How Does an AC Thermostat Work?

Thermostat Temperature Setting Does Not Change Air Temperature

 

Having owned Magic Touch Mechanical, an air conditioning company in Mesa, AZ for over twenty years, it’s easy to forget most people have no idea how central air conditioners work. Most of the time I get my blog article topics from listening to people outside of the HVAC (Heating, Ventilation, and Air Conditioning) Industry, and the questions they have about AC, heating, and home energy efficiency.

 

Which brings me to today’s topic: Turning the dial or turning your thermostat temperature setting up or down, does not change the temperature of the air coming out of your vents (air registers).

 

My sister-in-law is in town visiting and just by chance, it came out that she was under, what appears to be a common misconception, that there was a correlation between the thermostat temperature setting and the actual temperature of the air coming out. When I asked a few more people if they believed the same thing, I realized they all did. So, if you thought the same, you are not alone, and after reading this article you are now “in the know”.

 

Changing Your Thermostat Temperature Determines How Long Your AC Runs

 

If you thought adjusting your thermostat temperature to 75 degrees meant you would have 75-degree air coming out of your vent, it does not.

 

Changing the temperature of your AC thermostat simply determines if the “run-cycle” will be longer or shorter, based on the current temperature in the home, and what you set it to. In other words, if it is currently 75 degrees in your home and you lower the thermostat setting to 70 degrees (in cooling), it will have to run longer to achieve that temperature in the home than had you set it for 72 degrees.

 

Of course, there are other variables including; how efficient your home is (less efficient homes will most likely require the air conditioner to run longer), your air conditioners condition, the outdoor temperature, and how leaky or tight your air ducts are.

 

When measuring temperature, it will be colder in cooling, or warmer in heating, when checking closer to the unit than it will be when measuring it at a vent. This difference is due to energy waste known as duct loss. Duct loss is basically energy waste due to leaks, ductwork that isn’t insulated well, and having long lengths through unconditioned spaces (like attics, crawlspaces, and basements). This is why I’m a huge proponent of ductless air conditioners and heat pumps – which you can learn about by searching the word “ductless” in my blog’s search tool.

 

 

What Should the Temperature Coming Out of my Vents Be?

 

What should the temperature of my gas heating system be?

If you were to measure the temperature of heat coming out of a gas furnace directly at its output (where it connects to the ducts), you would likely see somewhere between 120-degrees and 140-degrees. When measuring it at the vents you will likely see a 20 to 40-degree lower reading because of the aforementioned duct loss.

 

What should the temperature of my heat pump system be?

If you were to measure the temperature of heat coming out of a heat pump directly at its output (where it connects to the ducts), you would likely see somewhere between 105-degrees and 130-degrees. When measuring it at the vents you will likely see a 20 to 40-degree lower reading because of the aforementioned duct loss.

NOTE: This does not mean a gas furnace will heat your home better than a heat pump…they just work differently. I own multiple heat pumps personally.

 

What should the temperature of my air conditioner be?

The temperature coming out of your vents should be about twenty-degrees lower than the temperature being sucked into your return register (where you put your air filters – which I know you change regularly!). The reason I didn’t use an actual temperature range as I did with heating, is; to understand this we need to get more in depth and I’m going to write a follow up article on it. I’ll place a link to that’s article here when it’s posted.

However, if your return and supply temperature measure roughly 20-degrees difference, that is ideal. Much less and you probably have a problem, much more you may also have a problem.

 

 

Don’t Turn Your Thermostat Temperature All the Way Down!

 

So now that we know that the temperature setting has nothing to do with the air temperature, we also know there is no reason to “bury the dial”. Turning your ac all the way down will only cause it to continue to run nonstop.

 

If your AC is running nonstop and your home is not at meat-locker like temperatures, there is something wrong and it’s time to call a professional. Maybe it’s a problem with the ac unit itself, but maybe it’s a problem with the home itself – which is where an energy auditor comes in. I recommend finding a company well versed in both but remember to do some research first or you may just get some bad advice and throw good money after bad.

 

Headed Down Pacific Coast Highway!

 

I’m eager to wrap this article up because I’m currently in CA where I just finished a few days of ductless air conditioning training at the Mitsubishi Electric training center. I have a new drop-top on my Jeep and am ready to go catch some coastal sunshine and take the long way back to Arizona!