Why the kitchen sink is different
A shower runs at a high flow for minutes at a time. A kitchen faucet is often opened a little for short bursts, such as rinsing a glass. Many tankless units need a minimum flow, commonly expressed in gallons per minute, before the burner lights. Below that, the unit never starts and the water stays cold.
Kitchen faucets also often have flow-restricting aerators, and the sink is often the fixture farthest from the heater. Each of those makes the problem worse.
Checks in a sensible order
- Open the tap fully. If the unit fires with the tap wide open and not when partly open, flow is the issue.
- Check the aerator. Unscrew it and look for sediment. Run the tap without it. If hot arrives, the aerator is limiting flow.
- Look at the other sinks. If the bathroom sink behaves the same way, the heater's minimum flow is the cause. If only the kitchen does, look at that fixture and line.
- Check the faucet cartridge. A mixing cartridge that is failing can let cold in or hold back hot.
- Look for a cross-connection. A faulty mixing valve or a bad check valve can feed cold into the hot line.
Still cold at the sink? Call for a plumber to test it.
Call (650) 860-1670| Cause | Sign | Fix |
|---|---|---|
| Flow below the unit's minimum | Works wide open, not partly open | Open further, change the aerator, or adjust the unit |
| Clogged aerator | Weak flow, cold water | Clean or replace it |
| Long pipe run | Long wait for heat | Consider a recirculation line |
| Failed faucet cartridge | Hot mixes with cold | Replace the cartridge |
| Cross-connection | Cold in hot lines elsewhere | Plumber to diagnose |
Long runs and recirculation
In many houses the kitchen is at the far end of the hot water run. A tankless heater does not remove the delay of hot water traveling through the pipe. A recirculation line, with a pump that keeps warm water moving, can shorten it. It is an added system with its own cost and energy use, so ask whether it fits your layout.
If the unit is working but the delay bothers you, the answer is probably recirculation, not repair. See tankless water heater service for the options, and faucet repair if the cartridge is at fault.
When to call
Call if the unit shows an error code, if no fixture gets hot water, if you smell gas or if you cannot find a simple cause. A plumber can measure flow at the unit and at the faucet and see where it drops.
Understanding minimum flow
Every tankless heater has an activation flow, the lowest rate at which it will fire. It is printed in the manual. Typical values are a fraction of a gallon per minute, but the number varies by model. Below it, the unit assumes no one needs hot water and stays off.
A kitchen faucet with an aerator can deliver a fraction of what a shower does. If you only crack the lever, the flow may sit below the activation point. Opening the tap more should light the unit. If it does, the heater is working and the question is how you use the tap.
Fixes ranked from cheapest to most involved
- Open the tap further, and use short full-flow bursts for rinsing.
- Clean or replace the aerator.
- Replace a worn faucet cartridge.
- Have a plumber check the unit's settings and the flow it measures.
- Add a recirculation line for faster hot water at distant fixtures.
Start at the top. Most households find the answer in the first two items.
Is it the heater or the faucet?
A quick comparison separates the two. If hot water arrives normally at a bathtub, shower or laundry tap but fails only at the kitchen sink, suspect the kitchen faucet, its aerator or its supply line. If all small taps struggle while the shower works, suspect minimum flow at the heater.
A third pattern is hot water that arrives, then fades to lukewarm after a few seconds. That points to the unit shutting off because flow dropped below its threshold, or to a cross-connection feeding cold water into the hot line. Note which pattern you see when you call. It saves a visit.
What a recirculation system adds
A recirculation loop keeps warm water moving through the hot line so it is close to the tap when you open it. Some systems use a dedicated return line. Others use the cold line as a return with a pump and a sensor valve under the farthest sink.
Trade-offs include a pump that uses power, some heat loss in the pipes and a cost that depends on the house. Timers and demand switches limit running time. Ask whether your tankless unit has a built-in recirculation mode, since some models do.
Water-conserving fixtures and tankless units
The City of Mountain View's Building Division cites California Civil Code sections 1101.1 to 1101.8, which require water-conserving plumbing fixtures. For single-family alterations with a permit, noncompliant fixtures must be replaced, and a signed flow rate certificate goes back to the Division before final inspection. Lower-flow fixtures mean lower flow at the heater, which is why a low-flow kitchen faucet can fall below a tankless unit's minimum.
Ask your plumber to check both when you pair a new faucet with a tankless unit. The same logic applies in Los Altos.
Our service area page lists the towns where a plumber can test flow at your heater and faucet.
Water-conserving fixture flow rates in California are set by the state's fixture rules, and the California Plumbing Code governs how the heater and the lines are installed.
Questions about this topic
Why does my tankless heater not make hot water at the kitchen sink?
The tap may flow below the unit's minimum. Open it further or check the aerator.
Can a tankless heater work with a low-flow kitchen faucet?
Often yes, but the faucet must flow enough to trip the unit. Check the specification.
Will a recirculation pump fix the delay?
It shortens the wait for hot water. It adds a pump and some energy use.
Is it dangerous if the heater does not fire?
Not usually. If you smell gas, leave and call from outside.
Do California fixture rules affect my tankless unit?
Water-conserving fixtures lower flow, which can affect when a unit fires. The Building Division can explain the rules that apply to a permitted project.