The older Bosch Aquastar model 125B LP L (Liquid Propane) or 125B NG L (Natural Gas) was 117,000 BTU and was a naturally drafting unit that was good for one major application at a time. The Aquastar 125BL unit was a low pressure unit design for non modulating applications for heating systems in conjunction with domestic hot water. The Bosch 125BL had a standing pilot light and took it's combustion air from the room it was installed in. The 125BL had to be vented with a 5" B-vent double wall venting going a minimum of 6' up through the roof vertical to a raincap. The 125BL unit could have been vented out the sidewall also with the AQ1 powervent kit (now discontinued). The Bosch Aquastar 125BL unit is discontinued now and has not been made for over 8 years or so. The replacement model for the older Bosch Aquastar 125BL would be the Bosch 345ESR or 450ESR recirculating units.
The gas tankless recirculating heaters, Bosch 345 ESR and the Bosch 450 ESR are temperature modulated and have electronic ignition. The ESR stands for Electronic ignition, Sealed combustion and Recirculating. These recirculating heaters are designed and approved for heating and recirculating potable water in conjunction with an indirect storage tank. The Bosch 345ESR and the Bosch 450ESR would be able to replace the older Aquastar 125BL unit but there would have to be significant changes such as the venting is stainless steel, combustion air taken from outside using 3" PVC and that the gas line sizing would have to be appropriate.
The 345/450 ESR are not approved or designed for closed loop heating applications. The ESR units are not a tankless heater as they have no flow switch to respond to flow. Instead it’s a recirculating water heater designed for tank loading applications such as the older Aquastar 125BL units were designed for. When an AquaStat (not included) hooked up to a tank tells the ESR that water needs to be brought back up to temperature, the ESR will tell a circulator (not included) to turn on to create flow through the heater to reheat the water in the tank. Please refer to the installation and applications manual. The basic setup for the indirect tank, aquastat, recirculation pump, check valves, etc. should be basically the same for the new ESR heaters.
The Bosch 345 ESR recirculating heater is a 95K BTU unit and can deliver 3.5 GPM at a 45 degree rise. Efficiency is 82%
The Bosch 450 ESR recirculating heater is a 120K BTU unit and can deliver 4.5 GPM at a 45 degree rise. Efficiency is 81%
The temperature control range for both heaters can be set anywhere from 115°F (45°C) - 190°F (90°C)
The 345/450 ESR comes with a standard ¾” pressure relief valve.
The 345/450 ESR has a special built-in freeze prevention cycle and is controlled by a sensor. Freeze protection is always assured by the operation of the appliance. If the primary NTC (temperature) sensor detects temperatures 9°F below the selected temperature, the appliance starts on minimum power until the sensor reads selected temperature. In the case of minimum set point temperature then it will start at 41°F. The use of non-potable additives like glycol is not approved.
The Bosch 345 ESR and the Bosch 450ESR must be installed indoors according to the provided installation manual. The 345/450 ESR is NOT approved for outdoor installations.
The Bosch 345/450 ESR recirculating heaters must be vented in accordance with the installation manual and require Category III 3” stainless steel sealed venting for the exhaust. These heaters are direct vent units and must be vented directly to the outside in accordance with the installation manual. These heaters cannot be combination vented with any other appliance. The intake should be taken from the outside and 3”PVC or other material can be used only for the intake air.
The Bosch 345/450 ESR recirculating heaters are gas specific so they cannot be converted from one gas type to another.