Heat Pump Not Heating in the Cold

Cold weather is when these codes show up

A heat pump heats by running the refrigeration cycle backwards. In cold weather the outdoor coil ices up, and the control has to defrost it. The indoor air can feel cool during that defrost, and the outdoor unit can steam. That part is normal. A code that stays on the outdoor board, or a house that never recovers after the cloud of steam, is the fault.

Read the amber status light before you open the disconnect. On the Carrier Infinity 25VNA8 and 24VNA9, short flashes are the first digit and long flashes are the second. Carrier 55 is 5 short and 5 long. Carrier 53 is 5 short and 3 long. Carrier 56 is 5 short and 6 long.

What Carrier says defrost is supposed to do

The 25VNA8 / 24VNA9 service manual only allows defrost below 50°F (10°C) outdoor ambient temperature. A defrost demand exists when the coil temperature is at or below 32°F (0°C) for 4 minutes during the selected interval. Defrost ends when the coil is warm enough: above 60°F if the outdoor air is warmer than 25°F, or above 45°F if the outdoor air is 25°F or colder, and never longer than 10 minutes. The outdoor fan turns on 10 seconds before the reversing valve switches back.

Two sensor failures change that on purpose:

• If the outdoor air thermistor fails, defrost is initiated from coil temperature and time. That failure is code 53.
• If the outdoor coil thermistor fails, defrost occurs at each time interval during heating and terminates after 2 minutes. That failure is code 55. A two-minute defrost, every interval, will not keep the coil clear in a hard freeze, and the indoor air stays lukewarm.

The same two failover sentences are in the Bryant 288BNV / 189BNV service manual. Bryant 53 is the outdoor air sensor. Bryant 55 is the coil sensor and the 2-minute defrost. Bryant 56 is the out-of-range event, which uses that same 2-minute defrost.

Code 56 is both thermistors out of range. The manual lists heating when cooling was demanded, a coil thermistor on the wrong tube, and an outdoor air sensor touching sheet metal. One action is to inspect the outdoor coil for obstructions. Do that with the disconnect and the breaker off. Leaves and a solid block of ice are visible without opening the electrical panel. The reversing valve, if the unit heats while the thermostat calls for cooling, is technician work.

Airflow still matters in heating

Low pressure in heating is often the outdoor coil, not an empty system you can top off. Carrier 32 points at the fault 83 list, which includes low outdoor airflow in heating. Carrier 83 is the 4-hour lockout after those trips. Goodman 01 / L1 is the low-pressure cutout on that outdoor control. The homeowner line on Goodman 01 is the thermostat setting and anything blocking indoor airflow. Charge is the technician half of the same sentence.

The filter is in that air path. The Department of Energy says to clean or replace furnace filters once a month or as recommended. ENERGY STAR says the same monthly check, and that a dirty filter can damage equipment. Use the size printed on the old filter. The pressure-code guide has the rest of the high-side and low-side list.

When the furnace is supposed to take over

Dual-fuel systems switch on purpose. The American Standard S9V2 installer’s guide describes a heat-pump compressor lockout temperature. Below it, the furnace is the primary heat. The menu default printed there is 45°F. That is a setting, not a fault code. If the gas furnace never starts below that temperature, the furnace has its own code. American Standard E3.4 is an open 2nd stage pressure switch: low stage can still run, and high stage locks out after three misses in the same call. The furnace pages are the next stop, not another refrigerant gauge.

Do not add refrigerant because the air feels cool. Do not hose an energized outdoor unit. If 53, 55 or 56 is on the board, or 32 has already become 83, call a licensed HVAC technician.

Safety first: Outdoor units carry line voltage even when the thermostat is off. Switch off the disconnect and the breaker before you touch the cabinet. Refrigerant, sensors and the reversing valve are technician work.