have said "...isn't going to buy you much..." in the way of fuel savings (which I had sort of presumed, since we were talking about what it takes do reduce the water heating bill.) Recovery time is a different performance parameter.

    Drainwater heat recovery will buy you more of both (fuel savings + reduced recovery time) for showers. In fact, a 50%+ drainwater heat recovery heat exchanger turns a typical gas-fired tank heater with a ~35KBTU burner into pretty much an "endless shower" heater at 2-2.5gpm, winter or summer. It won't buy you anything for tub filling though.

    The amount of capacity gained by going from 120F to 140F isn't huge, but the difference in standby loss is pretty significant.
  17. Scuba_Dave

    Scuba_Dave Extreme DIY Homeowner

    Messages:
    885
    Location:
    South of Boston, MA
    Unfortunately my WH is on the opposite side of the house as my drain
    To run some copper around it I would probably lose any heat gained in the added distance
    I do shut the drain while taking a shower to let the water heat the tub 1st
    Then I let it drain
  18. Scuba_Dave

    Scuba_Dave Extreme DIY Homeowner

    Messages:
    885
    Location:
    South of Boston, MA
    My drain also exits at the back of my house...where my greenhouse is located
    So drain water helps keep my greenhouse warm thru the winter
    Once I have my 4 solar panels setup for HW I will have almost 100% solar hot water from April thru at least September
    I spent $500 6 years ago on solar heat for my pool
    Pool topped out at 88 degrees this past summer.....just dipped below 70 recently, mainly because I shut the solar off
    It would cost more then $500 to heat my pool each year
    So payback period is 1 year
  19. Dana

    Dana In the trades

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    Huh?

    If 50-100 feet of copper could lose 30 degrees we'd all have to run our water heaters at 160F to be sure it was warm enough at the furthest tap!

    Serioiusly- insulating it with the cheap R2 pipe insulation from box stores would be MORE than enough to keep the distribution loss down to the "indetectible while flowing" range. We're talking maybe a minute's worth of flow in the round trip- how many degrees do you think 80F water in a copper pipe is going to give up to a 65F basement in 40-70 seconds, even without insulation? And if insulated?

    And what temp is the tub when you let it drain (presumably to retain some of the heat for space-heating?) I'm guessing it's still well above room temp, and WAY above incoming water temps, even after it's passed through the greenhouse. With drainwater heat recovery heat exchanger the greywater exits the house at temps several degrees above the incoming cold water temp, but well below room ambient in mid-winter. The total amount of heat recovered is much higher.

    Pool heating is an ideal solar application- always low-temp low delta-T from ambient, for highest collector efficiency, and used primarily during the seasons when the solar resource is greatest, collector losses lowest- it's a no-brainer kind of investment if you have the space to install it. But add another 30-40F on the collector side, and drop the operating ambient by 30F, for a shoulder-season DHW application and you're working less efficiently, and with a smaller seasonal solar resource.
  20. Scuba_Dave

    Scuba_Dave Extreme DIY Homeowner

    Messages:
    885
    Location:
    South of Boston, MA
    I'm really not interested in running 100' of copper pipe to the opposite side of the basement
    And then insulating it
    I'd much rather use that 100' of copper & build a solar system to heat my house
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