How can an A/C system consume coolant when in use?

Question

I drive a 2007 Subaru Outback. Recently, I've noticed that when I use the air conditioning, my coolant level drops ever so slightly. The rate of loss seems to be related to the amount of time the A/C is on.

I'd like to fix this myself before summer hits, so my question is, which part of the the AC system could be causing the coolant loss?

Zaid · Accepted Answer

Here is a possible way the two cooling systems are coupled:

With the AC on the vehicle has to reject additional heat to the atmosphere (sourced from the cabin).
This heat is rejected through the condenser, which is usually right next to the radiator.
The end result is that the coolant in the radiator is hotter than before, resulting in higher cooling system pressures. At a high enough pressure this can send some of the coolant to overflow, resulting in loss of coolant.

This behavior could be due to a number of factors, such as a radiator cap that isn't holding its rated pressure, or a clogged radiator but it could also occur just due to extreme outside heat.

I find this occurs on some of my vehicles on extremely hot days (45+ °C); I just need to top it up afterwards and make sure the level doesn't go too low.

Al_ · Answer

If you have automatic temperature control, the heater is also used to control the vent temperature. Try turning the heat alone on and see if it does the same. If yes, the heater core (or its piping) is probably leaking and has a solenoid valve that closes, slowing the leak when the heater is not in use.

Answered by Al_ on February 19, 2021

How can an A/C system consume coolant when in use?

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