The air onditioner leaks water, can't remember if

A common problem in the BMW 3 series of the 99 - 2002 year is that when there is a leak in the cooling system for what ever reason ( perished hoses leak in the 3 series a lot along with expansions bottles, radiators ) air leaks in as tiny amounts of water leave through the leak.
This air rises to the highest point in the cooling system which is the internal cabin heater matrix radiator in the 3 series models of the pre-said years.
As it fills with air over time the water pump can no longer circulate the coolant ( A pump designed for liquid will not move air only liquid ) leaving the internal cabin heater matrix radiator empty ,the coolant now takes the alternative route hence no heat.
A temporary solution is to put in more water or coolant remembering to ;

1 Loosen the air bleed screw or remove it when doing so.
2 Do it while the engine is running and cold and the pump is rotating.
3 Make sure the heating in the car is on full tempeture so as to allow the water to flow up and into the internal radiator.
4 Leave the bleed screw open for five minutes so to be sure all the air has purged.

After that tighten it and replace the filler cap and you should be good to go. if it does not fix the problem bring it to a garage to have the system pressure checked for leaks. they may also do a sniffer test to see if there is any combustion gases in there that should not be, this may indicate major machanical failure of the engine block head gasket or more common in the 2000 318 's is a cracked engine block head which needs total replacement.

So remember! practice and enjoy :)