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Community and Q&A

Understanding Relative Humidity Sensor Readings

Flytrappist | Posted in General Questions on

Hey folks.  I’ve learned a ton on GBA, you are a great resource!

I have a Santa Fe Compact 70 dehumidifier in the closed and well-sealed crawlspace of the 1200 sf house I am building for myself.  It is doing a good job of keeping relative humidity (RH) between 50-60% in the crawlspace.  We are in humid eastern NC (zone 3A).

For the past 3 months, I have been monitoring temperature and RH in the house and crawlspace with a couple “SensorPush” sensors.  In the as-yet uninsulated/unconditioned house, the relationship between temp and RH behaves as expected- temp goes up, RH goes down.  In the crawl, I consistently see the opposite- temp and RH rise and fall proportionally.  I’ve attached a couple screenshots so you can see what I mean.  I tried switching the sensors, and obtained the same results.

Can someone explain how dehumidifiers cycle on and off, and why RH rises as temp in the crawlspace rises?  I guess I should just be happy that the equipment is working, but I know there is probably a simple answer…

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  1. Expert Member
    NICK KEENAN | | #1

    The crawlspace chart looks fine, the humidity is in a narrow band, max 60.4%, average 57, min looks to be about 55%. Within the sensitivity of the sensor in the dehumidifier and your sensor I'd say that's essentially a flat line. The crawl space temperature looks to be fluctuating following the building temperature and I'd say the two sensors disagree on exactly how much temperature compensation to apply. Humidity sensors aren't typically high precision, within 5 percent is about what I would expect.

  2. maine_tyler | | #2

    There's probably sensor nuances and anomalies as DC says, but one other possible factor could be that since the crawlspace is relatively cooler than ambient and house-interior, a rise in ambient dewpoint (associated with a rise in ambient temp) may outpace the local (crawlspace) temp rise effect on the RH, causing the crawl RH to track temp rather than mirror. Not positive if this is what you're seeing, just a thought.

    Is the crawlspace well insulated as well?

    1. Flytrappist | | #3

      Crawl has R-10 XPS from ground to 3" below top of block, and 6" fiberglass against rim joists. Our design temps are 21 and 91 degrees. No insulation in floor- as mentioned, house is still uninsulated and unconditioned. I do have a 30 cfm continuous exhaust fan bringing fresh air from above, but dehumidifier is not close to either supply or exhaust.

      I was thinking the sensor is simply demonstrating the way the dehumidifier responds to temp and humidity, and I'm curious how it does that? I've asked Santa Fe, but haven't heard back. Will contact SensorPush as well. The consistency of the pattern over 3 months suggests to me that the data is not due to lack of sensitivity.

  3. charlie_sullivan | | #4

    Here's what I think is going on. The temperature in the basement is much closer to constant than the temperature in the house, because of all that thermal mass in contact with the earth. That small temperature variation could cause a variation in RH, absent other effects, but I think that infiltration of house air into the basement is the real cause. At the house temperature peak, with the lowest RH (68%) the dew point is 77 F. AT the RH humidity peak which is maybe 82%, at about 75F, the dew point is only 69. That means the actual amount of moisture in the house air is lower during that high humidity, low temperature condition, lower by about 22% as it turns out.

    If your software can plot the dew point in the house, I think you'd get a clearer idea of what's driving the situation in the basement.

    1. Flytrappist | | #7

      Interesting. Makes me wonder if the exhaust fan is a mistake? If I am pulling warm, moist air into the crawl from my half-built house, maybe I should leave the fan off until the house is insulated and conditioned?

      1. Expert Member
        NICK KEENAN | | #8

        Yeah, it's not doing you any good.

      2. charlie_sullivan | | #11

        Yes, definitely a mistake. Ventilating a cool basement in a hot, humid climate is a classic mistake that drives up humidity there.

  4. Expert Member
    BILL WICHERS | | #5

    It's worth pointing out that humidity is notoriously difficult to measure accurately. I wouldn't be surprised if your sensors are +/- 10 or 20%. This means it's difficult to reliably measure small changes, and you really can't directly compare measurements between different sensors if you haven't at least calibrated them against each other.

    Dehumifiers typically cycle based on the humidity levels and not temperatures. I think what you're seeing is that the crawlspace is maintaining a cool temperature, so it will see higher humidity levels based on what's going on outside. You may also find the crawl space humidity levels track soil moisture (this is what I've found in my own home), so humidity goes up after a rainstorm regardless of what the temperature does.


  5. Expert Member
    NICK KEENAN | | #6

    >Can someone explain how dehumidifiers cycle on and off.

    The control for a dehumidifier is a device called a humidistat. It's an electrical switch, it turns on when humidity is above the set point and turns off when it drops below. The dehumidifier itself is just on-off, it doesn't have speeds or anything. The way the humidistat controls how much drying you get is by how much of the time the dehumidifier is on, over time.

    Dehumidifiers have compressors in them, and stopping and starting is hard on compressors, so the humidistat is set to turn on a couple of points above the set point and turn off a couple of points below the set point, so run times and idle times are long. So the humidistat should keep humidity within a band around the set point. There are two cases where the humidifier will allow excursions outside of that band. The first is when the humidifier is undersized, it's running 100% and still not providing enough drying. The second is when the humidity naturally drops below the set point, at which point the dehumidifier shuts off and no longer has any impact.

    As Bill noted, these are not precision devices. Your chart looks to me like the humidity is being kept within a reasonable band.

    1. Expert Member
      BILL WICHERS | | #9

      >"As Bill noted, these are not precision devices. Your chart looks to me like the humidity is being kept within a reasonable band."

      Just to put some real numbers on this, we typically write our customer contracts for datacenter space saying that the humidity is maintained between 40% and 60%. The reason for the large(ish) band is that it's difficult to measure accurately, and it can vary with equipment operation and the seasons. We also specify in the contract where we will measure temperature and humidity (height above floor, relation to equipment, etc.), and we specify that only our own measurement equipment will be used for any contractual purposes (to avoid customers using cheap-o meters that are way off).

      Our usual actual range maintained by equipment is closer to 40-50%. We keep levels a bit high to keep static discharge down, which is what we care about -- electrostatic discharge (ESD) can damage electronic equipment. The facilities are designed to handle these humidity levels.


      1. charlie_sullivan | | #12

        I've heard horror stories about attempts to regulate humidity in data centers too tightly, ending up with a humidifier in one aisle fighting a dehumidifier in the next aisle. Those stories may have been exaggerated by the time they got to me.

        1. Expert Member
          BILL WICHERS | | #13

          Nope, probably not exaggerated -- that's a real issue. The units (CRACs, Computer Room Air Conditioners) are often independent, with seperate controllers in each unit. It's not uncommon to see one unit humidifying and the one right next to it dehumidifying. Obviously that is not good for system efficiency, so it's something we try to avoid. I've also seen one unit cooling and the one next to it heating, because someone set the temperature setpoints slightly differently.

          We don't normally heat datacenters, we just cool them year round. The computers provide all the heat. I usually have to design in a little bit of supplemental heating for the first winter or two before the facility is running at a certain minimum level to be able to just run cooling all the time, and I often use the electric reheat function of the dehumidifiers for this purpose, since it's a short-term thing.


          1. charlie_sullivan | | #14

            Thanks for confirming that sad truth that really happens.

    2. Flytrappist | | #10

      Thanks DC. The dehumidifier runs intermittently, and as you noted, it is doing a good job.

      As a side note to anyone in a humid climate sealing up a crawlspace- I strongly recommend getting the liner installed and taped asap. I was busy getting the house dried in, and didn't look in the crawl for a few weeks. Without vents, insulation, or a liner, I had a phenomenal mold factory under there! It was shocking how quickly the mold grew on my brand new floor system. There was condensation dripping from the floor joists! I immediately got the crawlspace sealed up, and in a week or so, all the mold had turned to dust. It is now so nice under there, I told my wife I'm going to put in a pool table. (Need to figure out what to do about the 36" ceiling height though.)

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