Tankless: No Hot Water at the Kitchen Sink

The Short Answer: A kitchen sink that stays cold while the shower is honest is almost never a dead tankless. It is a packed aerator, a single-handle cartridge dumping cold into hot, or that faucet sitting under the unit’s minimum GPM. Watch the in-use lamp. Do not descale a fixture problem. Hire a licensed plumber if the heater never sees the call.

Search Intent & The Real Problem

You opened the kitchen hot. Ice. You walked to the hall bath and got a real shower. That split is the diagnosis. A tankless only fires when flow through it clears activation — often ~0.4–0.6 GPM (read your manual). A 1.0 GPM kitchen aerator mixed toward cold is a trickle the board never counts. The distant shower has enough GPM, so it looks like “the heater hates the kitchen.”

Every fixture cold is a different job: Rinnai no hot water, Noritz no flow. Wait, then hot, is pipe lag: takes too long. Hot-cold-hot at the same tap is goes from hot to cold. This page is kitchen only.

What other guides get wrong

They sell a bigger tankless because the kitchen is cold. Capacity does not light a 0.3 GPM aerator. They vinegar-flush a unit whose in-use lamp never came on for that faucet. They skip the aerator, so you warranty a board for a screen full of Teflon and carrot peel. And they tell you to crank it to 140°F so “it stays on,” which scalds the next person who opens the shower.

Step-by-Step Troubleshooting

Step 1: Split kitchen-only from a dead heater

• What to look for: Closest other hot faucet as the control. Controller On. Photo of any code before you cycle power. Rinnai: In Use dark is the same no-call physics.
• Required tools: Phone, a helper at the sink, eyes on the controller.

Step 2: Kitchen aerator and the stops — the free save

• What to look for: 0.5 GPM “eco” kitchen aerator packed with solder balls, PEX shavings, or food grit. Spray head with a clogged puck. Hot stop under the sink not parallel to the supply. A 1/2” supply on a 3/4” house that only shows up at this fixture. Pull the aerator: if the stream jumps and the tankless finally fires, you found it.
• Required tools: Aerator key or pliers with tape, bucket, flashlight.

Do not leave the unscrewed aerator off as a “fix” that scalds and wastes. Clean it or fit a kitchen aerator that still clears your min GPM.

Step 3: Crossover at that faucet vs at the heater

• What to look for: Single-handle kitchen cartridge failed internally so cold dumps into hot. Dishwasher fill tee or a cheap under-sink recirc pump tying the two. Fixture test: close the kitchen cold stop, open kitchen hot — pressurized flow with cold isolated at that sink is a cartridge or local tee, not the tankless. Heater test: close cold to the heater, open a hot faucet — pressurized flow means a mixing valve or recirc check somewhere on the house.
• Required tools: Stops under the sink. Contact thermometer on the kitchen hot riser vs the heater hot outlet.

Lukewarm kitchen with 120°F at the heater outlet is a mixing valve or that cartridge — same complaint as not hot enough pinned to one fixture.

Step 4: Minimum flow, isolation valves, inlet screen

• What to look for: Display GPM sagging under activation while the kitchen is “full open.” Isolation valves at 45° after a flush. Inlet screen packed so the shower still clears min GPM and the kitchen does not. House static under ~40 psi at a hose bib will make a tankless ignore small draws; over ~80 psi is a PRV, not a kitchen setting.
• Required tools: Bucket (gallons in 60 seconds at the kitchen), hose-bib gauge, inlet-filter check with power/gas off.

A trickle through the heater is tankless pressure drop. Push-to-connect that did not seat on the kitchen branch can starve one fixture — SharkBite.

Step 5: Recirc, dishwasher, and when the heater is actually the problem

• What to look for: Comfort / recirc serving baths on a dedicated return while the kitchen is a dead-leg — that is wait time, not ice. Dishwasher on hot that never triggers the unit because the fill is too slow. Code on the display the moment you open the kitchen — that is lockout, not an aerator; photograph it. Wet cabinet: isolate water, kill power, stop diagnosing temperature.
• Required tools: Controller menus. Recirc off for one kitchen test.

Do not install a $90 under-sink crossover pump to “fix” kitchen wait. That is how you buy a real crossover.

When DIY Ends: Calling a Licensed Pro

Stop at a repeating flame-failure or overheat code, gas odor, a silent fan, a T&P stream, and any jumper on a flow sensor. You may clean the kitchen aerator, open stops and isolation valves, rinse a customer-accessible inlet screen, and run the two crossover tests. Shower-valve or kitchen-cartridge rebuilds, descaling without service valves, and gas work are licensed. A kitchen that never lights the in-use lamp is flow. A kitchen that lights it and stays cold is the heater path — hire that.

Frequently Asked Questions

Q: Why is there no hot water at the kitchen sink with a tankless? A: The kitchen draw is under minimum GPM, the aerator or cartridge is packed or crossing cold into hot, or a stop is closed. If every fixture is cold, the heater never fired — that is not a kitchen-only problem.

Q: Can a clogged aerator stop a tankless from heating? A: Yes. Many residential units need about 0.4–0.6 GPM to fire. A restricted kitchen aerator never makes the call, so the shower still works.

Q: Will turning the tankless up to 140°F fix a cold kitchen sink? A: No. It scalds when a real draw finally activates. Fix flow, the cartridge, or the inlet screen. Leave the setpoint at a safe 120°F.