Unfamiliar Territory Troubleshooting means running into unfamiliar equipment time after time. In a perfect world every piece of equipment would have a clearly layed out schematic and wiring diagram and sequence of events list. You rarely run into equipment with all 3. Over time, you have to develop the ability to follow through a schematic and figure out the intended sequence of events from the electrical schematic as well as your understanding of the HVAC/R task at hand. The above schematic of a roof top packaged heat/cool unit may look complex at first glance. However, we are going to go through the sequence of events with this equipment to give you a feel for the sorts of complexities that are designed into a control circuit. You cannot troubleshoot something if you don't have the foggiest idea whether a given control should be energized or whether a contact should be open or closed at particular times. You must dissect the operational steps in order to understand the intentions of the designers. Only then will you be in a position to diagnose faults. So before we start throwing faults at you, lets look at the normal operation of this unit. Button 1 This button returns you to the starting schematic. Button 2 This shows power on, with the unit standing by waiting for the T-Stat to call for heating or cooling. Electricity has got as far as many of the open contacts but can go no further unless some contacts close. The red wires are intended to show the logic of how the electricity flows. It is therefore important to understand that just because a wire or component is shown as black, this does not necessarily mean that there is no electrical potential at that location. With power on there is only one load that is operational. That is the compressor crankcase heater which is tied into constant power. The black dots and the numbered test points indicate junction points. If wires cross over each other with no junction dot then they are not joined electrically. Also note how the T-Stat is surrounded by a dashed line indicating the boundary of the T-Stat components. The T-Stat is poised in-between the heating and cooling position. If it "made" to the heating position nothing would happen because the T-Stat system switch is in the cooling mode. The vertical bar that joins the Y and Y1 terminals represents the T-Stat system switch. It would have to be slid to the left in order to enable the heat. The first circuit that we will examine is the indoor blower motor circuit. The customer has the option of operating the blower any time they want. Note the other "bar" on the right side of the T-Stat under the word AUTO. Switch the fan from the AUTO mode to the ON mode by clicking button 3. (if left in the AUTO mode, the fan would automatically come on when ever there was a call for cooling or heating)
Button 3 Moving the fan switch to the on position sent a signal through the G terminal to the K2 blower relay. The circuit was completed by returning to C, the common side of the 24 VAC control voltage transformer. The designation G is almost always used for the fan circuit. Click back and forth between button 2 and 3 and notice that the K2 relay has a pair of contacts which reverse positions. When the NO contacts close the HI speed fan is energized. We'll put the fan switch back to the AUTO position and see what happens if the T-Stat calls for cooling. Button 4 Even though the fan switch was in the AUTO position, when the T-Stat called for cooling the K2 Blower Relay still got energised (by way of the Y1 circuit) and the indoor blower motor started operating in high speed once again. That's proper, the AC mode requires full airflow. The Y (cooling circuit) also became energized. That pulled in 2 more relays, the K1 and K3 relays. K1 is the compressor contactor and it's 2 NO contacts K1-1 closed to bring on the compressor. K3 closed it's contact K3-1 which brought on the OFM (outdoor fan motor) which is of course the condenser fan motor. So on a call for cooling we energised the COMPR, OFM and IBM on HI speed. The PTC device in parallel with the COMPR run cap provides extra starting torque and switches itself out of the circuit after start up. Click button 2 to end the cooling call. Button 5 The T-Stat is calling for heat but nothing is happening. R1 made to W1 but that's as far as the signal got. That's because the T-Stat system switch is still in the cooling position and only cooling is enabled. Some T-Stats have an AUTO position not only for the fan but also for the system switch. That would enable the heating and cooling modes to automatically switch whenever required. Press button 6 to manually enable heating. Button 6 If the T-Stat system switch is in the heat position and the T-Stat calls for heat the K4 heat relay becomes energized. The squarish jagged design of the heat relay symbol indicates that it is a time delay relay. It sequences the closing of it's contacts over time (also known as a sequencer). The K4-1 contacts down by the IBM close bringing the fan into low speed operation. That is proper for the heating mode. Less airflow causes a higher supply air temperature, is less draughty and is also quieter. The heating mode does not require the large amount of airflow that the cooling mode does. The K4-2 contacts close bringing on the 1st bank of electric heat. Press buttons 7. Buttons 7, 8 and 9 After a short delay, the sequencer brings on the 2nd bank. Electric resistance heating elements draw a lot of power and it is customary to bring them on in sequence rather than all at once to avoid causing severe voltage drop and dimming the lights. The K4 (AUX) contacts also close and energise another time delay relay, the K5. After time delays, the 3rd and eventually the 4th banks of heat are sequenced on by the K5 relay by it's K5-2 and K5-3 contacts. Press buttons 8 and 9 to bring them on in sequence. Note the K5-1 contacts over by the IBM. They are in parallel with the K4-1 contacts and are there to insure that there is airflow over energised elements 3 and 4. For example, if the K4-1 contacts ever fail the K5-1 contacts will bring on the blower anyway. There is no sense in burning out elements from lack of airflow caused by a faulty contact when an extra electrical contact can reduce that risk. When the heating demand is satisfied the T-Stat breaks the heating control circuit. The control relays de-energize the elements in reverse order. The blower will continue operating until both fan contacts on the relays have opened. Summary You can see that there are a lot of things that take place in specific orders to provide heating and cooling from a simple 1 stage cool, 1 stage heat (4 sequenced banks) system. There are a few components that didn't get mentioned yet. The items marked CB are circuit breakers. Those and the high temperature safety limits are normally closed devices and don't come into play unless there is a fault. F1 through F5 are fuses. The PTC device in series with the start winding of the compressor is a solid state start assist device which takes itself out of the circuit automatically when the compressor gets up to sufficient speed. Now that you have an understanding of what is supposed to take place under normal conditions, you can attempt to diagnose some faults.