Bosch Water Heater Troubleshooting

The Short Answer: Bosch is four machines under one badge: Therm / Greentherm gas tankless, legacy AquaStar, electric Powerstar, and Tronic mini-tanks. Cold water is a packed inlet screen, no flow, a letter code (EA / C1 / E9), a dead 240 V leg, or a leak. Photograph the display. Do not paste Rinnai 11/12 onto it. If you smell gas, leave.

Search Intent & The Real Problem

The cover says Bosch. You opened a hot faucet and got a cold river, a click, or EA. People treat every Bosch like a 199,000 BTU Rinnai with 3-inch PVC, or like a 40-gallon dual-element tank. Wrong. Therm 830/940 ES and Greentherm 7000/9000 are fan-assist gas tankless with letter codes. AquaStar 125B is a standing-pilot water-valve box. Powerstar AE115/AE125 is a multi-breaker electric. Tronic is a point-of-use can. Split the plate first. Legacy gas: AquaStar. Electric whole-house: Powerstar. Cousin no-heat physics: Rinnai no hot water, Marey not igniting.

What other guides get wrong

They publish one Bosch ritual for 125B, 940 ES, Greentherm 9000, and Powerstar. Those boards do not share a chart. They tell you to bleed the gas line by cycling a faucet — that does not purge a gas pipe, and cracking the union is how garages fill — do not purge it. They skip the inlet screen, so you buy a gas valve for campground grit. They match-light a 120-volt Therm. Electronic units are not match machines — do not manually light.

Step-by-Step Troubleshooting

Step 1: Read the plate — then stop guessing

• What to look for: Model, NG vs LP, indoor vs outdoor. Photo of the A / C / E / F code and the label that defines it. Cold and hot on the stamped ports — reversed lines starve the water valve or the turbine.
• Required tools: Flashlight, phone camera.

Hot then cold mid-shower is flow, mixer, or scale — hot to cold — not a bigger Bosch.

Step 2: Flow, inlet screen, and the GPM floor

• What to look for: Isolation valves fully open. Inlet filter packed (the #1 Bosch “dead” call). Therm/Greentherm activation is often around 0.5 GPM — confirm the sticker. A 2.5 GPM shower on a small Therm 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. Maximum on many Bosch sheets is 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 dead: no hot to kitchen.

Step 3: Therm / Greentherm codes — one trial, then stop

Letter maps moved across Bosch years. The cabinet is law. Classic Therm 830/940 ES family (verify):

• What to look for: Fan spins before spark. Exhaust that smells like fuel: tankless exhaust gas. Whoosh / bang: delayed ignition — shut down — loud bang. Power-vent cousin physics: blower not working. Greentherm: condensate trap full, frozen outdoor drain, or scale that trips E1/E9 after a long shower.
• Required tools: The sticker. One reset after you fix flow or 120 V.

Do not keep clicking EA. That is how you fill a cabinet with gas. Manifold pressure is licensed: NG often ~3.5–7 in. w.c. (some Bosch incoming specs run higher — the plate wins) / LP ~10–11 in. w.c. No spark at all: piezo / igniter as cousins, not a match.

Step 4: Electric Bosch — Powerstar and Tronic

• What to look for: Powerstar: every labeled double-pole ON, neon vs no neon, flow transducer spinning, module thermal cut-out — cover off only at 0 V. Tronic mini-tank: dedicated breaker, tank full, ECO, then ohms that match that wattage — not a house-tank 12–13 Ω unless the plate is 4500 W / 240 V. Tronic C tankless: flow switch, inlet screen, one cut-out reset after you know why it opened. A 4500 W / 240 V house element is ~18.8 A; a 12 kW Tronic is a different amperage on a different breaker.
• Required tools: Breaker lock. Meter only with power verified dead.

Never energize an empty chamber — turned on without water. A breaker that will not reset is a short. Sister electrics: Seisco, Airthereal.

Step 5: Flue, leaks, T&P, and a dead exchanger

• What to look for: Draft-hood spillage or a disconnected B-vent on AquaStar-class. Indoor drip at a vent on a non-condensing Therm is not “condensation is fine” — exhaust drip, vent pipe. Greentherm: wet cabinet from a condensate hose, not automatically a cracked exchanger. Freeze split: isolate water, leave gas off — leaking after freeze. T&P dribble: expansion or >80 psi; never cap; typical 150 psi / 210°F. Wet foam on a Tronic storage can is a tank, not a thermostat.
• Required tools: Paper towel. Gas off. No epoxy on an exchanger.

Parts for 125B-era boxes are thin. A cracked Therm exchanger is replacement, not JB Weld.

When DIY Ends: Calling a Licensed Pro

You may clean an inlet screen with water off, confirm 120 V and a spinning fan, photograph a letter code, and verify every Powerstar breaker. Stop at gas fittings, a jumpered limiter, a silent blower, delayed-ignition bangs, repeating EA/E9, and live 240 V covers. A licensed plumber with a manometer and a combustion sense beats a fifth spark into a dark Bosch. Same-day diagnosis beats a 6 a.m. cold shower you keep resetting.

Frequently Asked Questions

Q: Why is my Bosch water heater not getting hot? A: Packed inlet screen, not enough flow, a letter lockout (EA, C1, E9), a silent fan, or a dead 240 V leg on Powerstar. Read the plate. It is not a Rinnai code 11.

Q: What does EA mean on a Bosch tankless? A: Failed ignition — no proven flame. Confirm the gas cock and LP, one trial, then a licensed tech. Do not keep clicking. EC is flame lost after it started.

Q: Can I light a Bosch with a match? A: Only if that jacket is a 125B-class standing-pilot and the card allows it. Therm, Greentherm, 125FX, and Powerstar are electronic. Match-lighting those is how you fill a closet with gas.