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What is this putty holding the sensor in drywell of indirect tank?

My aquastat was showing bad readings, upon inspection I found that the sensor was held in the drywell on the indirect water tank by some kind of putty that had dried and cracked letting the sensor come out a little and start reading the ambient air. How should the sensor be fixed in the drywell on the indirect tank? Is this putty the right stuff, if so - what is it so I can get some more to replace the cracked putty.

thanks in advance for your help,


  • HomerJSmith
    HomerJSmith Member Posts: 2,468
    It's hard to say. Is the putty malleable or brittle? The putty might be there just to keep the sensor in place.

    I would remove the putty and pull out the sensor, very carefully. If it is binding stop pulling as you might break the wires. A solvent like brake clean sprayed into the well to loosen the paste may help remove it. Clean the sensor and well. Use new thermo paste on the sensor and replace the sensor in the well. The thermo paste is the same kind of paste used on computer heat sinks.
  • Zman
    Zman Member Posts: 7,579
    I wouldn't pay much attention to the putty. They make clips which with do the same job. As mentioned, heat transfer paste on the sensor is way more important. It looks like someone has added some extensions to the well which may effect accuracy. I would pull the sensor out and see how far into the heater that well goes.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • mattmia2
    mattmia2 Member Posts: 9,754
    edited November 2019
    So someone just jammed it in there with plumber's putty or something because they didn't have the clip...

    Unless that well is really deep so it goes in to the tank at least a couple inches, that nipple and coupler they used because they didn't have a socket to tighten the well directly in to the tank could be placing the sensor where it isn't particularly close to the water.
  • hot_rod
    hot_rod Member Posts: 22,271
    The most common well looks like this, it takes a sensor 3/8" or 1/2" od.

    Most of the new boilers use sensors that are 6mm, about 1/4" od. A compression adapter is one way to reduce the large style well to accommodate the newer 6 mm thermistors common on mod cons.

    You can buy small squeeze tubes of heat transfer grease, I prefer the silicone based type, it seems to not harden like the metallic based compounds.

    Some versions has a tapered copper wedge that would push the sensor against the well wall, still good to add grease.

    Wire or zip tie the leads back against the well is a bit cleaner compared to plumbers putty if you don't have the official clip.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream