Thermostats in HVAC Systems

• Electrical-mechanical systems rely on energy conversion.
• Warm air furnaces use a fuel's chemical energy to produce heat, distributed by a fan motor powered by converted electrical energy.
• Controls monitor and direct energy related to pressure (gas, water, steam), temperature (air, water) and electricity.
• Controls are classified per application, action, sensing element.

Thermostat

• Thermostats are automatic operating controls that are normally open.
• Thermostats sense temperature changes in the room air.
• The heating of a bimetal sensor causes a mercury bulb to move and control a switch.
• Controls operate the gas valve and ignition control (where present).
• The thermostat is in series with the gas valve and ignition control.
• Thermostats operate in the 24-volt circuit.
• Heating thermostats have a heat anticipator in series with the load (gas valve).
• The heat anticipator fools the thermostat into opening sooner, moderating temperature swings.
• Thermostats should be located on an inside wall, 5 feet above the floor.
• Thermostats should be away from heat sources, and exposed to normal air circulation, within a room with average temperature.

High Limit

• High limits are automatic safety controls, normally closed.
• High limits sense air temperature around the furnace's heat exchanger.
• Klixon snap-action discs open the electrical circuit quickly when unsafe conditions are sensed. Fan-limit control involves heated bi-metal spirals causing a switch to open.
• Controls operate the gas valve and ignition control.
• The high limit will open and de-energize the gas valve upon sensing an unsafe high temperature.
• High limits can be wired in 24V or 120V circuits.
• Some high limits require manual resetting.

Fan Control

• Fan controls are automatic operating controls that are normally open.
• Fan controls sense temperature around the furnace heat exchanger.
• Klixon snap action discs close the switch and fan-limit controls use heated bi-metal spirals, causing the switch to close.
• Fan controls operate the furnace fan or a relay that controls the furnace fan.
• They can be wired in 24V or 120V circuits, and combination fan-limit controls exist.

Door Switch

• Door switches are automatic safety controls that are normally closed.
• Door switches sense the presence of the fan access panel.
• The switch mechanically closes when the fan access panel is correctly installed.
• Controls operate the furnace fan or a relay that controls the furnace fan.

Flame Rollout Switch

• Flame rollout switches are automatic safety controls that are normally closed.
• Flame rollout switches indirectly sense failure of the induced draft fan motor.
• The switch senses heat from combustion flame rolling back out of the combustion chamber.
• A capillary tube or klixon disk sensing element causes the switch to open.
• The switch de-energizes the main gas valve.
• Controls operate the main gas valve in an unsafe venting condition.

Induced Draft Pressure Switch

• Induced draft pressure switches are automatic safety controls that are normally open.
• The switch senses negative pressure developed by the induced fan motor.
• The switch will not close if the induced draft fan motor has failed or if there's a blockage in the venting system.
• The pressure-sensing element closes when the induced fan motor is energized, interlocking the ignition control sequence.
• If it opens during the firing sequence, the main gas valve will de-energize.
• Controls operate the main gas valve if an unsafe venting condition occurs.

Air Flow Sensing Switch

• Air flow sensing switches are automatic safety controls that are normally open.
• The switch senses airflow developed by an air-moving device.
• The switch closes when air moves through a duct.
• Controls depends upon the application.
• Air flow sensing switches are wired in series with electronic air cleaners, humidifiers, or electric heaters.
• These should operate with airflow; the air sensing switch closes when the furnace fan starts.

Fan Centers

• Fan centers are provided when a central air conditioning unit is installed.
• A fan center contains a 120-24 volt transformer and single pole/double throw relay.
• The relay energizes when there is a call for cooling.
• The closing of the normally open switch causes the furnace fan motor to start.
• When fan centers are installed, the center replaces the transformer in the furnace.
• Control of the furnace fan during the heating cycle is through the normally closed switch of the fan center.
• This switch is in series with the fan limit control.

Aquastat

• Aquastats are automatic operating controls, normally open.
• Aquastats sense water temperature in the boiler.
• The switch closes when the boiler water temperature falls below the set temperature.
• Controls operate the main gas valve or water pump.

Low Water Cut Off

• Low water cut offs are automatic safety controls, normally closed.
• The control senses the water level in the boiler.
• If the water level drops, the float switch opens the electrical circuit to the main gas valve.
• Controls operate the main gas valve.

Water Flow Switch

• Water flow switches are automatic safety controls that are normally open.
• Water flow switches sense water flow developed by the system pump.
• The paddle is activated by water movement and closes the flow switch.
• Controls operate the main gas valve in series with the flow switch.

Pressure Control Switch (Pressuretrol)

• The pressure control switch is an automatic operating control.
• It senses steam pressure in the boiler between high and low-pressure settings.
• It maintains proper operating pressure: the low-pressure switch closes to start the burner.
• The high-pressure switch opens to stop the burner when pressure reaches the top of its range.
• Controls steam pressure in the boiler.

High Pressure Control

• High-pressure controls are automatic safety controls, normally closed.
• They sense unsafe (high) steam pressure in the boiler.
• The switch opens when an unsafe steam pressure is sensed.
• Controls operate the main gas valve.
• The maximum operating pressure for a steam boiler in heating is 15 psi.

Continuous Pilot

• A pilot that stays lit all the time.

Intermittent Pilot

• A pilot that stays on during the firing cycle only.

Interrupted Pilot

• A pilot that is shut off after the main burner is ignited.

Thermocouples

• A thermocouple is an electrical generating device of two dissimilar metals joined at two junctions.
• Heating one junction produces a DC voltage.
• The temperature difference between hot and cold junctions determines the amount of DC voltage produced.
• Thermocouples are common on atmospheric burners.
• A typical thermocouple in a gas pilot flame produces 25 to 30 Milivolts DC.
• 1 Milivolt is equal to 1/1,000 volts.

Thermopile (Powerpile)

• A thermopile is a collection of thermocouples in series.
• Thermopiles generate 750 Milivolts.
• A thermopile packaged with a pilot is called a pilot generator.
• The voltage generated by a thermopile is sufficient to operate a Milivolt gas valve system, including its thermostat, safety limits, and gas valve.
• The maximum voltage drop across any safety or operating control is 10 Milivolts.
• A Thermocouple with an open circuit shows 20 to 30 mV indicating a good Thermocouple
• A Thermocouple on a closed circuit shows 10 to 15 mV indicating a good Thermocouple
• A Thermopile on an open circuit shows >750 mV indicating a good Thermopile.

Pilot Drop Out Test

• The Purpose of the pilot drop out test is to find the minimum voltage produced by the thermocouple.
• The pilot should be able to ignite the main burner smoothly within ninety seconds.

Flame Failure Response Test

• The test will ensure that the pilotstat will drop out within a sufficient amount of time after extinction of the pilot flame.
• The maximum amount of time is 90 seconds for residential Thermocouple / Thermopile systems.
• Expected Readings in a Thermopile Milivolt System includes open, and closed circuit tests with thermostats being either open or closed to receive an accurate reading
• High limit test should read 10 mV or less. Thermostat tests varies from 10 mV or less to 150 or less (with anticipator). While wiring loss tests should expect 50 mV or less.

Pilot Flame Rules

• Locate pilots where they can be observed easily and mounted rigidly.
• Pilot flame can not impinge on main burner or heat exchanger.
• The pilot needs to enough air supply in a location protected from ignition and extinguishment of the main burner.
• The pilot can not be subject to drafts.
• The thermocouples should be properly located within the pilot flame.

Flame Ionization

• Flame ionization is the process where molecules of air lose some of their electrons when heat is added to them.
• Molecules become positive ions, carrying negatively charged electrons.
• Atoms in air molecules become excited with added energy.

Flame Rectification

• The result is that the flame-detecting device will not see an alternating current signal but rather a pulsating Direct Current signal.
• A flame rod is a small-diameter metal rod supported by an insulator.
• The rod's tip projects about ½ inch into the flame.
• An Alternating Current signal is transmitted through the rod into the pilot flame.
• The flame rod uses a ground (usually) attached to the burner.
• The ground should be at least 4 times the surface area of the flame rod.

Intermittent Pilot Ignition System

• Intermittent pilot systems use a spark-ignited pilot to light the main burner, eliminating standing pilots.
• A pilot flame sensor monitors the pilot flame during main burner operation.
• Redundant gas valves are used in line with the main gas valve for safety.

White Rogers

• This is a capillary tube filled with mercury that expands and applies pressure to the diaphragm in the flame sensor assembly.
• This pressure makes or breaks a snap switch, which is in series with the main gas valve.

Ignition Control Modules

• Three types of ignition control modules: intermittent pilot, direct spark ignition, and hot surface ignition.

Solenoid Valves

• A fast-opening and closing valve.
• When solenoid coil is energized an electromagnet is created that provides opening and closing force for the valve.
• Solenoids are used on small and medium size burners.

Diaphragm Valves

• Uses upstream pressure to open the gas valve.
• Gas pressure is bled away from the diaphragm to close the valve.
• Used with atmospheric burners, rooftop units and water heaters.
• These valves are on/off with slow opening and fast closing functions.
• Opening and closing force is available gas pressure.

Combination Control Valves

• Most residential systems employ a gas valve containing all major components.

Drop Valve

• A ¼ turn lubricated manual shutoff valve is located in the system.

Auto Gas Valve

• Automatic gas valve used with a solenoid coil and a diaphragm valve.
• Main gas shut off valve.