How It Works: Glycol Make-Up Package 

My new expansion tank for Navien CH 240 heating system came precharged at 12 psi. What will happen to my systems operation if this tank pressure is not correct? I ask because I’ve seen some tanks recharged to higher pressures. Owners manual says nothing about recharge pressure. Also, is the glycol used to cool the heat exchanger? My glycol strengh is ver low...it is almost just water. Will this cause my heat exchanger to overheat? I am getting this error and the only thing that changed with my system is I flushed the system and in doing so lost most of the glycol so it is mostly water. The water sometimes boils within the heat exchanger ( this is the error code I get). Thanks.