Aquastar Water Heater Review

Review of Bosch tankless

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The Aquastar water heater is the Bosch trade name for the gas tankless water heaters and they are the most popular and reviewed (Amazon website) on demand gas appliances with several models that are Energy Star qualified.


Bosch in US offers several models that are designed for the outdoor and indoor installation, powered by natural gas or propane, used as a solar back-up, and built for smaller and larger homes.

These models include 2700ES, 2400ES, 2400E, 2400EO, 1600P, 1600H, 1600PS, 125BO, 125FX and 1000P and they come with the thermal efficiency from 80% to 87%, which qualifies, some models from Aquastar water heater series, Energy Star compliant.


Features and Benefits

aquastar water heaterThe following features and safety devices are standard on the majority of Bosch water heaters including the most popular Aquastar and 2700ES 2400ES models.

There are also models that are not equipped with such features; they have a slightly different features and less high-tech and sophisticate elements.

  • Every Bosch Aquastar water heater is suitable for potable water heating; bath and kitchen, and cannot be used for space heating.

  • As the burner output is proportionate to hot water flow demand, the power is usually controlled by modulation on the gas valve that further controls the constant gas/air ratio; or manually with a gas control slide button (like Aquastar 125BO or 1000P models).

  • Bosch Aquastar water heater supports variable flow applications, with modulating water valve or adjustable water flow restrictor (like 125BO models), for improved comfort and temperature control. Minimum water pressure is needed so the unit can be activated.

  • Key/Touch Pad interface control.
  • High quality copper heat exchanger provides a longer working life and it comes with 12 year warranty.
  • High efficiency Ceramat Burner.
  • Real-time diagnostics for troubleshooting.
  • Failure codes for easy diagnostics and repair.
  • Reset button.
  • Bosch Aquastar tankless water heaters are water resistant.
  • Built-in freeze protection, electrical power must be provided.
  • Natural gas and Propane, LP.
  • Aquastar tankless hot water heater is not intended for RV's, boats, mobile home or manufactured homes.
  • Not intended as a solar backup (read below for eligible models).
  • Aquastar tankless models 1600PS and 125BS can be used as the preheated water backup and bring the efficiency to your solar water heating system or woodstove. Before the installation, check the local codes to determine if you need a tempering valve.

Accessories review

  • Optional wireless remote control
  • Cascading kit - TLINK
  • Gas conversion kit (i.e. when converting from natural to LP gas)
  • Vertical and horizontal vent kit and stainless steel bird screen

Safety devices

  • Flame failure device (ionization flame rod sensor)
  • Safety thermocouple and piezo lighter for pilot light models
  • Pressure relief valve (included)
  • Temperature limiter - as the overheat prevention
  • Inlet and outlet temperature sensor
  • Fan speed monitoring (where applicable)
  • Backflow temperature sensor
  • Wireless remote for quick and easy temperature control. (TSTAT2)
emergency plumber

Model Types Review

The major differences between Aquastar tankless water heaters, which are also the key factor when planning to buy, are:

  • Hot water capacity
  • Venting installation
  • Ignition type
  • Efficiency

Selecting Aquastar based on the water capacity

How to properly size your tankless water heater?

The easiest method when choosing a water heater is to know how many plumbing applications; showers, dishwasher, washing machine and kitchen sink will use hot water. Based on that or your family size, you can decide about the capacity.

  • Aquastar 2700 series can provide enough hot water for up to three plumbing units simultaneously, delivering over 7 gallons per minute.

  • Bosch Aquastar 2400 models can be used for two major hot water applications at the time, providing around 6 gallons per minute.

  • Aquastar 1600, 125BO and 125FX models are good enough for one bathroom homes, providing around 4.3 gallons per minute, while 1000P is used for cottages and camp houses, delivering 2.6 gallons per minute.

Note: The suggestion is to use inlet and outlet pipes no less than 3/4" or 19.05 mm, so they can supply enough water for the full water heater capacity use.

Selecting Aquastar based on the ignition type

  • Bosch Aquastar water heater 1600H uses a hydro-generated ignition which makes the unit electrically-independent. A small turbine wheel is installed inside the unit and attached to an electromagnetic wire. When you open a hot water tap, the flow of the water spins the turbine wheel, which spins two dissimilar magnets that in turn sends a 1.5 volt DC current through the wire, and provides the spark to ignite the gas burners.
  • Bosch produces Aquastar models with the pilot light and piezo igniter: 1600PS, 1600P, 1000P, 125BS, 125BO and electronic pilot ignition 125FX model.
  • Electronic ignition electrode with the 120V plug-in supply is used for the next Aquastar models: 2700ES, 2400ES, 2400EO, 2400E.

When the burner does not ignite and when the hot water is on, the electrode might be one of the causes for the problem. The electrode might not produce the spark or one of the electrodes, called the flame sensor, doesn't detect the flame ionization.

Venting options

Room sealed combustion models

If your home requires direct venting option, the following Aquastar models are available; 2400ES and 2400E, and they come with a room-sealed combustion chamber and built-in power vent.

These models can be installed in the bathrooms, kitchen, even bedrooms, as they are indoor and room sealed, but only if using a twin pipe solution; one pipe for venting gases out and the other for fresh air intake.

If it is one pipe solution than ventilation has to be provided as the inside air is used for combustion. For these unsealed units there should be a dedicated room with proper venting.

Aquastar water heater 2700ES and 2400ES are equipped with a primary fan, mixer, and secondary fan, for better water heater performance and efficiency.

Another advantage is that they have the option to be installed outdoor, using the right vent kit (AQOK) but for the approved climate, while 2400E can be installed indoor only.

With their built-in power vent capability, you will have an option for easier installation as they are designed for vertical and horizontal vent termination.

Direct Vent / Power vent

Aquastar water heater 125FX is using power vent (exhaust fan) and is designed for horizontal venting only. 125BS, if using power vent kit can use a sidewall vent. With 1600PS, 1600P and 1600H sidewall termination is available as long as the unit uses the appropriate power vent kit.

1600P, 1600H, 1600PS and 125BS are models that can function as direct vent models also with a vertical termination using natural draft, while 1000P has to be used as a direct vent and vertical termination using natural draft.

Outdoor models

For the outdoor installation, there are two models from Aquastar water heater series available: 2400EO and 125BO with the great advantage; eliminating the need for a costly venting kit, freeing up the space in your home and reducing the problem and worry significantly of the potential gas and water leak.

Note: For freeze protection, electrical supply has to be provided and to plug in 120 V outlets.

The anti freeze kit is designed to provide protection for water heater, down to 5 F, but the suggestion is to drain the unit if exposed to long term freezing conditions.

High Efficiency models

Models Aquastar 2700ES and Aquastar 2400ES are the only two Bosch models that are EnergyStar awarded and high efficient gas tankless water heaters.

What makes these Aquastar water heater models high efficient (where EF factor is 0.82 or higher) is their efficient gas burning with high power pre-mix compact burner and the help it is getting from primary and secondary fans.



(Pictures courtesy of Bosch)

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