There are some talented individuals and mechanics on this forum, and I'm looking for opinions.

It was cold yesterday, teens and single digits but my heater worked fine while driving all day until the last 10 minutes as I was almost home. Started blowing cold air.

This morning was -4 and I started the truck, turned on the heater and nothing. Let it warm up for 20 minutes or so and still nothing. Won't blow any air - hot or cold. About 30 minutes into my commute, air finally kicks on and it's blowing good heat. On lunch break today it starts up and blows some heat, but not a normal load per se. It was warm/ slightly warm air.

Coolant levels are fine. What could be the issue ?

What kind of vehicle?

Sounds like you have two different issues....
1. Intermittent loss of blower motor
2. Intermittent loss of heat

Sound right?

Make/model and year of truck?

Do you have a temp gauge on the dash? If so what is the coolant temp doing?

Could be a lot of things causing this.

Apologies.. can't believe I left that out. 2013 F-150 XLT. 260k miles.

And yes, until yesterday I had no issues with the air or heat. But this morning it wasn't blowing at all. & when it blows it sometimes I get hot, or cold. Or somewhere in between.

I would suspect a bad thermostat. Does your temp gauge show the motor coming up to temp?

Can you hear the blower motor running or not? If so, I'd guess vacuum leak. If no motor running, bad motor.

on the triton 5.4 the thermostat can stick open or closed , 10 dollars and 20 minutes it would be my first item.

Blend doors in air box or thermostat possibly

I'm with gcp. My money is on tstat. Might as well replace the hoses too. 8 yo truck.

Just clarifying the symptoms to lead to a better diagnosis. You state that no air is coming out of the vents. Just like the blower motor aka squirrel cage fan is not working or working intermittently. Can you hear the fan running ?
If so and the motor IS running , then look to see if it is a problem in the heater plenum aka the boxy looking thing under the dash that has controls where the airflow is going - dash defroster/direct to face/down to feet.

Telling us more clues helps alot.

A few questions-
What size engine?
A/C or no A/C?
What does your temp gauge read when at operating temperature, normal, low or high ?

A few more comments-
If your temperature gauge is reading correct temperature when at operating temperature , feel the temperature at the radiator hoses. It should be fairly hot. Then check the heater hoses to see if they are BOTH hot. If one is hot and one is not , suspect a clogged heater core.
If the temp gauge and hoses are all kinda low , suspect a bad thermostat. Also this can also be caused by low coolant level and the heater core can become air bound if some was added recently. Another strange thing that can happen is possible freeze up in very cold weather ( you stated -4°) if your coolant is not properly mixed. Too much water or too much pure coolant can do this. Must be a correct 50/50 mix , more pure coolant is not better. Getting a cheap coolant hydrometer tester will tell you this.
Electrical gremlins are a cause too , lots of newer vehicles have climate control microprocessors and can go wonky.

And this is the beauty of Tman.

At work at the moment, but once I get to the truck I'll start diagnosing with a bit more info. It's a 5.0 V8. I really appreciate the replies thus far.

Blower motor.....next time it doesn't work, smack the blower motor with a hammer or whatever you happen to have on hand. If it comes on, you need to replace it. Most likely there's a dead spit in the motor, and when it gets jiggled to a working spot it comes on again.

Heat issue.....the temperature door on that truck is controlled by an electric motor, not vacuum. When the engine is hot but it's blowing cold, hop out and feel the heater hoses. They should both be hot and about the same temp. I

If they're not hot, you have an engine temp problem.

If one is hot and the other cool, you've got a clogged heater core.

If they're both hot, you have an HVAC temp control problem, most likely the temp blend door motor.

You'll need to feel the heater hoses next time the engine is hot and it's blowing cold in order to go any further with a diag.

Had this happen to my jeep long time ago

First off if the air does not blow its a fan problem not a water issue, check the heater fuse, the blower fuse block on blower, or a slight shock to the blower its self when powered up if it jumps into motion, its the blower, if it runs on a lower setting and now heat if the fuse block on the blower

Water issues, first off the biggest thing would be a plugged heater core, take the water lies off and flush them both out and watch for rusty water, I do this every year before winter and it always helps the heat in the jeep, bigger vehicles can plug up as well, simple fix, really

Bigger water problems, water pump, thermostat, or both, cheaper first, replace the stat, run it to heat if not real warm, could be the water pump.

Vacuum leaks are a pain hard to access and really need a helper to activate while you look, simple fix once found plug the hole or replace the lines to the doors, sticking doors are a pain to, most are just dirty, clean and will work.

Last You-Tube is your friend, everyone some where has had the problem on vid with a fix, you will be surprised !!

Listen to this guy.... and he means the heater hoses going through the firewall, not the radiator hoses up front.

vacuum leaks are found with an LP torch , start let the engine normalize then start following the vacuum hoses with a un lit torch if it sucks in lp the engine rpm go up