``Even though I turned on the heater, it doesn't warm up at all'' and ``The only thing that comes out is lukewarm air.'' - These are a lot of problems that come up in the winter.

Actually, however, most heating problems with air conditioners are not caused by malfunctions, but rather by minor causes. Misconfigured settings, dirty filters, problems with the outdoor unit...most of the cases can be easily fixed by yourself.

In this article, we will organize the reasons why the air conditioner does not work or produce only lukewarm air into six reasons, and provide easy-to-understand explanations for each of the possible solutions that you can try right away. Information as of February 2026.

Cause 1: The settings are incorrect in the first place (check the remote control)

You may think, "That can't be true," but this is surprisingly common. In this case, the operation mode of the remote control was set to ``cooling'' or ``ventilation''.

There are three checkpoints.

  • Driving mode: Is it set to "heating"? In "Auto" mode, it may switch to cooling depending on the outside temperature
  • Set temperature: Is the temperature set higher than room temperature (approximately 20-23℃)?
  • Air volume: If it is set to "light" or "quiet", it will be difficult for warm air to reach the room. We recommend that you first set it to Automatic

Be especially careful when using "autonomous driving mode." There are many cases where the air conditioner is automatically turned on in early spring or early fall, and then we go into winter without noticing that the air conditioner is still on.

Cause 2: The filter is clogged with dust

If dust accumulates on the air conditioner's filter, it will not be able to draw indoor air properly. As a result,heating capacity drops drastically.

According to Panasonic's official website, the recommended time to clean the filter is once every two weeks. In the winter, the heating is turned on at full capacity, so it's a good idea to clean it more frequently.

Steps for cleaning the filter:

  1. Turn off the air conditioner and unplug it
  2. Open the front panel and remove the filter
  3. Suck up the dust with a vacuum cleaner (the trick is to suck it from the surface)
  4. If it is very dirty, wash it with water and dry it thoroughly before putting it back

If you're wondering, "When was the last time you cleaned your filter?", check it out now. This alone can often lead to dramatic improvements.

Cause 3: Heating is temporarily stopped due to defrosting operation

In the winter, the air conditioner suddenly stops and starts again after a few minutes to over ten minutes.This is a normal operation called "defrosting operation." This is not a malfunction.

Roughly speaking, the heating system of an air conditioner works in that the outdoor unit collects heat from the outside air and sends it indoors. If the outside temperature is low, frost will build up on the outdoor unit's heat exchanger.

When frost forms, the efficiency of collecting heat decreases, so the air conditioner automatically temporarily stops heating and starts working to melt the frost. This is defrosting operation.

According to Daikin's official FAQ, defrosting operation usually takes about 5 to 15 minutes. This occurs more frequently when the outside temperature drops to around 0℃ or below.

Tips to reduce the frequency of defrosting operation:

  • Lower the set temperature by 1 to 2 degrees Celsius (reduces the load on the outdoor unit)
  • If snow has accumulated around the outdoor unit, remove it
  • Use a circulator or humidifier to increase the perceived temperature

Cause 4: There are obstacles around the outdoor unit

The outdoor unit has the important job of "collecting heat from the outside air." If there are any objects placed around this outdoor unit, it will cause poor air circulation and reduce heating efficiency.

Specifically, are you in a situation like this?

  • Putting a trash can or bicycle in front of the outdoor unit
  • Driving with the cover on
  • Snow has accumulated and blocks the air outlet
  • Leaves and debris are clogged in the suction port

Nojima's official support page advises to secure a space of at least 30 cm around the outdoor unit. Especially in the winter, check regularly to make sure the outdoor unit is not blocked by snow or ice.

Cause 5: Wind direction (louver) is facing upward

Warm air tends to accumulate at the top. If the air conditioner's airflow direction (louvers) are facing upwards, all of the warm air will accumulate near the ceiling, leaving your feet permanently cold.

The correct answer is to point the louvers downward when heating. By blowing warm air towards your feet, you can efficiently heat the entire room.

Even more effective is combination with a circulator. Placing the circulator on the floor and blowing air toward the ceiling will circulate the warm air that has accumulated near the ceiling, eliminating uneven temperatures in the room.

Cause 6: Refrigerant gas is leaking (repair required)

If the problem does not improve even after trying all of the above solutions, Refrigerant gas (a gas that carries heat that circulates inside the air conditioner) may be leaking.

If the refrigerant gas decreases, the outdoor unit will not be able to collect enough heat, and the heating capacity will decrease significantly. If you experience any of these symptoms, there is a high possibility that there is a refrigerant gas leak.

  • The air coming out of the indoor unit is lukewarm (it doesn't cool even when cooling)
  • There is frost on the outdoor unit piping
  • I hear unusual noises while driving

Refilling the refrigerant gas cannot be done by yourself. Please contact the manufacturer's support center or an air conditioner repair company for inspection. The estimated repair cost is around 15,000 to 30,000 yen (market price as of February 2026).

In addition, if it has been 10 years or more since the purchase, it is often cheaper to replace the equipment than to repair it, including the electricity bill.

You can try it now! 3 ways to increase heating efficiency

In addition to dealing with the causes, we will also introduce ways to improve heating efficiency.

1. Insulate cold air from windows

It is said that about 50 to 60% of the heat in a room escapes through windows. Just using insulation sheets and thick curtains can make a big difference in the perceived temperature. Even insulation sheets from 100 yen shops are effective.

2. Use a humidifier together

As the humidity increases, the perceived temperature also increases. Humidifying your room with a humidity level of 40 to 60% will help you feel warmer even if you lower the temperature setting, which will also save you money on electricity.

3. Circulate the air with a circulator

As mentioned above, warm air collects on the ceiling. Circulating air with a circulator or fan will evenly heat the entire room.

FAQ

How many minutes does it take to get warm after turning on the air conditioner?

Generally, the heating of the air conditioner approaches the set temperature in 15 to 30 minutes after it starts operating. It may take a little longer on days when the outside temperature is extremely low or if the room is large. We recommend using the timer function and turning it on 30 minutes before you wake up.

Do I need to do anything during defrosting operation?

Yes, it's OK to wait. Heating will automatically resume in about 5 to 15 minutes. Avoid turning the power off and on while defrosting, as this will actually reduce efficiency.

Which is better, air conditioner heating or fan heater?

Generally, running costs are lower for the latest air conditioners and heaters. However, heating efficiency decreases in extremely cold regions where the outside temperature is below 0℃, so it is effective to use it in conjunction with a fan heater or floor heating.

Which is better, “automatic operation” or “heating operation”?

In winter, it is best to manually select "heating mode". "Automatic operation" automatically switches modes depending on the difference between the room temperature and the set temperature, so it may switch to cooling or blowing air unintentionally.

Should I replace my old air conditioner? What is the guideline?

The average lifespan of an air conditioner is approximately 10 to 13 years. If you have been using a heating system for more than 10 years and it is not working properly, you can save 10,000 to 20,000 yen a year on your electricity bill by upgrading to the latest model. Considering the retention period of manufacturer's repair parts (approximately 10 years), 10 years is a good guideline.

References