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Why isn’t my sensor turning on my system when it is snowing outside?

Posted by on Aug 4, 2011 in Frequently Asked Questions | 0 comments

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When we say sensor, we are talking about a CIT-1, GIT-1 or a SIT-6E (or one of their predecessors). A sensor cannot switch a system on its own and must be installed with a control (or control panel). If your system is using a device that looks like a sensor, but directly switches a load, then you probably have a controller. GIT-3A, GIT-4, HSC-4, HSC-5, LCD-7, and other LCD models are all controllers; which combine a sensor with a switching relay. 

A sensor requires the following:

  1. Power from the control.
  2. Temperature below 38°F (3.3°C).
  3. Moisture on the moisture grid.
Warning: Danger of electrical shock. Only qualified individuals should perform troubleshooting and maintenance of branch circuit(s), heater(s), and control equipment.
 
Start by ensuring that the control has power. Regardless of the model of control your system has, there should be a green led indicator for SUPPLY power on the front panel. If the SUPPLY indicator is not lit, verify the supply voltage from the branch circuit running from circuit breaker. Check to ensure that the circuit breaker is not tripped. 

After verifying that the control has power, check the sensor supply voltage. Examine the wiring to the sensor. Your system may employ multiple sensors. Verify the integrity of the wiring and any slices. Verify that junction boxes, conduit, and related hardware is properly installed.  Independent of weather conditions, the functional operation of a sensor may be determined using a digital voltmeter (DVM) and a ten pound (4.54kg) bag of crushed ice or freeze spray, in the following manner:

  1. With the DVM set the 100 VAC range, verify the sensor supply voltage by connecting the negative (-) test lead to the sensor black wire and the positive (+) test lead to the sensor red wire. A DVM reading between 22-28 VAC or 24-30 VDC is acceptable. An AC component less than 28 VAC is allowed with a DC voltage in the proper range.
  2. With the negative (-) test lead remaining in place, reconnect the positive (+) test lead to the sensor blue wire and confirm there is a similar voltage  present (may be 1 to 2 volts less).
  3. For a pavement mounted sensor it is easiest to test proper operation with crushed ice. Place the entire supply of crushed ice atop the sensor and allow a time lapse in excess of 20 minutes before observing that a low voltage (2 volts or less) is indicated by the DVM. A nominal temperature below 38° F (3.3° C) must be detected before the sensor’s normally-open contact is closed. If the sensor surface temperature was excessive prior to commencing this procedure, it will be necessary to extend the cool down time interval by as much as an additional 20 minutes.
  4. For an aerial or gutter mounted sensor it is easiest to test proper operation with freeze spray. If you do not have access to freeze spray, place the entire supply of crushed ice atop the sensor or place the entire sensor in the crushed ice ensuring that the temperature probe (small, white, and opposite side of the sensor from the moisture grid) is covered with ice, and allow a time lapse in excess of 20 minutes before observing that a low voltage (2 volts or less) is indicated by the DVM. A nominal temperature below 38° F (3.3° C) must be detected before the sensor’s normally-open contact is closed. If the sensor surface temperature was excessive prior to commencing this procedure, it will be necessary to extend the cool down time interval by as much as an additional 20 minutes. If using freeze spray, spray directly on the temperature probe for 30-60 seconds. The probe has a layer of plastic and epoxy between you and the internal sensor so there may be a slight lag between cooling the sensor and the sensor’s normally-open contact closing.
Other things to look for:
  1. Obstructions: Overhanging trees, roofing, dormers, and so on, can cause issues by blocking falling snow and/or by falling debris.
  2. Heat sources: Chimneys, vents, other exhausts can artificially raise the temperature at the sensor. 
  3. Animals: Animals resting on a sensor may block snow/ice and artificially raise the temperature at the sensor. We have also taken services calls for system troubleshooting where the problem turned out to be a neighbors dog urinating on a sensor.
  4. High accumulations can bury the sensor and allow the moisture sensors to melt a cave in the snow, which will not allow snow to touch the grid. This igloo effect will render the sensor inoperative. This effect can be reduced by the use of a moisture cup on gutter sensors and by placing aerial sensors at a slight angle to allow gravity to help remove excess snow. A carefully aimed snowball or tennis ball could help in a pinch, but Environmental Technology assumes no liability for personal or property damage cause by errant projectiles.