Atmor Tankless Water Heater Troubleshooting
On this page (7 sections)
The Short Answer: An Atmor electric tankless heater that passes cold water is usually missing power on one of its circuits, not seeing enough flow to switch on, held back by a clogged inlet screen, or plumbed with hot and cold reversed. Warm but never hot, especially in winter, usually means the unit is too small for the flow. Check breakers, flow and settings; leave the inside to an electrician.
Small units and whole-house units behave differently
Atmor sells compact point-of-use models meant for one sink and larger whole-house models that use several heating chambers, each on its own circuit. The rating label shows the kilowatt (kW) rating and how many breakers it needs.
- Point-of-use units heat a small flow well. They will not supply a shower.
- Whole-house units heat in stages. If one circuit loses power, the heater still runs but at reduced output, which feels like “warm but not hot.”
How much a unit can do depends on its power and the temperature rise. Each kilowatt raises about 6.8 gallons per minute by 1°F. An 18 kW unit raising 50°F winter water to 115°F (a 65°F rise) can supply roughly 1.9 gallons per minute, about one shower. The same unit handles more flow in summer when the incoming water is warmer.
Match the symptom to the likely cause
| What you notice | Likely cause | Who handles it |
|---|---|---|
| No heat, nothing on the unit lights | Tripped breaker or no power | Owner checks breakers; electrician if a breaker trips again |
| Lukewarm at every flow on a whole-house unit | One of several breakers off or tripped, or a failed heating stage | Owner checks breakers; electrician |
| No heat at a low-flow tap, fine at the shower | Flow below the activation point | Owner: open the tap more, clean aerators |
| No heat after new plumbing work | Inlet and outlet reversed, or debris on the inlet screen | Installer |
| Hot, then cold, then hot | Flow near the activation point, or flow above what the unit can heat | Adjust flow; see below |
| Warm in summer, lukewarm in winter | Unit at its limit with colder incoming water | Reduce flow, or larger unit |
| Breaker trips when water runs | Element or wiring fault | Electrician |
| Water inside or under the unit | Fitting or chamber leak | Turn off power now; plumber |
What you can check safely
- Every breaker. Find all the breakers that feed the heater; whole-house units often use two to four double-pole breakers. Each must be fully on. Reset a tripped one once. If it trips again, leave it off.
- Settings. Some Atmor models have a temperature dial or display; others have switches that turn heating stages on and off. Make sure the setting or switches are not turned down or off.
- Flow. Open the hot tap fully. Rinse faucet aerators and shower heads. Very low-flow fixtures may not move enough water to switch the heater on.
- The inlet screen. If your manual describes cleaning the screen in the cold inlet as an owner task, do it with all breakers off and the water supply valve closed. Otherwise leave it for the installer.
- Fittings. Look for drips at the connections under the unit.
Do not close down the cold-water inlet valve to get hotter water. Restricting flow through the heater can overheat it.
Hot and cold cycling
Two different things cause it:
- Flow too low. If the tap is barely open, flow may hover around the activation point, so the heater keeps switching on and off. Open the tap more, or use the hot tap alone rather than mixing in cold.
- Flow too high. If the demand exceeds what the unit can heat, the water feels lukewarm, and some units’ overheat protection can cycle as conditions change. Reduce the flow.
More on both in electric tankless goes cold after a few minutes.
What the electrician will check
The electrician confirms each circuit delivers full voltage under load, that wire and breaker sizes match the rating label, and that terminal connections are tight and not heat-damaged. Undersized wiring is a common problem with owner-installed units. With power isolated, they test each heating element and the overheat protection, and check the flow sensor. A unit too small for the household is a sizing issue; no repair changes its output. Related: tankless water heater keeps tripping the breaker.
Repair or replace
Loose connections, a failed element on a model with replaceable elements, or a cleaned inlet screen are repairs. A cracked heating chamber, water-damaged electronics, or a unit that was never large enough usually means replacement.
Frequently Asked Questions
Q: Can my Atmor run two showers at once? A: Only if its kW rating supports the combined flow at your winter temperature rise. Use the calculation above; most units that handle one shower comfortably will run lukewarm with two.
Q: My Atmor stopped heating right after a storm. What happened? A: A power disturbance can trip one of several breakers, leaving some stages without power. Check every breaker that feeds the unit. If all are on and it still does not heat, call an electrician.
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.
- 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.
- 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.
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