Bosch Therm Tankless Water Heaters

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? Models reviewed: 830 ES, 940 ES, and 1210 ES.

The Bosch Therm series is the new line of gas-powered tankless water heaters that are more advanced than the previous models.

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 or point-of-use, while the largest water heaters are for multi-bath homes or whole-house.

Model review

Bosch Therm tankless water heaterBosch Therm

The following models are designed for residential, potable water heating, on-demand, and continuous flow, using natural gas or propane. They are mainly built as non-condensing, while only one model, C1210, is condensing. Considered as the best model, it can maintain efficiency high over its lifetime, which is often over 20 years.

All of the Bosch tankless heaters have a compact design that allows easy and wall mount installation with flexible and multiple venting options.

  • 330 PN
  • 520 PN
  • 520 HN
  • 830 ES
  • 940 ES
  • C 1210 ES

Series 330 and 520 are made as indoor models only, and they come with the simplest design, lowest flow rate and power, and economy components, which are the reasons for the shorter warranty and lower prices.

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 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, making it ideal for larger homes with 2-4 baths or more utilizing the cascading system.

A modulating water valve and gas burner with the constant gas-air ratio improves the combustion, reduces the emission of greenhouses gases, and increases comfort and temperature stability.

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

Bosch Therm 830 ES

This is a smaller version of the above 940 ES model - it is built with the same size and features. It can achieve the same temperature and efficiency, but it comes with reduced power of 175,000 BTU and a lower water flow rate of 8.3 GPM, also a lower price tag, making it an excellent buy for using in homes with 2-3 baths.

Bosch Therm 1210 ES

Bosch Therm 1210 ES is comes from the condensing line, and it is the most energy-efficient and highest performing unit from all Bosch tankless water heaters. A model C 1210 ES is designed with an ultra-high thermal efficiency of 98%, providing great savings, and it complies with the Ultra-Low NOx requirements, making the unit Eco-friendly. 

Due to the high gas input of 225,000 BTU and capacity of 12.1 GPM, the water heater can be used in residential and commercial applications, in applications with the high hot water demand.

With the Intelligent Cascading feature, when even more power/water flow is needed, up to 24 units can be combined into one system.

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, ensuring longer life and improved efficiency.

Due to the higher gas input than the maximum used in the evaluation, the unit is not Energy Star rated.

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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 (PC board), protected with waterproof plastic.

The PC board is the brain of the heater, and it allows 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.

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

With the High-Temperature Kit, the outgoing water temperature can be increased to 185 F for use in commercial applications, but only on Therm 830 and 940 models.

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 helpful info, such as the set, inlet and outlet temperature, water flow, fan speed, and 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.

Up to 24 units can be combined into one system when using the Intelligent Cascade System. The system is not applicable to 520 and 330 models.


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

Condensing models provide more flexibility, utilizing PVC, CPVC, ABS, and stainless steel pipes.

The outdoor model is usually mounted on the external wall or in the recess box, which is an optional accessory. Outdoor models require an outdoor installation kit.

Safety features

  • The 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.

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 the warranty of models from other manufacturers; it is 15 years for the heat exchanger and 5 for other elements. According to the experts, these tankless water heaters can last more than 20 years.

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