This blog entry compares the Bosch Therm series 520PN & 520HN models to the Bosch Therm 660EF heater. The Bosch Therm 520PN & 520HN gas tankless water heaters are 117,000 BTUs as the Bosch Therm 660EF is 140,000 BTUs. These tankless water heaters are designed for 1 major application at a time such as 1 shower at a time or maybe 2 small sinks at once. They all are available in natural gas or propane models.
The Bosch Therm 520 PN, 520HN and the 660EF all take thier combustion air from the space where they are installed in. You will want to make sure you refer to the installation manual for these units and make sure you have enough combustion air for the area where these are to be installed. You can always consult a HVAC technician to calculate and make sure you have enough air supply. These units should NEVER be installed in a bathroom or bedroom where the door is normally kept closed as the heaters while running would deplete the oxygen from the room causing illness and death. This statement is on the front of the heaters and also noted in the installation manual. The National Fuel Gas Code states this also but the installer would also want to go by local code.
The Bosch Therm 520PN has a standing pilot light and is a naturally drafting unit as the P stands for standing pilot and the N stands for naturally drafting. The Bosch Therm 520HN has a hydro generator ignition and is a naturally drafting unit as the H stands for Hydro generated and the N stands for naturally drafting. The Bosch Therm 660EF requires power and plugs into 120V outlet. The 660EF has electronic ignition and is fan assisted which makes it a direct vent unit and the E stands for electronic ignition and the F stands for fan assisted.
The Bosch Therm 520PN and the Bosch Therm 520HN both use an exhaust vent of 5" diameter double wall type B-vent and should be vented vertically up through the roof for a minimum of 6' to the raincap. There is a vertical vent kit called the H/P VVT Vertical Vent kit and has all the material for penatrating the roof. If venting the 520PN or 520HN up through the roof is not an option then the heaters could be vented horizontally out the sidewall but MUST use the AQ4 Powervent kit. The 660EF is a direct vent unit and requires to be vented verically or horizontally with 3" stainless steel sealed vent pipe. The 660EF requires a 660EF adapter first on top of the heater to accomadate the venting.
There is also the Bosch Therm 660EFO Outdoor unit and it is just the same as the 660EF but is designed specifically for outdoor installations. The 660EFO has a special top that it exhausts out of and also has the electronic ignition and is fan assisted. The Bosch Therm 660EF/EFO heaters plug into a 120V outlet.