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appropriate high temperature limit for the storage tank?

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  • appropriate high temperature limit for the storage tank?

    I’m continuing to learn more about my DHW solar thermal system that came with my house. It’s got two Steibel Eltron Sol 25 Plus collectors on the roof and a SBB-300 Plus tank in the basement and has been operating pretty much fine for the past 7+ years. However, the controller (Goldline TL-30) shuts off the circulator when the tank temperature reaches about 145 deg F (measured on the hot water pipe leaving the tank, well before the mixing valve). I’ve got the TL-30 high limit dialed to its max (supposedly 220 deg F). So two questions: In general, what’s an appropriate high temperature limit for the storage tank? And, why might the TL-30 high limit shutoff be kicking in at such a low temperature?

    One important detail - the storage tank sensor (a GLSB-10k best as I can tell) is not in the lower thermal well as expected, but rather simply taped to the metal cleanout port cover near the bottom of the storage tank. This port is covered with a cap with 2” of foam insulation, so it seems like the sensor should be getting a decent temperature reading from the lower part of the storage tank. If anything, I think the sensor would be underestimating the temperature in the storage tank and therefore allowing the tank to reach a higher temperature before the controller’s high limit kicks in. I’ve made a few quick measurements and the 10k sensor changes resistance with temperature roughly as expected. Could the precise temperature-resistance response of the sensor degrade over time?

  • #2
    Originally posted by sroof View Post
    I’m continuing to learn more about my DHW solar thermal system that came with my house. It’s got two Steibel Eltron Sol 25 Plus collectors on the roof and a SBB-300 Plus tank in the basement and has been operating pretty much fine for the past 7+ years. However, the controller (Goldline TL-30) shuts off the circulator when the tank temperature reaches about 145 deg F (measured on the hot water pipe leaving the tank, well before the mixing valve). I’ve got the TL-30 high limit dialed to its max (supposedly 220 deg F). So two questions: In general, what’s an appropriate high temperature limit for the storage tank? And, why might the TL-30 high limit shutoff be kicking in at such a low temperature?

    One important detail - the storage tank sensor (a GLSB-10k best as I can tell) is not in the lower thermal well as expected, but rather simply taped to the metal cleanout port cover near the bottom of the storage tank. This port is covered with a cap with 2” of foam insulation, so it seems like the sensor should be getting a decent temperature reading from the lower part of the storage tank. If anything, I think the sensor would be underestimating the temperature in the storage tank and therefore allowing the tank to reach a higher temperature before the controller’s high limit kicks in. I’ve made a few quick measurements and the 10k sensor changes resistance with temperature roughly as expected. Could the precise temperature-resistance response of the sensor degrade over time?
    Safety first. I'd check the tank nameplate for any limits on temp./pressure. Also, check the relief valve. Most of the U.S. requires safety devices that relieve (open, "lift", or operate) at +150 PSI and/or +210 deg. F. As a practical matter, most residential hot water storage tanks are set at ~~+140 to +160 deg. F. +/- a lot due to generally cheap and usually very inaccurate thermostats. Solar sensor quality is a bit better than that.

    Next, make sure the controller is functioning properly. They're usually quite robust, but don't last forever. Replace it if necessary and don't cheap out, but don't get more bells/whistles than necessary. As a practical matter it may be as cost effective to replace it. They're ~~$100 -$200 or so depending.

    With solar thermal sensors, the placement, attachment and insulation are often overlooked in terms of their importance, beginning with the idea that 2 differential sensors give better service and closer approximations of reality if they are attached to similar equipment or devices in an identical fashion with equal high levels of insulation (both attached to pipes with metal bands and thermal grease for example, rather than one to a pipe with a metal band and the other one taped to a flat surface, or better yet, both placed in in identical thermowells, etc.). Also, I'd move the tank sensor to the return line from the tank to the collector as close to the tank as possible. I'd use thermowells for both, filled w/ thermal grease and heavily insulated - again, both to the same degree in the same fashion. Remember, they need to be accessible and serviceable. If attached to a pipe, use metal clips, thermal grease and lots of equal levels of weatherproof insulation around each sensor, as waterproof as possible. The high insulation for a more accurate temp. and treating them the same for precision and a better differential temp. reading.

    Keep in mind, the temp. that the sensor reports is no more than an approximation of reality. The better the thermal contact between the sensor and the surface it's attached to, the more accurate the reading. Or, to be slightly more correct about it, the lower the ratio of: (the thermal resistance between the sensor and what it's attached to)/(the total thermal resistance between the fluid and the surrounding outside environment), the more accurate the temp. reading will likely be .

    Bottom line: Make sure the controller and sensors are working nominally, put both sensors as close to the collector or tank outlets respectively as possible, attached to piping with good thermal contact or in in thermowells if possible, with heavy, weatherproof insulation.

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    • #3
      Temperature setting depend on the construction type of storage tank. Glass lined steel tanks limit out at 140 degrees. SS steel tanks can go higher.
      Expansion and contraction of the tank can chip the glass liner allowing the steel to rust.

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      • #4
        appropriate high temperature for storage tank

        A storage tank is used to collect the heated water. Its true that there are different models of storage tank and temperature limit also vary. Glass lined storage tank maintains the hot water by allowing heat losses of less than 1 degree Fahrenheit per hour. I too had solar water heater installed by Mod note - a company that is too cheap to pay for advertising.

        Nice try but you are banned
        Last edited by russ; 09-27-2014, 11:30 AM.

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        • #5
          too hot for flesh?

          There's too hot for the tank/system and then there's too hot for flesh.

          If you were getting 100% (temperature) out of your solar at 220 degrees and you tossed your baby into bath water that hot the doctors at the emergency room would be obligated to call the police.

          That and the fact that water boils at 214 degrees.

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          • #6
            Originally posted by cherrick View Post
            There's too hot for the tank/system and then there's too hot for flesh.

            If you were getting 100% (temperature) out of your solar at 220 degrees and you tossed your baby into bath water that hot the doctors at the emergency room would be obligated to call the police.

            That and the fact that water boils at 214 degrees.
            Actually that is 212° F at sea level.
            [SIGPIC][/SIGPIC]

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            • #7
              212

              Originally posted by russ View Post
              Actually that is 212° F at sea level.
              ugh. My bad. And I even had it up in Google in a browser just to make sure. I still typed it in wrong.

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              • #8
                So Death Valley, at an elevation of -282' at the lowest point, is in fact sooo hot that water even boils at a higher temperature.
                And a pressurized DHW system could in fact get to a storage temperature as high as 220F.
                SunnyBoy 3000 US, 18 BP Solar 175B panels.

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                • #9
                  OP here - thanks for all the advice.
                  I've got a good mixing valve, so even when my storage tank reaches 140 deg F, the water at my faucets is cooler (and we're careful with the baby's bath water!). I finally found more detailed specs for my tank and learned that it is glass-lined steel and has a recommended temperature setting of 176 to 185º F. That's hotter than I thought! I think the tank has never been hotter than 145º F, as measured at the upper temperature sensor port.

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                  • #10
                    Originally posted by inetdog View Post
                    So Death Valley, at an elevation of -282' at the lowest point, is in fact sooo hot that water even boils at a higher temperature.
                    And a pressurized DHW system could in fact get to a storage temperature as high as 220F.
                    A set of steam tables will easily give the saturation pressure of water as f(temp.) or sat. temp. as f(pressure).

                    FWIW, sat. (boiling) temp. at 55 PSIA ( ~ 40+14.7), a not uncommon household water pressure = +287 F.

                    A tank at the bottom of an open loop water system w/ say 20 ft. of additional total elevation on top of it will probably have a sat. (boiling) temp. ~~ +230 F. or maybe a bit less depending on the decrease due to pump suction/velocity head reductions.

                    Hopefully, pressure/temp. safety devices will be set to operate before equipment design limits are reached.

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                    • #11
                      Originally posted by J.P.M. View Post
                      A tank at the bottom of an open loop water system w/ say 20 ft. of additional total elevation on top of it will probably have a sat. (boiling) temp. ~~ +230 F. or maybe a bit less depending on the decrease due to pump suction/velocity head reductions.
                      But water in the tank at that temperature would boil as it circulated through the upper parts of the thermal loop. So that temperature could only be reached through aux heating at the bottom of the tank and with both pumped and thermosiphon circulation prevented.
                      SunnyBoy 3000 US, 18 BP Solar 175B panels.

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