Bosch Aquastar tankless 1600 water heater fires on even when hot water not used

[ChicagoSuburban]

Bosch Aquastar tankless 1600 water heater fires on even when hot water not used. Then the unit automatically shuts off the gas. I have to re-lit the pilot and then the unit stays on for a minute and then shuts off gas again.

This is what happened. The pilot went out yesterday so I re-lit the pilot. I pressed the gas inlet button to light the pilot, and let it go after 30 seconds and the pilot stayed lit, however, the unit fired on right away as if someone was using the hot water. No one was using the water, so the unit should not fire on. Then after a minute or so I heard a click and the gas automatically shut off. I had to reset the gas slider to off, wait a few minutes and then re-lit the pilot. The whole cycle repeated again and again. The unit fires on when the gas slider knob is at pilot-lighting position or anywhere "right" of that position. If I slide the gas knob slightly to the "left" of the pilot-lighting position, the pilot stays lit, but the unit does not automatically turn the fire on when I turn on a faucet.

Any ideas?

 

[ChicagoSuburban]

The unit stays on, that is the fire keeps on burning even after the hot water faucet is turned off. Then the unit clicks loudly and turns off the gas supply altogether. I have to let it cool down and then re-light the pilot. It has been happening a lot lately. Is the water flow sensor or the gas flow sensor bad?

 

[SHEPLMBR]

Has this unit been properly flushed and maintenanced?

 

[Caduceus]

From the Bosch users manual. "6.9 Burners ignite without water flow • 1. The water valve may be dirty and sticking, prohibiting it from fully closing the gas valve. This part requires periodic maintenance (every 2-5 years depending on water quality and use). Visit www.boschhotwater.com to purchase water valve rebuild kit to service the water valve

 

[ChicagoSuburban]

It sounds like I have to remove and descale the "water valve" in order to fix this problem. I searched for how-to videos on youtube and Bosch web sites and found no such videos. I downloaded the service bulletins from
http://www.bosch-climate.us/bosch-hot-water/aquastar-service-bulletins/
and they have a few photos and illustrations. But, for a weekend warrior like me I need an actual video or a step-by-step guide. As an example, how to remove the copper pipe that attaches to the water valve - that information is missing in the bulletins. Furthermore, how to quickly clean the filter that is on the bottom side of the water valve, without dismantling the whole water valve etc.
Can anyone help please?

 

[ChicagoSuburban]

I found a solution while searching different forums. Here is the solution and according to my research it works for several models and it fixed all of my problems - all at once.

Solution: Lower the incoming gas pressure. There is a sweet spot for gas pressure that these Bosch tankless HWHs like. Play with the red lever on the gas line coming to the unit. Turn it all the way on (straight up, full on). Slowly turn it clockwise so the gas pressure decreases. There will be a point when you'll see the pilot fire is becoming smaller. Stop right there. If you turn the red knob anymore, the pilot light/fire will start to flicker and eventually turn off and you you have to re-light the pilot. That is the sweet spot. Use a marker and put a mark on the gas pipe so you'll remember that spot.

It works like a champ for me. The pilot haven't gone off for 2 months now. It even stopped the water dripping.