Quick answer
A tankless water heater with no hot water has usually shut itself off and shows an error code. Read the code, then check that other gas appliances work, that the unit has power, that the hot tap is open wide enough to reach the minimum flow, and that the inlet filter is clean. Scale on the heat exchanger causes overheating and cold bursts in hard water areas. HomeAdvisor puts tankless repairs at $75–$1,300, about $600. Call (888) 217-6781 to reach a plumbing company taking calls in your ZIP.
Follow the short menu, then your 5-digit ZIP. The line goes quiet for a few seconds while it connects; stay on. If it does not connect, call once more: companies taking calls change minute to minute.
Written & reviewed by Marcus ReedIICRC WRT
Reviewed November 4, 2026· Next review May 2027
A tankless water heater that gives no hot water has usually shut itself off for a reason it will show you: an error code on the display. Before anything else, read that code and check gas, power and water flow, because most "no hot water" calls on tankless units turn out to be one of those three.
Check These First
- 1Read the error code. It is on the front panel or the remote controller. Write it down along with the brand; the manual (or the brand's website) lists what it means.
- 2Check other gas appliances. If the stove will not light either, the gas supply is off or low. On propane, check the tank level.
- 3Check power. Tankless units need electricity even on gas. A tripped breaker or GFCI turns the unit off.
- 4Open the hot tap wider. Tankless heaters need a minimum flow to fire. A low-flow faucet or a clogged aerator can stay under it.
- 5Check the inlet filter. A small screen where cold water enters the unit traps sediment and cuts flow.
Common Causes
| What you notice | Likely cause |
|---|---|
| Error code for ignition failure | Gas supply, gas valve, igniter or flame sensor |
| Hot water, then cold bursts | Flow dropping below the minimum, or scale on the heat exchanger |
| Lukewarm at best | Scale buildup, undersized unit for the demand, mixing valve |
| Unit dead, no display | Power, a blown fuse on the board, or a failed board |
| Works for one tap, not two | Demand above the unit's capacity |
Talk to a plumber about this
One call reaches a plumbing company taking calls in your ZIP.
Scale and Descaling
In hard water areas, minerals build up on the heat exchanger and the unit overheats, throws codes and shuts down. Most manufacturers call for flushing with a descaling solution about once a year. See tankless water heater repair.
What a Repair Costs
HomeAdvisor puts tankless repairs at $75–$1,300, about $600 on average; Fixr's national range is $100–$650. A descale or a flame sensor is at the low end; a heat exchanger or board at the high end.
When to Call
Call with the brand and the error code once gas, power and flow are ruled out, or if you smell gas. (888) 217-6781 connects you to a plumbing company taking calls in your ZIP. Follow the short menu, enter your 5-digit ZIP and stay on while it connects; if it does not connect, call once more, because the companies taking calls change minute to minute.
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Frequently Asked Questions
Why is my tankless water heater not producing hot water?
Most often the gas supply, power, a flow below the unit's minimum, or a safety shutdown that shows an error code. Read the code first.
Why does my tankless water heater go hot then cold?
Flow dropping below the minimum at a low-flow tap, or scale on the heat exchanger. Descaling about once a year is the usual fix in hard water.
How much does tankless water heater repair cost?
HomeAdvisor puts tankless repairs at $75–$1,300, about $600 on average; Fixr's national range is $100–$650.
Sources
Methodology: How we source and verify data · Report an error
Disclaimer: HearthDry is an independent educational resource. This article is for informational purposes only and does not constitute professional, legal, or insurance advice. Consult licensed professionals before making decisions about your property or insurance claims.