Gas Furnace question

As you can tell, I am very green to home ownership. I do have another question, however. The previous owner had purchased a furnace in 2005, so it's relatively new. When we put the heat on above 74 degrees and it's running for a bit, the furnace emits large amount of steam..as if the pressure gets too high. When this happens, I check the psi gauge and it's only around 5 or 10 psi. Any idea as to what could be causing this? I'll post any additional information if needed.