Old Carrier Box Air Con Has Issue

[Carriercon]

Hi, the old Carrier box air con in my place, when running on max heat for a while, turns off and stops heating. I'm not sure if this is how it regulates temperature or if it's a problem. Cooling works fine and doesn't do that. What could be the issue? Possibly overheating or need a clean? Or is some component failing. It heats and it heats up, but after prolonged use it stops and needs to restart. Here's some pictures. It's old but in pretty good condition I think. I also have no idea what the model is or the power so I wonder if anyone can help me identify that, as all the details are missing on the exterior of the unit

https://ibb.co/7RcWHND
https://ibb.co/dKGMgjC
https://ibb.co/tpPCQTW
https://ibb.co/sRF5nQ5
https://ibb.co/pv1J42w
https://ibb.co/HgS32vN

[Brian_UK]

Heat pump units do need to stop and defrost from time to time.

There may be a problem with the defrost timer circuit.

Next time it stops have a look at the outdoor coil to see if it is frozen over.