Water Leaking From Car When Heater Is On – Solution

Water Leaking From Car When Heater Is On – Solution

Nothing irritates more than finding wet spots due to water leaking from the car when the heater is on.

It’s not hard to find water in your vehicle as it collects at the lowest areas, but it can also run along wires. It can also travel up the fabric and cardboard surfaces.

Water leaking from your car when the heater is on

Does the water leak from the car when the heater is on a result of car washing or rainstorm?

It’s always good to get the immediate cause of the leakage to know how to handle it in good time.

What Causes Water Leaking From The Car When The Heater Is On

Your heater is part of the car’s cooling system. It acts and looks like a miniature version of the radiator.

Your heater core circulates the coolant via the radiating heat and tiny tubes into the cabin. The heater ensures the proper functioning of the defroster.

When the engine warms, the coolant absorbs the heat and takes it around the car engine. It then takes it to the radiator for cooling below the boiling point.

A thermostat maintains a constant temperature throughout the entire process.

When you turn the heat on in your car, air blows over a heater core to get warm and comes to the cabin.

The coolant has corrosion inhibitors coating the inside surfaces of the cooling system. They also do the same to your heater core.

The depletion of the corrosion inhibitors leads to a corroded cooling system. It gets filled up with contaminants that might start to leak.

What Causes Water Leakages When The Heater Is On?

A car heater is a smaller version of the car radiator.

Though it works like the radiator, your heater core uses the coolant heat to warm your car’s interior instead of cooling the vehicle coolant.

The hot coolant from the vehicle engine goes through a coiled heater core tube that exchanges the coolant and the cabin air.

The tube fins increase the heat transfer surface area to the fan-forced air through the fins. It results in heating the car passenger compartment.

When you own a car, you face the reality that each car component has its problems. The same applies to the heater core.

The following are the major problems of the heater core:

  • Heater core leaking
  • Heater core clogging
  • Electrolysis – results in severe corrosion and bursting of your heater core

Heater Core Leaking

The antifreeze or coolant has corrosive inhibitors coating the inside surfaces of the vehicle’s cooling system. The same applies to your heater core.

When the inhibitors get consumed, the vehicle cooling system corrodes and then filled with contaminants. They all lead to heater core leaking.

The car engine is in great danger when the coolant level in the system is low due to a leaking heater core. The leaking will make the engine seize and overheat.

It’s a significant cause of mechanical breakdown.


It’s a chemical reaction occurring between the coolant and the metal surfaces.

In the case of electrolysis, significant corrosion leads to the rupture of the heater core. The coolant will spray into the car passenger compartment.

White smoke will fill the cabin of the vehicle. The electrolysis is expected in the latest car models with an aluminum heater core and radiator.

Since aluminum is a soft metal than iron, it’s very reactive to electric currents and acids present in your coolant.

The heater core electrolysis corrosion occurs when:

  • You fail to change the coolant regularly.
  • The stray electric currents flow via the coolant because of the loose, corroded, or absence of the ground straps of your engine.

Electrolysis Corrosion Effects

  • Heater core leakages
  • Tiny black heater core pinholes
  • Radiator coolant leaking
  • Manifold gasket coolant leakages at the intake connections

If the coolant is very low, the temperature gauge and a warning light might fail to indicate the problem.

It’s because it’s hard for the temperature gauge to read the temperature of the coolant when the pipes of the heater core system are empty.

Water leaking from your car when the heater is on causes

Signs That Your Heater Core Has Problems

Since the car warning light in the dashboard can’t alert you of a leakage in the heater core, you should know the common signs that your heater core is leaking.

The following are significant indicators to look out for in your vehicle:

Little Or No Heat Inside The Vehicle

A heater stops to give warm air, but the engine is hot with a cold cabin.

Fog Up On Windows

Windows become foggy without any reason. The fog is a warm, moist condensation that is not mild.

The coolant fluid leaks into the car cabin and evaporates into the steam that condenses on the colder parts of the vehicle.

A Drop In The Coolant Level Or A Very Hot Engine

Don’t take overheating lightly, as it will land you in serious mechanical repairs. Overheating leads to breakdown and wear of the vehicle’s major components.

Ensure the coolant level is on the right level and top up anytime it drops beyond the needed level.

Coolant Smell Inside The Car

A sweet smell from the car vents indicates heater core leaks and radiator fluid scent. Don’t sit back to enjoy the scent.

Quick Reduction Of The Coolant

The vehicle devoured the coolant quicker than expected.

How To Fix Water Leaking From Car When The Heater Is On

A malfunctioned heater core needs immediate attention to prevent potential damages.

Water leaking from the car when the heater is on can be that the hoses have a problem. It can also be that the entire heater core has a problem.

It might force you to replace or repair the hoses or the whole system.

Accessing the units might be tricky since it’s hard for this component to be reachable. In some vehicles, you may have to remove the car dashboard first.

In some vehicles, you can reach the heater core via the hood without great hustle.

How To Maintain Your Car To Prevent Water Leaking From The Car When The Heater Is On

Maintain The Coolant

The coolant isn’t for air conditioning alone but also for heating. The fluid circulates via the radiator into the heater core to heat the air blown into your car.

You need to check the coolant levels monthly to top it off to maintain the required level.

Set Annual Inspections

Invest in regular service every time before temperatures drop in a year in the winter. It’s the best time to prepare your vehicle for the upcoming harsh weather condition.

While servicing your car, check the following:

  • Blower fan
  • Heating core
  • Radiator
  • Belts
  • Thermostat
  • Hoses

They all need to be in good condition. Make sure you replace any part that needs spare parts to prevent breakage later on in the months to come.

Check For Leaks

A radiator can cause the heater core to malfunction. Leaking will drip the radiator fluid on the heater core, leaving the system with less coolant.

If you spot any leaking signs under your car or smell it from the car vents, there is a leak that you need to address.

Take the vehicle to a mechanic to replace or fix the condition on the heater core.


Poor cooling system maintenance may destroy your car. Change your coolant after 24,000 miles or two years.

It will depend on the manufacturer’s recommendations. Dashboard indicators might fail to show if there is a leakage.

Be keen on these signs to ensure no water leaks from the car when the heater is on.