High Efficient Bosch Therm Tankless Water Heaters

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An expert review of Bosch Therm water heaters. What does the new Therm series offer, what are the best tankless models and benefits for the homeowners.

Bosch Therm series is the new line of gas-powered tankless water heaters that is more advanced than the discontinued Aquastar series. The Therm series includes models for every budget; from the small and simple to Energy Star and high-efficiency condensing water heaters. The smallest units are designed mainly for smaller apartments, cottages and condos while the largest water heaters for multi-bath homes.

Model review

The following models are designed for residential, potable water heating, on demand and in continuous flow, using natural gas or propane. They are mainly built as non-condensing, while only two models; C1050 and C1210 are condensing.
  • 330 PN

  • 520 PN

  • 520 HN

  • 660 EFO

  • 830 ES

  • 940 ES and 940 ESO

  • C 1050 ES

  • C 1210 ES

Series 330 and 520 are made as the indoor models only, and they come with the simplest design, lowest flow rate and power, and with the economy components, which are the reason for shorter warranty and cheaper price. Also, their energy factor does not meet the Energy Star standards. These models are not made for space heating, and they do not support solar thermal heating systems and recirculation.

Bosch Therm 940

bosch therm

Bosch Therm 940 is the largest and most powerful non-condensing water heater. Thanks to the modulating burner, its power goes from the minimum of 19,900 to the maximum of 199,000 Btu and provides a high water flow of 9.4 GPM.

Modulating water valve and gas burner with the constant gas-air ratio improves the combustion, reduces the emission and increases the comfort and temperature stability.

The Therm 940 ES is the indoor model while the Therm 940 ESO is an outdoor unit that comes with all the necessary elements for the outdoor installation.

Bosch condensing

The Therm condensing line consists of the most energy-efficient units from all Bosch tankless water heaters. The model C 1210 ES was designed with the highest thermal efficiency of 98%, and it complies with the Ultra Low NOx requirements. These ultra-efficient models include an even flame pattern on the gas burner, which reduces the hot spot and extreme thermal stress on the heat exchanger, for longer life and improved efficiency.

Features and benefits of the high efficient Therm models


Every process inside the Bosch Therm water heaters is controlled and monitored by the electronic circuit board, which is protected with the waterproof plastic. The PC board is the brain of the heater, and it allows the real-time diagnostics, programming, spark ignition, modulation and other functions for superior accuracy, safety, comfort, performance and efficiency. The software on the PC board has been updated for the enhanced performance, lower activation rate and the implementation of the cascade system.

emergency plumber

control panel of the Bosch Therm models

The built-in digital control panel at the front of the unit allows you to change the temperature settings, which goes from the 100 F to 140 F, with the increments of 2 F, for better accuracy, higher safety and energy savings.

The LCD display shows the error codes for easy troubleshooting, and useful indicators such as the power bar or flame ON/OFF. The diagnostic mode will give you all kinds of useful info such as the set, inlet and outlet temperature, water flow, fan speed, burner maximum power...

The fan speed is adjustable through the control panel, and it is beneficial if planning to use the unit at different elevations. The unit was tested up to 8000 ft of altitude.

All these commands are also available on the wireless remote control that is not included in the package, but is optional, and it gives you even more convenience.


Bosch Therm tankless water heaters can work together through the Intelligent Cascade system and provide even more hot water, especially in high demanding homes, and can complement other systems, such as hydronic floor heating, solar heating, recirculating and space heating.


The indoor Therm water heaters are designed as sealed combustion units that are using the twin or concentric vent pipe to move the exhaust gas outside. Every model must have the stainless steel, category III vent piping to transfer the gases out, while the air side of the vent system can use the PVC.

The outdoor model is usually mounted on the external wall or in the recess box, which is an optional accessory. Keep in mind that model 940 ESO cannot be used as an outdoor model unless the outdoor installation kit was installed.

Safety features

  • Built in freeze prevention protects the water heater down to approximately 5 F, but only for short terms and in the areas where the temperature doesn't go below freezing.

  • Flame failure ionization flame rod sensor.

  • Heat exchanger overheat sensor.

  • Inlet and outlet temperature sensors.

  • Backflow temperature sensor.

Today, the Bosch Therm line is the most innovative tankless series, built strong with quality materials and decent features. This is why the warranty on the heat exchanger and other parts is longer than with other manufacturers, it is 15 years for the HE and 5 for other elements. They are estimated to last at least 20 years.

(Pictures courtesy of Bosch)