My Bosch Aquastar water heater has a reoccuring problem where when the hot water is turned on the pilot igniter clicks for several seconds and when the pilot is finally lit, I get a large fireball at the base of the unit. After that, the hot water starts flowing and everything is fine, however the fireball is a bit unnerving. Is this simply an adjustment issue? A service tech came out a cleaned the unit, the problem stopped and then came back a couple weeks later. Suggestions?

If your getting a spark the generator should be ok I would clean the pilot injector and re try

Tankless?... call service technician

Tank? clean arrestor screen, replace gas valve thermostat
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html

http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html#intellivent

Gene

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Call Bosch
Gas Products Technical Support: 800-642-3111

That model number is only shown at one website, and that model number is not shown at Bosch.
Open following link for Bosch:
http://www.boschwaterheater.com/BoschHotWatercomHome/GasProducts/tabid/331/Default.aspx
Search of Bosch for that model number shows no result.

Some water heaters do not have a pilot light. Instead they use hot surface igniter rod that glows red hot
The igniter rod ignites the gas after solenoid valve opens and gas flows to combustion chamber.
The igniter rod and heat sensor have to be operational, and have to be positioned correctly for ignition to take place.
If igniter rod is not working, or if heat sensor does not detect flame, then gas solenoid valve shuts down and water heater does not heat water.

Before calling service technician, open following links for Bosch troubleshooting resources:
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html
Resources at Bosch
http://www.boschhotwater.com/HelpfulResources/TechnicalSupport/BoschAquaStarModels/tabid/370/Default.aspx
Typical Bosch Aquastar manual:
http://waterheatertimer.org/pdf/Bosch-aquastar-170vp-manual.pdf

All tankless and tank-type gas water heaters require regular cleaning and maintenance.
It is unknown if your unit needs maintenance or repair parts, but catching up on maintenance using information from manual is good place to start.
Gas appliances are increasingly complicated and experienced technician is good resource to have in local area before installing tankless.

I know from you info that you don't want to work on it any more but this will help if you decide to try again.

Troubleshoot
Q. No Spark At Pilot? Q. Spark Appears When Hot Water Is Turned On, But Pilot And Burners Will Not Ignite? Q. Pilot Sparks Continuously With No Water Running? Q. Pilot Lights But Burners Will Not Come On? Q. Main Burners Go Out During Hot Water Use? Q. Hot Water Temperature Fluctuates At Tap? Q. Water Is Too Hot? Q. Water Is Not Hot Enough? Q. Burners Do Not Burn Cleanly; Yellow Flames When Operating? Q. Burners ignite without water flow? Q. Low hot water pressure? Q. Noise when the heater is running?

That particular tankless water heater has hydro-generated ignition. In order for it to function, it requires a certain water pressure and velocity. You can try turning on the hot water in two location in your house to see if that gets it started. If it does, try getting a higher flow shower head, or removing the limiters in your faucets.

i have an aqua star 125 ng an the pilot lights and stays lit but the main burners wont fire up when i turn the water on i have good water pressure so i kind of stumped and i just rebuilt the diaphram in it also

ok make sure u turn everthing thing off then wait 5 min then turn the dail that controls the heat on it to pilot light then turn the gas nob to pilot then ignit it once lit wait a few seconds and turn gas nob to on pisition then turn heat dail to were the arrow is not to a b or c cause the gas is to high it will blow it self out

The manual says to hold a match near the pilot while the ignitor is trying. It says that if the unit lights the pilot orifice may be dirty or clogged. Then the manual says to check the voltage on the igniter. It should be 1.3 volts DC minimum if it is not means the electronic control box is defective.

Let me know the status ...Kevin

Aquastar 1600H NG Natural Gas Tankless Water Heater Bosch AquaStar Model 1600H NG Natural Gas Whole house tankless gas water heater, 117,000 Btu input, natural draft with hydro-generated ignition. Provides endless hot water for one major application at a time . Can replace 40-gallon storage tank water heater. Optional Accessory: Model AQ1- Power vent with built-in... More freeze protection when venting horizontally. Capacity: 1 to 2 Bathrooms Dimensions: 8.75D x 18.25 W x 29.75H Weight: 40 Max GPM @ 45F Rise: 4.2 Energy Factor: NG .80 LP .84 Minimum Input: 28,000 Maximum Input: 117,000 Vent Size: 5 Gas Connection: 3/4 Water Connections: 1/2 Warranty: 12-year warranty on heat exchanger, 2-year on parts Features: Endless hot water Can save up to 50% off utility costs annually Perfect for one major application at a time No standing pilot, hydro generated