The thermostat sends a low-voltage signal on a dedicated wire that selects heating or cooling. A wiring error, a wrong configuration setting or a dead coil on the changeover device leaves the system in the wrong arrangement.
Service · Alhambra, CA
Heat Pump Not Heating or Cooling in Alhambra, CA
When a heat pump fails, the most useful fact is which mode fails. A system that cools but will not heat points at different parts than one that heats but will not cool, and one that does neither points at the outdoor unit itself.
What you are seeing
Three patterns and what each suggests
- Cools, will not heat
The changeover never happens, the thermostat is not asking for heat, or the outdoor coil is failing to gather it.
- Heats, will not cool
The system stays in its heating arrangement, or the outdoor fan or charge has failed.
- Neither mode works
The outdoor unit has lost power or the compressor is not running, so the fault is not mode specific.
How it works
How the thermostat and the valve choose the mode
A system in the wrong mode can look like a refrigerant problem. The pressures and temperatures tell the technician which it is.
Pressures are the second clue. A system stuck in the wrong arrangement gives readings that look like lost charge, so the technician compares both mode readings before drawing a conclusion.
Property type
Old thermostat wiring in older Alhambra homes
Replacing a thermostat in an older bungalow is a common cause of a heat pump that suddenly heats instead of cools. The wire that commands the valve may have been placed on the wrong terminal, or the new thermostat set for a furnace system.
- Wrong valve terminal
The thermostat commands the opposite mode.
- Furnace configuration
A thermostat set for gas heat never calls the heat pump properly.
- Thin cable
A run with too few conductors forces shared wires and mistakes.
Technique
How the failing mode is traced
Check configuration
The thermostat's system type and its reversing valve setting.
Confirm the signal
Voltage at the valve coil when each mode is called.
Compare the registers
Supply air temperature in each mode shows whether the changeover occurred.
Measure the circuit
Gauge readings show whether the charge has dropped.
Check the outdoor unit
Compressor and fan run in both modes.
Tell the call which mode fails and whether the outdoor unit runs, and the likely cause can be narrowed.
(626) 687-0603Phase order
Settings before parts
A thermostat setting takes no parts to correct, so it is checked first. Parts are replaced only after the signal path has been proven, and sealed-system work comes last.
Local detail
The mode problems that appear at season change
In Alhambra, the first warm days of spring and the first cool nights of autumn are when a stuck mode shows up, because the system switches duty for the first time in months. Los Angeles County homeowners usually learn of the fault then.
Triage order
What each finding leads to
- Wrong configuration
The thermostat is reprogrammed or rewired.
- Dead coil
The solenoid coil is replaced without opening the circuit.
- Mechanical changeover failure
Handled as sealed-system work, covered on the reversing valve page.
- Low charge
The leak is located and repaired, then the circuit is charged by weight.
Measured results
Proof that both modes work
- Mode change
Supply air swaps between warm and cool within moments of the thermostat changing mode.
- Supply temperature
Air at the registers reaches the expected level in each mode.
- Steady operation
The unit satisfies the thermostat without short cycling.
- Auxiliary heat
The strips stay off until the heat pump alone falls behind.
Safety protocol
Simple checks you can make
- Mode and setpoint
Confirm Heat or Cool and a setpoint that demands it.
- Emergency heat
Avoid leaving it on, as it runs the strips continuously.
- Filter
Replace a clogged filter.
- Leave the wiring
The thermostat base and air handler hold live low-voltage and strip circuits.
Scope: Emergency diagnostic dispatch, whole system replacement and ductwork replacement only. Scope and pricing are provided directly by independent licensed contractors.
Common questions
Questions about a heat pump that is not heating or cooling in Alhambra
Does this cover heat pump mode faults in 91801, 91803 and 91804?
Yes, it applies to Alhambra homes in those three ZIP codes.
Why does my heat pump cool but not heat?
A stuck reversing valve, a thermostat configured wrongly or a failing outdoor coil are the usual causes, and the line temperatures tell them apart.
Can a new thermostat break a heat pump?
It can if the valve wire is on the wrong terminal or the system type is set incorrectly. The system then runs the opposite mode.
Is emergency heat a fix?
No. It runs electric strips only and bypasses the heat pump, which is a stopgap, not a repair.
A heating or cooling fault rarely fixes itself.
Call and describe the problem in Alhambra: what the system is doing, what it did first and how old the equipment is.