Bosch Powerstar Tankless Troubleshooting
The Short Answer: A Bosch Powerstar (AE115 / AE125, also WH17 / WH27 cousins) that will not heat is a dead 240-volt leg, flow below activation, a stuck flow transducer, a tripped module thermal cut-out, or reversed inlet/outlet. Neon off with cold water is power or flow. Neon on with lukewarm water is too much GPM or a missing circuit. Lock every double-pole off before the cover. Sister electrics: Seisco, SioGreen.
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Search Intent & The Real Problem
The box on the wall is passing cold water. This is a high-kW electric tankless — not a gas Bosch Therm, not a 40-gallon dual-element tank. AE115 is the ~17 kW two-module unit; AE125 is the larger three-circuit cousin. Each module has its own thermal cut-out. The flow transducer has to spin or the neon never comes on. One breaker off and the unit can look “alive” while it cannot make temperature rise.
What other guides get wrong
They treat it like a 4500 W tank: one 30-amp, two screw-in elements, a red ECO. A Powerstar is tens of kilowatts. They skip the other breakers. They blow the inlet with compressed air and destroy the flow transducer. They tell you to “just turn the knob up” when one 40-amp is sitting in the middle. And they paste gas tankless 11/12 codes onto a neon lamp.
Step-by-Step Troubleshooting
Step 1: Identify the platform and kill every circuit
| Family | kW class (typical) | What you are looking at |
|---|---|---|
| AE115 / WH17 / RP17 | ~17 kW at 240 V | Two heating modules, two double-poles, neon, flow transducer |
| AE125 / WH27 / RP27 | ~27 kW at 240 V | Three circuits / more modules — still 240 V |
| Point-of-use smaller Powerstar | Lower kW | Still prove dead; not a 120 V plug-in |
- What to look for: Model on the rating plate. Every breaker labeled for the heater ON or tripped. Inlet marked blue, not swapped. Mounted with the required clearances — that is already done; do not re-plumb it as a diagnosis.
- Required tools: Flashlight, phone photo of the plate. Panel from a dry location.
A 4500 W / 240 V tank element is ~18.8 A and ~12–13 Ω. Powerstar modules are several times that load. Compare ohms only with all breakers locked off.
Step 2: Neon off vs neon on — that is the split
| Symptom | First checks |
|---|---|
| Cold water, neon off | Breakers, reversed inlet/outlet, flow below activation, transducer not spinning, module cut-out |
| Water too cold, neon on | Flow too high for the kW, one circuit off, voltage sag, temp knob down, inlet water dropped |
| No water moving | Isolation valves, inlet screen, house supply — not the heater |
- What to look for: Activation is low but real: about 0.6 GPM on AE115, about 0.8 GPM on AE125 (confirm the sticker). A 2.5 GPM shower on a 17 kW box in 50°F inlet water will be lukewarm with the neon on. That is physics, not a failed element.
- Required tools: Eyes on the neon. A watch and a gallon jug if you need flow.
A breaker that will not reset is a bolted short. Do not hold it.
Step 3: Water path — fill, screen, transducer, no dry fire
- What to look for: Isolation valves open. Inlet screen packed. Plumbing crossover (close cold to the heater, open a hot faucet — pressurized flow means a mixing valve or cartridge is dumping cold into hot). Flow transducer must spin with water running — flashlight through the housing with power off is the safe look; live observation is licensed.
- Required tools: Faucet, valves. Never an air line. Bosch says compressed air kills the transducer.
Never energize an empty chamber — same physics as turned on without water.
Step 4: Module thermal cut-outs — reset is not the repair
- What to look for: Cover off only at 0 V. Reset buttons on top of the heater module(s). A single press after you know why it opened: no flow, scale, a stuck triac, dry fire. Replacing nothing and walking away is how it opens again. All modules must be reset or you get half-heat with the neon on.
- Required tools: Breakers locked. Finger on a cool module.
Do not jumper a cut-out. Do not keep clicking it like a tank ECO — repeated resets cook the next part.
Step 5: Elements, voltage, and replace vs service
- What to look for: With every breaker locked and wires lifted, module element ohms per that kW — not 12–13 Ω unless the math on the plate says so. Any sheath-to-chassis continuity is a ground. AE115 / AE125 are rated 240 V and sag hard at 208 V (output drops). One leg missing: neon may still glow. A wet board or a leaking module is replacement more often than a $12 part.
- Required tools: Digital meter, all breakers locked. Clamp-amp checks are licensed live work.
Lukewarm with all modules drawing nameplate amps is undersized flow, not a mystery sensor. Restrict the fixture or resize. After a storm, restore power only with water in the unit — beeping / outage cousins.
When DIY Ends: Calling a Licensed Pro
You may confirm every breaker, purge air with power off, photograph the neon state, and clean an inlet screen. Stop at live lugs, module replacement, triac/board work, a repeating cut-out, and any leak into the cabinet. Multiple 240-volt circuits and a wet electric tankless are licensed electrician or plumber work. A licensed tech with the Bosch sheet beats a second used board from the internet.
Frequently Asked Questions
Q: Why is my Bosch Powerstar not producing hot water? A: A breaker off, flow below ~0.6–0.8 GPM, a stuck flow transducer, reversed inlet, or a tripped module cut-out. Neon off is power or flow. Neon on and lukewarm is too much GPM or a missing 240 V leg.
Q: Where is the reset on a Powerstar tankless? A: Thermal cut-outs on top of each heater module, under the cover. Prove 0 V first. Press once after you know why it opened. There is no tank-style ECO on the jacket.
Q: Can I replace a Powerstar heating module myself? A: Only with every breaker locked off, 0 V proven, and the unit drained. If you cannot prove dead, or the cabinet is wet, hire a licensed electrician. Never work it live.