My Honda S1 heater rebuild is almost finished but reworked unit has pipes in a different position. 
Question…does it matter which way the water circulates through the heat exchanger ? 
Incoming air filter added to crash box aperture to keep out squirrels/mice road kill/leaves and creepy crawlies/ 
I should have 3 speeds and hot AND cold air 
…what happens when the air gets into the pipes within the dash is another problem methinks .