Residencial hvac heating problem (water based)

I had a new ac built to order that replaced my old one 3 years ago. Right after warranty was off (2 years in) a/c started to act wierd. During winters from time to time, it starts to blow cold air and with no reason goes back to normal. During summer, all is ok though. This year again same thing happened. I manually overrided actuator to open position few times. After sometime spring inside it broke, so i bought new unit (erie ag13b020) and replaced whole head, hoping that problem will disappear. After 24h however, problem came back. During the day a/c blows hot air, stops automatically and starts right back working all good, but at night though set unchanged to 80 degrees, will start blowing cold air continiously. Only way to get hot air, is again manually opening water walve, which again will make it work great during the day, but fails at night though untouched. What seems to be the problem? Also what's that part on the pripe near erie looking like oversize "watch" battery seen on last picture? Can it be responsible for the on and off cold air?







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