Caveat: I know very little about HVAC systems but am willing to learn. We have a Honeywell central furnace that came with the house, built in 2011. The model number is N9MPD100J20A2. The serial number is A093359675. It has been working all winter just fine, but just yesterday it started intermittently blowing unheated air into the house. This happened a few years ago as well when the heater turned on for the first time in the winter, but it seemed to have resolved on its own, and we never bothered to look into it. This time the problem seems to be more persistent. We changed the air filter, but that didn't fix anything. The problem hasn't happened long enough for us to be positive about a pattern, but it seems that the first time the heat comes on after not having been on for a few hours (or, once, when we powered off the Ecobee4 thermostat and powered it back on again), the furnace does blow hot air. When the space reaches the desired temperature, the furnace turns off as expected. The next time the temperature drops below the set temperature, we get cold air. We have peeked into the furnace (through the peepholes) when it is operating normally, and we see a flame. When it is blowing cold air, there is no flame. I wanted to open up the panel to see what's going on inside as I've read others on the forum have done this, but my husband is wary of opening up the panel of a gas furnace, and the manual says that the power to the furnace will automatically shut off if the panel is open, so we're not sure it would work anyway. The indicator light is flashing green. We don't see any error codes. It seems like others who have encountered this problem have fixed it by cleaning a flame sensor. Could that be the culprit in our case, too? Does anyone have any other suggestions? We prefer to troubleshoot this as much as possible before calling in a professional. Thanks in advance for your help.

It's a Whirlpool model NUGE105KG01. It's old, but my father has replaced many of the parts over the past few years(he's an electrical engineer). But he's recently had a stroke, so is recovering and can't really help with this. The problem is that the blower NEVER goes off. No matter whether you turn the heat off or not. It just runs 24/7 unless I uplug the door(which shuts the whole machine off). The heat works fine. I've read some of the other threads here, plus some online stuff. The thermostat is about 8 years old and appears to work. I tapped on some of the components inside the heater, but the fan didn't shut down. I've attached pictures of the unit plus the electrical diagram my father blew up to use when he worked on it. Any help is appreciated.