Gas Tankless Water Heaters FAQ

Yes, as with all such products there are certain actions that must be taken at regular intervals to ensure that the water heater remains working in a safe and efficient manner while also minimizing the chance of a component failure. Full details regarding the maintenance requirements of this product can be found in the printed manual that can supplied with the water heater, and so please have it to hand when conducting any maintenance steps. If you no longer have the manual, an electronic version can is availalbe for download on this page in the table titled Bosch Aquastar Models.

Safety Notice - This is a gas-fired product, and only professionals who are qualified to work on live gas equipment should do so.

Annual Maintenance

  • Inspect Water Filter Screen - Located inside the cold water inlet port within the water valve is a water filter screen. The filter prevents solid particles in the water supply from entering the water valve and causing damage, and over time it does mean that the filter itself will begin to plug up. This will eventually reduce the water flow rate through the water heater, potentially resulting in water temperature issues or even prevent the water heater from activating.
  • Once a year remove the filter screen and clean it under fresh running water. If lime scale is present in the filter we recommend soaking it in a little white vinegar until the scale dissolves. If the filter is visibly damaged, then it should be replaced immediately (refer to parts list (PDF 0.2 MB) for part#).
  • Inspect Pilot Flame Assembly - The pilot flame should be blue and fully engulf the thermocouple. If the pilot flame appears to be unstable, or yellow, then the pilot orifice or the pilot tube may be dirty. Refer to the "Maintenance And Service" section of the product manual for directions on cleaning.
  • Inspect Main Burners - When lit, the main burners should be blue and steady. If they are yellow or unstable this may be an indication that the burner orifices are clogged (typically with dust or even spider webs). We recommend that a qualified gas service person be used to clean the burners.
  • Exhaust Vent System - Correct venting of the exhaust gases is essential for the correct and safe operation of the water heater. Shut off the gas and water supplies to the water heater and check the entire vent system for damage or blockages. The entire length of the vent must be clear and damage-free.

Every Two Years

  • Lubricate The Water Valve - The gas valve is operated by a mechanical connection to the water valve. There is a pushrod inside the top of the water valve, and it should be lubricated with silicon grease every two years to ensure that it slides back and forth through the o-rings correctly. If this is not done then over time you will likely see a reduced operation of the water valve, which turn will reduce the output of the gas valve and burners and the output water temperature will not rise as much as it did when the water heater was new.
If the pushrod jams completely due to lack of maintenance you could also experience the following two major symptoms:
  • When you use a hot faucet to call for hot water, the burners in the water heater may not activate at all.
  • The burners could ignite correctly on a call for hot water, but they may stay lit AFTER the call for hot water has stopped. If this happens, shut the gas control slider to the Off position immediately!
As you can see from the above description, correct maintenance of the water valve is absolutely essential to the safe and correct operation of the water heater.

Every Three To Five Years

  • Rebuild the water valve - The duration in which this is needed will vary based on quality and chemistry of the water supplied to the product, but we do recommend that it is carried out every three to five years. It involves purchasing and installing a new pushrod and o-rings for the water valve.
  • Important Note! If the water valve has been installed for longer than five years and has never been serviced, it may be prudent to skip the cleaning/rebuild altogether and instead purchase a new water valve assembly. A universal water valve kit (part# 8707002499) is available that is compatible with all versions and production ages of the 125B water heater.

Additional Maintenance Tip – Limescale

Heat Exchanger - Over time it may be possible that limescale builds up inside the water tubing of the heat exchanger. The likelihood of it happening will of course be dependent on the chemistry of the water that is supplied to the water heater and the temperature at which the product is set to. To combat the build-up of scale we recommend the following procedure be carried out annually;

  • Turn off the gas supply to the water heater, and also slide the gas control knob on the water heater to the Off position.
  • Close the water supply valve to the water heater.
  • Open up a hot water fixture in the house to relieve the pressure.
  • Place a bucket under the plumbing connections to and from the water heater to catch any remaining water, and remove the connections.
  • Once the water has finished draining, you will need to setup a submersible pump in a bucket of white vinegar and use hoses to circulate the vinegar through the water heater. If this process is done annually then you may need to circulate the vinegar for a few minutes, but if it's been a number of years, or if scale is a problem in your location, then it may be prudent to circulate the vinegar for half an hour.
  • If the white vinegar that it returning back into the bucket is very cloudy, then pour it out and repeat the process with fresh white vinegar.
  • When you have finished the white vinegar flushing, reconnect the water connections to and from the water heater, relight the pilot, and put the product back into operation.
  • Open a hot water faucet to ensure that the water heater fires up and runs correctly, and leave the hot water flowing for a few minutes to flush any white vinegar from the water heater.

For LP:

Static = 11" to 14" water column.

Operating = 10.5" minimum.

For NG:

Static = 7" to 14" water column.

Operating = 5.7" minimum.

1.) Is the pilot flame lit and steady? If not, light the pilot.

2.) Is the water flowing at a high enough rate through the water heater?

  • If the temperature control knob is turned all the way clockwise, the 125B requires an absolute minimum flow rate of 0.5gpm to activate the burners.
  • If the temperature knob is rotated all the way counter-clockwise, the minimum flow rate needed to activate the burners rises to 1.1gpm.
  • If the flow rate does not exceed the minimum required to activate the burners, they will not turn on. Look into increasing the flow rate from that fixture.

3.) If the pilot is lit and the burners are not activating despite the water flow rate being higher than the minimum required, you may have a crossover in the plumbing in the home somewhere (typically it could be anywhere where hot and cold water are blended together; such as shower valves, single-handled sink faucets, and even washing machines).

To check for a plumbing crossover, do the following:

  • Turn the cold water supply to the water heater off (do NOT shut the water supply to the house off!).
  • Go around the house and open all hot water faucets. Make certain that all single-handled faucets are set to HOT only.
  • Wait for 5 minutes.
  • Go around and check all of the outlets. Water should not be flowing from any of them (remember, you have shut off the water supply to the water heater, and as that feeds the home's hot water fixtures you should have no water flowing from them now).
  • If you do find water flowing from any hot fixture while the water supply to the water heater is still turned off, then you have a plumbing a crossover somewhere in the home. A plumber should be called in to investigate and resolve it. A plumbing crossover can cause activation, and water temperature stability issues, with any tankless water heater.

4.) If there is no plumbing crossover but the burners will still not activate with sufficient water flow, then you may be looking at a dirty, or jammed, pushrod in the water valve. Please refer to the "Regular Maintenance" FAQ at the top of this page for further details.