Here’s Why Your Car Heater Is Only Blowing Cold Air

by | Feb 16, 2021

Picture this: it’s a bitter winter morning, the kind that freezes your hair if it’s still wet while it chills you to the bone. As you hurry into your car, your only thought is getting warm as quickly as possible. As you start it up, you immediately max out the heat.

Freezing your butt off on the way to work? Nah. You’re not playing these games.

Ain’t nobody got time for that.

Then the heater starts blowing air. Relief seems to be only a minute or two away.

But after a few minutes of driving, your heater is only blowing cold air.

At this point, there’s only 1 or 2 things you can realistically do: call your car every name in the book for letting you down in your time of need, or resign yourself to sitting in the cold until you get to work.

Or, you know, you could read this rest of this article and fix the problem.

But that’s probably why you’re here, right?

So here’s the good news: your heater only blowing cold air is a very common complaint cold-weather car owners have. Usually, the fix is relatively simple (even if it can be a bit expensive).

So let’s get to the bottom of this.

But first…

Here’s How Your Car’s Heater Works

Just to be clear, there are 3 types of car heating systems out there. Heating for water cooled engines, heating for air cooled engines, and heating for electric engines.

Each work a little bit differently, but chances are the vehicle you are having problems with is a water cooled engine, so we will stick with the heating systems for a water cooled engine.

Water cooled engine heating systems are relatively simple and easy to understand. As your engine heats up, it generates heat. If it generates too much heat, your engine will overheat.

Fortunately, water-cooled engines have a coolant that absorbs some of that heat from the engine, which keeps it from overheating.

When the coolant is heated up (often to up to 200 degrees), some of it is forced through a special valve which is generally called the thermostat into the radiator, which then reduces its temperature.

Part of this coolant is then forced through what is essentially a mini-radiator called the heater core, which you can see below.

car heater core
The two hoses allow hot coolant to flow in while cold coolant flows back to the engine.

The heater core (much like the radiator) dissipates heat by dramatically increasing the surface area of the heated coolant. Once the hot coolant is in the heater core, a fan blows the hot air around the little metal bits which heats the air up.

This hot air then ends up in your cabin.

So to summarize how it works: your engine gets hot, and then the hot coolant heats up the heater core, when then transfers the heat to your cabin through a fan.

Now that you understand how your heater core (and heating system) work, it should be pretty easy for us to explain one of the most common questions we’re asked about car heaters.

Why Does My Car Heater Only Work When I’m Driving?

For your car’s heater to work, your engine has to be hot enough to heat up your engine coolant.

Most engines reach operating temperature within about 30-45 seconds after you start them.

Depending on how cold it is outside, your engine coolant will take anywhere between 2-10 minutes to heat up all the way. As we’ve already learned, your heater needs the engine coolant to be hot in order for it to blow hot air.

The faster your drive your car, the more heat your engine produces. The more heat your engine produces, the faster your coolant heats up. The faster your coolant heats up, the faster you heat up!

We’re not trying to tell you to drive like your Vin Diesel, but the faster you can drive, the faster your car will heat up.

So if you’re asking yourself “why the heck is my car only blowing cold air when I’m idling and warm air when driving” there’s your answer.

Feel the need for speed, and you’ll feel the heat.

That aside though – what if your car isn’t blowing any heat at all?

The Reasons Why Your Car Is Blowing Only Cold Air When The Heat Is On

Assuming your car is blowing air, we can safely rule out that your fan is the cause of the problems. Other than that, there are 6 primary reasons why your car is only blowing cold air when the heat is on.

1: Low Coolant Level

As we discussed before, coolant is the key to making your heater work. Without coolant, your heater will only be blowing cold air when the heat is on.

Most heater issues in our experience can be linked back to low coolant levels. Depending on how low it is, your coolant warning light may come on. In some cases, that means you just have to add more coolant to the coolant resoviour.

It’s also possible that this could mean more serious issues.

Here are just a few of the most common:

  • Your radiator hoses are cracked or their clamps are loose, causing coolant to leak
  • Your radiator cap is bad
  • Your heater core is cracked or leaking
  • Your head gasket is bad
  • Your intake manifold is bad

2: Issues With The Thermostat

Take a look at your vehicle’s thermostat on the dashboard. If you notice that it continously stays at C even after the engine has had enough time to heat up, you may have a busted thermostat.

The thermostat is responsible for telling your vehicle that the engine is warm. If it isn’t working, the heated coolant won’t be sent over to the heater core. Without that heated coolant circulating through the heater core, your heater will just blow cool air.

The good news is that a thermostat fix is relatively simple, and won’t cost you an arm and a leg to fix.

3: Heater Core Issues

The heater core itself is essentially a small radiator, and the same set of problems that can plague your radiator can also plague your heater core.

Your heater core typically sits right behind your dashboard, and is made up of brass or aluminum tubing that allows the hot coolant to release its heat which is blown into the cabin.

As we talked about before, that hot coolant comes directly from the engine and circulates through the heater core before flowing back to the engine once it’s sufficiently cool.

If this flow is interrupted for any reason, your car’s heater may stop working. Common issues include coolant not traveling through the heater core properly, a block in one of the tubes, or air from the fan not reaching the heater core.

In our experience, heater cores can get clogged, especially with rust or other contaminents. This commonly happens when the coolant hasn’t been changed or flushed in a long time.

4: Air Bubbles In The Cooling System

Your vehicle’s cooling system is a closed-loop, liquid cooling system. Coolant is designed to flow through it with no air blockages, which is why the system must be sealed at all times, so that air is not allowed to get in.

This can have a number of causes, with a faulty radiator cap being the most common. If you notice your engine is overheating, it also could be a blown head gasket, which is much more serious.

Air bubbles in the cooling system can cause your heater to blow cold air, but they can do much worse if left unchecked. They can cause your engine to overheat, which can cost you some serious dollars to fix.

If there are air bubbles in your cooling system, they will need to be flushed out to fix the problem.

5: Jammed Blend Door

Your blend door is what allows the air to move from your vehicle’s heating system into your cabin. Sometimes the blend door can get stuck or jammed, which will leave your vehicle blowing only cold air.

Sometimes this is caused by a bad blend door actuator, but in other cases the door can just get jammed completely.

Getting your blend door fixed is a bit of a pain. If it’s just the actuator, you can likely perform the fix at home. However if your blend door is properly jammed, it’s better not the DIY it unless you have experience. Take your vehicle to an experienced mechanic.

6: Water Leaks

Leaks can happen – and they are always worth looking out for when anything goes bad with your cooling system. Radiator hoses (which are made of rubber) can dry out and crack. Seals can be punctured, or just lose their seal over time. Your water pump could be damaged.

If you notice your vehicle isn’t blowing hot air when the heater is on, pay attention to the underside of your vehicle when it’s parked. Do you notice any liquid pooling? If so, you may have a leak. Take your vehicle to a qualified mechanic to diagnose what’s wrong, because leaks can come from a number of different places.

Generally speaking, it’s relatively cheap to fix leaks provided it’s not from a major seal like the head gasket, or a busted water pump.

7: Heater Control Issues

Although this is uncommon, we have seen cases where the entire heating system is working perfectly fine, it’s just that the buttons on the console aren’t working. Over time, the buttons (or the wiring and electronics that support them) can get be damaged. The controls themselves can get stuck, clogged, or broken. The wiring can also short out.

If this is the case, you’ll want to have these replaced with new ones. Depending on the type of vehicle you have, these costs could vary widely.

My Car Heater Is Blowing Warm Air But Not Hot. Why Isn’t It Getting Hot Enough?

Another common problem we often see is where a car heater is blowing warm air but not hot air. For those of you asking “why is my car heater is not getting hot enough” the answer is generally that some part of your heating system is partly plugged.

In a lot of cases, this is because the thermostat itself is stuck in a partially open position. The thermostat regulates the flow of coolant into the heater core. If your thermostat is partially stuck, some hot coolant will flow into the heater core. This is the warm air you feel.

But, because the thermostat is stuck your heater core is not getting enough coolant to provide enough heat for the air in your cabin to get hot.

You can usually fix this with some penetrating oil as well as manually working the thermostat back open.

The other issue that we commonly see is a blockage in the heater core itself. As we mentioned earlier, older heater cores can get plugged by rust or other gunk. This can prevent coolant from fully circulating through the heater core, which warms the air.

To fix this issue, you’ll likely need a coolant change or flush, which you should be getting regularly anyway. Clean coolant helps keep blocks from happening.

Heating System Repairs Can Be Expensive, But They Don’t Have To Be

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About Protect My Car

Protect My Car is an extended auto warranty company. Our goal here at Protect My Car is to eliminate your worry of being financially responsible for an expensive mechanical breakdown. With our extended auto warranty, you don’t have to worry about being fully burdened with the cost of a covered repair.

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