Tip & How-To about Cars & Trucks

Car Heater Not Blowing Hot

My car heater isn't blowing hot
There's a few things that can cause this.

  • Air in the coolant system
  • Blocked Heater Matrix
  • Thermostat stuck in the 'open' position
Air can enter the cooling system either after a coolant change or via a leaking/split hose anywhere the coolant passes through.
If you're confident that you have no coolant leaks and have just changed the coolant, try bleeding the system:
  1. Set your heater control to hot
  2. Remove the radiator cap/coolant bottle cap
  3. Start the engine and let it tickover for a few minutes until warm. In normal circumstances this would allow air to escape.

The Heater Matrix
It's like a mini-radiator under the dashboard somewhere. As coolant passes through it
can leave deposits of limescale, dirt and corrosion particles inside the matrix. Over time it forms layers and eventually, a blockage. Coolant cannot circulate through the matrix.

If you suspect a blocked matrix your first task should be to add some type of coolant flush to try and shift it, then change the coolant if successful. However, unblocking a heater matrix isn't often successful. Replacing a blocked heater matrix isn't an easy task either.

When you start your car your engine is cold. The thermostat is in the closed position. As you drive your car the coolant in the engine begins to warm up and become hot - the thermostat opens at a pre-determined temperature and then allows the coolant to enter the radiator via the top hose. The coolant then circulates through the radiator in a downward direction losing excess heat before returning into the engine via the bottom radiator hose.

A thermostat often fails in the open position. That simply means coolant is being pumped around the engine from the word go. There's nothing regulating the flow of coolant into the radiator and it can take many miles of motoring before your engine reaches it's true operating temperature.

In normal running it would take approximately 5 miles/5 minutes for a thermostat to begin opening and a further 5 miles/5 minutes before your engine reached its ideal operating temperature.

If you're doing short trips your engine just isn't reaching its ideal operating temperature.

  1. Try bleeding the air from the cooling system.
  2. Use something to flush the cooling system
  3. Change the thermostat - it's far easier and cheaper - than replacing a heater matrix
If none of the above work - suspect a blocked heater matrix


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