Averaging Thermistors and How They Work

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 its 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 than 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 Ohm's 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 the resistance increases when you connect resistors in SERIES (out of one into the next). 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 halved (2,500 ohms).

This property of Ohm's law and parallel/series circuits means that we can easily average out thermistor temperatures so long as they are all the same, 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 number of sensors in parallel that you have in series, and Ohm's 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 four averaging sensors to as many as you want, so long as there are the same numbers of series and parallel sensors. That means that the total number of sensors will always be a square of a whole number.

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.




Related Tech Tips

Sometimes, Experience Can Hold You Back
When working in a business with people, you have two options: 1. Become bitter as you experience people, life, and issues. 2. Grow from experiencing people, life, and issues. I’m not proud to say it, but I realize now that when I started my business and had bad experiences, I would run to option #1 […]
Read more
Solenoid Facts
Do you know how a solenoid valve works? Really? On the surface, I think we all understand how a solenoid valve works. The coil energizes, creating an electromagnet. That temporary magnetism lifts an iron plunger within the valve allowing refrigerant to flow. But is it really that simple? It turns out that the answer isn't […]
Read more
Density of Air & Water Part 2
Here is part 2 from Michael Housh from Housh Home Energy in Ohio. Thanks, Michael! This is part two in a series on a deeper look into the sensible heat rate equations. You can find the first article here HERE if you missed it. This article will dive deeper into the density of “standard air” […]
Read more

One response to “Averaging Thermistors and How They Work”

  1. Is there a device or controller that can accept multiple thermistors and get the average??
    Just wondering because if one of the thermistors goes bad you will loose the whole signal

Leave a Reply

Your email address will not be published.

This site uses Akismet to reduce spam. Learn how your comment data is processed.


To continue you need to agree to our terms.

The HVAC School site, podcast and daily tech tips
Made possible by Generous support from