Tankless Water Heater Takes a Long Time to Heat Up
On this page (7 sections)
The Short Answer: Most of the wait for hot water is the cold water already sitting in the pipe between the heater and the faucet; it has to run out first. A tankless heater also needs a few seconds to fire and reach temperature. If the nearest faucet heats quickly and distant ones lag, the pipes are the cause. If every faucet is slow or ends up only warm, look at the heater.
Time it
Use a phone timer at two faucets: the one closest to the heater and the farthest one. Open each fully to hot.
| Result | Meaning |
|---|---|
| Close faucet hot in a few seconds, far faucet much longer | Pipe volume; normal for the layout |
| Both slow, both reach full temperature | Heater slow to start, or low flow delaying activation |
| Both slow and only lukewarm | Heater problem: scale, gas supply, sizing, or a crossover |
| Hot, then a burst of cold, then hot again | Normal “cold water sandwich” after a short pause between uses |
| Faucet at a trickle never gets hot | Flow below the heater’s minimum |
How much water is in the pipe?
As a rough guide, a 50-foot run of half-inch copper holds a little over half a gallon. At a typical faucet flow of 1.5 gallons per minute, emptying that takes around 20 to 25 seconds before hot water can arrive, regardless of what heater is installed. Larger pipes and longer runs hold more. Moving from a tank in the middle of the house to a tankless on an outside wall often adds pipe length, which is why the wait can seem longer after a switch.
Heater-side causes
- Low flow at the faucet. The heater does not start until flow passes its minimum. Low-flow aerators and handles set halfway can delay or prevent firing.
- Scale. In hard water, scale in the heat exchanger slows heating and lowers output.
- Gas supply. An undersized gas line can limit firing rate, especially in winter.
- Crossover. A faulty mixing valve or recirculation check valve can blend cold into the hot line so the water never gets fully hot.
The cold water sandwich
When hot water is turned off and on again within a short time, the water that sat in the heater stays hot, but a slug of unheated water passes through while the burner restarts. You feel hot, then a brief cold burst, then hot. It is a normal trait of tankless heaters. Some models offer a small built-in buffer tank or recirculation to reduce it.
What helps
- Recirculation. A dedicated return line with a pump, or a manufacturer-approved recirculation setup, keeps hot water near the fixtures. Running it on demand (with a button or motion sensor) or on a schedule saves energy compared with running all day. A plumber should design it; some under-sink systems use the cold line as the return, which can make the cold tap run warm.
- Insulating hot pipes slows cooling between uses.
- Point-of-use heater at a far fixture, if a long wait is a daily problem there.
Raising the temperature setting does not make hot water arrive sooner.
What the plumber will check
If every faucet is slow or lukewarm, a plumber will check the heater’s inlet filter, flow at activation, gas supply, and scale, and test for crossovers. If the delay is pipe-related, they can advise on recirculation options.
Related guides: Rinnai takes too long, Navien DHW wait, and no hot water at the kitchen sink.
Frequently Asked Questions
Q: How long should I wait for hot water from a tankless? A: It depends almost entirely on the distance and pipe size. Near the heater, a few seconds; at a far bathroom without recirculation, 30 seconds to a minute can be normal.
Q: Is the wait longer than with my old tank? A: Often slightly, because the heater needs a few seconds to fire, and its location may add pipe length. The bigger factor is usually where the heater sits relative to the fixtures.
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.
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