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Is My Solar Water Heater Working???

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Posted by Thomas Flannigan on April 28, 2006, 3:45 pm
On Wednesday we finally switched on a solar water heater installed on my
roof in north suburban Chicago.  As far as I can tell we are the first
in our town to have such a device.  I think it is working.  Can someone
tell me if I am rights?
     The line into the Rheem Solaraide was running at about 140 degrees
Fahrenheit.  The line out, about 70 degrees.  The last two days I have
run the dish washer and washing machine on time delay so the old backup
tank, linked to the solar tank, would demand water.  When I got home the
  line in was running at 70, then shut off when sun stopped striking the
20 watt PV panel driving the water pump.  The line from the Solaraide
tank to the backup gas tank was hot.  It seemed to be working.
    But last night I started a load of laundry at 8 PM and the gas
heater on the gas backup tank kicked on.  Should't two days of sunshine
have produced enought hot water without requirirng the gas element to
kick in?
   We do 20 loads of laundry a week which probably uses about 300
gallons off the water tank, plus showers for 2 adults, bubble baths for
2 small kids and the dish washer 7-10 times per week.
Nervous in Chicago

Posted by SJC on April 28, 2006, 5:55 pm

  I am trying to envision the layout of your system. It seems that the =
solar tank
is a "preheater" to the regular water heater. It depends on what the =
water temperature
in the solar tank is.
  If the line into the solar tank is 140F and coming out 70F then it is =
putting that much
heat into the solar tank. That still does not tell us how hot the water =
is in the solar tank
nor how much water is in the solar tank. We would have to know how big =
your collectors
are and how big your tank is and what flow rate your pump is running at.
    If the line between solar and regular is say 70F, then that is the =
of the water entering the regular tank. If the regular tank is set to =
come on below 140F
then it stands to reason that the water will have to be heated from 70F =
to 140F in the
regular tank.

Posted by Thomas Flannigan on April 28, 2006, 6:06 pm
 Thanks for the reply.  We have 3 collectors, 12 feet by 20 inches.  Each
water tank is 80 gallons.  The old gas tank, now a backup, is set at the
lowest setting, 110 degrees.  I don't know how how hot the water in the
Solaraide tank is.
   The reason I ask this is I have read that you can turn off your
backup unit during the summer.  I also live in a place where nobody
knows anything about this.   You go to the hardware store to buy a drill
pump or tubing and they look at you like you are a apace alien. I have
no other place to turn to.
   A plumber installed the system with me helping. He had never done it
before but he is very good.
   I imagine 140 in and 70 out means the heat exchanger is working on
the Solaraide.
  Confused in Chicago.

Posted by SJC on April 28, 2006, 6:15 pm

   The drop from 140F to 70F seems like a lot. That would say that the
solar tank is a great heat sink and the heat exchanger inside is =
However, the fact that you can get the water back up from 70F to 140F
says that you have some of the most super collectors known, or that your
pump flow is slow.
  The 12 feet by 20 inches sounds like they may be Fireball collectors.
These are piped in series as I recall from their cutaway illustration, =
you would have to confirm what kind of collectors. The more conventional
4 foot by 8 foot collectors made by Sunearth and others are a parallel =
configuration, which would not product this kind of rise in temperature.
   Just from what I have heard, I would say your pump is either =
is blocked, or is getting insufficient power to operate with enough flow =
transfer enough BTUs. You are getting such a high temperature rise, I =
say that where you should be getting 2 gpm flow you are getting more =
1/2 gpm. Just a guess with minimal information.

Posted by daestrom on April 29, 2006, 3:05 pm

I have to agree with SJC.  The outlet temperature of 70F means the tank
temperature *must* be 70F or less.  A very slow flow would mean the hx
outlet is almost exactly the same as the tank temperature.  And it would
explain why the inlet temperature is 140F.

Some folks are fooled thinking that since a slower flow raises the collector
outlet temperature (say, to 140F in this case), that a lower flow somehow
improves throughput.  Completely wrong.  A higher flow will result in a
lower collector outlet temperature at first, but will transfer more heat to
the tank and warm the tank up more quickly.  Eventually, with a hotter tank,
and hotter hx outlet (and therefore collector inlet) temperature, the
collector outlet will rise and you'll end up with a tank of hot water.

But as long as the hx outlet temperature stays low (70F in this case), all
you have is a tank of tepid water and the backup heater will still have to
heat the water from 70F up to its setpoint.


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