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solar expansion tank

Brendan_2 Member Posts: 14
when the expansion tank fails and circulation is lost will thermomax tubes be damaged from stagnation , and what is the best way to prevent this

Thanks for your all you do at this site


  • hot_rod
    hot_rod Member Posts: 19,289
    good question for the manufacturer

    of the tubes. All systems have the potential to go into stagnation conditions. It happens in power outage, pump or controller failure. Also when the tank is up to temperature the pump shuts down and the collectors "cook".

    The main think with evac tubes is to use a high temperature glycol that can withstand those stagnation temperatures. Tyfocor is one brand that offers very high temperature solar fluids. Osterwalder is another German fluid designed to 230 c temperatures.

    But you really don't want the collectors going into these extreme over heat conditions, use the energy somehow, or install a dump zone. Or plate collectors :)

    If the expansion tank has failed it could be caused by excessive temperatures. Sometimes a cool down tank is installed up line from the bladder tank to protect the high temperature fluid from destroying the bladder expansion tank. Zilmet offers these tank in the US now, nipple out both ends to pipe inline with the bladder tank.

    Most of the new solar controllers have an evac tube setting OTC (option tube collector) It watches the collector temperature and pulses the pump to try and keep the fluid from over-heating. it runs the pump at 100% speed for 10 second periods when the max. temperature is exceeded.

    But, of course you still need some place to send that energy. Setting the storage max. temperature above 180F shortens the tank life and could pop the T&P valve when the tank stratifies.

    Be sure to install a listed, quality thermostatic valve on the tank if you plan on running those elevated temperatures.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Brendan_2
    Brendan_2 Member Posts: 14
    solar servicing

    Thanks Hotrod for your detailed response

    Does anyone have experience with Thermomax/Viessmann heatpipe tubes they have a touted overheat protection mechanism in the condenser how effective is this

    I had some defective tubes replaced by Kingspan Solar free of charge but they requested a closeup photo of the condenser of each tube for whatever reason

    Also what is the useful life of the expansion tank under normal conditionns

    Thanks again
  • hot_rod
    hot_rod Member Posts: 19,289
    as for exp tanks

    their life has a lot to do with the job you assign them.

    First the sizing is critical. Solar thermal systems work over a much wider temperature range, size them with a solar sizing formula, not a hydronic sizer.

    Ideally they are installed on the coldest fluid side of the system, this being the piping going to the collector. Keep them away from potential stagnation temperatures, by not installing them on the hot from the collector piping.

    Still, temperature in solar, especially with evac tube systems is the concern. I agree with the European approach of a cooldown tank before the bladder tank, and a solar specific tank, not a standard hydronic expansion tank.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • michael_34
    michael_34 Member Posts: 303
    can you post the photos

    of the condensers? I would like to see what they look like. I have seen Vacuum tubes go bad. When they do, they get a creamy white color at the condenser point on the glass.


  • Brendan_2
    Brendan_2 Member Posts: 14
    thermomax solar tubes


    Sorry i don't have the pictures now but the defective tubes i had were defective out of the carton apparently they had a batch number that had no vacuum, the glass was warm to the touch but the silver coating was intact and looked perfectivly normal

    it is my understanding that the actual condenser the copper bulb can get distorted if stagnated too long

    hope this helps Brendan
  • Kevin_in_Denver_2
    Kevin_in_Denver_2 Member Posts: 588
    Thermomax quality is Suspect

    A thread from a few years ago:


    There are three different failure modes:

    1. loss of vacuum in glass tube

    2. Loss of partial vacuum (or loss of vapor) in the heat pipe

    3. Failure of heat limiting device (a snap disc I think)

    Stagnation temperatures (400F+) can certainly distort copper components and turn them black. It probably only takes one day of full sun. A distorted condenser bulb isn't a failure, but it can make it impossible to replace if it does fail because you can't remove it from the header assembly.
    Superinsulated Passive solar house, Buderus in floor backup heat by Mark Eatherton, 3KW grid-tied PV system, various solar thermal experiments
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