5-1 Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Puffett, Hossack, Stone, Burn & Miles, Gasfitting 2e Chapter 5 THERMOSTATS

5-2 Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Puffett, Hossack, Stone, Burn & Miles, Gasfitting 2e Functions of a thermostat 1.To regulate the temperature of a substance or space by controlling the flow of gas to the burner. 2.To add to the convenience, utility, economy and, in some cases, safety of the appliance.

Types of thermostats 1.Rod and tube thermostats 2.Liquid expansion thermostats 3.Vapour pressure thermostats 4.Bimetal thermostats 5-3 Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Puffett, Hossack, Stone, Burn & Miles, Gasfitting 2e

Thermostat function Snap acting Gas flows at full rate then shuts off when temperature is reached. Cooling may take place before thermostat comes back on. Modulating Heat gradually increases and will begin to reduce gas flow as temperature is reached. Gas flow will then maintain temperature. 5-4 Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Puffett, Hossack, Stone, Burn & Miles, Gasfitting 2e

Rod and tube thermostats (1) Operation Rod and tube has different coefficient of expansion. Outer tube and inner rod fixed at one end. 5-5 Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Puffett, Hossack, Stone, Burn & Miles, Gasfitting 2e

Rod and tube thermostats (2) 5-6 Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Puffett, Hossack, Stone, Burn & Miles, Gasfitting 2e

Liquid expansion thermostats (1) Liquids expand when heated and contract when cooled. A bellows is opened and closed by this expansion and contraction. By attaching a gas valve to the bellows gas flow can be controlled. 5-7 Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Puffett, Hossack, Stone, Burn & Miles, Gasfitting 2e

Liquid expansion thermostats (2) 5-8 Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Puffett, Hossack, Stone, Burn & Miles, Gasfitting 2e

Vapour pressure thermostats Sensing bulb of thermostat is filled with gas. When temperature increases, the gas pressure in the bulb increases. This increases the bellows acting on the gas valve. 5-9 Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Puffett, Hossack, Stone, Burn & Miles, Gasfitting 2e

Bimetal thermostats If two different metals are bonded together and an increase in temperature takes place, the expansion will cause the bimetal strip to bend upwards or downwards depending on the location of the metal with the higher expansion rate. Bimetal thermostats are: (i) used to control the fan of a duct air heater; or (ii) used as room thermostats Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Puffett, Hossack, Stone, Burn & Miles, Gasfitting 2e

Bimetal thermostats—central heaters 5-11 Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Puffett, Hossack, Stone, Burn & Miles, Gasfitting 2e

Bimetal room thermostats 5-12 Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Puffett, Hossack, Stone, Burn & Miles, Gasfitting 2e

Thermostats—Summary Purpose: To regulate the temperature of a substance or space by controlling the flow of gas to the burner. Thermostat types 1. Rod and tube 2. Liquid expansion 3. Vapour pressure 4. Bimetal Thermostat function –Snap acting –Modulating 5-13 Copyright  2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Puffett, Hossack, Stone, Burn & Miles, Gasfitting 2e