Missing coolant when turning the heater on?

Hi, i have a bmw 320sport 2003. i changed the expansion tank about a month ago as it was cracked and recently noticed that the water light in the dash keeps coming on. the 1st few times the light came on, i checked the water levels and it was half empty so refilled. took it to a garage but they couldnt see any leaks. then i noticed that whenever i put the heating on, thats when the water light comes on. if i dont put the heater on, then the water light doesnt come on. any ideas what is causing this?

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This post was auto-generated based upon a question asked on our tech article page here: Pelican Technical Article: BMW 3-Series E36 Cooling System Flush

[Wayne 962]

Well, you could have a leak in one of the hoses to your heater core. If you put the heater on, and let the car idle, do you see any coolant leaking out the bottom of the car? Or, you could just have some extra air in the system. The coolant system needs to be bled with the heater turned on - if it's not, then you might have some issues like these.

- Wayne