Thermostat Controls

• Electrical-mechanical systems rely on energy conversion.
• Warm air furnaces use fuel, distribute heat via a fan motor, and convert electrical energy.
• Controls monitor and direct energy, sensing pressure, temperature, and electricity.
• Controls are classified by application, action, and sensing elements.

Thermostat

• An automatic operating control, normally open.
• Senses temperature changes in the room air.
• Action involves a bimetal sensor moving a mercury bulb to open/close a control switch.
• Controls gas valve and ignition (if present).
• Thermostat types include millivolt (750mV), low voltage (24V), and line voltage (120/240V).
• The thermostat is in series with the gas valve and ignition control.
• Heating thermostats have a heat anticipator in series with the gas valve load.
• The heat anticipator heats up, causing the thermostat to open sooner.
• This modulates the temperature swing in a heated room.
• Thermostat installation guidelines:
  • On an inside wall
  • About 5 feet above the floor
  • Away from heat sources like lamps and direct sun
  • In a location with normal air circulation
  • In a room representing average building temperature

High Limit

• An automatic safety control, normally closed.
• Senses the air temperature around the furnace heat exchanger.
• Utilizes a Klixon snap-action disc that opens an electrical circuit when unsafe conditions are sensed.
• Uses a fan-limit control where heated bi-metal spiral causes a switch to open.
• Controls the gas valve and ignition.
• De-energizes the gas valve if unsafe high temperatures are detected.
• Can be wired in 24V or 120V circuits.
• Some high limits require 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 the switch to close
• Has a fan-limit control where heated bi-metal spiral causes switch to close
• Controls the operation of the furnace fan or a relay to control the fan.
• Can be wired in 24V (energizing a 24V fan relay coil) or 120V (directly energizing the fan motor).

Door Switch

• An automatic safety control (normally closed).
• Senses the presence of the fan access panel.
• The switch mechanically closes 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.
• Indirectly senses induced draft fan failure or spillage in the venting system.
• Senses heat from combustion flame rolling out of the combustion chamber.
• A capillary tube or Klixon disk causes the switch to open and de-energize the main gas valve.
• Controls operation of the main gas valve during unsafe venting.

Induced Draft Pressure Switch

• An automatic safety control (normally open).
• Senses the negative pressure developed by the induced fan motor.
• Won't close if the induced draft fan motor fails or if there's venting system blockage.
• Action involves the pressure sensing element closing when the induced fan motor is energized.
• Interlocks the ignition control sequence.
• De-energizes the main gas valve if the switch opens during firing.
• Controls the main gas valve when unsafe venting conditions occur.

Air Flow Sensing Switch

• An automatic safety control (normally open).
• Senses airflow made by an air-moving device.
• The switch closes when air moves through a duct.
• Controls depend on use, wired in series with devices like electronic air cleaners, humidifiers, and electric heaters.
• Ensures that devices operate only when there is airflow.

Fan Centers

• Devices used when a central air conditioning unit is installed.
• Contains a 120-24V transformer and a single pole/double throw relay.
• The relay energizes on a cooling call, closing the normally open switch to start the furnace fan motor.
• The transformer in the center replaces the transformer in the furnace when they are installed.
• Furnace fan control during heating occurs via the normally closed switch of the fan center, in series with the fan limit control.

Aquastat

• Automatic operating control (normally open) used in hot water boilers.
• Senses water temperature of the boiler.
• Closes when water temp falls below the present setting.
• Controls the main gas valve or water pump.

Low Water Cut Off

• Automatic safety control (normally closed) in hot water boilers.
• Senses water level in the boiler.
• A float senses level; if it drops, the float switch opens the electrical circuit to the main gas valve.
• Controls the main gas valve.
• Note: Only licensed plumbers or steam fitters should change it.

Water Flow Switch

• Automatic safety control (normally open).
• Senses water flow developed by the system pump.
• A paddle activates by water movement, closing the flow switch.
• Controls main gas valve in series with the flow switch.

Pressure Control Switch (Pressuretrol)

• Automatic operating control in steam boilers.
• Senses steam pressure in the boiler between high and low settings.
• Maintains proper operating pressure, closing the low-pressure switch to start the burner when pressure drops.
• The high-pressure switch opens to stop the burner when pressure reaches its upper limit.
• Controls steam pressure in the boiler.

High Pressure Control

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

Types of Pilots

• Pilots ignite the main burner and generate electricity via the thermocouple/thermopile to prove pilot operation.
• Types:
  • Continuous Pilot: Always lit.
  • Intermittent Pilot: Lit only during the firing cycle.
  • Interrupted Pilot: Shuts off after the main burner ignites.

Thermocouples

• Electrical generating device of two dissimilar metals joined at two junctions.
• Heating one junction produces a DC voltage.
• The temperature difference determines DC voltage quantity.
• Common on atmospheric burners.
• A typical thermocouple in a gas pilot flame produces 25-30 mV DC.
• Keeps a solenoid, such as a pilotstat, energized.
• Used to prove the presence of a pilot flame.
• If the pilot flame is extinguished, the voltage will fall and de-energize the electro-magnetic solenoid of the pilotstat.
• When the pilotstat is de-energized, gas through the main valve will no longer flow and the burner will not operate.

Thermopile (Powerpile)

• A collection of thermocouples in series.
• Generates 750 mV.
• Packaged with a pilot.
• Sufficient to operate a millivolt gas valve system.
• The system includes its own thermostat, safety limits, and gas valve.
• It is important that all wiring connections are secure and tight, the generated voltage is less than 1 volt.
• A maximum voltage drop is 10 mV across any safety/operating control.

Pilot Drop Out Test

• Finds the minimum voltage that energizes the pilotstat.
• At this voltage, the pilot should ignite the main burner within one second.
• This is usually between 2-4 mV.

Flame Failure Response Test

• Ensures pilotstat drops out in a sufficient time 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 where easily observed.
• Mount rigidly.
• Must not touch the main burner/heat exchanger.
• Must have enough air.
• Must be protected from ignition/extinguishing of the main burner.
• Locate so it is not subject to drafts.
• The thermocouple must properly be positioned within the pilot flame.

Causes and Remedies for Pilot Flame Issues

• Lazy Yellow flame:
  • Dirty lint screen or primary air opening: Clean pilot as required, remove lint screen
  • Orifice too big: Replace orifice inlet fitting
• Waving Blue Flame:
  • Excessive draft at pilot location: Relocate pilot
  • Products of combustion: Install protecting baffle
• Small Blue Flame:
  • Low gas pressure: Increase pressure to normal
  • Clogged pilot-burner orifice: Clean pilot burner orifice
  • Clogged pilot line filter: Clean filter
  • Improper orifice: Install correct orifice
• Noisy Lifting Blowing Flame:
  • High gas pressure: Reduce pressure
• Hard Sharp Flame:
  • Manufactured, butane-air, and propane-air: Install correct orifice inlet fitting
  • Orifice too small: Install correct orifice inlet fitting
• Normal Flame:
  • Proper Installation: None needed

Flame Ionization

• Molecules of air lose electrons when heated.
• Molecules become positive ions, carrying negatively charged electrons.
• Electrons escape atomic bonds, allowing the flame to conduct electrical current.
• Directing an AC signal will conduct charge to ground.
• The ground is connected to the burner and thus is connected to the flame

Flame Rectification

• Carbon deposits at electrode base cause short circuits, fooling flame detectors.
• Ground is made much larger than the spark electrode.
• More difficult for current to travel, but easier from electrode to ground.
• Flame-detecting device sees a pulsating Direct Current signal, not AC

Flame Rod

• A small metal rod supported by an insulator.
• The tip of the rod projects around ½ inch into the flame.
• An Alternating Current signal is transmitted through the rod into the pilot flame.
• The flame rod uses a ground attached to the burner.
• The ground must be at least 4 times surface area of the flame rod.
• The general ground area is usually 10 to 1.
• The flame flows from flame rod to ground
• The result is a pulsating direct current is established
• Verified by a flame safeguard control

Flame Rod Rectification

• Conditions such as a high-resistance short to ground due to a cracked or carbonized flame rod will produce an AC voltage.
• This voltage will be rejected by flame safeguard control, and may cause it to lock out.

Intermittent Pilot Ignition Systems

• Use a pilot ignited by a spark.
• Eliminates standing pilots.
• A pilot flame sensor monitors the pilot flame during main-burner operation.
• Uses a redundant gas valve in line with the main gas valve for safety.
• Flame failure response time of 0.8 seconds

Pilot Flame Rectification

• Uses an electrode directing a high-voltage spark to a ground electrode.
• The flame is sensed via flame rectification.
• Flame failure response time is 0.8 seconds.
• Trial for ignition with safety lockout between 15-90 seconds.
• Disconnects the ignition spark when the flame is sensed.
• Spark gap must be 1/8-3/16 inches.
• May have pre-purge up to 45 seconds.
• Common problems are poor grounds and excessive temps permitting electrical current leak to ground

Direct Spark Ignition

• Directly ignites the main burner without a pilot.
• Uses an electrode directing a high-voltage spark to ground.
• The flame is sensed using flame rectification.
• Ignition control is a microcomputer:
• Safe start check ensures is no false flame
  • Transformer to generate the ignition spark
  • Control to disconnect the ignition spark when the flame is sensed.
• Trial is 4 seconds.
• Failure response is 0.8 seconds.
• System has a redundant (second) gas valve in line to the main gas valve for safety

Hot Surface Ignition

• Uses a silicon carbide igniter to ignite the main burner flame after heating to a hot surface.
• The flame is sensed by a flame rod.
• Ignition control is a Microcomputer:
  • Pre-purge to vent combustible gases before ignition
• Retry of flame ignition sequence
• Recycle of flame ignition sequence if lost during burner run
• Trial for ignition around 4 seconds
• Failure response around 0.8 seconds
• Redundant second valve in line

Ignition Control Modules

• There are three types of ignition control modules:
  • Intermittent Pilot
  • Direct spark Spition
  • Hot surface Ignition
• The basic sequence of operation to 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
• Coil enegergies the valve, providing opening and closing
• They are used in small and medium burners of as safety shut off

Diaphragm Valve

• Uses upstream gas pressure on the valve diaphragm to open gas valve
• Pressure is bled away from the diagram to close it
• Used with atmospheric burners, rooftop units and water heaters
• Valve open and close slowly to fast
• Opening and closing force uses available gas pressure.

Combination Control Valve

• Most residential systems employ a gas valve with major components for automatic firing
  • Pilotstat - Coil energized by the thermocouple millivolt voltage
  • Themocouple can hole solenoid, but not strong enough to pull. Usually done manually.
  • Automatic Gas Valve - used with solenoid and diaphragm
  • Main gas and pilot regulators - adjust gas pressure to correct amount
  • Main gas shut off valve

Manual Gas Valve

• Drop Valve - 1/4 turn, shut off main valve line
• Test Openings are 1/8 - capped nipples used to check gas pressure