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Sierra AC Services works on central air conditioning, furnaces, heat pumps, ductwork and thermostats for homes and small businesses in Sierra Madre, California. This page is the same reference sheet we work from: what each system does, what usually goes wrong, and what actually gets checked.
Component index
01
How a residential split system divides the work between an outdoor unit, an indoor unit and the duct path that carries the air.
A split system moves heat in two places. The outdoor unit holds the compressor, condenser coil and fan, and rejects heat to outdoor air. The indoor unit is either an air handler with an evaporator coil or a furnace with a heat exchanger; it holds the blower that pushes air through the ducts. Refrigerant travels between the two through a matched pair of copper lines called the line set.
Airflow closes the loop. Return air is pulled back to the unit, passes through the filter and across the coil, then leaves through supply ducts and registers. Most comfort complaints come down to one of four things: not enough airflow, a refrigerant problem, an electrical or control fault, or water where it should not be. Finding which one is the job.
02
Cooling work from the thermostat down to the line set: diagnosis, refrigerant, drainage and the starting components.
AC.01
Supply and return air temperatures are measured at the registers, static pressure is checked across the filter and coil, and the condenser coil is inspected. Superheat and subcooling readings help decide whether the refrigerant charge is the cause or the symptom.
AC.02
Current equipment commonly runs R-410A or R-454B. Recovering, evacuating and charging requires the correct refrigerant and a vacuum pulled on the line set. A leak search comes before any top-off, because a system that loses charge once will lose it again.
AC.03
The evaporator coil pulls moisture out of the air, so the condensate pan, drain line and float switch all have to stay clear. A plugged drain, a slime-filled pan or a frozen coil are the usual reasons water shows up at a ceiling stain or a utility closet floor.
AC.04
A run capacitor, a contactor and the compressor make up the start circuit. Capacitors drift out of tolerance, contactor points pit and weld, and a weak capacitor is a common reason an outdoor unit hums but never turns the compressor.
NoteSEER2 is a published cooling efficiency rating for a specific model, so a higher number means more cooling delivered per watt of electricity. The rating belongs to the equipment nameplate, not to a promise about how any particular house will run.
03
Gas and electric furnace work: ignition sequence, airflow limits, venting and heat exchanger condition.
A gas furnace ignites fuel at the burner and pushes air across a heat exchanger; an electric furnace uses resistance elements and the same blower. Both depend on airflow being high enough to carry heat out of the cabinet, and on the flue staying open. When airflow drops, the temperature inside the cabinet rises and the limit switch shuts the burner down — which is exactly what it is there to do.
The safety chain is checked before anything else on a no-heat call: induced draft, pressure switch, hot surface ignitor, flame sensor, limit switch and rollout switches around the burner box.
NoteAFUE expresses how much of a furnace's fuel energy actually becomes delivered heat. It is a published figure for a specific model and says nothing about duct condition, airflow or how the equipment was installed.
04
One refrigerant loop, two jobs, and a reversing valve that decides which direction the heat moves.
A heat pump does not make heat, it moves it. On a heating call the reversing valve sends hot refrigerant gas to the outdoor coil so the unit gives heat to outside air, while the indoor coil absorbs heat from the house. Reverse the valve and the same components run the cooling cycle. One blower, one thermostat and one set of ducts serve both.
Because the outdoor coil runs cold in heating mode, moisture freezes on it and a defrost cycle periodically reverses the system to clear the ice. Ductless mini-splits use the same cycle for rooms that have no ductwork, with an indoor head mounted on the wall and a short line set running to the outdoor unit.
HP.01
A solenoid shifts the valve on a mode change. When it will not shift, the system cools fine and refuses to heat, or heats only with the backup element doing the work.
HP.02
Board, timer or sensor driven. Ice left on the outdoor coil blocks airflow through the fins and cuts heat exchange, so a defrost fault shows up as slow, steady comfort loss.
HP.03
An electric supplemental element helps during defrost and in cold weather. The emergency heat setting on the thermostat forces that element, so it is a test position rather than an everyday mode.
NoteHSPF is the published seasonal heating rating for a heat pump model. Like SEER2, it describes the equipment on a rating condition, not the output of an installed system on a particular day.
05
The delivery path decides whether the equipment can move enough air to do what it was sized to do.
Return air comes back to the unit through large ducts, gets filtered, crosses the coil, and returns to the rooms through supply ducts and registers. Every supply outlet should deliver air and every return path should stay open enough to let the blower pull. When one is missing, the whole system makes up for it noisily.
Flex duct is common for branch runs because it routes easily in tight construction, and it is easy to crush, kink or pinch during later work in an attic or crawl space. A collapsed branch shows up as one warm room, a whistling register, or a system that moves plenty of air at the cabinet and nothing at the grille.
06
Thermostats, low-voltage wiring and the safety switches that stop an unsafe state before it becomes damage.
A thermostat is a user interface, not the deciding device. It reports demand over a low-voltage bus, and the control board in the furnace or air handler decides whether the compressor, blower, burner or reversing valve responds. That is why swapping a thermostat can change nothing, and why a blank screen often traces back to a transformer fuse rather than the thermostat itself.
Interlocks are the quiet reason equipment stays safe. A float switch stops the blower when the condensate pan fills, pressure switches open when refrigerant pressure leaves a safe range, and a low-voltage transformer feeds the control circuit that runs the whole call.
NoteA setpoint inside roughly 68–72°F in both seasons keeps the equipment cycling normally instead of overworking against an extreme request.
07
A seasonal routine that keeps airflow, drainage and heat exchange working the way the equipment was set up.
Before any service visit
Turn the system off at the electrical disconnect beside the outdoor unit and set the thermostat to off. Drainage work needs power off as well, and refrigerant work is done with recovery equipment by qualified technicians.
RequestTo book a maintenance visit, call +1(626)621-9977 or email [email protected] with the equipment location and system type.
08
Symptom, likely area, what gets checked. General field reference — it narrows the search, it does not replace diagnosis on site.
Work through the list before scheduling if you can do it safely. Filters, register positions, a tripped breaker, a thermostat left in the wrong mode and a plugged condensate drain resolve a large share of calls without any parts.
Showing all 9 troubleshooting entries.
CoolingUnit runs but the air coming out is not cold
Likely area
What is checked
WaterWater staining a ceiling or pooling at the unit
Likely area
What is checked
AirflowAirflow is weak in some rooms only
Likely area
What is checked
ElectricalBreaker trips when the outdoor unit starts
Likely area
What is checked
HeatingFurnace runs but there is no heat, or it locks out
Likely area
What is checked
HeatingHeat pump runs but the house does not warm up
Likely area
What is checked
CycleSystem short cycles on and off constantly
Likely area
What is checked
ControlsThermostat screen blank or no response at the wall
Likely area
What is checked
CoolingIce on the suction line or the evaporator coil
Likely area
What is checked
No entries match that filter. Clear the search box or choose another category.
ScopeEverything above is general field information. Diagnosis depends on measurement, access and the specific equipment installed, and refrigerant work requires qualified technicians with recovery equipment.
Schedule a service visit09
Describe the symptom and the system type. That is usually enough to plan the right visit.
Sierra AC Services
Include when you call