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Using a remote meat thermometer to measure HVAC duct temperature.

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I needed to get an accurate reading of the air temperature in my ducts when the furnace was running and during the cool down process. 

Diagnosing the problem.

I was using a Thermomist spray humidifier in my forced air gas furnace. I hadn’t used it for a couple of years and it died. I replaced it with a Trion Mister Mister unit. That was a little over $100. 

The Trion unit is fairly simple. It works off 24 volts and consists of:

  • A humidistat
  • A Selenoid valve
  • A thermostat

I will detail the issues in another post but the short take is that the thermostat is supposed to close at 110 +/- 5 Fahrenheit and open at 95 +/- 5 F.

There were some incorrect settings on my 40 year old Singer furnace with regard to the fan on/off settings.  I needed to get an accurate reading of the temperature inside the duct since the humidifier is triggered on and off by the temperature (and humidity). 

I had a hole that will be covered over with a plate from the old unit. I could check that with a laser thermometer but that requires opening the duct while the fan is running.  

The solution – a remote meat thermometer.

I purchased a remote meat thermometer for about 10 dollars at Walmart. This is a simple unit with a probe. For it’s intended use, you keep the readout outside your oven and run the probe to the meat. They are very accurate.

Most of these attach magnetically which is great for the old style sheet metal ducting. I simply drilled a hole slightly larger than the probe just a little upstream of the thermostat. Be careful not to drill into anything such as air conditioning coils or wires that might be running in the duct.

Results

The results were pretty impressive. I was able to watch the temperature rise and fall with the heater’s cycle. (That is about as exciting as watching paint drying)

I was able to confirm that the fan on/off settings on my furnace were WAY off. The fan was shutting off around 100 degrees. That left 100 degree air in the duct. First of all, that is ineffecient and secondly, a thermostat designed to open at 95 degrees would still be closed resulting in a puddle under the duct.

After adjusting the fan on/off switch, I was able to get the duct temperature down to about 80 degrees at the end of the heating cycle. However, I found that the thermostat was not opening as designed. It would open at 80 degrees.

USE CAUTION IF YOU ADJUST THE FAN SWITCHES. If you rotate the fan/limit switch dial you can damage the bimetal sensor. That can be an expensive repair or possible result in a fire or damage to your furnace.

I contacted Trion’s support and they are sending out a replacement unit.

In short, having an accurate reading of the duct temperature allowed me to more easily diagnose issues with the furnace and the humidifier.

When you are done you can seal the hole for the probe with a sheet metal screw.  

If I can’t get this working, I may go back to my humidifier hack

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