We’re having a heat wave

“I can’t explain it, don’t understand it…It’s like a heat wave”

I hope Linda Ronstadt isn’t reading this, but I’ve contorted her words to capture a common misperception about thermostats.

While we experience a week-long, or longer, summer heat wave here in Northern California and other parts of the country, I want to focus on thermostats.

Prefer a visual? This is now available as a YouTube video, too!

What is a thermostat?

I think everyone knows that a thermostat is an electrical (at least) or electronic (more likely now) device to regulate heating and air conditioning systems.

In the industry lingo, thermostats are a vital component of any HVAC (for heating, ventilation, and air conditioning) system.

But what does regulate actually mean?

Basic functions of any thermostat

The thermostat is wired to the control switches or control processor of the HVAC system and allows turning it on, turning it off, and changing its settings.

Whether you have separate heating and cooling systems, including boilers and compressors, or an integrated system, such as a heat pump, it doesn’t matter.

Modern thermostats provide a universal interface to control the operation of the environmental air handlers in your home or building.

Turn it on or turn it off – these are very basic functions. If that is all that thermostats did, they would be nothing more than a standard switch.

The “secret sauce” of every thermostat is the ability to control the resulting room temperature automatically.

Set a desired target temperature or “set point,” and the thermostat will automatically control the heating, cooling, or both to reach that desired temperature level.

How temperature control works

Temperature control adds one crucial function to every thermostat to make this work.

Early thermostats, which have been around for years, used a special bi-metallic strip of two different metals physically joined together into a coil.

Probably not my intent to delve into the minutiae of the physics involved, but through the magic of dissimilar thermal expansion of different kinds of metals, the thermostat measures the air temperature in the room by expanding and contracting.

The resulting change in shape of the metallic coil opens or closes a dry contact electrical switch or relay which turns the HVAC system on or off.

Programmable and smart thermostats

Now, making a thermostat programmable, or even adding further smarts, isn’t all that complicated.

Additional control logic in hardware, software, or a combination of both allows for the automation of the thermostat.

One or more schedules can adjust the thermostat automatically based on the time of day, the day of the week, season of the year, the arrival and departure time of the residents, the occupancy/vacancy status of the home, and other factors.

Machine Learning and Artificial Intelligence

More advanced smart thermostats with Internet connectivity can be controlled remotely for convenience, and some can consult historical and real-time weather databases to make adjustments in programming on the fly.

Yeah, nowadays, it is sexy to call that processing artificial intelligence (AI) or even simply machine learning (ML), but it isn’t that complicated to understand.

The ubiquitous Nest Thermostat was arguably the first smart home device to marry beautiful aesthetics with elegant, functional controls while throwing in a sprinkling of ML programming.

So, what’s the problem?

The dirty little secret about HVAC systems is they are simple – much simpler than most of us think.

Automated scheduling, computerized controls, fancy mechanical buttons or dials, and even mysterious bi-metallic temperature sensing strips are all hiding one indisputable fact:

At their core, thermostats are just an on/off switch, not a speed control.

What I mean is – it doesn’t matter what target temperature you choose; you can’t make your HVAC system run faster or slower just by setting a lower or higher temperature further away from the current room temp.

Too hot? Too cold? Change the temperature on the thermostat and wait.

That bears repeating – Whether you have a fifty-dollar basic one or the fanciest programmable smart home thermostat for several hundred dollars, you can do nothing to make your HVAC run faster – other than buying a bigger/more powerful unit.

Burning up and want results faster?

It doesn’t matter if you set the temperature to 20 degrees or just 1 degree lower; it cools down at the same rate.

Freezing? Set the temperature above the current room temperature—any setting—and the heat will warm you up—at the same rate.

Target temperature – what it really does

The target temperature you set on your thermostat does only one thing – it sets the final desired temperature and automatically shuts the system off when that is achieved.

It doesn’t change the speed or power. You can’t make it cool down or heat faster by choosing a different setting.

So, the thermostat for an HVAC system is very different from the accelerator pedal in a car!

What’s the problem?

So what’s the big deal?

The placebo effect of setting a higher or lower temperature setting to try and command your HVAC system to work faster has, in my opinion, one drawback: If you forget that you have done this, especially if you are making the change before going to sleep, you might be waking up to a freezing cold room in the summer or a burning hot room in the winter.

There’s also a secondary problem – Because the physics of thermostats and the counter-intuitive rules of thermodynamics are not widely understood, you still might have trouble convincing your spouse or family members that they needn’t bother futzing with the thermostat constantly if they feel too cold or too hot.

Just throw in the towel and let them twiddle the dial or push the buttons as much as they like, but remember to change it back to a rational setting when they are not obsessing about it so you don’t freeze or boil.

One Last thing

The heating and air conditioning system in a car works the same way, but with one nuanced difference that is worth knowing.

When you select “Max heat” or “Max cool”, many cars will disengage the thermostat and keep the cooling and heating systems and the blower fans running at full throttle.

So in a car, choosing the “Max” setting does make a difference. The calibration of the temperature sensors and/or their physical placement can be unreliable so bypassing the thermostat and simply using “full on” or “full off” controls may work better.