I have a shelter sf1000. Tonight got real hot top of fire box door was red hot. My duct blower will only run for 10 seconds to a minute then shut off periodically. House struggles to get to 70 degrees which is not right. Fire box is definitely getting hot enough to where I cant even put my face in front of it. I’m new to this. Right now I have a wood burning and propane running and as some of you may know that’s eating the wallet. Why is my duct blower not running the adequate amount of time to heat my house when firebox is hot enough?