Check valve doesn't work with Solar Drainback system.

Discussion in 'Solar and Geothermal Water Heating Forum' started by politics123, Jun 27, 2010.

  1. politics123

    politics123 New Member

    Messages:
    2
    Location:
    virginia
    Hi --

    Looking all over the place for a solution to my problem. Even though I've installed a check-valve, my system "flows" the wrong way at night, sucking hot water out of the tank.

    I have a Radco DBX type drainback system connected with AET panels on the roof. The Radco is a drainback tank with integrated heat exchanger, mounted on the floor next to the water heater, 2 pump setup -- one on the collector loop, one circulating water from the HW heater to the heat exchanger.

    During the day, the system works perfectly. Sun heats the collectors, collector-loop pump works like it should, drainback tank gets hot, circulatory pumps circulate domestic water through the heat exchanger and it heats up. At night, however, the flow reverses... how water leaves the HW heater, circulates the reverse way through the circulatory loop (out the top of the HW heater, through the heat exchanger, into the bottom of the HW heater).

    In other words, at night, I lose some of the BTUs I gathered during the day. I've installed various swing/spring checks, but they don't always shut-off the reverse flow (and, eventually, they go bad and get stuck in the open-position... i guess oxygenated hot-water is a bit corrosive)

    Anyone have an idea how to stop the reverse flow? Do I need to mount the drainback tank above the HW heater?


    Thanks!
    Michael

    PS -- just to be clear -- no flow at night through the collectors. Thermo-syphoning is only occuring on the HW --> heat exchanger loop. Solar Hot Water Drawing v2.jpg solar_bug.jpg
    Last edited: Jun 27, 2010
  2. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,691
    Location:
    Cave Creek, Arizona
    INsulate the pipes if they aren't already. IF you are getting circulation, it is because the water in the piping is cooling and creating convection in the pipes. OTHERWISE, the hot water would rise to the top of the loop where the checkvalve is and STAY THERE, since hot water rises. It will ONLY drop down to the bottom of the tank when it cools down.
  3. politics123

    politics123 New Member

    Messages:
    2
    Location:
    virginia
    The pipes are well insulated, but I think it's impossible to prevent any cooling in the pipe... Do you think a couple of vertical loops solve the circulation problem? How high would they need to be?
  4. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,691
    Location:
    Cave Creek, Arizona
    You already HAVE a vertical loop/heat trap. IT is automatic as soon as the pipe turns downward. Water either goes upward when it is heated or downward when it is cooling. IT CANNOT be hot and also flowing downhill.
  5. frankflynn

    frankflynn New Member

    Messages:
    8
    Location:
    Palo Alto, CA.
    If your heat exchanger / drain back tank is not lower than the bottom of the hot water heater then you could have some flow trying to equalize the temp.

    I have a somewhat similar system but it is the older direct, drain back (no heat exchanger, domestic hot water circulates in the panels during the day and at night a mechanical valve seals the hot water tank and drains the panels). I also have a check valve which has never given me any problems. It is unusual compared to others I've seen - it looks like a fat bit of pipe, inside is a ball (about an inch diameter - looks like a 'super ball' that we used to play with as kids) and a weak spring on top of it pushing the ball down (closed). Perhaps because the ball os not metal? Perhaps I'm lucky but it's been replaced once in 20 years because the plumber was servicing the big drain back valve (HG Sunspool) and said we should replace everything. It looked pristine to me.

    Finally - are you sure you are seeing backwards flow? In my system flow at night is impossible and I know that it does cool off even when I know no one is home and no hot water is being used.
  6. hickory

    hickory New Member

    Messages:
    1
    Location:
    Rock Hill SC
    Solar hot water

    Hi,
    I just had a new solar hot water kit installed. However the back up element seems not to be kicking in and giving me hot water when I need it for a shower. The tank reads 83 F degrees now 11pm EST. and the solar panel on the roof read 43F degrees.
    The way it was designed was to give me hot water whenever I needed it and not have to wait when the sun shines during the day time.
    Any suggestions on what went wrong? How to trouble shoot?
    Thanks,
    JB
  7. Thatguy

    Thatguy Homeowner

    Messages:
    1,459
    Location:
    MD
    An active control system vs. a passive one.
    I.e., one or more temp. sensors or a flowmeter that detects the reverse flow and controls an electrically operated valve through an electronic interface.
    Temp. sensors & interface parts, $20. Labor, one weekend depending your skill level.

    A single transistor driving the valve may do it. To reduce false alarms we may want to use an AND function with the sensor output and a photocell indicating that the sun has gone down.

    BTW, Jason Bourne I'm not, but with your lat/long given to the precision you gave it in your link, 0.001 degree, I can find your location within +/- 300' or so.

    How many watts, how many volts? Even better, post a schematic.
    Last edited: Nov 30, 2010
  8. MikeP860

    MikeP860 New Member

    Messages:
    2
    Location:
    Brooklyn, CT
    Hey Michael! There are two types of check valves. Unless you are powering your circulating pumps through a PV DC (solar panel), you should use the spring type check valves for AC circulating pumps. Another hint is that you should not install a swing type check valve vertically upside down because they have a tendency to hang open. If you are using household current to power the circulating pumps, switch to the "spring" type check valve. Make sure your arrows follow the flow. :)
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