Seisco Electric Tankless Troubleshooting
On this page (6 sections)
The Short Answer: A Seisco electric tankless that will not heat most often has one of its several breakers off, air trapped in the heating chambers after work on the plumbing, too little flow to activate, or more demand than its kilowatt rating can heat. A flashing red light is a diagnostic code; its meaning depends on your model’s documentation. Breaker checks are owner-level; anything inside the cover is electrician work.
How a Seisco is built
Seisco whole-house units heat water in several separate chambers, each with its own heating element and usually its own double-pole breaker. Sensors measure water temperature, and the control board sets power to the elements. That design explains a common symptom: if one breaker is off, the display can still light and the water still warms, just not enough.
Owner checks
- Every breaker for the heater. Look for two to four double-pole breakers labeled for the water heater. Reset any tripped breaker once. If it trips again, leave it off and call an electrician.
- The status light. If the light is flashing red or the unit is beeping, count the pattern twice and write it down, including any pause between groups of flashes.
- Air in the system. After a plumbing repair, a water outage, or a new installation, air can stay in the chambers. Run a hot tap until the water flows steadily without spitting. Electric tankless elements can be damaged if powered without water around them, so if you shut off the breakers before plumbing work, restore water first and power last.
- Flow. Open a hot tap fully. A barely open faucet may not move enough water to activate heating.
- Inlet screen. If your manual describes cleaning an inlet filter, do it with the breakers off and the water shut off.
About Seisco error codes
Seisco boards report three-digit (and on some models two-part) codes through flashing lights and beeps. We were not able to confirm a current, published Seisco code list from the manufacturer, and code meanings vary between models and board versions. Use the service sticker inside your unit’s documentation or contact the installer or Seisco support with the model and the code you counted. Do not rely on code tables written for other brands.
What is safe to say in general: codes that refer to a high-temperature limit, a leak or moisture sensor, or an element are reasons to leave the power off until a technician inspects the unit.
Capacity: why it may be working but lukewarm
Electric tankless heaters are limited by their kilowatt rating. As a rough guide, each kilowatt can raise about 1 gallon per minute of water by about 6.8°F. An 18 kW unit raising 50°F winter water to 110°F (a 60°F rise) can supply roughly 2 gallons per minute, about one shower. In summer, with warmer inlet water, the same unit does much more. If the water is lukewarm only when several fixtures run, or only in winter, the unit may be working correctly but undersized for that demand.
What the electrician or technician checks
They measure voltage at each circuit, test the elements and temperature sensors with power isolated, inspect the high-limit and any leak sensor, and check wiring connections for heat damage. A loose or overheated connection is a common failure on high-amperage heaters and is a reason for prompt service.
Element and sensor replacements are part repairs. Scorched wiring or a damaged board may lead to replacement, particularly on an older unit if parts are hard to find.
Related: tankless water heater keeps tripping the breaker, electric tankless goes cold after a few minutes, and EcoSmart tankless troubleshooting.
Frequently Asked Questions
Q: My Seisco works in summer but not in winter. Is it failing? A: Probably not. Colder inlet water needs a bigger temperature rise, which reduces how many gallons per minute the same kilowatts can heat. Reducing flow or simultaneous use helps.
Q: Can I turn the breakers off when I am away? A: Yes, if the unit is indoors and not at risk of freezing. When you return, run water through the heater before turning the breakers back on.
Manuals and supporting references
These links distinguish general safety information from model-specific documentation. A manual directory helps you locate your unit’s instructions; it does not establish that every model uses the same parts, codes, or test values.
- Stiebel Eltron: Tempra engineering specifications (opens in a new tab)Flow, temperature-rise and power relationship; product ratings apply only to the named Tempra models.
- Bosch: electric water heater troubleshooting FAQ (opens in a new tab)Flow and heating-capacity behavior of the Bosch models listed there. Other models use their own limits.
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