Instead of using a Raspberry Pi for some toy, why not create a multi-zone thermostat. This month we are going to discuss how to implement a thermostat using a raspberry pi, relays, and temperature sensors.

Next meeting we will create a web interface for the thermostat so it can be controlled by any computer, tablet, or phone in the house.

Introducing the Thermostat

Before we start discussing how to implement a thermostat with a micro controller, how about we discuss how a thermostat works. We will discuss a typical thermostat for both heating and cooling.

How a bimetallic thermostat switches on and off

  1. An outer dial enables you to set the temperature at which the thermostat switches on and off.

  2. The dial is connected through a circuit to the temperature sensor (a bimetal strip, shown here colored red and blue), which switches an electrical circuit on and off by bending more or less.

  3. The bimetal (“two metal”) strip is made of two separate metal strips fastened together: a piece of brass (blue) bolted to a piece of iron (red).

  4. Iron expands less than brass as it gets hotter, so the bimetal strip curves inward as the temperature rises.

  5. The bimetal strip forms part of an electrical circuit (gray path). When the strip is cool, it’s straight, so it acts as a bridge through which electricity can flow. The circuit is on and so is the heating. When the strip is hotter, it bends and breaks the circuit, so no electricity can flow. Now the circuit is off.

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Bimetal with a mercury switch

This thermostat uses a longer bimetal strip, but the switch uses a small blob of mercury to short the contacts. The basic operation is the same.

How the furnace works

Now that we understand the thermostat works, lets look at a furnace with multiple zones.

This is a circulating hot water heating system. Often this type of system is divided into multiple zones. Each zone has it’s own thermostat and heats some part of the house.

Furnace Types

This shows the most common furnace types. Some forced draft heating systems also contain air conditioning. But basically all these system have one function, to heat (or cool) our homes.

Furnace control

Furnace control is not our topic today, but all of these systems have one common feature. The thermostat does it’s job by opening and closing a switch. So for our purpose we going to control the system by using relay to simulate the opening and closing of the thermostat switches.

To be continued

Written by John F. Moore

Last Revised: Thu Oct 5 17:25:13 EDT 2017

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