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New solar install with flaws!

BenBen Member Posts: 18
So here we go..... I pay $3000 a year for winter heat in a newer home. I decided in November to install 120 evac tubes on the garage roof. I was doing this on a tight budget, $4400 invested so far.

 I use the hot water provided to preheat my DHW. My storage tanks also have a top coil that I return my radiant floors (my only heat source) through as long as the tanks are greater then the floor return temp. Then to the internal coil in my propane DHWT.

I used 3/4" copper for the panels BUT then I switched to PEX to get down the wall, outside (buried) and into the basement. I plan on using my controller to shutdown at 175F to stagnate the panels. My 2 expansion tanks(4gal) are on the copper side mounted in the top of the garage about 6 feet below the panels. One supply and one return side. system pressure is 15psi.

I am looking for a few things from you helpful people.

1. Bets on how the PEX will last

2. System efficiency

3. How would you add another storage tank and plumb in if I was going to get a 80gal non working hot water tank(no exchanger). I have lots of open controller function yet.

Thanks to all,



  • hot rodhot rod Member Posts: 6,999

    Start with a simulation program, RET Screen, F-Chart, T-sol, etc. this will help determine the potential of that array at your location under the conditions you have. It would help you determine the amount of storage that makes sense.

    Pex is a gamble in any solar system. The potential to send a slug of 400f or higher fluid from an evac tube array is risky at best.

    Not reason to lock out the array if the temperature exceeds the pex temperature. if a load is present you should continue to harvest.

    All systems have the potential to stagnate. Suppose the power goes out mid summer day. When it comes back up that fluid will hit the pex.

    I'm curious why two expansion tanks connected in different location? By connecting in different places you have established multiple points of no pressure change or reference for the pumps.

    In solar piping it is common to see the expansion tank connected to the line going up to the collector to put it in the coolest fluid.

    Some solar expansion tank manufacturers suggest a cool down tank connected to the expansion tank, especially on evac tube systems. This is to prevent the tank bladder from seeing excessive temperatures.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • BenBen Member Posts: 18
    my reason, your thoughts?

    The reason for the 2 exspansion tanks, one on each side of the collector is for the emptying characteristics. There is a check on the supply side and I did not want to push all the hot liquid one way due to using the pex. If the pump is off, this should allow stagnation empting of the collectors equally both way reduing the risk of super heated fluid reaching the pex. I only have to empy the 6 headers and 40' of 3/4" copper, hopefully half each way. I have a 15' copper lead into the expansion tank. I hope with this the temp will not hurt them.

    How would you add another storage tank if it was just a pressure vessel (old hot water tank)?

  • BenBen Member Posts: 18
    edited February 2012
    Any help?

    Anyone please? You opinions count.

    Is it going to cause a pump head issue if I have the tank 50' upstream of the pump before the tubes?
  • Karl_NorthwindKarl_Northwind Member Posts: 140

    i would consider putting a tank inline with the return from the collectors and pulling out thru a mixing valve, set to 180F.  a 10 gal electric water heater in an insulated box will help.  that will keep your pex from seeing over 180F.  

    I would leave the expansion tank alone for now, and potentially add a HX to dump heat directly to the floor loops.  I think you might be a bit undersized for 120 tubes with the HX's in your tanks. 

    is the floor concrete?  you can dump quite a few BTUS into the concrete before you heat it too much.  I assume because the lines are buried the tubes are on a ground rack, do you want to cover half or more of them in the summer?

  • BenBen Member Posts: 18
    Tube location

    The tubes are mounted on the 45' of my garage roof and I can get some crazy wind here. I hope to avoid covering them! So here is something to throw around.

    Plz check IMG.

    Would this work to dump heat into my garage floor? Set the mix valve at 100 and kick the pump on when the copper just infront of the pex gets to 160? Would I kill this pump 5' before I would let my panels stagnate?
  • joseph annonjoseph annon Member Posts: 33
    pex and solar

    Did a job where the customer insisted on using pex on a ground mount array for money reasons.

    Had a problem with the pump and the pex which was about 20' from the collector was superheated and failed. It also means that you will have to careful when charging the system to make sure that the temperatures in the collectors are cool enough for the pex.

    SO as long as there are no problems and everything works perfectly you will have no problems.
  • Kevin_in_DenverKevin_in_Denver Member Posts: 588
    PEX in the Collector Loop

    I've had PEX survive 230F at 20 psi. You probably have enough expansion to always keep things 20 psi or below. Make sure the proper air pressure is in the expansion tanks. The factory pre-charge pressure of 12psi is probably about right.

    Be super careful about the pump starting during a stagnation event. At 60psi, I've had the PEX blow right in front of me at about 210F. A tiny, weak pump is probably unable to push a steam bubble out of the collectors, so that's one possible strategy.
    Superinsulated Passive solar house, Buderus in floor backup heat by Mark Eatherton, 3KW grid-tied PV system, various solar thermal experiments
  • NRT_RobNRT_Rob Member Posts: 1,009
    If possible

    maybe you could put a temperature limiting valve prior to the pex on the return. Then it would not stop flow from the panels but would "sip" heat from them until cool enough to allow for full flow?

    would mix with the solar panel supply water.
    Rob Brown
    Designer for Rockport Mechanical
    in beautiful Rockport Maine.
  • GSEGSE Member Posts: 9
    too hot

    Solar is supposed to be a 30-40 year product.  Starting out with materials already exceeding their design parameters ie the pex is not smart.

    While I'm not a fan at all of ETs, they will hold the heat longer than a flat plate and you're going to see higher stagnation temperatures/pressures, further straining your PEX.

    Space heating systems need either a well designed dump loop, secondary summer heat load, or be in drain back.  Otherwise it's a risk in itself.
  • zacmobilezacmobile Member Posts: 211
    pex & solar

    We did this....once. It was our first solar install (which was incidentally the first viessmann collectors installed in canada) the tubes were mounted on a hill next to the house & we ran 1-1/4" pex through two 80ft. long 4" pvc conduits. The rep came by and was checking out the system and closed & opened some valves which caused the fluid to become super heated then when it hit the pex... well you know the rest. I wasn't there but the other guys said it sounded like a bomb went off. I kept the piece of pex around for many years until an over-zealous apprentice threw it out. We ended up pulling 3/4" soft copper through the conduits with a come-along. Lesson learned.
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