I have 2 30yo collectors that I want to hook up direct to the cold water to the Water

Discussion in 'Solar and Geothermal Water Heating Forum' started by randrew, Aug 26, 2012.

  1. randrew

    randrew New Member

    Messages:
    4
    Location:
    Pollock Pines, California
    Can I do that? lol I have 2 - 4ft x 8ft solar water collectors that are about 30 years old now that I want to use to cut down on my electric bill - I was hoping I could just put in a loop out to the first collector, hook it up directly to the scond collector, and then back into the house to the water heater - sort of a hot water pre-heater system if you will. I would put in 3 shutoff valves (so I can bypass and drain collectors in the winter) - one on each of the lines going to and from the collectors and one in between the two lines in and out to the collectors - in the main (old line) to the water heater. My concerns are over heating in the summer - would that be an issue? I live in a forrested area of Northern california between Sacramento and South Lake Tahoe - summer temps are around the 80-100f range, there are some shade issues from the 60+ft pine trees but not too much... everything I have seen online had a heat exchanger tank - didnt find anything like what im wanting to do. are there other issues that KI should be thinking about?

    Any Help is appreciated!

    -Randy
  2. Dana

    Dana In the trades

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    2,930
    Location:
    01609
    What you're proposing probably won't meet code, and carries a real risk of a steam explosion if allowed to stagnate. To just plumb it in-series with the hot water heater (no pumping loop) you have very little thermal mass in the panel system, so you WILL hit very high temps, and you get very little energy benefit without the water flowing when the sun is out. There are batch heaters outer there that can be plumbed that way, but they have a few 10s of gallons of water in the collector which both stores the collected energy and keeps the collector temps down, but most aren't suitable for places with high freeze risk.

    If the aging collectors designed such a way that they will drain reliably via gravity (many are, some need to be mounted with a bit of tilt), it's safer to run them at atmospheric pressure, and build yourself an insulated tank where can use a big coil of PEX containing the potable water as the heat exchanger as pre-heat to your electric tank.

    It would be similar to Gary Reysa's system, except that you have the benefit of starting with salvaged thermal panels. (He lives at altitude in Montana, so his summer/winter temperature issues are similar to yours.)
  3. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,869
    Location:
    Cave Creek, Arizona
    Solar collectors work by circulating the water through them many times increasing the temperature slightly each time. Just running the water through them, except for the initial blast of extremely hot wate due to no circulation, will give very little "preheating" to the water heater. Have you tested the collectors to be sure they have not been damaged by freezing?
  4. randrew

    randrew New Member

    Messages:
    4
    Location:
    Pollock Pines, California
    Thanks. that is very helpful. My concern of doing this was that the water in the collectors might sit in there for an extended time during the day when no one uses much hot water... I need to investigate an inexpensive solution for a storage take with heat exchnager... :) I was planning on draining the panels for the months of Dec-Feb... it doesnt get real cold here normally but it can dip into the high 20's form time to time... The collectors are encased in a nice aluminum frame with glass - tempered im guessing - its the kind you see in bathroom windows with bumps on one side that is not see-through. the copper on the inside looks a lot like Gary Reysa's system - with aluminum fins im guessing...
  5. randrew

    randrew New Member

    Messages:
    4
    Location:
    Pollock Pines, California
    I have no t tested them yet but they came from an Uncle and were known to be in good shape when I got them over 10 years ago.
  6. Dana

    Dana In the trades

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    2,930
    Location:
    01609
    Without recirculation there isn't enough thermal mass in the water contained in the collector's tubing to keep it from going over boiling temp. But even if it stagnates at 200F instead of blowing up under steam pressure when it boils (that DOES happen with solar collectors, most systems have pressure relief valving to keep that from being a permanently destructive event should the recirculation pump fail on a hot sunny day.) And as hj points out, once that slug of superheated water is purged in an in-line flow the rate of heating drops to a very low number during flow.

    Most commercial collector glazing is a low-iron soda glass for higher collection efficiency. Reysa's DIY collectors usually use polycarbonate plastic greenhouse glazing- he's more interested in cost per BTU-bagged than BTU per square foot efficiency. But the concept is sound, and his cheap DIY thermal storage tanks work too. Using copper plumbed collectors you may have a slight amount of lime-up using it at atmoshperic pressure in drain-down mode, but not enough to matter for your intended purposes. (They were probably designed for closed loop glycol running 12-20psi.)
  7. protech

    protech Master plumber

    Messages:
    53
    Location:
    Central Fl
    As others have already stated, running the panels in series with the tank will result in very poor solar fraction (probably 10% or less). You need to install a differential controller and circulation pump if you want to get any benefit from the panels. You will also want to install a tank big enough to store the heat without kicking the backup heating on all the time. The thermocline in the tank needs to remain below the thermostats(s) in the tank so the backup heating doesn't come on unless there is unusually high demand.

    www.winterhavenplumber.com
  8. randrew

    randrew New Member

    Messages:
    4
    Location:
    Pollock Pines, California
    Thanks for the additional information. I have decided to hold off trying to install them for now. I am now thinking about building a pergola for the panels to sit on, and moving my existing water heater to a new enclosure on the exterior of the hosue which I will build big enough for the WH and the Thermal HE Storage Tank - then install everything properly. In the meantime (over the winter) I will test each panel with air pressure to make sure there are no leaks in the panels.
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Solar and Geothermal Water Heating Forum ABS for solar collectors May 10, 2012

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