Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Adding Elec. HWH to Solar Storage

Saggs Member Posts: 174
I'm looking to install a 3 panel solar system for a fellow who also wants elec backup and additional storage. The cost difference to go from a 80 gal solar tank to a 120 gal w/ elec backup is like $3,000. Does anyone have a simple piping layout that would enable the solar tank to circulate its hot water thru the elec tank when it is hotter than the elec's set point? Maybe have a bronze circulator send water thru the elec from the solar when it reaches 140 and the elec is set at 120? Thx in advance.


  • Big Ed_4
    Big Ed_4 Member Posts: 2,706

    A pump can be use or you might want to just go gravity. First you twin in the two tanks , reverse return ,Cold in one tank , hot out of other . Then tee hot and cold to the other tanks . Now If you connect the two drain ports with a pipe and add a common drain. Two ball valves on either side of drain to control flushing. The hotter tank will flow to the cooler tank ...

    Now if you just want to dump solar to electric you would need a pump to control. Delete the common drain pipe and install the pump on the hot off the solar pumping to the electric hwh. After twining in tanks. Use a auto mix on both set ups and after the pump.
    I have enough experience to know , that I dont know it all
  • hot_rod
    hot_rod Member Posts: 21,874
    how many

    people? 80 gallons should be plenty of solar for a family of 4, especially with a back up tank.

    You really don't want to "mix" the solar tank. It wants to be stratified so the coldest water, where the solar senor is located, isa always at the bottom of the tank.

    Look for a 1.5 to 2 gallons of storage per square foot of panel.

    A 120 gallon tank with upper element would provide solar and 50 or so gallons via the element if ther is no solar. Maybe a one tank, 120 would be enough. But a dual tank will provide more harvest than one tank that has the upper element activated.

    With RET Screen, F-Chart or other solar design software you can run different system combonations and get solar fractions and payback nunmers.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Larry Weingarten
    Larry Weingarten Member Posts: 3,247
    Big tanks...

    ... usually cost more per gallon of storage than smaller tanks. I'd listen to what HR and Big Ed have said and add the possibility of creating a bigger tank by tying two tanks together both top and bottom. This, done right, allows gravity to equalize temps in the tanks and not mess up stratification. You wind up with more, less expensive, easier to work on solar storage. Always a good thing :~)

    Yours, Larry
  • Larry C_13
    Larry C_13 Member Posts: 94
    Solar collectors rated for POTABLE water?

    Are the collectors rated for use with potable water? Or are you using a separate loop for the solar collectors. How do you deal with freeze protection and stagnation?

    Larry C
  • Steven Eayrs
    Steven Eayrs Member Posts: 33
    a dedicated solar tank.......

    which never is heated electrically would be my choise. Give the max. solar gain. This tank would simply be piped in series, before a smaller electric tank. Doesn't take much to keep a little bit of water hot w/ electric, and the solar would supply hot water to it. IMHO
  • kpc_73
    kpc_73 Member Posts: 26
    I guess..

    I don't understand my "mixing" is bad. how else do you add capacity to an existing solar set up... I realize I have A LOT to learn here. Any books to read up more on this? ty,kpc
  • Mech E
    Mech E Member Posts: 10

    I believe that an 80 gallon solar tank piped in series with a much smaller electric tank would be the best option. You could also install a recirc line between the two tanks to allow more solar Btu's to be transferred into the electric tank (making your solar system more efficient). I found two nice schematics of the systems which I am talking about (thanks to Viessmann). Please see them below.
  • Saggs
    Saggs Member Posts: 174

    Thx to everyone for chiming in. So it looks like I should use the 80 gallon w/ the dual coils (one solar, one boiler) and take the hot out of the solar into the cold of the elec (maybe 40 gals?) then run all domestic out the hot side of the elec thru a mix valve. The viesman drawings look like they are "twinning" them or are they more in "series?" I like the idea of gravity doing the work for me. Will the gravity work better if the elec. tank is smaller than the solar?
  • hot_rod
    hot_rod Member Posts: 21,874
    do you want

    solar and boiler on one tank via a dual coil, AND and electric tank? So a total of 120 gallons of DHW?

    That seems like a lot of DHW.

    If you have a boiler, with at least 80,000 input, and you want to use a dual coil tank with boiler input on the top, I don't see a need for the electric "backup" tank.

    A boiler with proper piping, DHW priority and a properly sized circ should provide a good DHW flow.

    My system has a 60 gallon Heat-Flo dual coil tank. Solar input at the bottom coil, and a Lochinvar 80 Knight feeding the top coil.

    That setup can run a shower non stop at about a 2 gpm flow and never run out of DHW.

    In the summer months the solar provided 90&% of the DHW, a 60% solar fraction across 12 months.

    I'd rather a single coil solar tank feeding over to the electric backup tank. You would harvest much more if the entire 80 gallon tank was just solar supplied. As soon as you heat the top half with a boiler you reduce the solar potential.

    I don't know where the job is located but a 3 panel 80 gallon system should have a 55- 75% solar fraction.

    Unless the electric cost are very high that 25- 45% should be do-able with the electric backup tank.

    Regardless of the final answer the tanks are just piped in series, cold into the solar "pre-heat" tank, feeding to the electric tank. A good, listed 3 way thermostatic on the output of the second tank. No need to pump across the two tanks.

    If DHW re-circ is desired it just flows from the second tank, you don't want to mix the solar tank, it works best when it is allowed to stratify.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Saggs
    Saggs Member Posts: 174

    HR, he has a steam boiler for home heating and it didnt make sense to keep that big block hot to assist his summer/ spring DHW needs. So if the tanks are piped in series will the natural stratification keep the elec. satisfied on a good sunny day? Sounds simple enough, Thankyou
  • EJ hoffman
    EJ hoffman Member Posts: 126

    Use tekmar differential controllers. Though you might want to add a relay or timer to shutoff electricity to the elements when pump is on, or just a timer to shut off elements for a few hours during the day while the solar is collecting and every one is off at work or school. Have not messed with a taco oo vdt yet but this could also work.
  • Mike C._4
    Mike C._4 Member Posts: 56
    another idea

    I have essentially the same setup-a solar preheat with an indirect smaller tank. I also have Taco 006 pump with an arrangment of valves to recirculate between the tanks. I just use it with an appliance timer to move the heated water over to the indirect tank. I have shut the indirect zone off and set the timer to start cycles late in the morning when the water is usually heated. The differential control is a better way to go though.
  • hot_rod
    hot_rod Member Posts: 21,874
    yet another method

    is to use a 3 way valve and temperature control. The flow is from the solar until it drops in temperature, say 110F for example, then it shuttles to the backup source. In this picture from Caleffi Idronics 3 it is shown with a tankless as the backup. No reason that couldn't ba an electric tank style heater.

    It will require a 3 way valve with a high psi shutoff of course, typically a zone valve will not have a high enough shutoff. A 3 way motorizied ball valve would be a good choice, watch the Cv rating.

    It does add some cost and complexity to the installation. The other drawing shows a basic in series dual tank arrangement.

    Caleffi does build a non electric valve to do this also, but the flow rates are a bit low for a typical U.S. system, it's a European only product at this point.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
This discussion has been closed.