Warm Air Furnace - Thermostat

• Electrical-mechanical systems use or convert energy.
• Warm air furnaces use natural gas to produce heat, distributing it via a fan motor powered by electrical energy.
• Controls monitor and direct energy in warm air furnaces, sensing pressure, temperature, and electricity.
• Controls classify by application, action, and sensing elements.

Warm Air Furnace - Thermostat

• Type: Automatic operating control (normally open).
• Senses: Temperature changes.
• Action: A bimetal sensor heats, moving a mercury bulb to close/open a control switch.
• Controls: Operation of gas valve and ignition.
• The thermostat is in series with the gas valve and ignition control.
• Thermostats usually operate in a 24-volt circuit.
• Heating thermostats have a heat anticipator in series with the load that is a small heater that moderates temperature swings.
• Thermostat location considerations:
  • Inside wall
  • ~5 feet above the floor
  • Away from heat sources
  • Exposed to normal air circulation
  • In a room with average building temperature

High Limit

• Type: Automatic safety control (normally closed).
• Senses: Temperature of the air surrounding the furnace heat exchanger.
• Action: A snap action disc (Klixon) opens the electrical circuit quickly.
• Fan-Limit control: Action is a heated bi-metal spiral that causes the switch to open.
• Controls: Operation of gas valve and ignition control.
• The high limit opens and de-energizes the gas valve when unsafe temps are sensed.
• Can be wired in 24V or 120V circuits.
• Some high limits are manual reset.

Fan Control

• Type: Automatic operating control (normally open).
• Senses: Temperature of the air surrounding the furnace heat exchanger.
• Action: A snap action disc (Klixon) causes the switch to close.
• Fan-Limit control: Action is a heated bi-metal spiral that causes the switch to close.
• Controls: Operation of the furnace fan or a relay that controls the fan.
• Can be wired in 24V (energizing a 24V fan relay coil) or 120V (energizing the furnace fan directly).

Door Switch

• Type: Automatic safety control (normally closed).
• Senses: Presence of the fan access panel.
• Action: Mechanically closes the switch when the fan access panel is correctly installed.
• Controls: Operation of the furnace fan or relay.

Flame Rollout Switch

• Type: Automatic safety control (normally closed).
• Senses: Failure of the induced draft fan motor or spillage in the venting system.
• Action: A sensing element (capillary tube or Klixon disk) causes the switch to snap open and de-energize the main gas valve.
• Controls: Operation of the main gas valve during unsafe venting conditions.

Induced Draft Pressure Switch

• Type: Automatic safety control (normally open).
• Senses: Negative pressure developed by the induced fan motor.
• Action: Pressure sensing element closes when the induced fan motor energizes to interlock the ignition.
• If the switch opens during firing, the main gas valve de-energizes.
• Controls: Operation of main gas valve if an unsafe venting condition occurs.

Air Flow Sensing Switch

• Type: Automatic safety control (normally open).
• Senses: Airflow developed by an air-moving device.
• Action: The switch closes when air moves through a duct.
• Controls: Depends on the application.
• The switch will be wired in series with devices like an electronic air cleaner, humidifier, or electric heater.
• These devices should not operate without airflow, activating the switch when the furnace fan starts.

Fan Centers

• Fan centers are devices used when a central air conditioning unit is installed.
• They contain a 120-24 volt transformer and a single pole/double throw relay.
• The relay energizes during a call for cooling, causing the furnace fan motor to start.
• When fan centers are installed, the transformer in the center replaces the transformer in the furnace.
• During heating, control of the furnace fan occurs via the fan center's normally closed switch, which is in series with the fan limit control.

Hot Water Boilers - Aquastat

• Type: Automatic operating control (normally open).
• Senses: Water temperature of the boiler.
• Action: Switch closes when the water temperature falls below the present temperature setting.
• Controls: Main gas valve or water pump.

Low Water Cut Off

• Type: Automatic safety control (normally closed).
• Senses: Water level in the boiler.
• Action: A float senses the water level. If the water level drops, the float switch opens the electrical circuit to the main gas valve.
• Controls: Main gas valve.
• Only licensed plumbers/steam fitters may change this control.

Water Flow Switch

• Type: Automatic safety control (normally open).
• Senses: Water flow developed by the system pump.
• Action: Paddle is activated by water movement.
• Controls: Main gas valve wired in series with the flow switch.

Steam Boilers - Pressure Control Switch (Pressuretrol)

• Type: Automatic operating control.
• Senses: Steam pressure in the boiler between a high and low pressure setting.
• Action: The low-pressure switch closes to start the burner when pressure drops.
• Controls: Steam pressure in the boiler.
• The high-pressure switch opens to stop the burner when pressure reaches the top of its operating range.

High Pressure Control

• Type: Automatic safety control (normally closed).
• Senses: Unsafe (high) steam pressure in the boiler.
• Action: Opens the switch when unsafe steam pressure is sensed.
• Controls: Main gas valve.
• The maximum operating pressure for a steam boiler used in heating is 15 psi.

Flame Control Systems - Types of Pilots

• A pilot flame has two functions:
  • Safely ignite the main burner.
  • Generate electricity for the thermocouple/thermopile system to prove the presence of the pilot flame.
• There are three types of pilot ignition systems:
  • Continuous Pilot: Always lit.
  • Intermittent Pilot: stays lit during the firing cycle only.
  • Interrupted Pilot: Shuts 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 DC voltage.
• The temperature difference between the junctions determines the DC voltage quantity.
• Thermocouples are common on atmospheric burners.
• Typical thermocouples heated in gas pilot flames produce 25 to 30 mV DC.
• 1mV = 1/1000 V, enough to keep a pilotstat energized.
• If the pilot flame is extinguished, the voltage produced by the cooling then the thermocouple will fall and is no longer able to energize the electro-magnetic solenoid of the pilotstat.
• A de-energized pilotstat stops gas flow, and the burner will not work.
• The thermocouple is screwed into the pilotstat coil and should be tightened 1/4 turn beyond finger tight.

Thermopile (Powerpile)

• A thermopile is a collection of thermocouples in series.
• The voltage range of a typical thermopile output is 750 mV.
• When packaged with a pilot, it is called a pilot generator.
• Thermopiles generate enough voltage to operate a millivolt gas valve system.
• Such a system would have its own thermostat, safety limits, and gas valve.
• Important to keep all the wiring connections secure and tight to minimize voltage drop due to thermopiles being less than 1V
• The maximum voltage drop across any safety or operating control is 10mV.

Thermocouple/Thermopile Testing

• Thermocouple Open Circuit: Should read ~20 to 30mV to determine if the thermocouple is good.
• Thermocouple Closed Circuit: Should read ~10 to 15mV to determine if the thermocouple is good.
• Thermopile Open Circuit: Should read >750mV to determine if the thermopile is good.

Pilot Drop Out Test

• Determines the minimum voltage to energize the pilotstat.
• The pilot should light the main burner within one second at this voltage.
• This is usually between 2 to 4mV.

Flame Failure Response Test

• Ensures the pilotstat drops out quick enough after pilot flame extinction.
• The maximum time is 90 seconds for residential thermocouple/thermopile systems.

Expected Readings in a Thermopile Millivolt System

• Open circuit test: 700-800 mV
• Closed circuit test (Thermostat open): 600 mV
• Closed circuit test (Thermostat closed): 400-450 mV
• High limit (closed): 10 mV or less
• Thermostat (without anticipator): 10 mV or less
• Thermostat (with anticipator): 150 mV or less
• Wiring Loss: 50 mV or less

Pilot Flame Rules

• Locate pilot so it can be observed easily.
• Mount the pilot burner rigidly.
• The pilot flame cannot impinge on the main burner or heat exchanger.
• Pilot flame must have sufficient air.
• Protect the pilot from the ignition and extinguishing of the main burner.
• Locate so it won't be subject to drafts.
• Locate the thermocouple properly within the pilot flame.

Pilot Flame Issues

• Lazy Yellow Flame
  • Causes: Dirty lint screen, orifice too big
  • Remedies: Clean the lint screen or replace the orifice fitting
• Waving Blue Flame
  • Causes: Excessive draft, recirculating products of combustion
  • Remedies: Relocate pilot, install a baffle.
• Small Blue Flame
  • Causes: low gas pressure, plugged pilot burner orifice, plugged pilot line filter, improper orifice.
  • Remedies: Reduce high gas pressure, Clean Burner Orifice, Clean Filter, Install correct Orifice
• Noisy Lifting Blowing Flame
  • Causes: high gas pressure
  • Remedies: Reduce Pressure
• Hard Sharp Flame
  • Causes: bad manufactured orifice
  • Remedies: Install proper oriface inlet fitting.
• Normal Blue Flame
  • Cause: good install
  • Remedies: None

Flame Ionization

• Process where air molecules lose electrons when heated.
• Makes molecules into positive ions, capable of carrying negatively charged electrons.
• When atoms from air molecules in a flame are affected by enough heat electrons will escape atoms.
• At this stage the flame can conduct electricity.
• If an AC signal is directed to an electrode immersed in the flame, the flame will conduct electrical charge to the ground.
• Ground is connected to the burner.
• Carbon deposits on the electrode base could cause a short to ground, fooling flame detectors.
• Solve this by making the ground larger than the electrode.

Flame Rod

• Rod which the tip projects a 1/2 inch into the electrode and is made of metal and supported by an insulator
• An AC signal is directed to pilot flame
• Flame Rod uses a ground attached to the burner with a surface area 4 times larger than the flame rod
• The signal will flow easier from the flame rod than the ground
• Resulting in a pulsating Direct Current that can be verified by a Flame Safeguard Control

Flame Rod Rectification

• High resistance short to ground due to a carbonized flame rod cause an AC signal
• Rejected by Flame Safeguard Control and will lock out

Intermittent Pilot Ignition Systems

• Systems where the pilot is ignited by a spark
• These systems removes standing pilots
• Monitors pilot flames during main burner operation
• Uses redundant gas valve in line with main gas and can shut off after 0.8 seconds

White Rogers Flame Sensor

• It is a capillary tube filled with mercury
• The pilot heats mercury making it expand pushing against the diaphragm and making or breaking a switch in series with the gas valve

Pilot Flame Rectification

• 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.
• May have pre-purge up to 45 seconds.
• Common problems are:
  • Poor ground connections
  • Excessive temperatures at the ceramic flame rod insulator permits electrical current to leak to ground

Direct Spark Ignition

• Directly ignites the main burner without using a pilot.
• Uses an electrode that directs a high voltage 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:
  • Safe start check to ensure that a false flame condition is not present.
  • Transformer to generate the ignition spark.
  • Control to disconnect the ignition spark when the flame is sensed.
• Trial for ignition is usually 4 seconds.
• Flame failure response time is 0.8 seconds.
• System has a redundant (second) gas valve in line with the main gas valve for safety.

Hot Surface Ignition

• Utilizes silicon carbide igniters to ignite
• Pre-purge of excess gasses may be performed by the system before igniting
• Relies on flame rectification from a 1DC µ amp signal for ignition
• The ignition process is controlled by a microcomputer and utilizes pre and retry functions

Ignition Control Modules

• Intermittent Pilot
• Direct spark ignition
• Hot Surface Ignition

Solenoid Valve

• The solenoid code in the gas valve is energized which provides opening and closing force.
• Used in small to medium sized burners

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.

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.

Other Valves

• Automatic Gas Valves are used with solenoid coils and a diaphragm valve
• Manual Drop valves that are one-quarter turn valves
• Test openings are 1/8 capped nipples on the valve train used to test gas pressure