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  1. #1
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    Thermistors, a Trick or a Treat

    We have an 8 year old AquaSnap 30RBA060 and a 17 year old 80Ton Flowtronics Chiller. These two chillers are connected in a common loop with the 80Ton set 5 degrees higher as a backup to the 60Ton. Weve been using the 80Ton a lot lately because the 60Ton throws a P.14 and a P.97 Alarm code, which shut down this unit.
    The main problem is the P.97 alarm, (LWT is higher then EWT by 6 degrees ), this problem has been intermittent, it may not showup for a couple of months or it could be a weekly occurrence.
    We thought it was the Thermistor Molex connector at the Computer Mother Board so we would shut the unit down and clean this connector, this would always fix this problem, but only for a short time before the P.97 alarm shut the unit down again. The other day with the P.97 code shutting down unit I decided to just shut the unit off. I waited about 4 minutes powered the unit back up and low and behold the LWT temp drop back down. I cleared both Alarms and the unit started and ran until 4 PM, then the LWT temp started to climb slowly, then after about 6 hours of spastic operation the unit shut down. ((We can watch a graph of the units display on the Web, a trick our IT department came up with.)) We were thinking it was a problem with the main computer board.$$$$$$

    We found the real problem today ((I'm crossing my fingers here)).
    The answer to this problem I think is the way Carrier mounted the LWT and EWT Thermistor probes on top of the chilled water pipe connections to the bundle. They are both covered with foam trapping moisture around the Thermistor cable at the mounting nut. I believe this moisture over time (years) was creating our intermittent resistance changes in the wire ( the lower the resistance the higher the recorded temperature ), causing strange temperature readings on the LWT computer port.

    Hope this is a treat and helps someone chasing the AquaSnap gremlins. 8>)

    Cleon

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  3. #2
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    Thanks for sharing!
    There is not better place for the working men than the union! 100% UA the only HVAC union!

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  5. #3
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    Quote Originally Posted by valdelocc View Post
    Thanks for sharing!
    Your more then welcome.
    I don't know how you guys service these computer controlled units.
    My first experience with this was with the Carrier FlowTronic 80Ton unit, fortunately the IT group figured out how to get the information from its computer so I could figure out what it was trying to do when it would fail.
    I attached a graph of the LWT Thermistor failing. The top left is the expansion valves for the two circuits.
    At 1600 hours you can see on the bottom graph that the LWT Thermistor jumped up (orange) the top (blue) is the EWT. At 2300 hours the unit shut down and the 80Ton started.
    Attached Images Attached Images

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  7. #4
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    Did you change the thermistors location?
    There is not better place for the working men than the union! 100% UA the only HVAC union!

  8. #5
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    Thread Starter
    Quote Originally Posted by valdelocc View Post
    Did you change the thermistors location?
    No
    The Thermistors are in dry wells so you would need to drain the system and drill and tap the chilled water piping for the new well placements.
    Heres a link to the 30RBA manual that shows a Thermistor and its dry well.

    https://www.manualslib.com/manual/74...page=72#manual

    The main point to remember is, these computer controlled units can trick you.
    I know a tech that changed out a computer board at big $$$$$ and still had the original problem.

  9. #6
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    Good info! By any chance, did you replace the thermistors?

  10. #7
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    Quote Originally Posted by R123 View Post
    Good info! By any chance, did you replace the thermistors?
    Yes
    After I cycled the power to the unit and saw the LWT drop back down very close to the EWT (remember the chiller has been in an alarm P.14 and P.97 shut down mode for over 12 hours) that the problem was either on the computer board or a strange problem with the LWT Thermistor that would ((sort of fix itself)) with a power cycle. Go figure, talk about a tech nightmare.

    Ive attached a graph of the unit properly working with both the EWT and the LWT Thermistors changed.
    By the way, this chilled water system has evolved over 30 years and is not correctly engineered .
    .
    Attached Images Attached Images

  11. #8
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    Quote Originally Posted by Cleon_ View Post
    Your more then welcome.
    I don't know how you guys service these computer controlled units.
    My first experience with this was with the Carrier FlowTronic 80Ton unit, fortunately the IT group figured out how to get the information from its computer so I could figure out what it was trying to do when it would fail.
    I attached a graph of the LWT Thermistor failing. The top left is the expansion valves for the two circuits.
    At 1600 hours you can see on the bottom graph that the LWT Thermistor jumped up (orange) the top (blue) is the EWT. At 2300 hours the unit shut down and the 80Ton started.
    Did you mean to say that GREEN is the Entering Water Temperature? If not, then can you explain, again?
    Also, are these temperatures taken from the chiller's sensors?

  12. #9
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    Quote Originally Posted by Nuclrchiller View Post
    Did you mean to say that GREEN is the Entering Water Temperature? If not, then can you explain, again?
    Also, are these temperatures taken from the chiller's sensors?
    My error, yes Green is the EWT.
    The data for the graphs is taken from the data port on the front panel of the unit and the IT department took this data stream and created the graphs. The graphing software is a free download on the web.

  13. #10
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    Over the years I've encountered a similar problem with the Honeywell probes VF20T or -L, if they are exposed to condensing water in chilled water applications. After some years they begin to change the value e.g. instead of 8C they show 12 C. Very tricky...

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