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Why Your Heat Works During the Day (but Not at Night)

There are two kinds of problems if your heat works during the day, but not at night. The first set of issues is about nighttime factors, such as freezing temperatures or high heating needs. The second set of problems persists all the time, but the signs become evident only at night.

If your heat works during the day but not at night, the cause could be thermostat issues, airflow problems, clogged flue, frozen condensate line, furnace overheating, and cold air draft. Your heating system may also be inefficient or failing.

When your heat works during the day but not at night, verify if the heating system is efficiently effective in the daytime. The specific problem depends on the system you have, i.e., heat pump, furnace, etc. Many variables are at play, so I’ll try to explain as many reasons as I can.

Reasons Why Heat Works During the Day, but Not at Night

You are probably encountering one of the following scenarios:

  • Your gas furnace or air source heat pump runs all night, but your house isn’t desirably warm.
  • Your heating system stops abruptly before attaining the temperature you’ve set on the thermostat.
  • You have uneven heating in your house, causing cold and hot spots.

I’m segregating these scenarios because the causes and triggering factors are different. This approach will help you filter the possibilities and detect the relevant problem in your situation.

Your Heating System May Be Inadequate or Inefficient

Here are a few facts about gas furnaces and air source heat pumps:

  • A gas furnace with less capacity (i.e., BTU, sometimes Ton) than what you need for your property will run all night but not heat your house to the desired temperature.
  • An air source heat pump may fail to warm your house to the selected temperature in sub-freezing conditions. The only exception is a cold climate air source heat pump.
  • Inefficient gas furnaces (low AFUE rating) and air source heat pumps (poor SEER and HSPF ratings) are unlikely to provide adequate heating at night in colder climates.

You can’t do much about the capacity, efficiency, and efficacy other than changing the heating system. Bear in mind that ancient gas furnaces and smaller air source heat pumps may not perform well on freezing nights.

Let me now highlight the typical manageable causes of heat working during the day but not at night.

Your Thermostat’s Settings May Be Incorrect

Programmable thermostats are great. Smart thermostats are even better, but all the excellent features and intuitive technologies can be a bane when a thermostat misregulates your heat.

If your heat doesn’t work at night, disable all the personalized and automated settings on your thermostat. Try the following steps:

  1. Turn off the schedules, learning, eco mode, and everything else interfering with the straightforward heat setting.
  2. Ensure your thermostat has power and that it turns your heating system on and off.
  3. Select the heat mode on your thermostat and choose the temperature you desire.
  4. Observe if your furnace continues to run till it is desirably warm or shuts down abruptly.

The furnace may continue to operate but not attain the temperature set on your thermostat. In this case, you’ve a problem with the heating system.

If the thermostat shuts down the heating prematurely, the device may have a problem, probably a malfunctioning sensor. In other cases, the thermostat may be too close to the air supply vent or duct.

The Heating System’s Airflow Could Be Clogged

One of the common reasons for insufficient heating at night is poor airflow in the system.

Clogged or restricted airflow could be due to several reasons, including:

  • Unclean air filters on return and supply vents.
  • Obstructions at and around the return vents.
  • Blockage in the return and supply ducts.
  • Inefficient or malfunctioning air handler units.

All HVAC air filters are important. However, the filters on the return vents are critical for heating because gas furnaces use combustion. There is also an air filter on the main return duct going into your furnace or HVAC. You must regularly check and clean these air filters, vents, and ducts.

A gas furnace cannot perform efficiently or effectively if the airflow is limited since combustion requires air. Insufficient airflow can also shut down the furnace if the pressure switch senses any negative pressure caused by the draft inducer motor.

The Gas Furnace’s Flue Is Probably Blocked

The heat settings during the day aren’t as high as at night. Thus, gas furnaces produce more exhaust as they operate longer at night, so the flue shouldn’t be blocked, even partially.

A blocked flue increases the volume and concentration of combustible exhaust gasses inside the furnace. Hence, the temperature inside the furnace spikes, the airflow is adversely affected, and a flame rollout is possible.

Gas furnaces have a flame rollout switch that shuts down the appliance if the combustion or fire spreads beyond the burners.

A blocked flue may spike the temperature in your furnace. High temperatures can trigger the limit switch, which will shut down the unit as a safety measure to prevent damage. Thus, the furnace will stop before heating your house to the temperature set on your thermostat.

The Condensate Line Might Be Frozen

Like flue ducts, the condensate line of a furnace should be clear for optimal heating. However, a condensate line can freeze during cold nights. Debris may also clog the condensate pipe. Both scenarios will shut down your heating, and the furnace will not turn on again.

In some regions, a condensate line can freeze at night and show no worrying signs during the day. Likewise, the exterior outlet of the flue duct may be blocked by snow at night. These triggering factors will cause a furnace to underperform and even shut down.

The Furnace Blower Might Be Dusty

The heat exchanger of a furnace can’t operate efficiently or effectively if there’s a lot of dust, debris, and other interferences causing overheating. For instance, a dusty blower can increase the temperature in a furnace and impair the heat exchanger’s efficiency.

A severe heat spike in the furnace will trigger its high temperature limit switch. Thus, the unit will shut down, irrespective of the thermostat’s heat setting and your real-time room temperature.

A dusty blower is an issue at all times. However, the impact on the heat exchanger and furnace temperature becomes consequential only when the thermostat demands greater output at night.

Your House May Have Cold Air Draft

Poor insulation and leaks in weatherstripping can impair the efficacy of all heating systems. This problem is not evident during the day because the sun’s heat supplements the furnace. However, you may have a cold air draft in your house at night when the ambient heat is no longer available.

Air draft or varying pressure is a common problem that causes uneven heating, creating hot and cold spots. As a result, a furnace or heating system may continue to operate all night without attaining the temperature you set on the thermostat.

Final Thoughts

You may have other problems leading to inadequate heating at night, such as short cycling, ignition failure, and low natural gas or propane. However, there will be telltale signs of these issues. I hope the reasons explained in this article will help you detect the actual cause.


  • Chris Hewitt

    Chris is a Texas-based freelance writer who loves the outdoors and working in his garage. When he's not enjoying the Texas sun, he can be found tinkering with all sorts of things in his workshop.

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