VisionPro Outdoor Sensor Reading inaccurate
 

Hi. I have a VisionPro TH8320 thermostat with an outdoor temperature sensor installed. For about 1 1/2 years the outdoor sensor reading was fairly accurate. I noticed over the summer, it was reading high (I have another thermometer close to the sensor). Figuring the sensor went bad, I replaced it. The temperature reading is still 20 degrees too high. The temperature reading moves up and down, but reads consistently 20 degrees above the actual temperature. Unfortunately, I use this sensor to change from the heat pump to my back up heat source. Any idea what could be causing this error?

Was any new electrical wiring done, that may run near the sensors wires oming back to the thermostat.

If you know how to use a multimeter you can ohm out the sensor to see if it's accurate. You should be able to find the resistance tables on the Honeywell website. It's possible that you got a bad second sensor but not likely. You can ohm it to see. If the sensor is ok then I would be looking at an internal circuitry problem with the thermostat assuming there have been no other electrical changes that could influence the wiring.

The resistance chart is in the install manual of the thermostat.

One more thing: Make sure all the wiring connections for the outdoor sensor are tightly connected and not corroded. A bad connection can increase the resistance of the circuit and cause the reading to be inaccurate.

A higher temp reading indicates that the circuit is shorted somewhere. If you were to take the sensor leads and touch them together, you would read max temperature. Take your sensor off and connect it directly to the thermostat inside. See if it reads the same as the indoor temp after a few minutes. If it does you will need to replace the sensor wire. I would recommend shielded cable.
If the sensor reading is still high take your resistance reading. You should be around 10k ohms at 77 degrees F. If that's ok then its time for a new stat.

[QUOTEA higher temp reading indicates that the circuit is shorted somewhere. If you were to take the sensor leads and touch them together, you would read max temperature.][/QUOTE]

They are negative coefficient. So shorting them together reads the lowest temp.

A thermistor loses resistance the higher the temperature goes. Not the other way around..

There are positive and negative coefficient thermistors. But your right, HW does use the negative ones. The controls I'm working on at a place currently, is using positive coefficient. Getting which ones I'm talking about confused.

PTC - Thermistor, Positive - the resistance increases with increasing temperature.

NTC - Thermistor, Negative - the resistance decreases with increasing temperature. :thumbsup:

Thank you.

Yes I found this out after I made the heat come on in july..no one in the building thought it was funny...

LOL... Brought the heat on in a place where i was using 20,000 and 10,000 ohm sensors on the same stats. I wired them up wrong the first time. Ooops.

Hey guys, thanks for the information. Originally, I thought it might have had something to do with the way the builder wired it. On the sensor instructions it tells you to run dedicated wire directly to the tstat and not through the tstat wires. It also says to keep it away from any electrical lines and electric motors. Upon tracing the sensor wires, I realized that they ran the wires into the furnace (near the blower motor) and joined them up with the tstat wires and ran them back to the tstat. But, alas, I multimetered the tstat and the OHM readings were accurate for the outside temp. It appears the issue is with the tstat. Unfortunately, this is the second time I had this problem. The first time, I was still under the builders warranty, so it got fixed on their dime. This time it is all me. Not sure why I keep having this issue, but am thinking about upgrading to a tstat with a wireless sensor. Any thoughts?

Shielded wire