A technician arrives to find the evaporator coil and suction line encased in ice with the system still running. What should be done before any refrigerant readings are taken?
- Add refrigerant until the suction pressure rises to its normal range
- Raise the blower speed and take readings right away
- Recover the charge and weigh in the nameplate amount
- Shut off the compressor and run the blower until the coil thaws
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D. Shut off the compressor and run the blower until the coil thaws
Ice blocks airflow and insulates the coil, so every pressure and temperature taken on an iced coil is misleading. Adding refrigerant because suction is low is the classic mistake: the low pressure may be caused by airflow, and the system could end up overcharged.
Reference: Service procedures; frozen evaporator diagnosis
A furnace's rating plate lists a maximum external static pressure of 0.50 in. w.c. The measured total external static pressure is 0.95 in. w.c. What does this tell the technician?
- The blower is moving more air than the duct was designed for
- The duct system is too restrictive and airflow is below design
- The furnace is operating normally within its allowable range
- The supply ducts are leaking and losing pressure to the attic space
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B. The duct system is too restrictive and airflow is below design
Static pressure is the resistance the blower works against. Nearly double the rated maximum means undersized or blocked ducts, a restrictive filter or a dirty coil, and with a PSC blower airflow falls well below design. Leaky ducts lower static pressure rather than raise it.
Reference: Total external static pressure; blower performance
Why are gas burners set up with more air than the exact amount needed for complete combustion?
- To raise the temperature of the burner flame as high as possible
- To increase the carbon dioxide percentage in the flue gas
- To keep the flue gas from rising too quickly in the vent
- To make sure all the fuel finds oxygen and burns completely
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D. To make sure all the fuel finds oxygen and burns completely
Mixing in a real burner is never perfect, so some excess air is supplied as a safety margin against carbon monoxide. It comes at a cost: extra air carries heat up the vent and lowers the flame temperature, so too much of it reduces efficiency.
Reference: Combustion fundamentals; excess air
The outdoor unit will not start from the thermostat. When the technician jumpers R to Y at the thermostat subbase, the outdoor unit starts and runs normally. What is at fault?
- The contactor
- The thermostat cable
- The thermostat
- The transformer
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C. The thermostat
A jumper at the subbase does exactly what the thermostat's cooling contact should do, using the same wires. Since the unit runs, the cable, transformer and contactor are all proven good and the thermostat is not closing R to Y.
Reference: Thermostat troubleshooting; jumper test
Why should a component be disconnected from the circuit before its resistance is measured?
- Parallel paths through other parts can give a false reading
- The ohmmeter's battery can damage the control board
- Resistance cannot be measured on a cold component
- The meter needs line voltage present to read ohms
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A. Parallel paths through other parts can give a false reading
An ohmmeter reads whatever paths exist between its leads. If the component is still wired in, current can also flow through other devices in parallel, so a good reading may hide an open part. Power must also be off, since an ohmmeter must never be used on a live circuit.
Reference: Electrical troubleshooting; resistance measurement