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Munchkin outdoor sensor

Joe_103 Member Posts: 4
I emailed HTP online support and told them of my problem. I will let you know the outcome.


  • Joe_103
    Joe_103 Member Posts: 4
    Munchkin outdoor sensor

    I have an 80M Munchkin (natural gas) I installed last year with the Vision 1 and a Superstore indirect water heater. Been making domestic hot water just fine since installation, and the fintube baseboard heated the house just fine all last winter.

    I finally turned the thermostat up today to start my heating season- house was 60º and 45º outside- but when I did that I didn't hear any of the relays kick in down in the basement that should normally make themselves heard.

    I went down and checked and sure enough there was no "flame on" indication on the control panel, no faults, no pump running- nothing. Just the "Actual Temp" indication of 85º (the thing had heated the superstore back up about 45 minutes ago).

    I checked all external connections, wiring harness connections to the 925 controller, continuity of the thermostat wiring- everything I could see. Shut the power off to the unit, let it set a minute and repowered- no change in status. Pressed the reset button 2 or 3 different times, and still nothing.

    I finally looked in the "operating manual" on page 36, section F concerning the Status menu and read that. Did what it said, and as I was reading the values displayed I got to "d5", which is ~Actual temp from the outdoor sensor~ and it read 156º !!! I double and triple checked it to be sure I was checking the right value. I sure was. I went outside and took the weather cover off that sensor and wiggled the wires a bit, nothing out of the ordinary- it worked all last year with no problem. I don't think it was damp- no visible water drops under the cover.
    I shut the power off, disconnected one leg of the outdoor sensor, powered the 80M back up, and shazaam~ the unit started firing! I am glad I checked the things I did and found that "problem".

    Now, what could've made the outdoor sensor give a reading of 156º when it is 50º outside? What resistance reading should I get on a digital Ohm meter from the leads going to the sensor.

    I guess I'll see if HTP can get me a new sensor if I can't get this one to come out of it.
  • Plumb Bob
    Plumb Bob Member Posts: 97

    The sensor is a thermistor and its resistance is lower when the temperature is higher.

    When the sensor resistance goes to zero, the Munchkin thinks the temperature is 156F. Actually what you probably have is a short that has caused the resistance as seen by the Munchkin to be zero. Could be a bad sensor, but it's quite likely to be in the wiring somewhere.

    if the above explanation made sense, you are probably handy enough to trace the short with an ohmmeter. If it is the sensor, you should get it replaced, but not before confirming that it's not in the wiring.

    The Munchkin uses a 1.2Kohm thermistor. At 50F, it should be somewhat higher than 1200 ohms, but when you measure it across the leads near the boiler, you'll find it's less than 1 ohm.
  • Plumb Bob
    Plumb Bob Member Posts: 97

    Sorry to state the obvious, but just in case you are not real familiar with electricity:

    (a) disconnect wires at Munchkin and measure the resistance across leads. It will be in the single digits rather than kilo-ohms.

    (b) go outside and disconnect leads from sensor, and only then measure sensor resistance. If single digits, replace sensor. If normal (kilo-ohms), then measure resistance at the disconnected leads. If it is in the single digits, the short is somewhere in the wiring.

    Again I apologize if this stuff is obvious to you.
  • radmix
    radmix Member Posts: 194
    munchkin senser

    I have had a series of munchkin sensers go it is probably not a short.
  • Munchkin OAS failure

    I thought that Munchkin/HTP took care of their sensor problem! Now we're seeing more bad outdoor sensors. What's the deal?
  • tim smith
    tim smith Member Posts: 2,752
    outdoor sensors

    I have had a couple that water gets into casing, now I use silicone and seal up case well to stop this.
  • WaterHeaterGuy
    WaterHeaterGuy Member Posts: 80
    They did...

    ... who knows how old his boiler is, or more specificly how old his Vision I kit is.
  • Bryan_16
    Bryan_16 Member Posts: 262

    I installed one last Thanksgiving, worked great. When customer started it backup this fall the sensor was shot. Thought it was 156 degrees out! 4th one bad in 4 years, out of 25 installed. Sure sounds like a problem to me.
  • Joe_103
    Joe_103 Member Posts: 4

    Well, my unit was bought in late '05 and installed in Oct. 06.

    I took the sensor off the wall and used a digital tester to measure the resistance. It was all over the scale on the 2000 ohm range. Varied up and down from 150~1400. I wasn't sure if it was my tester or what, so I took it to a friend of mine who has a well equipped shop and he couldn't get me a defininte answer to whether the thing was good or not.

    Anyway, I printed off a page of these discussions and showed him what you all said the thermister was rated at, and we looked in electronics parts catalogs and couldn't find one rated at 1200 ohm- they're either 100, 500, 1000 or more- no 1200's. He says I could series a couple of 100's and a 1000 and it would work. Also said I could try a 1000 and see what the sensor tells the Munchkin.

    The reason I want to try to do it this way is because when I called the place HTP told me to call (in Kansas City) about parts for this thing, they quoted me $63.00 for this jewel. I can get 63 bushel baskets of thermistors for 63 dollars. HTP tech desk also told me the thing will run without the sensor, but I realize that the unit will fire more on the heavy side without its' input.

    I also asked HTP and the place in KC about what I've read about a series of bad "sensors" and he said for me not to believe everything I read. I explained that I value what I read on here because no one stands to gain anything by lying, so why should they not be telling the truth?
    He didn't have much to say about that, and didn't volunteer to send me a new sensor. Like I told him, it hasn't been in service even a year yet, so why would it go bad? The only thing he would allude to is that the people they buy the sensors from MAY have had a run of bad ones, but that was early in the product development and would not probably be the same batch as the one I have---- yeah right. Anyway, I think I will try putting three of them in series and see how it works..wish me luck. Joe

  • Plumb Bob
    Plumb Bob Member Posts: 97

    Fluctuating resistance reading = bad sensor.

    Yes, the 1.2K thermistor is an unusual item. I have no idea why HTP chose this particular sensor.

    You can use a 1K sensor and it will work except the temperatures will be several degrees off (problem is worse at low temperatures, but still better than running without sensor). Putting 1K + 100x2 in series to get 1.2K will work perfectly. Just check that the outdoor temperature is read correctly by the Munchkin (within 1 or 2 degrees).

    HTP is pretty good about sending replacement parts for free although they absolutely hate to pay for labor. I don't know why they wouldn't do so in your case. Sending e-mail may be better than calling. Mention that you posted a question on the Wall, that gets their attention, no one likes bad publicity.
  • radiantcomfort
    radiantcomfort Member Posts: 1

    when the sensors go bad, they read 150F. I have had 4 of these. you need a new one.
  • Ted_9
    Ted_9 Member Posts: 1,718

    If it's a year old or less, return it under warranty.
  • Joe_103
    Joe_103 Member Posts: 4

    The outcome is good. I was contacted by Bryan of HTP and told him the story and he is shipping a new sensor, free of charge, to replace the original.
    I plan on sealing this unit with silicone caulk or something similar.
    KUDOS to HTP, and thanks for the information from this site!
  • Bryan_16
    Bryan_16 Member Posts: 262
    another one

    Had another od sensor bad on seasonal start-up. 2nd one this year 2nd one on this job in 5 years. So much for never having any trouble with them.. "We've never heard of that problem before. You're the first one to have a problem" Bryan
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