Central Heat Not Working But AC Is: What’s The Reason And How To Troubleshoot?  

Ever had an air conditioner work perfectly all summer, but the moment you switch the thermostat to heat, nothing happens? 

The system turns on, you hear the fan running, air comes out of the vents, but the house stays cold.  

This is one of the most frustrating HVAC problems homeowners deal with, and it usually shows up at the worst possible time. 

In fact, this is a very common issue in New Jersey where HVAC systems sit idle between seasons and then suddenly get pushed back into action.

And here’s the most confusing part: If your AC works, it feels like the whole system is fine. 

But heating and cooling don’t always use the same parts, which means one can fail while the other keeps working normally.  

In this guide, I will walk you through some of the most common reasons why your heater is not turning on but AC works. Also, I will tell you how you can safely troubleshoot the system. 

So, let’s get started. 

Key takeaways  

• If your AC works but heat doesn’t, the problem is usually on the heating side only
  
• Thermostat issues are the most common and easiest fix
  
• A dirty air filter can shut heat off completely
  
• Heat pumps can cool fine but fail to heat due to valve or AUX heat issues
  
• Safety sensors can stop heat while the fan still runs
  
• Start with simple checks before calling a professional
  
• If you smell gas or breakers keep tripping, stop and call a technician 

First thing first: What type of HVAC system do you have?

Before you start flipping breakers or adjusting the thermostat, it’s important to figure out what heating system you’re using in your home. 

This matters more than you might realize, because the reason your heat isn’t working depends heavily on the type of system you have.

For instance, most New Jersey homes use one of these:

• A gas furnace with central air conditioning
• An electric furnace
• A heat pump (with or without auxiliary heat)
• A dual-fuel system that uses a heat pump with gas backup

Now, here’s where things get confusing for homeowners. 

In homes with gas or electric furnaces, the air conditioner and the heater are controlled by different internal components. This means your AC can work perfectly while the heating side completely fails.

If you have a heat pump, the same system handles both heating and cooling, it just runs in reverse. Meaning, even in this system, when something goes wrong, cooling might still work while heat doesn’t.

Here are the most common signs of a central heating problem 

In a situation when the heater is not turning on but AC works, the HVAC system seems to be totally okay. You hear it running, air is moving, and nothing appears broken. 

But no matter how long you wait, your home never warms up. 

Now, below are a few signs that you’re dealing with a heating problem.

1. Thermostat is set to heat, but the air is cold

This is one of the most common signs. 

You set the thermostat to heat, raise the temperature, and expect warm air. But what comes out of the vents feels cool or room temperature. 

This usually means the system is responding to the thermostat, but the heating components themselves aren’t turning on or aren’t able to produce heat.

2. System turns on, but no warm air comes out

In this case, the system starts up like normal. You might hear the blower fan and feel air moving through the vents, but it never gets warm. 

This can trick homeowners into thinking the system just needs more time. But in reality the heater isn’t firing at all or is shutting down before it can heat the air.

3. Fan runs continuously, but heat never kicks in

When the fan keeps running without heat, it’s often a sign that the system is stuck in a safety mode. 

This commonly happens when a sensor detects a problem and shuts the heat off to prevent damage or unsafe operation.

4. Outdoor unit runs in summer, but not in winter

A lot of homeowners notice this after switching seasons. The AC worked great all summer, but once winter arrives, the outdoor unit doesn’t turn on at all. 

Depending on your system type, this can point to a heat pump issue, a reversing valve problem, or auxiliary heat not engaging when it should.

5. Heat worked last season, now it doesn’t

This one is especially frustrating. Everything was fine last winter, but now the heat won’t turn on at all. 

This mainly happens because seasonal changeovers can expose worn parts, dirty sensors, or electrical issues that weren’t obvious before. 

In short, here are some common signs that there is a heating problem:

Here are the most common reasons why central heat is not working but AC is

When your AC works but your heat doesn’t, it almost always means the problem is isolated to the heating side of the system. 

The good news? 

Many of these issues are common, seasonal, and fixable, especially if they’re caught early. 

All in all, here are the most common reasons why your central AC works but not heat: 

1. Thermostat problems 

This may sound too simple, but thermostat problems are the number one reason heat won’t turn on. 

Even when the screen looks normal, the thermostat might not be sending the right signal to the heating system.

Also, since thermostats control heating and cooling separately, one side can fail while the other still works perfectly. 

2. Tripped breaker or power issue 

This one catches a lot of homeowners off guard. 

Your heating system and air conditioner don’t always share the same power source. So, it is very possible for the AC to work while the heating side has lost power.

You see, ACs rely heavily on the outdoor unit, while heat often depends on indoor components like the furnace or electric heat strips. This means if one breaker (on the heating side) trips, the other (on the cooling side) can keep running.

3. Dirty air filter blocking heat operation

A clogged air filter can absolutely shut your heat down. And it happens more often than people think. 

That’s because when airflow is restricted, the HVAC system overheats, and built-in safety switches shut the heat off to protect the equipment.

4. Furnace ignition or pilot light problems (in gas furnaces)

If you have a gas furnace, ignition issues are a major suspect. 

You see, modern furnaces are designed to shut themselves down at the first sign of anything unsafe, which often leaves homes with a running fan but no heat.

And possible causes of this include a pilot light that’s gone out, a cracked or failed igniter, a gas valve that isn’t opening, or a dirty flame sensor. 

5. Dirty flame sensor 

A dirty flame sensor is one of the most common reasons a furnace won’t stay on. Here, the furnace might ignite normally, the flame starts, and then everything shuts off after a few seconds.

What’s happening behind the scenes is simple: The furnace doesn’t see the flame, even though it’s there. And for safety reasons, it shuts the gas off to prevent a dangerous situation.

6. Heat pump reversing valve failure (in homes with heat pump)

If your home uses a heat pump, the reversing valve plays a critical role. It is the component that switches the system between cooling and heating modes.

Now, when the reversing valve fails, the system can get stuck in cooling mode. That’s exactly why the AC works perfectly in summer, but when you switch to heat, cold air blows out, or nothing happens at all.

7. Auxiliary or emergency heat problems

In colder New Jersey winters, heat pumps rely on auxiliary or emergency heat to keep homes warm. 

If this backup heat isn’t working, the system might run constantly but never actually heat the space.

Common reasons for this include burned-out heat strips, failed relays, or a thermostat that isn’t calling for auxiliary heat when it should. This becomes especially noticeable during freezing temperatures, when the heat pump alone can’t keep up.

When aux heat fails, homeowners often think the entire system is broken, when in reality the backup heat just isn’t engaging.

8. Safety switches and sensors shutting the heat down

Heating systems are packed with safety controls designed to prevent fires, gas leaks, and overheating. These include high-limit switches, rollout switches, and pressure switches.

If any of these sensors detect a problem, the system will shut the heat off immediately, while still allowing the fan to run. This is why it can feel like the system is working when it really isn’t.

Air conditioning doesn’t rely on these same safety switches, which is why AC can continue working normally even when heat is locked out.

9. Control board or wiring issues

If everything else is working fine, the issue might come down to electrical or communication problems. 

After all, loose thermostat wires, failed relays, corroded connections, or a damaged control board can all prevent the heating system from turning on.

These problems often show up after seasonal changeovers, when systems sit unused for months and then suddenly need to work again. 

In short, here are the reasons why heat doesn’t turn on even if AC works:

Heater not blowing hot air in house: Here’s how to troubleshoot

If the heater is running but the air coming out isn’t warm, you might assume the system is broken. 

But in reality, many heating problems are caused by small issues that are easily overlooked. 

And before you decide to call a professional, you can safely check and troubleshoot these problems. 

These checks won’t damage your system and often solve the problem without any tools or technical experience.

Let’s see how.

Step 1: Check thermostat settings and batteries

Start with the thermostat, because it controls everything. Make sure it’s set to HEAT and not AUTO or COOL. 

Then, raise the temperature a few degrees above the current room temperature and wait a minute to see if the system responds. 

If your thermostat uses batteries, replace them even if the screen still works. That’s because weak batteries can prevent the heat signal from being sent properly. 

Step 2: Replace or clean the air filter

A dirty air filter can quietly shut your heat down. That’s because when airflow is restricted, the system can overheat and trigger safety switches that stop heat production. 

I suggest you pull the filter out and check it against a light. If you can’t see through it, it’s time for an air filter replacement. 

This is one of the most common and easily fixable causes of heating problems, especially if the filter hasn’t been changed in a while.

Step 3: Reset HVAC breakers

Next, check your electrical panel. Your heating system often has its own breaker, separate from the air conditioner. 

Look for breakers labeled furnace, heat, air handler, or HVAC. 

Even if a breaker doesn’t look fully tripped, flip it off and back on. But if the breaker trips again after resetting, don’t keep flipping it, as that’s a sign of a larger electrical issue that needs professional attention.

Step 4: Make sure the gas valve is on 

If you have a gas furnace, confirm that the gas valve is in the ON position. 

This valve is usually located near the furnace and can sometimes get turned off during maintenance or accidentally bumped. 

Step 5: Check for error codes on the furnace

Many modern furnaces have a small viewing window with a blinking light that displays error codes. 

These flashes can indicate what the system is struggling with, such as ignition issues or airflow problems. 

While you might not be able to fix the issue yourself, noting the error code can be very helpful if you end up calling a professional.

Step 6: Confirm that the vents are open and unobstructed

Finally, walk through your home and make sure all supply vents and return vents are open and not blocked by furniture, rugs, or curtains. 

Closed or blocked vents can disrupt airflow and cause the system to overheat or shut the heat off entirely. Good airflow is essential for proper heating.

In short, here’s what you should do if heater isn’t working: 

Here’s when you should not troubleshoot yourself and call in a professional: 

• You smell gas
• Breakers keep tripping
• Furnace ignites then shuts off
• Heat pump blows cold air in heat mode
• System worked last winter but not now
• Error codes won’t clear

You might also want to read: How Much Does HVAC System Replacement Cost?

Final words

By now, I am sure you know why your central heat isn’t working but your AC still is. Also, you know to what extent you can troubleshoot by yourself and when you should call in a professional.

You see, if your heater isn’t working, it doesn’t automatically mean you’re facing a major or expensive repair. 

In many cases, the cause is something simple, like a seasonal issue, a tripped breaker, a dirty sensor, or a small electrical problem.

Got more questions or need to schedule air filter replacement?

You can get in touch with us!
At Clean Air Technologies, we offer both one-time and recurring air filter replacement plans. We install high-quality MERV filters so that you keep breathing fresh air and your HVAC system stays in top shape. 

Frequently asked questions 

Why does my AC work but my heat doesn’t?

This usually happens because your heating and cooling systems use different components.

Your AC relies on the outdoor unit, while heat depends on parts like the furnace, heat pump, or auxiliary heat. 

If a sensor, breaker, or heating component fails, the AC can still work normally while heat completely stops. It’s common after seasonal changeovers from summer to winter.

Can a bad thermostat cause heat to stop working?

Yes, absolutely. A thermostat can fail on the heating side while still working for cooling. 

Dead batteries, incorrect settings, wiring issues, or a faulty thermostat can all prevent heat from turning on. 

Can a dirty air filter stop my heat from working?

Yes, and this happens more often than people realize. A clogged air filter restricts airflow, causing the heating system to overheat. 

When that happens, built-in safety switches shut the heat off to prevent damage. 

The fan might still run, which makes it confusing. Changing your air filter regularly can prevent this issue entirely.

Why does my furnace turn on but then shut off quickly?

This is often caused by a dirty flame sensor, overheating, or a safety switch being triggered. The furnace might ignite briefly, then shut down as a precaution. 

Even though it feels serious, the fix is sometimes simple. A professional can clean or replace the sensor and make sure everything is operating safely and correctly.

Why does my heat pump blow cold air in heat mode?

When a heat pump blows cold air in heating mode, it’s usually due to a reversing valve issue or auxiliary heat not working properly. 

Heat pumps rely on these components to switch between heating and cooling. If they fail, the system can’t produce warm air. 

This isn’t a DIY fix and needs professional diagnosis.

Is it safe to keep running my system if there’s no heat?

It’s best not to. 

Running the system without heat can stress components, especially if safety switches are being triggered. 

If you notice repeated shutdowns, strange smells, or breakers tripping, turn the system off and call a professional. 

Continuing to run it might lead to more expensive repairs or even unsafe conditions.