Measuring the Heat

Have you ever wondered how your heating system knows to turn on? Or to turn off? You could say “the furnace controller tells it to” and “the thermostat tells it to,” but that’s not the whole picture, is it? We need a way to measure the temperature inside the furnace and inside our homes. It has to be durable, reliable, and affordable. It doesn’t have to be precise, but it must be right every time it’s measured.


A Complex Web of Technology

There are a staggering number of ways to control a furnace through temperature input. A brief and nowhere near all-inclusive list of techniques include: Gas Expansion Tubes, Bi-Metal Switches, Bi-Metal Coils, Thermocouples Driven by a Pilot-Light, Thermistors, and of course modern IR Temperature Sensors found in your enthusiast-chef’s kitchen. These devices are all in some way sensitive to the heat.

Bi-Metal systems expand as temperatures change. Measuring the expansion reads the approximate temperature.

Gas Expansion Tubes have an internal change in pressure as temperature changes. The pressure can be used to calculate temperature.

Thermocouples generate an electrical current when they’re heated. Measuring the current allows you to determine the temperature. Inside a furnace, they’re often heated directly by the pilot light or burner to read flame temperatures.

Infrared Thermometers measure “Blackbody (Wikipedia Link)” radiation, but aren’t all that effective around metals or the air.

And lastly, we have the humble Thermistor, which varies it’s resistance based on it’s temperature. Reading this electrical current, like the Thermocouple, allows us to determine the temperature.


What Do We Use Today?

The popular systems in today’s micro-chip, smart-thermostat-driven world often use Thermistors and Thermocouples. These are accurate, small, cheap,  durable, and reliable. They use well understood physics, can be replaced when damaged in many applications, and can be put basically anywhere. This flexibility allows us to put these inside the furnace, on the hot water tank, and in your home, all using essentially the same parts, with some modifications during manufacturing to better survive in the furnace.


Let’s Answer the Question

Finally, we can answer how the furnace knows to turn on, from an electrical perspective. We measure the temperature in a few key places, using either resistance or the thermo-electric effect. In your thermostat, we push a volt of electricity into the thermistor, if we get half a volt out, we know it’s cold. If we get an entire volt we know it’s quite warm. Our thermostat processes this and determines what action to take. Maybe it needed to see 0.6 volts or it needed 0.7, so it signals the furnace.

Outside hot water tank, we repeat the process and send some voltage to our thermistor. The result comes back and it’s time to check on the furnace. Another thermocouple or thermistor inside confirms the furnaces status and it’s signaled to start up. We can measure the internal temperature and kick on the circulatory system to get things moving. In a forced-air system, you would skip measuring the tank temperature.

Without these little silicon miracles, we would have to use a purely-mechanical system. At best a bi-metal solution or some pressure regulated system. These were used in the past, but largely cannot match today’s durability, reliability, and low costs.

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