Design of a Comfortable Home HVAC Thermostat

M. Haidekker, July 2014


The project in this article is described for educational purposes only. Do not attempt to build or duplicate the device described herein. Do not use this information to build a similar device. YOU MAY CAUSE PROPERTY DAMAGE, FIRE, INJURY, OR DEATH. Your home HVAC system should be serviced by qualified personnel only. The author of this article assumes no warranty that the device is fit for any purpose, nor assumes any liability for damages caused by the use, proper or improper, of the device described herein, or any methods or devices derived thereof.

If you perform any of the steps described in this article, you do so entirely at your own risk.


Why would I ever want to build my own thermostat? Low-priced ones can be obtained for a few dozen bucks at any home improvement store, more sophisticated programmable thermostats range up to perhaps eighty dollars, and the real deluxe devices, supposedly "learning" and featured with a web interface (so a hacker can toast you remotely, or cryogenically preserve your body) -- perhaps 200 to 250 dollars. Wouldn't this cover the entire gamut of what I could ever want?

Nope. Without exception (at least to my knowledge), all thermostats use two-point control. This means, basically, that you have two "trip points": your desired temperature (setpoint) and a second, usually lower temperature, by about 0.5 to 1 degree C. Let's say you want to heat to 21 degrees C (70F) and your home is colder than that. The thermostat kicks on the furnace and the temperature increases until it just exceeds 21C, which is when the furnace is turned off. Now your home starts to cool again until the temperature drops below 20C (i.e., the second point in the two-point control) and the furnace is turned on once again.

For more information about two-point controls, I'll refer to Chapter 2 of my book Linear Feedback Controls -- The Essentials.

For now, let's say that this system works well when the outside temperature is massively different from your inside temperature, because it causes the furnace (or the compressor in summer) to turn on periodically. In consequence, the air in your home is distributed fairly well. The situation is entirely different if only a little heating (or cooling) is needed. To stay with the example above, the furnace would kick in and raise the temperature to 21C. Now, your home would start to cool, albeit very slowly. Therefore, it takes a long time until the furnace goes on the next time. Until then, peripheral rooms would cool more than the center -- this is usually where the thermostat is located. Warm air is no longer evenly distributed, and some rooms get uncomfortably chilly or even clammy.

The situation is worse in larger homes with two HVAC systems and two thermostats. In such a case, the ground floor heating system causes warm air to rise in the hallway, and the upper thermostat reads an inaccurately high temperature, exacerbating the problem for the upper rooms.

It is noteworthy that this problem is inherent in the two-point control principle, and even the most expensive thermostat won't solve this problem. Most often, people would compensate by turning the temperature up in winter and down in summer, leading to higher energy costs.

What can be done? A proportional-integral (PI) controller would be ideal (see Chapter 6 of my book), but this is practically impossible as a conventional PI controller would require some sort of continuously adjustable heater/cooler -- such as, for example, variable flame strength. Since this is not practical, we can discard any linear control system. The HVAC system is on/off only.


But wait. Isn't pulse-width modulation (PWM) on-off, too? Yes, it is, but PWM is most frequently used at relatively high frequencies, most often in the kHz range. So what about slowing it down to a frequency that would make sense to a HVAC system?

The central idea on which my own thermostat is built is the need for frequent air recirculation. In other words, I want the thermostat to run the heater or compressor periodically, for example, once every 30 minutes. When the outside temperature is very different from your inside temperature, the action of this thermostat is similar to the conventional two-point thermostat. However, when only a little heating or cooling is needed, the thermostat will still run every 30 minutes, but stay on for a very short time only. Obviously, air circulation is improved, and a little heat is added more frequently (or humidity removed when you are cooling).

This leads us to a little bit of easy math. On to Part 2.