I smell coolant, burning when my heat is on.
There is no leaks, like the heater core is not working, no fogging, no puddles.

But when I turn on my heater, there is a sweet coolant smell that will make you sick. I have to open up the windows. But when I have the a/c on the smell goes away.

What to do?

Robin:uhoh:

Welcome to the forum!
If you smell coolant in the interior, the heater core is probably developing a leak. As the leak gets larger, the windows will fog, you may notice steam from coming from the registers, then coolant dripping from the heater case onto the passenger side floor, then puddles on the carpet. When using the A/C, the blend door is closed so you won't smell it, if you move the temperature selector to "warm", you'll probably notice the odor. Time to replace the heater core.

W When using the A/C, the blend door is closed so you won't smell it, if you move the temperature selector to "warm", you'll probably notice the odor. Time to replace the heater core.

the blend door does not automatiacly close when the a/c is on. the temp of the a/c is variable by blending warm air with the a/c stream.

As said the heater core is leaking.
Anti freeze/coolant fumes can be dangerous to your health.
Can cause kidney damage and other health problems.
Do not drive it like that unless all windows are down and well vented.
Fix it or bypass it as soon as you can.
MT