Bosch AquaStar Tankless Troubleshooting
The Short Answer: AquaStar is Bosch tankless — 125B standing-pilot, 125X/FX hydro-gen or battery spark, 2400E/2700ES 120-volt fan-assist. Cold water is no flow, a packed inlet screen, a dead pilot or igniter, a flue limiter, or gas pressure. Do not paste Rinnai 11/12 onto it. If you smell gas, leave.
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Search Intent & The Real Problem
The cover says AquaStar or Bosch AquaStar. You opened a hot faucet and got a cold river, a click, or a letter code. People treat it like a 199,000 BTU Rinnai with 3-inch PVC. A 125B is a compact, non-condensing box with a water valve that has to lift before gas happens. 2400E/2700ES class is 120-volt with a fan and a display. Cousin no-heat physics: Rinnai no hot water, Marey not igniting, Camplux. Electronic units are not match-light machines — do not manually light.
What other guides get wrong
They tell you to “bleed air from the gas line” by cycling a hot faucet. That does not purge a gas pipe, and cracking the union is how garages fill — do not purge it. They paste Rinnai code 11 onto a 125B that has no display. They skip the inlet screen, so you buy a gas valve for campground grit. They send you after a standing-pilot thermocouple on a 2400E that has none.
Step-by-Step Troubleshooting
Step 1: Read the plate — 125B vs FX vs 2400E class
| What you have | Ignition / vent | This page? |
|---|---|---|
| 125B / 125BS, view-port pilot, temperature knob | Standing pilot, draft hood | Yes — pilot first |
| 125X / 125FX, spark when water flows, often no house power | Hydro-gen or batteries, water valve / microswitch | Yes |
| 2400E / 2700ES / later Bosch, 120 V, display, fan | DSI + blower, letter codes | Yes — code against that sticker |
| Condensing Bosch with PVC vent, modern Greentherm | Different platform | Photograph the plate; do not guess 125B |
| Click, no flame | Spark without gas, or lockout | Stay |
| Hot then cold mid-shower | Flow, mixer, or scale | Hot to cold |
- What to look for: Model, NG vs LP, indoor vs outdoor. Photo of any A / C / E code and the label that defines it. Cold and hot on the stamped ports — reversed lines are a dead water valve.
- Required tools: Flashlight, phone camera.
Step 2: Flow, inlet screen, and the GPM floor
- What to look for: Isolation valves open. Inlet filter packed (the #1 AquaStar “dead” call). 125B activation is often around 0.5 GPM with the knob toward hot; cranked toward cold it needs more flow. A 2.5 GPM shower on a small AquaStar in 40°F inlet water is lukewarm with a healthy burner — math, not a failed exchanger. House static over ~80 psi belongs on a PRV, not on the temperature knob. Minimum supply on many Bosch sheets is well below that; maximum is often 150 psi.
- Required tools: Faucet, water off before the screen. Jug and a watch if you need GPM.
Crossover: close cold to the heater, open a hot faucet. Pressurized flow means a mixing cartridge is dumping cold into hot. Kitchen-only: no hot to kitchen. Lukewarm with fire: Rinnai not hot enough as physics.
Step 3: 125B pilot, or FX spark — one try
- What to look for: 125B: pilot lit and wrapping the thermocouple before you expect main burners. Lights while held, dies on release: thermocouple ~20–30 mV, not a tenth match. No spark on FX: water valve not lifting, dead hydro-gen, dead batteries, packed screen. Spark, no flame: gas cock closed, empty LP, or a solenoid that never opens. No spark at all: piezo / igniter, Honeywell no spark as cousins. Whoosh / bang: delayed ignition — shut down — loud bang.
- Required tools: Jacket lighting card. View of the burner with gas off. Manometer work is licensed: NG often ~3.5–7 in. w.c. (some Bosch 125 sheets want higher incoming — the plate wins) / LP ~10–11 in. w.c.
Do not match-light a 2400E. Odor during a relight: smell gas after lighting — leave.
Step 4: 2400E-class codes — confirm the sticker
Letter maps moved across Bosch years. The cabinet is law.
| Code (classic 2400E / 2700ES family — verify) | Meaning | First move |
|---|---|---|
| A2 | Flue-gas limiter | Termination, exchanger seals — licensed. Cool-down, then one reset |
| A7 / A9 | Hot-water sensor | Wiring, clip on the pipe, then licensed |
| C1 / C6 / C7 | Fan rotation / speed | Flue, 120 V, blower — do not fire a silent fan |
| E1 | Over-temp on the hot pipe | Scale, low flow, then licensed |
| E2 | Cold sensor | Wiring, then licensed |
| E9 | Temperature limiter / ECO open | Overheat or dry fire — do not jumper |
| EA / EC | No flame / flame lost | Gas, spark, one trial — then licensed |
| FA | Gas-valve circuit | Shut gas. Licensed. No wrenches |
- What to look for: Fan spins before spark. Exhaust that smells like fuel: tankless exhaust gas. Power-vent cousin: blower not working, pressure switch open as prove physics.
- Required tools: The sticker. One reset after you fix flow or 120 V.
Step 5: Flue, leaks, T&P, and a dead exchanger
- What to look for: Draft-hood spillage, bird nest, or a disconnected B-vent on a 125. Indoor drip at a vent is not “condensation is fine” on a non-condensing AquaStar — exhaust drip, vent pipe. Wet cabinet, green copper, freeze split: isolate water, leave gas off — leaking after freeze, Takagi leaking as physics. T&P dribble: expansion or >80 psi; never cap; typical 150 psi / 210°F — Rinnai T&P as the relief lecture, Bosch parts as parts.
- Required tools: Paper towel. Gas off. No epoxy on an exchanger.
Parts for 125B-era boxes are thin. A cracked exchanger is replacement, not a rebuild.
When DIY Ends: Calling a Licensed Pro
You may clean an inlet screen with water off, confirm a 125B pilot against the card once, verify 120 V and a spinning fan on a 2400E, and photograph a code. Stop at gas fittings, a jumpered limiter, a silent blower, delayed-ignition bangs, and repeating EA/E9. A licensed plumber with a manometer and a combustion sense beats a fifth spark into a dark AquaStar.
Frequently Asked Questions
Q: Why is my AquaStar tankless not getting hot? A: Packed inlet screen, not enough flow to open the water valve, a dead standing pilot or spark, a 120-volt fan that never proves, or gas pressure. Read the plate. It is not a Rinnai code 11.
Q: Why does my AquaStar spark but never light? A: The igniter is working and gas is not arriving, or the board is in lockout. Do not keep clicking. One trial after you confirm the cock and LP, then a licensed tech.
Q: Can I light a Bosch AquaStar with a match? A: Only if that jacket is a 125B-class standing-pilot and the card allows it. 125FX and 2400E-class units are electronic. Match-lighting those is how you fill a closet with gas.