Tag: thermistor

There was a question in the Facebook group a few days ago about averaging sensors. There are two common configurations/methods used for averaging. The first is simply a setting in a thermostat or control where it reads separate sensors and then the thermostat itself averages out the readings using software.

For example, if the onboard sensor is being averaged with a remote sensor it could look like this:

Onboard Sensor = 78°
Remote Sensor = 82°

78° + 82° = 160

160 ÷ 2 = 80°

So the average temperature is 80° between the onboard sensor and the remote sensor. This could be handy if the remote sensor is in one room with a different solar or equipment load that the other but there is no automatic damper to separate the zones.

The other strategy is to simply wire sensors as averaging which has nothing to do with the thermostat or control and everything to do with Ohms law and the nature of parallel and series circuits.

A thermistor (temperature sensor) is a type of resistor that changes resistance based on temperature. There are many different types of thermistors but for this strategy to work, they all need to have EXACTLY the same thermistor properties.

You probably already know that when you connect resistors together in SERIES (Out of one into the next) that the resistance increases. So if you connect a 5,000-ohm resistor in series with another 5,000-ohm resistor they would have a resistance of 10,000 ohms.

What you may not know is that when you connect two resistors in PARALLEL you give the electrical current two paths which decreases the resistance. In fact, if you connect two 5,000 ohm resistors in parallel the total resistance would be half or 2,500 ohms.

This property of ohms law and parallel/series circuits means that we can easily average out thermistor temperatures so long as they are all the same and all the connections are good and we don’t have runs that are too long, as this will add in resistance and throw off the readings.

Take a look at the image at the top.

All you need to do is have the same # of sensors in parallel that you have in series and ohms law does the work. We don’t need to have the thermostat do the math because the series sensors add together and the parallel sensors divide.

This means you can have a few as 4 averaging sensors to as many as you want so long as there are the same # of series and parallel sensors. This means that the total # of sensors will always be a square of a whole #.

2×2 = 4
4×4 = 16
5×5 = 25

So on and so forth…

This can come in handy when conditioning a large room with a single zone but it is also somewhat troublesome because if any sensor fails the thermostat or control will read incorrectly.

— Bryan



In HVAC we work with thermocouples for temperature measurement all the time and we may not understand the difference between a thermocouple and other types of temperature measurement sensors.

A thermocouple is just two different types of metal connected together at a “junction” that generate a small voltage with a change in temperature. Older furnaces would often use a thermocouple as a sort of flame sensor that would be immersed in the pilot flame and would generate a small control voltage that would “prove” that the pilot flame was lit. You will still often see this sort of millivolt control system on water heaters, fireplaces and some pool heaters.

We often use a “K-type” thermocouple for line and air temperature measurements. This thermocouple uses one wire made of a chromium/nickel alloy called chromel and another made of an aluminum/nickel alloy called alumel. The voltage across this junction changes as the temperature changes and the thermometer is calibrated to track this voltage change and display a temperature readout.

A thermocouple can often be confused with a thermistor which is a resistor that changes in resistance with a temperature change but they do not function in the same way at all.

Thermocouple sensors must be calibrated to the thermometer or meter they are plugged into using and ice bath to be accurate and they tend to be slightly less accurate than thermistors.

— Bryan


When you work on a heat pump system and you want to test defrost there are so many different test procedures to follow to test the board and sensors. Most involve “forcing” a defrost by shorting out pins on the board or advancing the time of the defrost initiation and installing a factory provided pin jumper.

Lots of pins and jumping involved.

But one thing to need to be able to distinguish is whether the system uses sensors or thermostats to initiate and terminate defrost.

A thermostat is an open and closed switch, they are usually round in shape like the one shown above and they open within a set temp range and they close within a set temp range. The one shown above is a Carrier Defrost Thermostat and it closes at 30 degrees +/- 3 degrees and it opens at 65 degrees +/- 5 degrees. In this case, because this particular sensor closes in colder than 32-degree temps you can’t even use an ice bath to test it. If it is below 32 outside it is easy to test (duh) otherwise you can just run it in heat mode with the cond fan off and see when it closes by using an Ohmmeter.

On a defrost thermostat you can also easily jump it out to test the board since it is just open an closed.

A defrost sensor is a thermistor. A thermistor changes resistance based on the temperature it is exposed to. In order to test you can measure the ambient temperature, make the sensor is removed and acclimated, measure the Ohms of resistance and compare to the manufacturer chart.


You CANNOT jump out a thermistor.

— Bryan

P.S. – A podcast about Heat pumps is available HERE

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