I have hydronic baseboard heat and had trouble keeping house warm in January. I have a gas boiler but now use an outdoor wood furnace to heat my water. I noticed the pressure was low so I recharged system it seems one zone is not heating as good I think its air in lines but how high should pressure read? I pressurized to 15 psi and it stays there even when its hot

Fifteen psig is more than enough for a single story & most two story houses.