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  1. #1

    Questions Regarding TXV on Geothermal Unit With Desuperheater


    I'm new to the site and my knowledge of HVAC is very limited, almost non-existent before this recent incident so I'll try to give as much background info as I can. A few days ago I received a call from the housekeeper saying there was water coming from everywhere. I rushed home to find water pouring out of the walls and ceiling. After we got the water turned off and cleaned up, I started looking around to find the source. Turns out that the DHW input line on my WFI geothermal unit ruptured at the Pex/copper line junction. There was about 2ft of copper coming out from the front of the unit and then pex the rest of the way. After making some calls, my installer told me that they now recommend at least 5ft of copper on the unit side. I assumed the desuperheater line just got too hot and ruptured. As I looked more in depth to the system, I noticed that the desuperheater loop was plumbed incorrectly. Instead of DHW IN teeing into the cold water input of my hot water heater and DHW OUT going to the boiler drain, DHW IN was teed to a random domestic cold water line and DHW OUT was piped directly to the cold input of my HW Heater (making it the only input source for my HW heater). Even I, with my VERY limited knowledge of HVAC realized this was incorrect. Its not even a loop and really only works as a pre-heater if the HW heater is being drained frequently. As this is a vacation home, the HW heater very seldom gets used and the heated water from the desuperheater just sits in the line.(to my understanding). I checked the other four units we have on the property and they are all plumbed the same way. After the units dried out, our HVAC guys turned the affected unit back on and it would only run for 2 or 3 mins at a time. After some diagnostics, they found a problem with the TXV. To my understanding that is a metering device that controls the flow of liquid refrigerant. OK, now for the questions...

    1. Did the incorrect plumbing of the desuperheater and the lack of use of the HW heater cause the temperature in the desuperheater lines to become to high and make the pipe rupture.

    2. Had we had 5ft of copper installed on the DHW in and out lines, would the heat have dissapated enough to cause the line not to have ruptured.

    3. I've been led to believe that TXVs don't malfunction very often, but assuming it did, could it have caused enough heat buildup to cause the pipe to rupture.
    ---(I know more heat can be created by them causing the unit to run for extended periods, but our unit seemed to only kick on for 2-3 mins and then off for several minutes)

    4. Could the unit being doused with that amount of water (I removed upwards of 300 gallons from the house) have caused the TXV to fail.

    I feel that improper plumbing is the main cause of this issue, but my HVAC knowledge is very limited. Regardless of what caused the problem I need to have all of the desuperheaters re-plumbed correctly, but y'all know how it is when something goes wrong. Everyone is trying to pass the blame. My main objective is to get the original plumbing contractor to cover the re-install, and I need a little info in case I have to get ugly with them. In my opinion they installed something incorrectly and they need to make it right. Our HVAC contractors are stand up guys and if I have to threaten the plumbing company with enlightening our homeowners insurance about the incorrect plumbing I want to make sure it doesn't get tossed back in our HVAC contractor's lap.

    Thanks in advance for any information y'all can pass my way,

    ps. sorry for such a long post, I just don't know enough to be any more concise.

  2. #2
    Join Date
    Jan 2013
    Frederick, MD
    "Hot Water High Limit (Domestic Hot Water Option)
    This mode occurs when the hot water input temperature
    is at or above 130F for 30 continuous seconds. The DHW limit status LED on the unit illuminates and the hot water pump de-energizes. Hot water pump operations resume on the next compressor cycle or after 15 minutes of continuous compressor operation during the current thermostat demand cycle."

    This should answer question #1 for you. this is right out of the Envision installation manual. (assuming that is what unit you have)

    I would say no to all the other questions also. The pipe rupture is probably a defect in the material or faulty installation. Water damage to the unit would not cause the TXV to malfunction. With water damage my concern would be anything electrical. And with a WaterFurnace thats a lot.

    The way the water for the DHW was piped it would not even have moved water unless hot water was being used in the house. With the ruptured pipe on the inlet side of the DHW this would have a cold water supply normally. Since water was not moving through the DHW circuit it would just try to heat the stationary water in the pipe until the heat made its way back to the inlet temperature sensor and shut the pump off. Im sure that was not good for the pump as it is liquid cooled and lubricated. But that inlet pipe should not see water above 130 degrees and I'm not a plumber but I hope even pex pipe would have no problem with 130 degree water.

    Now if your unit was kicking on for 2-3 min and then back off. That could be the result of a bad TXV witch would cause the unit to shut down on one of the many built in safeties.

  3. #3
    Join Date
    May 2013
    Winnipeg, Manitoba
    For the most part I agree with Fredrick_tech.

    I have had the pex line rupture on one of my units and this was why.

    1.The Desiperheater (DSH) pump failed. Same result as your piping problem. No flow.

    2. It was heating season and the home owner let the air filter get extremely dirty.

    Remember the DSH water is heated with the super heated vapor directly out of the compressor and the dirty air filter will jack the head pressure and temp way up. Now this pump had been failed probably for awhile by it was a really cold night so the run times were probably really long.

    The system is designed to have water in the desuperheater heater and no flow(like when the DSH tank is at temp) when another external problem shows up that's when you may have problems.

    Good luck I hope you get this figure out with out too much difficulty.

  4. #4
    Join Date
    Feb 2014
    Westport, PA
    Hi, yesterday I had my geothermal with DHW system burst the pex tubing. Mine is plumbed correctly, accept it only has about 2 inches of copper protruding from the unit. 6 days prior to this event my installer replaced my air coil, and Freon filter. I was wondering if you ever got any answer to the pex getting too hot.

  5. #5
    Join Date
    Feb 2014
    Memphis TN area
    Is it pex with brass connections and copper rings? or plastic rings? and is the burst in the plastic pipe or at a joint?

  6. #6
    Join Date
    Feb 2014
    Westport, PA
    Pex tubing is what burst, the tubing is connected to a brass fitting with copper, or a metal crimp ring similar to water plumbing in my home.

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