Hysteresis Thermostat – What is it and What Does it Do?

What is a hysteresis thermostat and what does it do?  At All Systems Mechanical air conditioning Santa Clarita, we specialize in the installation and repair of residential and commercial air conditioning and heating units, but we are also retained for our consultation services all across the United States.  Typically, I base these posts on the questions that I get asked most often by HVAC consumers, both commercial and residential.  Today, however, is a bit different in that we will be addressing a subject that is only rarely talked about – hysteresis.  In today’s post, we will briefly discuss what hysteresis is, what a hysteresis thermostat is, and what it does for you.

What is Hysteresis?

People get wrapped up on some of these complex terms like hysteresis (myself included) and I’ve always found that, more often than not, they are an overly-complex explanation of something very simple.  Likewise, hysteresis is an engineering term used to describe something very simple – hysteresis can be thought of as “lag,” in its most simple form.  How a reaction lags an action is hysteresis.  Let’s look at a basic example:

Squeeze a yoga mat or a piece of foam.  When you release it, does it pop back instantly?  No, of course not.  Yet, you are no longer squeezing it…the foam is slowly returning to it’s original shape, but it is lagging the action of you removing your hand.  This is a basic example of hysteresis.

However, in its most common uses hysteresis applies to electronic and magnetic systems, one of which is your thermostat, which we will discuss in the next section.  If you want to nerd out on the technical definitions and applications of hysteresis, take a look at the wikipedia link: hysteresis.

What is a Hysteresis Thermostat?

Now that you know what hysteresis is, you can probably figure out what a hysteresis thermostat is too – a hysteresis thermostat is a thermostat that is designed to lag the inputs from the environment for the purposes of saving you energy and saving your air conditioner or furnace wear and tear from turning on and off frequently.  Let’s take a look at an example of what a hysteresis thermostat is and how it helps you out:

You are living in a poorly insulated house in Santa Clarita, California (let’s just say that, hypothetically, your builder skimped on the insulation to try and make up money after the latest housing market collapse – read more in Los Angeles Housing Bubble).  But it is a scorching 100-degree day outside.  How long will it take that heat to penetrate your nicely air conditioned house?  Not very long.  So, your HVAC system (what does HVAC mean?) cools your house to a comfortable 72 degrees, but since your insulation is so poor, it is only a minute or so before your temperature raises to 73 degrees and your AC kicks on again.  This cycle repeats itself, over and over as your poorly insulated house overworks your AC in a poor attempt to maintain 72 degrees.  This thermostat is not a hysteresis thermostat – it lacks hysteresis in its programming and therefore turns on and off constantly.

Now, imagine that you have a hysteresis thermostat, and you set a temperature of 72 – your thermostat will let the temperature raise in your house until it is 73 or 74 degrees, then cool it down to 71 degrees, and allow your system (and your energy bill) to relax for a bit, saving you wear and tear on your system.

A hysteresis thermostat really is that simple.

Advantages of Hysteresis Programming.

Hysteresis provides a few key benefits to you and your air conditioning and heating system:

  • It saves your HVAC system from undue wear and tear – anyone who has worked with cars or other machines, knows that start-up is the most stressful time on your system.  By decreasing the frequency with which this happens, you decrease wear and tear on your system.
  • A hysteresis thermostat saves you energy by keeping your system from turning on and off frequently – your system will run more efficiently if it runs through a proper cycle.

Where Can I Find a Hysteresis Thermostat?

Here’s the hard part – figuring out which thermostat has hysteresis programming can be difficult.  Most companies don’t advertise this and consider it proprietary knowledge.  That being said, you can safely assume that if your thermostat was manufactured within the last 5-10 years, it will be a hysteresis thermostat or have something comparable.  Many companies use deadband programming, which is similar, but not the same.  One thermostat people seem to like is the White-Rodgers UP400 Touchscreen Thermostat, which has a narrow hysteresis band of only a few degrees, which some people feel more comfortable with.  I myself am a big fan of the Honeywell T6000 – it is basic, reliable and programmable.

Final Thoughts on Hysteresis Thermostats.

If you want my honest opinion, unless you like nerding out on how your HVAC system works, don’t worry too much about whether or not you have a hysteresis thermostat.  Almost all modern thermostats have hysteresis programming or something comparable.  If you landed on this page because you suspect that this is the problem with your system, I’d suspect that this is not so and would recommend that you take a look at our ASM Air Conditioning Blog for more troubleshooting suggestions for your problem, such as: Air Conditioner Won’t Cool, and Furnace Won’t Turn Off.  If you live in Santa Clarita or the Greater Los Angeles area, please don’t hesitate to give us a call – we cover most of Southern California and would be happy to help you!  If you don’t, please feel free to shoot us an email anyway – we will help in any way we can.

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