The older Bosch Aquastar model 125FX LP (Liquid Propane) was 117,000 BTU and was a direct vent unit that was good for one major application at a time. The 125FX LP had electronic ignition and was fan assisted and took it's combustion air from the room it was installed in. The heater had a built in fan so the heater could be vented right out the sidewall of the building. The 125FX LP had to be vented with a 4" sealed stainless steel venting going a maximum of 15' out the sidewall of the building into the FXHood. The FX Hood was a termination hood specifically designed for the 125FX LP and had a built in flapper to keep cold air from penetrating the vent pipe and freezing the heater. The Bosch Aquastar 125FX LP is discontinued now and has not been made for over 8 years or so. The replacement model for the older 125FX LP would be the Bosch 660EF LP.
Bosch 660EF LP
The Bosch 660EF LP can be used for a direct replacement for the older Bosch Aquastar 125FX LP. The 660EF Lp is also a direct vent unit, it is 140,000 BTU and has electronic ignition and is fan assisted to vent right out the sidewall of the building. The 660EF LP has 3/4" water and gas connections on the bottom of the heater. The 660EF LP also takes it's combustion air from the area where it is installed. The 660EF LP is designed for 3" stainless steel venting and has a special 660EF adapter required for the top of the unit to attach the venting to. You would then need to use a 3" to 4" increaser so you could go to 4" stainless steel venting and can then be adapted to hook up right to the existing FXHood.
If the older Bosch Aquastar 125FX LP was originally installed with 4" galvanized venting and there was no FX hood in place then the installer could use the AQ5EF sidewall vent for the new Bosch 660EF LP. The AQ5EF sidewall vent kit comes with the 660EF adapter, 3"x90 degree elbow, 3oz tube of high temp sealant, wall thimble, termination box, a 3"x12" piece of stainless pipe and a 3"x18" piece of stainless pipe.