Tankless: No Hot Water at the Kitchen Sink
On this page (5 sections)
The Short Answer: When a tankless heater supplies hot water to the showers but not the kitchen sink, the heater is working. The kitchen faucet is either not drawing enough flow to switch the heater on, or cold water is mixing into the hot line at that faucet. Clean the aerator, open the faucet fully to hot, and test again before calling for a heater repair.
Why one faucet can be cold
A tankless heater starts only when flow through it rises above a minimum, which varies by model and is listed in the manual. Kitchen faucets often have flow-limiting aerators, and many people run them at a trickle or with the handle halfway between hot and cold. That hot-side flow can be below the heater’s starting point, so the heater never fires. A shower valve opened wide draws more hot water and starts it easily.
Narrow it down
| What you see | Most likely cause |
|---|---|
| Cold at a trickle, hot when the faucet is wide open on full hot | Flow below the heater’s minimum |
| Weak flow from both hot and cold at the kitchen | Clogged aerator or sprayer |
| Strong cold, weak hot at the kitchen only | Debris in the hot supply stop or faucet cartridge |
| Lukewarm even when wide open, other taps hot | Faucet cartridge or a nearby device mixing cold into hot |
| Hot for a few seconds, then cold, then hot | Flow near the minimum; heater cycling on and off |
| Kitchen and all other taps cold | Not a kitchen problem; see the heater’s troubleshooting |
| Only cold for the first half-minute, then hot | Water in the pipes cooling; see takes a long time to heat up |
What you can check
- Run it fully hot. Push the handle all the way to hot and open it fully for at least 30 seconds. If it gets hot, the problem is low flow at normal settings.
- Clean the aerator. Unscrew it from the spout, rinse out grit and mineral buildup, and reinstall. Pull-down sprayer heads also have a screen that clogs.
- Check the shut-off valves under the sink. Make sure the hot valve is fully open.
- Watch the heater (or its display) while someone opens the kitchen tap. If the heater does not react, flow is too low. If it fires and the water still stays cool, look for mixing.
- Compare with a nearby tap. If a bathroom faucet near the kitchen gets hot but the kitchen stays lukewarm even fully open on hot, the kitchen faucet’s cartridge is a likely culprit.
Do not raise the heater’s temperature setting to compensate. It does not change the starting flow and raises scald risk at other taps.
What a plumber will check
A plumber will check the faucet cartridge and replace it if it leaks internally, look for crossovers from under-sink recirculation valves or other mixing devices, and measure the flow at the kitchen. If the heater’s inlet filter is partly clogged, all fixtures will have reduced flow, which can push the kitchen below the minimum first. The plumber can also confirm the heater’s minimum flow from the manual for your model and compare it with what the kitchen faucet delivers.
If the kitchen has low pressure on both hot and cold after a new heater installation, see low water pressure after installing a water heater. Brand-specific no-heat pages include Rinnai In Use light not on.
Frequently Asked Questions
Q: Should I take the aerator off permanently? A: No. Clean it or replace it with one that still meets your local requirements and gives enough flow to start the heater. Leaving it off can cause splashing and does not fix a cartridge problem.
Q: Will a bigger tankless heater fix it? A: No. A larger heater does not start at a lower flow, and some have a higher minimum. The fix is at the faucet or in the plumbing.
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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