Warm Air Furnace: Thermostat

• Electrical-mechanical systems rely on energy conversion.
• Warm air furnaces use chemical energy from fuels like natural gas to produce heat.
• A fan motor distributes the heat throughout a house using mechanical energy.
• Electrical energy is converted by the furnace's electrical distribution system to power the fan.
• Controls monitor and direct energy in warm air furnaces.
• Controls sense pressure (gas, water, steam), temperature (air, water), and electricity.
• Controls are classified based on application, action, and sensing elements.

Thermostat (Warm Air Furnace)

• An automatic operating control, normally open.
• Thermostats sense temperature changes in the ambient air.
• Heating of a bimetal sensor causes the movement of a mercury bulb, closing or opening a control switch.
• Thermostats control gas valve and ignition.
• The thermostat is connected in series with the gas valve and ignition control.
• Typically operates in a 24-volt circuit.
• Heating thermostats include a heat anticipator in series with the gas valve load.
• Heat anticipators fool the thermostat into opening sooner using a small resistance heater.
• This helps moderate temperature swings.
• Thermostats should be located on an inside wall, about 5 feet above the floor.
• Avoid locations close to heat sources or direct sunlight.
• The location should have normal air circulation, and average building temperature.

High Limit

• An automatic safety control, normally closed.
• Senses the temperature of air around the furnace heat exchanger.
• Uses a Klixon snap action disc to quickly open the electrical circuit when unsafe conditions are sensed.
• The action of a heated bimetal spiral causes switch to open
• Controls the gas valve and ignition.
• The high limit will open and de-energize the gas valve.
• Can be wired in 24V or 120V circuit.
• Some high limits are manual reset.

Fan Control

• An automatic operating control, normally open.
• Senses the temperature of the air surrounding the furnace heat exchanger.
• Klixon snap action disc causes switch to close.
• Fan-Limit control: Action of heated bi-metal spiral causes switch to close.
• Controls the operation of the furnace fan or relay that controls the fan.
• Can be wired in a 24V circuit (energizes a 24V fan relay coil) or a 120V circuit.
• 120V directly energizes the furnace fan.

Door Switch

• An automatic safety control, normally closed.
• Senses the presence of the fan access panel.
• Closes mechanically when the fan access panel is correctly installed.
• Controls the operation of the furnace fan or relay.

Flame Rollout Switch

• An automatic safety control, normally closed.
• Senses failure of the induced draft fan motor or spillage in the venting system.
• Detects heat from combustion flame rolling back out of the combustion chamber.
• The sensing element (capillary tube or Klixon disk) causes the switch to open and de-energize the main gas valve.
• Controls the operation of the main gas valve in an unsafe venting condition.

Induced Draft Pressure Switch

• An automatic safety control, normally open.
• Senses negative pressure developed by the induced fan motor.
• Will not close if the induced draft fan motor fails or the venting system is blocked.
• The pressure-sensing element closes when the induced fan motor is energized, interlocking the ignition sequence.
• The main gas valve will be de-energized if the induced draft pressure switch opens during firing.
• Controls operation of the main gas valve if there's an unsafe venting condition.

Air Flow Sensing Switch

• An automatic safety control, normally open.
• Senses airflow developed by an air-moving device.
• Switch closes when air moves through a duct.
• Controls depend on the application.
• An airflow sensing switch will be wired in series with an electronic air cleaner, humidifier, or electric heater.
• None of these devices should operate without 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 a single pole/double throw relay.
• The relay is energized 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 transformer in 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 (Hot Water Boilers)

• Automatic operating control, normally open.
• Senses water temperature of the boiler.
• Switch closes when the boiler water temperature falls below the preset temperature setting.
• Controls the main gas valve or water pump.

Low Water Cut Off

• An automatic safety control, normally closed.
• Senses water level in the boiler.
• A float in the control senses the water level.
• If the water level drops, the float switch opens the electrical circuit to the main gas valve.
• Controls the main gas valve.
• Only a licensed plumber or steam fitter may change a low water cut-off control.

Water Flow Switch

• Automatic safety control (normally open).
• Senses water flow developed by the system pump.
• The paddle is activated by water movement.
• Activated paddle closes the flow switch.
• The main gas valve is in series with the flow switch.

Pressure Control Switch (Pressuretrol)

• Automatic operating control.
• Senses steam pressure in the boiler between a high and low pressure setting.
• Maintains proper operating pressure in a steam boiler.
• Low-pressure switch closes to start the burner when pressure drops.
• High-pressure switch opens to stop the burner when pressure reaches the top of its operating range.
• Controls steam pressure in the boiler.

High Pressure Control

• Automatic safety control (normally closed).
• Senses unsafe (high) steam pressure in the boiler.
• Opens the switch when an unsafe steam pressure is sensed.
• Controls the main gas valve.
• The maximum operating pressure for a steam boiler in a heating application is 15 psi.

Types of Pilots

• A flame with two functions: to ignite the main burner safely and to generate electricity to prove the presence of an operating pilot.
• 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

• An electrical generating device with two dissimilar metals joined at two junctions.
• Heating one junction produces a DC voltage.
• Voltage quantity depends on the temperature difference between the hot and cold junctions.
• Common in atmospheric burners.
• A typical thermocouple heated in a gas pilot flame produces 25-30 millivolts DC.
• This small voltage keeps a solenoid, such as a pilotstat, energized.
• Proves the presence of a pilot flame.
• Generating 25-30 mV keeps the pilotstat energized.
• If the pilot flame is extinguished, the voltage falls and de-energizes the pilotstat.
• When de-energized, the gas through the main valve stops.
• The thermocouple is screwed into the pilotstat coil and tightened ¼ turn beyond finger tight.

Thermopile (Powerpile)

• A collection of thermocouples in series.
• Generates 750 millivolts.
• Packaged with a pilot, called a pilot generator.
• The voltage generated runs a millivolt gas valve system.
• A thermopile system includes its own thermostat, safety limits, and a gas valve.
• With voltage still under 1 volt, wiring connections must be secure.
• The maximum voltage drop across safety or operating control is 10 mV.
• Open Circuit for Thermocouple: 20-30 mV indicates a good thermocouple.
• Closed Circuit for Thermocouple: 10-15 mV indicates a good thermocouple.
• Open Circuit for Thermopile: >750 mV indicates a good thermopile.

Pilot Drop Out Test

• Determines the minimum voltage required by the thermocouple to energize the pilotstat.
• At this voltage, the pilot should be able to smoothly ignite the main burner within one second.
• Usually between 2 to 4 millivolts.

Flame Failure Response Test

• Ensures that the pilotstat drops out in time after the pilot flame goes out.
• The maximum time is 90 seconds for residential thermocouple/thermopile systems.
• Expected readings in a thermopile millivolt system (mV): Open circuit test: 700-800, Closed-circuit test (thermostat open): 600, Closed-circuit test (thermostat closed): 400-450, High Limit (closed): 10 or less, Thermostat (without anticipator): 10 or less, Thermostat (with anticipator): 150 or less, Wiring loss: 50 or less.

Pilot Flame Rules

• Locate the pilot where it can be observed easily.
• Mount the pilot burner rigidly.
• The pilot flame must not impinge on main burner or heat exchanger.
• Sufficient air supply is required.
• Protect the pilot from the ignition and extinguishing of the main burner.
• Protect the pilot from drafts.
• Properly locate the thermocouple within the pilot flame, so that 3/8 to 1/2 of pilot should be on the thermocouple tip.
• Remedies for lazy yellow flame includes cleaning burner and replace oriface inlet fitting
• Remedies for waving blue flame includes relocate pilot, or install protecting baffle.
• Remedies for small blue flame includes increasing pressure to normal, clean pilot burner orifice, clean filter and install correct orifice
• Remedy for noisy lifting blowing flame is to reduce pressure
• Hard sharp flame requires correct orifice inlet fitting. Normal flame dont require any.

Flame Ionization

• Flame ionization occurs when air molecules lose electrons due to added heat.
• Molecules become positive ions and carry negative electrons.
• Atoms in air molecules become excited with added heat.
• Electrons escape atomic bonds with enough energy, allowing the flame to conduct electricity.
• Directing an AC signal to an electrode immersed in the flame causes electrical charge conduction across to ground.
• The ground is connected to the burner.

Flame Rectification

• A carbon deposit at the base of the electrode could short-circuit to ground.
• A larger ground area helps electrical current travel from the electrode to ground, and not vice versa.

Flame Rod

• A small metal rod supported by an insulator.
• Projects about 1/2 inch into the flame.
• An AC signal transmits into the pilot flame, using flame ionization to conduct the signal.
• The flame rod uses a ground (usually) attached to the burner.
• The ground must be at least 4 times the surface area of the flame rod.
• A Pulsating Direct Current is established and the result is that flame-detecting device will not see an alternating current signal but rather a pulsating direct current signal.
• Cracked or carbonized flame rod causes will produce AC signal and be rejected by the Flame Safeguard Control.

Intermittent Pilot Ignition Systems.

• Uses a pilot that has been spark-ignited to light the main burner.
• Eliminates standing pilots.
• Pilot flame sensor monitors the pilot flame during main burner operation.
• Uses redundant gas valve in line with main gas valve for safety.
• Flame failure response time is 0.8 seconds.
• Flame sensor is a capillary tube filled with mercury.
• The ignited pilot flame will heat the mercury causing it to expand and apply pressure to the diaphragm in the flame sensor assembly.
• Uses an electrode that directs a high voltage spark to a ground electrode.
• The flame is sensed using flame rectification.
• Flame failure response time is 0.8 seconds.
• Trial for ignition with safety lock out can be between 15 and 90 seconds.
• Disconnects the ignition spark when the flame is sensed.
• Spark gap must be from 1/8 to 3/16 inches.
• Common problems are poor ground connections, or excessive temperatures at the ceramic flame rod insulator.

Direct Spark Ignition

• Directly ignites the main burner without using a pilot.
• Uses an electrode to create a spark to a ground electrode.
• The flame is sensed using flame rectification.
• Minimum flame current is about one DC µ amp.
• Ignition control is a microcomputer, containing a safe start check, transformer, and spark control.
• Trial for ignition is usually 4 seconds.
• The flame failure response time is 0.8 seconds.
• A redundant gas valve is used in line with the main gas valve.

Hot Surface Ignition

• Uses a silicon carbide igniter to ignite the main burner flame after it is heated.
• The flame is sensed by a flame rod.
• Minimum flame current of one DC µ amp.
• Microcomputer ignition control can purge, retry, and recycle flame ignition sequences.
• Trial for ignition is usually 4 seconds.
• The flame failure response time is 0.8 seconds.
• System has a redundant (second) gas valve.

Ignition Control Modules

• There are three types
• Intermittent Pilot
• Direct Spark Ignition
• Hot Surface Ignition
• The basic sequence of operation for an ignition control module is
• Safe start check
• Ignite flame
• Energize the main gas valve
• Prove the pilot or main gas flame
• Monitor the flame during the running cycle.

Solenoid Valve

• Is a fast opening and fast closing valve.
• When the solenoid coil in the gas valve is energized, is creates an electromagnet that provides opening and closing force for the valve.
• The solenoid is used on small and medium size burners or as the safety shut off valve for the pilot valve train.

Diaphragm Valve

• Uses upstream gas pressure on the valve diaphragm to open the gas valve.
• The gas pressure is bled away from the diaphragm to close the valve.
• It is 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 Valve

• Most residential systems employ a gas valve that contains the major components for automatic firing in one valve
• Pilotstat is a solenoid coil energized by the thermocouple millivoltage. The thermocouple will produce a current that can hold the solenoid in place. It is not strong enough to pull in the solenoid. This is done manually when the pilot valve is depressed and opened on initial light off.
• Automatic gas valve is used with a solenoid coil and a diaphragm valve.
• Main gas and pilot regulators are used to adjust gas pressure to proper amounts.
• Main gas shut off valve.

Drop Valve

• Is a ¼ turn lubricated manual shut off valve that is the main shut off for the valve train.
• It is usually located in the piping drop at the appliance.
• Test Openings are 1/8 - capped nipples located on the valve train and are used to check gas pressure at a specific point.