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Steca Differential Controller Problems

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by sbleiweiss, Jan 10, 2010.

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  1. sbleiweiss

    sbleiweiss New Member

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    I use a Steca solar controller (TR0301U) as a differential temperature controller to decide when to charge storage. It has a sensor in the tank and another on the wood boiler outlet. When the boiler is hotter than storage by the set threshold the controller energizes the zone valve to the storage tank heat exchanger.

    I have had a problem with this controller ever since I set this system up. I wanted to compare notes with others that have this controller in their system before I seek a warranty replacement. The problem I have is that the temperature readings on the Steca jump around. The readings will change by as much as 5 degrees in the matter of a few seconds. It only seems to do this after it has been charging the tank for a while. When the system is idle, it will usually read +/- 1 degree. The large jumps when charging will cause it to stop charging the tank when it should not.

    I am wondering if this is normal for these controllers of if I have a dud. I have tried to isolate some external cause of this problem, but it appears to be the controller itself.
    Thanks,
    Scott B.

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  2. tom in maine

    tom in maine Minister of Fire

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    I have used two of the Steca 301's. One was on a solar system and the other is on my wood boiler.
    The solar system one had erratic collector readings. I was ready to just replace the sensor when I found that the readout was erratic also.
    I got another unit under warranty and have had no problems since.

    The wood boiler unit is fine.

    One thing that I did after the problem, was to solder the sensor connections to the wire that I used to run back to the controller.

    I wound up buying another controller so I could have a replacement.
    Gary at builditsolar.com had some odd problems with his Steca controller. Different issue, but he took it out.

    I hope they work out okay. I like the control.
  3. jebatty

    jebatty Minister of Fire

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    I use the Steca and have not experienced any problem at all. Your unit sounds defective to me, but be sure to check all sensor connection to make sure they are tight.
  4. ewdudley

    ewdudley Minister of Fire

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    Since your readings are stable when pump not running, perhaps there is interference being introduced by the sensor wires being too close to the pump power wires.

    For the purposes of debugging you might try temporarily running separate sensor wires by a different route, across the floor or whatever and see if that helps. If it does, figure out where the sensor wires might be picking up interference and change routings to see if you can cure it.

    Also since the pump relays in these types of units are often marginally engineered, you might want to -- as a matter of policy and as a means of curing your problem -- use the controller pump relay as a 24VAC control current relay and then use it to switch the 110VAC somewhere near the pump.

    Cheers --ewd
  5. sbleiweiss

    sbleiweiss New Member

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    Thank you all for the feedback.
    Eliot, we think alike. I have tried changing sensor wiring to twisted pair cat-5 wire, moving the control away from a 24V xformer in the zone control panel, checking for interference from the boiler draft fan motor and system circulators. I have moved the Steca to a different circuit and even replaced sensors with fixed resistors. I have not found any way to affect the problem.
    Overall I think this control is pretty well suited to this application. I will pursue getting a replacement.

    There is a hidden menu in these controls that allows adjusting the threshold values. It is not a very well kept secret. See :
    http://www.solarhotusa.com/support/literature/files/StecaTR0301UHiddenMenu.pdf

    Thanks again,
    Scott B.
  6. ewdudley

    ewdudley Minister of Fire

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    Scott, it sure sounds like Steca owes you a new controller. You've certainly earned one with your troubleshooting exercise, especially with Tom's evidence that there is such a thing as a bad controller when there shouldn't be more than a coupla-two-tree in a million.

    I'm really looking forward to taking advantage of the low-cost self-contained PID controllers and differential temperature controllers now available, but it sounds like less-than-industrial-grade quality control may be at least one of the costs of low price.

    --ewd
  7. Nofossil

    Nofossil Moderator Emeritus

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    In all fairness to the Steca, I've had an NFCS installation that experienced bizarre electrical issues including sensor readings that changed depending on whether a variable speed pump was running. Turned out to be a ground problem between buildings combined with running long cables next to each other. The Steca may be defective, but there may also be installation characteristics that affect the system. Part of an 'industrial grade' installation is careful attention to power, ground, and signal routing. Despite being a control systems engineer, I'm still learning.
  8. ewdudley

    ewdudley Minister of Fire

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    Yes, the last thing I would suspect is the controller itself. Once to convince some non-believers I had to fly to Fresno just to sketch an as-installed schematic, convert a found-on-site extension cord into a ground bus, and fly back home. No it was not our subsystem, no it was not my firmware.

    But once in a while it sure can be the controller, and this sounds like one time it might could be.

    --ewd
  9. sbleiweiss

    sbleiweiss New Member

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    I was able to close the loop with Steca. Apparently some units are sensitive to power line frequency variations. There is a software fix and new units are not affected. I tried checking the power frequency, but my counter will only read to 1/10 Hz and takes 10 seconds per update, so it can't really measure it well. I did see a range from 59.9 to 60.2 which is more than I would have thought. Voltage was good at 117 VAC. Who knows... Anyway, this company does stand behind their products and it has been easy to deal with their US distributor Sunearth Inc: www.sunearthinc.com
  10. ewdudley

    ewdudley Minister of Fire

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    I doubt they mean line frequency variations from one line cycle to the next, the U.S. grids are as rock-steady as you could ask for on a day-to-day basis, cascading blackouts not withstanding.

    They probably mean frequency variations from one continent to another.

    It would be fair to speculate that in order to cancel out line frequency noise they were doing AtoD sampling at some power-of-two multiple of line frequency and averaging-out symmetrical line frequency noise. It's a nice way of compensating for a less-than-robust board design. Except they probably were assuming 50 Hz and things went downhill from there.

    --ewd
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