Friedrich Wallmaster

I just bought a house with a Friedrich Wallmaster heater and A/C wall unit. I though that it was broken becuase it would not blow warm air. The repair guy said that it adjusts to the outside temp so if it is cold it will blow hot air and if it is hot outside it will blow cold air. This doesn't make sense to me becuase if this is the case why have a temp dial on the unit or even have warm and cool air settings. Also, today I turned it on as it was 30 degrees out and the room was cold. The unit blew hot air for 15 minutes then shut off. I was able to turn it back on after it sat for awhile.
Can someone tell me how these things work becuase I don't have a manual and could not find one online. It sounds like the thermostat in the unit is broken to me.
Thanks
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