Smelly heater core?

[25psi]

I'm in southwestern FL so I don't use the heater all too often but when I do it puts off this very musty and thick smell, it doesn't "smell bad" per se but it's no fun breathing it. It's only when the heater core is being used, outside air or A/C is fine.

[timebuilder]

Find a shop that does a lot of AC work, which should not be difficult in Florida. Many shops are equipped to apply a spray bacteriostat and fungicide to the evaporator to prevent odors, and there is no reason those chemicals could not be applied to the heater core, as well.

[toyomoho]

You could have had moisture buildup and then mold over the years. Make sure the A/C evaporator drain tube is unplugged. This tube drains any condensation from A/C usage.