Brrr! A car heater blowing cold air once things start to get cooler outside is not good. Let’s get that taken care of so your morning commute isn’t any more miserable than it needs to be.
Basic in design, heaters are usually trouble free. When they go on the fritz the fix is normally simple. Let’s look at how heaters operate and why they can stop working.
Understand the Basic Theory First
So exactly how does your car produce the heat that is supposed to come out of the vents? It comes from the engine. That’s why you don’t have any heat until the engine warms up first.
The main part of the heating system is called the heater core, which looks just like a little radiator. You’ll probably never see it though, because the heater core is buried deep inside the dash components of the car.
Hot coolant (water and antifreeze) from the engine flows through the heater core and then back to the engine through rubber hoses which are called, that’s right, heater hoses. Like I said, things relating to heaters are typically simple.
A fan blows air through the heater core, where it gets hot, and then into your car. The fan is not called a heater fan, sorry to disappoint you, because it blows air into the car for the air conditioner as well. So, it’s called a blower fan. Again, very simple. Now for some common issues for a car heater blowing cold air.
The Car Is Very Low on Coolant
Never overlook the simple things, no matter how unlikely they might be. If the engine’s cooling system is drastically low on coolant the heater will starve for water and won’t work.
The heater is not your big problem, the severe loss of coolant is. You could have a leaky radiator or another problem. Find the leak and repair it before the engine gets ruined from overheating.
The Heater Control Valve Is Stuck Closed
Some cars have a valve in the rubber hose that supplies coolant to the heater core. It closes when the heater is not in use and when the air conditioner is on. That helps keep things cooler under the dash.
On most cars that do have a heater control valve, you will see it under the hood. Watch to see if it moves from closed to open when the heater is turned on. If not, then you can typically move it open by hand. Replace it at your earliest convenience.
The Blend Door Is Stuck on Cold
This is a little more complex. Remember we said the blower fan pushes air through both the heater and air conditioner? When you select a temperature on the dash control, a door moves to blend hot and cold air together to meet your request.
Often you can see that door under the dash on the passenger side of the car. If it doesn’t move as you operate the temperature adjustment, that’s the problem.
Don’t force it to move, as some newer cars control that door by electric motors that are both delicate and expensive. Have a qualified shop find the cause. It’s money well spent.
The Thermostat Is Stuck Open
Just like your house has a thermostat for its heating and cooling system, your car’s engine has one too. Unlike your home’s thermostat, the one in the engine is not adjustable, which is good, because you can’t argue over its setting with your spouse.
The engine’s thermostat is closed when the engine is cold. It stays closed and restricts the flow of coolant to the radiator until the engine warms up, which is typically about 200 degrees Fahrenheit, give or take a few degrees. Then the thermostat will open, usually quite suddenly, to allow water to circulate through the radiator.
If the thermostat sticks open the engine will take a long time to warm up, and in winter it might never reach its full normal operating temperature. So, you will have very little heat, which won’t be delivered until very late in your drive to work. Thermostats are inexpensive, and sometimes easy to replace, but don’t let that fool you.
Unlike your old resto mod, modern vehicles require rather complicated and critical steps to burp air out of their cooling systems after repairs have been made. If you don’t properly refill the cooling system, and take the time to burp out all the trapped air, the resulting low level of coolant can cause your expensive engine to overheat.
If you are not sure about how to do all that, and if you don’t trust your neighbor who says he does, pay a shop to fix it for you.
The Heater Core Is Clogged with Corrosion
Well, we ran out of simple things to check for a car heater blowing cold air. A clogged heater core is rare and expensive to fix. If an engine’s cooling system is poorly maintained corrosion will form inside it and circulate to the narrow passages inside the heater core, clogging them up. And no, you can’t have bypass surgery done on your car’s heater.
Once a heater core is clogged so badly that it restricts the flow of coolant through it, you can’t typically flush it out without breaking it. Then it has to be replaced. That requires disassembling the dashboard to access the heater core.
Remember, we said it is buried deep under the dash inside your car. That is an expensive, awkward, and not fun job to do. So maintain your cooling system properly and avoid all those problems.