Thermostats: Good, Better, Best, and Good Enough (Part 1)

By Bradford E. White, CEM

Thermostats! Evocative of hearth and home? Hardly. But these essentially simple devices have come a long way from what our grandparents used.

A Short History

Just before the 1890s, when most homes were heated by coal, the thermostat was a revolutionary device. Prior to thermostats, one had to shovel coal into a central furnace, shake the grate, and adjust the combustion air by hand. If you were really cutting-edge, you might have a chain and handle in the living space to save a trip to the basement. This was a vast improvement over small coal stoves in most rooms.

The first modern thermostat was a bimetallic coil: two different metals that expand and contract at different rates, but cut into strips and bonded together. When heated or cooled, this coil would move. This motive force was put to work, adjusting the air shutter on the furnace, allowing more or less combustion air into the firebox to vary the heat produced in response to space temperature. (However, there was not enough motive force to shovel the coal and cart away the ashes!)

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Photo by Earle Neil Kinder via starmanseries on Flickr Creative Commons

Later, this same bimetallic coil was connected to a glass bulb with a dollop of mercury inside. As the coil wound or unwound, the bulb would tip and make or break an electrical circuit to start a burner and possibly a pump—an electrical marvel circa 1928. Other versions made a direct electrical contact to start the heating system. Simple! Most of us have grown up with thermostats, often the ubiquitous “Honeywell round one,” AKA the T87, but also known as the “hockey puck.”

The Basics

At its core, a simple thermostat is really an on/off switch; on when the room temperature falls below the set temperature and off when the room temperature is at or above the set temperature. You could do this manually, but you have a meaningful and examined life with many better things to do. Plus, you could forget to turn it off or on. Turning a thermostat WAY UP will not give you more heat faster, but will just keep the system running until it gets to whatever gawdawful set-point seemed like a good idea at the time. You feel satisfied. Righteous. Hot. And aggrieved when the fuel bills arrive.

A device called an anticipator was introduced in the 1950s to shut off the system before the temperature overshot the mark. Older systems with cast iron radiators and high mass often noticeably overheated a space, so this made sense.

To further improve on this, a time clock was added to allow different temperature settings day or night. Next came mechanical, then electronic, timers. Better. In fact, this represented a plateau of technology for nearly 40 years. No more jumping out of bed to turn up the thermostat then cowering under the quilt for a half-hour more.

(So far, these thermostats worked nicely on a simple two-wire cable, connecting a circuit. One could remove the thermostat, touch the wires together, and get heat. Many of these thermostats work for both heating and cooling and will, by necessity, use five or more wires to operate their systems. Historically, heating only is typically two-wire operation, but the current crop of thermostats often require a three-wire cable with that third “C” or common wire used to extract power from the heating system to power the thermostat. )

Experiencing Setbacks: Not a Bad Thing

Many homeowners wonder to what extent they should set back the temperature at night or when they are away during the day. The answer is highly individual and specific to the house itself.

A study performed in Germany a dozen or so years ago recommended a setback from 20 degrees C down to 15 degrees C (68 degrees F to 59 degrees F). Of course, one has to consider that most homes in Europe are built to higher standards of insulation, air sealing, solar advantage, and so on, and the houses would often never drop to the cooler setback temperature even on very cold nights.

A house with high mass, such as a brick townhouse, will be slow to cool, but also slow to heat up once it has cooled. A low-mass framed house may cool rapidly, but also heat more rapidly. With daytime setbacks, solar exposure can play a significant role.

The largest single heat loss component in most existing homes (dating from the 1800s to today) is air leakage. If the house is drafty, one may feel a descending chill after 15 or 20 minutes, or even sooner. A well-sealed and well-insulated house might not drop more than one degree in an hour and may not even reach the 60-degree setback temperature overnight.

I suggest experimenting with your home. A data logger (a device that can record temperature and other variables over time) can be helpful with this. Start by dropping the temperature, at night or during the day, by 10 degrees. See how fast the house cools and how fast from a cold start the home recovers to comfort again.

New Technologies

While most homes were making do with conventional thermostats, the commercial and institutional world developed in wider and deeper directions, because there was more energy and costs to be saved. Direct digital controls—electronics of all kinds controlling large buildings—became the norm. Technology improved and became less expensive. Some of these improvements became so inexpensive that using them in homes became possible, and then expected.

So, what is the difference? Now, instead of an on/off switch, electronics allow:

• Anticipating the time your heating system will get your home comfortable. Instead of your guessing, “oh, start it an hour ahead of when I get up,” many thermostats today will learn how fast your home heats from a cold start and adjust the start time accordingly so you wake up to a warm house at the time you want. This is commonly known as “smart recovery,” but also several derivations of the term, licensed to several manufacturers. “Meet me there, at that temperature I want, at that time that I set.”
• Gauging how far from set-point your space temperature is, and then adjusting the heating system output accordingly. (Note that heating systems also improved here, with modulating versus on/off combustion, allowing variable output and making this all possible.) This is also known as “deviation from set-point” and is very much like your car accelerator. You see your car slowing as you start up a hill, so you press the accelerator. When you crest the hill, you take your foot off the gas, all to maintain a constant, safe, prudent, and sensible speed.

As with your car, nothing stands still. Technology’s progress has yielded a new crop of devices that use Wi-Fi and incorporate advanced features. We will cover these in detail in Part 2.

Bradford E. White is an HVAC engineer in Boston with more than 30 years of experience in the design of HVAC systems. Brad is a LEED Accredited Professional (BD+C), a Certified Energy Manager and Certified Energy Auditor (CEM and CEA) with the Association of Energy Engineers, and has been a valued BBR workshop teacher for many years.

© 2015 by Bradford E. White, CEM. Published with permission by Boston Building Resources.