I have been reading a lot of posts trying to figure out my problem

so I replaced heater core the other day and everything was working great until the next day the heat was only on floor nothing else so I disconnected the battery waited a few minutes hooked back up and everything worked perfectly drove all day and now here it is today and have to do the same thing again I can’t keep disconnecting the battery as I’m trying to get it inspected but I need heat any help would be appreciated