HVAC BASICS

              Contactors                

Line voltage is supplied to the condensor through the contactor.The contacts of the control are normally open.When the thermostat calls for cooling, the 24volt coil in the control is energized and this closes the contacts and the line voltage is distributed to the remainder of the condensing unit.

To assist you in finding the correct replacement you need to do the following;

There are two basic types of motor capacitors most commonly used in HVAC applications today. The run capacitor and the start capacitor. Motor capacitors are used to either increase run efficiency of the motor or to give it an initial start boost in torque to a motor.  Motor capacitors are only used for single-phase motors that generally see heavy use throughout the HVAC industry. Common applications of single-phase motors which use commonly use run capacitors are air handler blower motors, induced draft or forced draft fan motors on high efficiency furnaces, condenser fan motors, compressors, and pumps for hot water and chilled water. Common applications for the start capacitor are generally limited to compressors and only to start the compressor. Typically, when a start capacitor is used, a special relay accompanies it in the circuit to disengage the start capacitor after the compressor motor gets it's initial boost in torque from the start capacitor. The most common relay for use in disengaging the start capacitor is the potential relay and the least common is the current relay.

Capacitor videos    Capacitors 1   Capacitors 2  Capacitors 3

                       

                                    Transformers 

A step down transformer is a magnetic device which can take the line voltage and through a series of coils, deliver a lower voltage(24volts) to operate the control circuit.The 24 volts comes from the transformer. Most of the time the transformer is located inside the furnace, although sometimes (about 10%) it is located inside the outdoor AC unit. The transformer produces the 24 volts which goes up to your thermostat then out to your air conditioning unit. You should probably try to turn the thermostat down so the air conditioner is supposed to be coming on. Pull your high voltage disconnect so there isn't a chance of you getting shocked. Test and see if you are getting 24 volts straight out of the thermostat wires that come into your outdoor AC unit. If you are getting 24 volts then one of your safety controls on your air conditioner is not allowing the contactor to kick in. A lot of the times air conditioning units have low pressure (low refrigerant) switches on them. If you do not have enough Freon (refrigerant) in the system it will not allow the unit to come on. If you aren't getting 24 volts straight out of the thermostat wires, then you have a thermostat or wiring problem.

Transformers 

         Start Relays

Open or close electrical current to motor windings. The relay will allow start windings to be energized until the motor reaches about two thirds of its rated speed.

There are generally 4 types of start relays:

Current (magnetic) electromagnetic much like a solenoid .Either a weight or spring spring holds the start winding contact points open when the system is idling. When relay is energized the solenoid is heavily magnetized, overcoming the spring pressure closing the contact.

Potential(magnetic)(votage relays) (used with high torque capacitor motors)(Look similar to current relays)contacts remain closed while unit is in idle.As motor reaches its rated speed , the relay opens the contacts to start winding.

Thermal  Uses two bimetal strips to control contacts one for start windings the other for run windings, both contacts normal closed. A  resistance wire is mounted near the bimetal strips in series with both the start and run bimetal strips.When the motor reaches its proper speed, the resister heats up causing the start strips to open the line to start windings. An alternate type  resistance wire is wired in series.The tension wire when cold or unit idling keeps both contacts closed to both windings. When motor reaches a proper operating speed it will heat up the resistance wire causing it to open the start windings.                                                                                                                                                                          Solid state   Changes in motor voltage  as it starts and then gathers speed opens the start windings at the correct time.                                   Understanding Relays video               Relays

Sequencers

Purpose of the Fan Center Relay: The purpose of the fan center relay is to provide low voltage control (24 Volts) of line voltage circuits (110 volts) and can be used to control two-speed blower motors and auxiliary circuits in heating or cooling equipment. Many of the newer furnaces do not have fan center relays, but many of the older furnaces and some new furnaces still do. You would need to look inside your furnace to determine if you have a fan center relay. Some of the relays are attached to a junction box where the power comes into the furnace, and others are located in the blower compartment attached to the blower housing. You might not have the relay that we sell below. Many of the newer furnaces are control by electronic circuit boards.

In the trade we refer to them as, "1 stack," 2 stack," "3 stack."These contacts close and open within specified timings and are used to control the furnace fan and heating elements on demand from a room thermostat. For example, H1-20 C40-110 means the ON time for heating is 1 to 20 seconds and the OFF time for cooling is 40 to 110 seconds. The first contacts turn ON first and turn of OFF last." Refer to large image above. When 24 volts is applied across the two bottom connections on this sequencer you should get continuity (a closed circuit) between the two terminals the one on the left M1 and the one on the right M2. You should also have continuity between M3 & M4. This might take 30 to 90 seconds after the 24 volts is applied before you get the continuity. If you do not get continuity then you might want to purchase one of the sequencers we have below.

The fan limit control provides high limit control of the burners and control of the fan motor according to the furnace plenum temperature. When your furnace burners come on the fan limit control measures the temperature inside the furnace's plenum. When the temperature of your furnace gets warm enough and to the preset temperature on the fan limit control, the control clicks on and causes the furnaces blower fan to kick on and circulate air throughout your home. When your thermostat is satisfied the burners kick off and the fan continues to run until your furnace's plenum cools down to the fan OFF set point on your fan limit control. The fan limit control turns the fan off. The fan limit control is an important safety device. Let's say that your furnace's burners come on but the blower motor is bad and will not start. Without the fan limit control your furnace could over heat and cause a fire! The fan limit control shuts the gas burners down so the furnace will not over heat and cause a fire. The Honeywell fan limit controls that we sell below have a manual fan switch that can be used to override the fan set point and keep the fan running continuously.

    

"2PDT" means Two Pole Double Throw. When the 24 volt coil is energized (through the bottom two terminals) terminals #1 and #3 close the circuit at the top and terminals #4 and #6 close at the bottom. These terminals would be called "NO" (Normally Open) because these terminals are normally open until the 24 volts are applied to the coil.  When the coil is not energized terminals #1 and #2 are closed and terminals #4 and #5 are closed. These terminals would be called "NC" (Normally Closed) because these terminals stay closed until the 24 volt coil is energized. I hope this explanation will help you understand what the markings on the relay represent. The specifications on this relay are: 24 volt coil, rated at 12 FLA (Full load amps) for 125 volts, 6 FLA (Full Load amps) at 277 volts. 60 LRA (Locked Rotor Amps) at 125 volts and 35 LRA at 277 volts.

A flame sensor is a safety device. When your furnace's main burners ignite (light Up) the flame sensor picks up the heat from the furnace's main burning and says, "YES,"  and sends a signal back to the main furnace control board saying that it is OK to let the main burner stay on.  Or if the main burners do not ignite, the flame sensor does not pick up the heat and says, "NO" to the main control board, Shut That Gas Valve and Main Burners Down! Thank God we have safety controls like the flame sensor.  Most of the time, after turning your furnace's power off, you may clean the flame sensors with light sand cloth, emery cloth or steel wool. Reinstall the flame sensor and you are back in business! Over the years flame sensors can build up a light coating of burned-in dust and dirt. This insulates the flame sensor so that it doesn't sense the heat of the flame well enough, so by cleaning it you have almost restored it to its new condition. Seldom, but sometimes they do go out completely. Below we have various furnace name brand flame sensors with an opportunity for you to purchase. It might be worth having a spare on hand.

Pressure Switches

   

A high pressure switch (HPS) and a low pressure switch (LPS) are protective devices for the compressor and refrigeration circuit. The high pressure switch monitors the system for an inoperative outdoor motor, and/or a dirty/restricted condenser (outdoor) coil. The low pressure switch monitors the refrigeration system for a loss of refrigerant charge, and may also be helpful in stopping the evaporator (indoor) coil from freezing up due to a dirty filter or low airflow over the coil. I say "may" because the low pressure switch cut-out setting could be set too low (15-20 lbs.) to be of any help. An evaporator coil could start to form ice at 30 lbs. low pressure charge.If either one of these switches trip, the HVAC unit will shut down. The control needs to be reset to resume operation. Some switches are reset only from inside the HVAC unit. Others may be reset from the thermostat by turning the thermostat subbase "system" switch off, and back on again, or by doing the same to the HVAC unit's main panel circuit breaker.

Motor Hard Start Capacitors

                                                                                  If you have a new or old compressor that isn't starting, try installing this hard-start capacitor before you do anything else. The parts inside a compressor might be too physically tight to move, or on systems with a TXV where the refrigerant pressures are higher on start-up, and need an extra "kick" of energy to get started. This hard-start capacitor provides that extra jolt of energy. Capacitor is very easy to install (takes about 1 minute) and will remain permanently in the equipment.

    Hard start capacitors

Metering Device

Meters refrigerant into the evaporator and provides a pressure drop which separates the high side from the low-side

Operation:                                                                                                                                                                                                          Refrigerant enters the metering device as a high pressure medium temp. subcooled liquid
The liquid refrigerant will then flow through a restricting orifice where the pressure will be decreased, as a result
Refrigerant will leave the metering device as a low pressure low temperature
Saturated liquid vapor mix 25% vapor 75% liquid

Fixed Orifice capillary tube,  accurator
Automatic Expansion
Thermostatic Expansion internally equalized ,external equalized
 

Thermal expnasion valves p 1  Thermal expansion valves p 2

Gas heater pumps

  

Gas valves are used to control the flow of gas only when it is safe to open and ignite the gas. There are two major styles of gas valves; intermittent and standing pilot. A intermittent gas valve has a electronic coil that opens the valve when it is energized. These valves are redundant. Meaning that when the valve opens, there are actually two ports that open at the same time. So if the valve were ever to fail in the open position, a gas leak would occur only if both ports went bad at the exact same time. This is a very unlikely event.
A standing pilot gas valve has a pilot light that heats up a thermal coupler. When the thermal coupler is heated, there is a 24VAC electrical signal that is sent to the gas valve. When the electrical signal is recieved by the valve, the valve is safe to open and the gas will be ignited. The standing pilot gas valve is also redundant, so there is a minimal chance of the valve opening and leaking gas.