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Thread: Different Solar Water Heater Options

  1. #16
    In the trades Dana's Avatar
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    Quote Originally Posted by Thatguy View Post
    The average insolation for San Antonio is 5.3 kwh per sq. meter per day, so a one square meter panel at 80% eff. would give you 160w all the time, on average.
    http://www.builditsolar.com/Referenc...or/ColEfic.htm

    You'd also need to know some kind of percentile figure for cloud cover to decide the size of the storage tank. 120 gallons (1100# of water) raised 90F stores ~1 therm = ~30 kWH of energy. Your incoming water temp in summer might be 70F.



    Only unglazed collectors at VERY low delta-T between the water and ambient air temp come anywhere near 80%. I wouldn't use that in any kind of calc. Assume 50% for an annualized average for a single-glazed panel & reasonable DHW temps. It'll be somewhat higher efficiency with a bigger storage/collector-area ratio (the 120gallons/40sf panel) somewhat lower for the lower ratio unit (80gallons/64sf), but probably no higher than 60% or lower than 40% for either.


    Quote Originally Posted by Thatguy View Post
    Two of us use 35 therms/mon of NG for heating water. With a 75% efficient heater this is 1100w all the time.

    This helps to keep all the units straight.
    http://www.onlineconversion.com/


    That seems unusually high monthly-average NG usage for simply DHW for two people. What are you using for a hot water heater, and is 75% your presumed efficiency, or the presumed efficiency of the solar/electric water heater?

    Actual use patterns make a HUGE difference in actual efficiency. The EF number doesn't mean squat with NG fired tank heaters- it's all about total volume uses, but tankless on-demands tend to come in no worse than 10-15% below their EF in worst-case real-world patterns. Worst case patterns for tankless is draws of 2 gallons or less, with very few draws of over 10 gallons. For NG tanks, it's primarily total volume that counts- EF tests are run at ~60gallons/day- if you use more than that the efficiency goes up assymtotically to a hard maximum of ~79-80% efficiency @ 100% burner duty-cycle. But at 28 gallons/day NG tanks drop below 50% efficiency.

    In short, a tankless with an EF of 0.82 can run anywhere from 0.67-0.82 under any real world conditions, but a "high efficiency" NG tank heater with a 0.68EF could run anywhere from 0.40-0.75 in the real world, depending on volume of load. (Used in a combi space-heating/DHW load situation it might go as high as 0.78.)

    Quote Originally Posted by Thatguy View Post
    Mr. Mikey, if this
    http://en.wikipedia.org/wiki/Analysis_of_variance
    can't tell a difference between your old and new setups to some reasonable confidence level, I'd have to wonder what's going on.
    I have also heard that people who changed to high eff. furnaces (let's say 80% to 95%) didn't notice anything, either.
    It's strange.
    With heating equipment the largest immediate factor in getting anything like the rated AFUE-type efficiency out of it is having the burner properly sized to the actual peak "design day" load. It's common to find furnaces 100-300% oversized, especially in older housing that has seen a number of upgrades to the building envelope (better insulation, better windows & doors, etc.). An 80% AFUE type furnace that is 300% oversized doesn't dramatically underperform a 90% furnace that's 300% oversized- they both SUCK compared to what they would do if better matched to the load. A properly sized or undersized 80%AFUE furnace can/will outperform a 300% oversized 90%AFUE furnace.

    To optimize the efficiency of whatever you're installing, any time you're replacing a furnace or hydronic boiler, insist on a careful whole house heat-loss calculation (ACCA Manual-J method or similar), and don't oversize the unit by more than 10% (in fact, undersizing it by 15% is usually "safe", in terms of actually ever feeling cold.) AFUE numbers are based on about a 33% average duty cycle, which means on the coldest days of the year the thing will run almost constantly. If that's not happening, odds are it's significantly oversized, and you can subtract 10-15% (or more) off the rated efficiency, eaten up in cycling losses.

    The majority of replacement furnaces that get installed are either the same or one size larger in output (to be on the "safe" side, as in, "The customer will never call the installer screaming that they're too cold 'cuz it's not keeping up.") This is the WRONG thing to do, 100% of the time. Most of the time that "safe" factor was budgeted in by the original contractor, who only ball-parked it in the first place, and in the intervening decades the building envelope has been improved, lowering the heat load.

    There are similar cycling loss issues related to oversized AC equipment too- Manual-J or similar load calculations need to be applied here as well.

  2. #17
    Homeowner Thatguy's Avatar
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    Quote Originally Posted by Dana View Post
    Assume 50% for an annualized average for a single-glazed panel & reasonable DHW temps.

    That seems unusually high monthly-average NG usage for simply DHW for two people. What are you using for a hot water heater, and is 75% your presumed efficiency, or the presumed efficiency of the solar/electric water heater?
    Presumed; we use a total of 160 gal/day.


    Actual use patterns make a HUGE difference in actual efficiency.
    Didn't know that.

    It's common to find furnaces 100-300% oversized, especially in older housing that has seen a number of upgrades to the building envelope (better insulation, better windows & doors, etc.).
    Our 83% furnace runs 1/4th of the time in winter.
    We did get better windows, but I should scrounge up the NG bills from before the windows were put in.


    A properly sized or undersized 80%AFUE furnace can/will outperform a 300% oversized 90%AFUE furnace.
    Maybe that's why people who switch to high eff. don't see it on their bill.

    don't oversize the unit by more than 10% (in fact, undersizing it by 15% is usually "safe", in terms of actually ever feeling cold.)
    I've heard -10%

    AFUE numbers are based on about a 33% average duty cycle, which means on the coldest days of the year the thing will run almost constantly. If that's not happening, odds are it's significantly oversized, and you can subtract 10-15% (or more) off the rated efficiency, eaten up in cycling losses.
    Good. That means my house is not as leaky as I thought, based on HDD and input BTU/hr.

    The majority of replacement furnaces that get installed are either the same or one size larger in output (to be on the "safe" side, as in, "The customer will never call the installer screaming that they're too cold 'cuz it's not keeping up.") This is the WRONG thing to do, 100% of the time. Most of the time that "safe" factor was budgeted in by the original contractor, who only ball-parked it in the first place, and in the intervening decades the building envelope has been improved, lowering the heat load.
    Thanks for the benchmarks.
    Last edited by Thatguy; 04-13-2009 at 04:05 PM.

  3. #18
    Computer Programmer Bill Arden's Avatar
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    Quote Originally Posted by Thatguy View Post
    I wired a 120v 4w analog electric clock to my 240v water heater element through a dropping resistor.
    Before insulating the tank it ran 5 min out of 5 hrs. After, 5 min out of 7 hrs.
    You can use that to calculate the standby power needed per month.
    Overtime you can also measure your average power usage.

    Assuming it's a 4500watt element you have to multiply the time(in hours) by 4500.

    so 5 min every 5 hours would be...
    (5 / 60) * (24/5) = .4 hours per day.
    .4 * 31 days = 12.4 hours per month
    12.4 hours * 4500 watts = 55.8Kwh per month.
    at $0.10 per Kwh that's $5.58 per month.

    so 5 min every 7 hours would be...
    (5 / 60) * (24/7) = .28 hours per day.
    .28 * 31 days = 8.85 hours per month
    8.85 hours * 4500 watts = 39.8Kwh per month.
    at $0.10 per Kwh that's $4 per month.

    For the ROI calculations you can use the difference since only the standby power is reduced.
    $5.58 - $4 = $1.58 per month savings or $18.96 per year.

    if I remember right... a blanket costs about $20 so the ROI would be.
    $20 / $18.96 = ~1 year
    Important note I dont know man made laws, just laws of physics
    Disclaimer: I'm a big fan of Darwin awards.

  4. #19

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    First let me say that Fafco's system is trash. Stay away if you know what's good for ya.

    As far as the estimates you received, I'm going to assume that they were drain back systems and not direct(open loop) systems. If thats the case I would recommend a 120gallon tank with 2 4x8(32 square foot) collectors for a decent solar fraction. You should get 85-90% with that setup for your area.

    Quote Originally Posted by utexas2001nc View Post
    Hi!

    I'm not a plumber, just a consumer, trying to weigh the best options for a new solar water heater for my home. We have 4 people in the house (2 bathrooms) and currently a 50 gallon electric tank. I do not run my clothes in hot water. We run the dishwasher about once per day, and could do this in the afternoon to catch the most energy-efficiency. We are in Texas, so the weather is hot and sunny and we don't freeze very often.

    I have talked to multiple installers about their different products, and frankly it's a little bit overwhelming because they all recommend different set-ups.

    One recommends an 80 gallon solar tank (which gives about 35 gallons of electric-heated water) but they say that even on the cloudiest of days we could expect to get 10 gallons from the solar panels, giving 45 gallons of hot water (almost what we have now with our 50 gallon tank.) They put this tank with 2 4x8 glass collectors (not sure of the brand) and a drainback system.

    One person said that 80 gallons is definitely not enough, and we would have to get a 120 gallon tank. This is going to be difficult to fit into my water heater closet. This quote was going to include 1 4x10 Chromagen collector on a closed loop with heat exchanger to the 120 gallon solar tank, no drainback system.

    2 other companies sell the "Sungrabber" system by FAFCO, which has 2 2x12 polymer collectors with a drainback system. This gets hooked up to a standard electric tank (80 gallons).

    My questions are: is there a specific type/brand/system that works best for energy efficiency/maximum hot water? Has anyone had experience with Sungrabber? Any other general thoughts on the subject?

    Thanks so much for any input you may have! I really appreciate it.

  5. #20
    DIY Junior Member ARSolar's Avatar
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    Having researched solar hot water for quite some time, I've found that www.heliodyne.com and www.aetsolar.com are two of the best systems out there. Heliodyne has the best solar panel as far as ratings but AET supposibly has a very good panel as well.

    Both drainback systems and glycol systems are used very sucessfully in this country for closed loop systems.

    If I were installing a new system in my home, I would demand the best such as Heliodyne in either a drainback or glycol configuration. With your incentives get the best!!!

    Have you settled on a system?

    Good Luck,
    AR

  6. #21

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    I install AET almost exclusively. I can vouch for their product. I have serviced many systems with AET panels that were far older than a decade and those panels were still cranking out heat.

  7. #22
    DIY Junior Member ARSolar's Avatar
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    Quote Originally Posted by protech View Post
    I install AET almost exclusively. I can vouch for their product. I have serviced many systems with AET panels that were far older than a decade and those panels were still cranking out heat.
    Good to see someone working in the solar thermal industry! What is your preference, drainback or glycol systems?

    Do you have any opinions of the new Heliodyne Helio-Pak closed loop glycol systems using AC Wilo pumps?

    Thanks,
    AR

  8. #23

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    All DHW systems here in central Fl are direct. I have never seen an indirect system here as there is no reason to have them here.

  9. #24

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    For commercial spa systems I use ethylene glycol with glazed collectors. For residential pools and spas I use polymer drain back systems.

  10. #25
    DIY Junior Member Stevenc's Avatar
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    Default Solar heater options -

    Just a question here - sounds like there are space constraint issues with the tank and then the conventional water heater also. The question is - why not get the oversized tank to meet your needs and hold the solar heater water and add a tankless water heater to address the additional heat needed on the cloudly days?

    Steven

  11. #26

  12. #27
    DIY Junior Member Alphacarina's Avatar
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    I've had a 4 X 10 AET panel on my roof for about 4 years and I can also vouch for their quality and efficiency - VERY good product. I paid a little less than $1,000 for mine from a local dealer

    For tankage, I used two conventional Sears Best 55 gallon electric water heaters - 3 inches of polyfoam insulation in them. The outside metal skin on them remains cold to the touch (they're in the garage) even when they're both full of 160 degree water

    My system is a direct one also and the only electricity I have applied is a single connection to the upper element in the tank which directly feeds to the house and that is through a timer so it is only on for about 45 minutes in the early morning just before we take our showers. The electric power to the timer is only switched on for 2 or 3 months in the winter - Other than those few months, all the electricity is shut off and solar supplies 100% our hot water needs for 9 or 10 months in a row - We live on the Mississippi Gulf Coast. I *do* have to drain my system a couple of times each winter when we get a hard freeze . . . . If I neglect to drain the panel, it's no big deal as my controller protects it from freezing by circulating some of my hot water though the panel overnight . . . . but then I have little or no solar hot water left come morning

    The AET factory representative told me that 110 gallons of tankage was 'oversize' for a single 40 sq ft panel . . . . but the proof is in the pudding as they say. Assuming you have very well insulated tankage (so you aren't losing a significant about of heat overnight) the large, 'oversized' tankage is actually more efficient since you are storing up excess heated water from day to day . . . . which sure comes in handy when you have a cloudy day

    My wife and I built and installed the system ourselves and we've been very happy with it over the 3 1/2 to 4 years we've had it - With the variuous rebates, it's already paid for itself and now we have essentially 'free' hot water for life

    Don
    Last edited by Alphacarina; 11-28-2009 at 09:19 AM.

  13. #28

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    I'm surprised that you haven't had the collector freeze. I don't work that far north but the general consensus among solar professionals is that you are to far north for a direct low mass system. Do you manually drain the collector down in a freeze event? I wouldn't trust an Eaton freeze valve that far north.

  14. #29
    DIY Junior Member Alphacarina's Avatar
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    Yes, I manuallly drain it whenever we have sub 30 degree temps forecasted for overnight . . . . 5 or 6 times last year - I close two ball valves in the basement at the water heaters (on the two lines going up to the collector) and then open two valve and that drains both lines - There's an air vent at the top of the collector. The collector and the piping going to it only hold a couple of gallons total

    My controller will recirculate water through the collector every few minutes when it detects freezing temps if I don't drain it . . . . but that wastes lots of warm water. If there's any danger of freezing and we're going to be out of town, we just leave it drained

    An indirect gycol system would be slightly less hassle, but it would have cost us 2X more and it's not as efficient as the system we have . . . . and it works well for us. No regrets

    Don

  15. #30
    DIY Senior Member Runs with bison's Avatar
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    Quote Originally Posted by protech View Post
    I'm surprised that you haven't had the collector freeze. I don't work that far north but the general consensus among solar professionals is that you are to far north for a direct low mass system. Do you manually drain the collector down in a freeze event? I wouldn't trust an Eaton freeze valve that far north.

    Since when is Biloxi "that far north". Heck, it's difficult to get much farther south than that without getting wet...

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