Why Is My Tankless Water Heater Running Out of Hot Water?

Graphic: summary of this guide, made by our team from the cited sources.

Match what you notice to the likely cause

What you notice	Most likely cause	First thing to try
Lukewarm only when a second fixture or the dishwasher runs	Demand above the unit's flow at your temperature rise	Stagger the loads, then check the unit's flow chart
Fine in summer, weak in winter	Colder inlet water needs a bigger temperature rise	Compare winter rise with the manual's flow curve
Weak hot flow that slowly got worse	Clogged cold water inlet filter	Clean the inlet filter per the manual
LC or SS on the display	Scale in the heat exchanger or a service reminder	Book a flush and descale
Cuts out when the furnace or range is also on	Gas line, meter or regulator too small	Have a gas fitter check line sizing
Never quite hot at one tap, even alone	Hot and cold crossing over in a mixing valve	Ask a plumber for a crossover test
Cold burst for a few seconds, then hot again	Cold water sandwich, not a capacity problem	Usually normal; see below

It is a flow limit, not an empty tank

A tankless heater is rated in gallons per minute, not in gallons, and that rating moves with the weather. ENERGY STAR's demand water heater bulletin says output capacity is partly determined by groundwater temperature, so colder climates generally need higher-capacity units (ENERGY STAR).

The math is simple once you see it. The Building America Solution Center gives a Seattle example: groundwater averages about 52°F, so reaching 120°F takes a 68°F rise, and that household needs a model that can supply at least 4.5 gallons per minute at that rise (Building America Solution Center). A DOE Building America guide uses a milder case, 65°F water heated to 105°F for a 2.0 gallon per minute shower, which needs a 40°F rise at 2.0 gpm (DOE measure guideline). The same burner that comfortably handles the second case in September can fall short in February.

Your manual shows exactly where your unit sits. Rinnai's RE series manual lists a maximum flow of 5.3 gpm for the RE140 and 9.8 gpm for the RE199, but those maximums apply at small temperature rises; its water flow curve shows the deliverable flow dropping as the rise increases (Rinnai RE manual). Read the curve at your winter rise, not the headline number on the box. Our tankless water heater size calculator does the temperature-rise arithmetic for you.

Add up what is running at the same time

Often the "running out" moment lines up with two or three fixtures running at once. The Solution Center's sizing tool lists ASHRAE mixed flow rates: showers 0.8 to 2.5 gpm, dishwasher fill 1.0 to 3.0 gpm, bathtub fill 1.0 to 6.0 gpm and clothes washer fill 1.0 to 6.0 gpm (BASC sizing tool). The same table puts a typical whole-house recurring peak at 3.0 to 4.0 gpm and a severe peak at 6.0 to 8.0 gpm. Building Science Education says high-efficiency tankless units often provide 2 to 5 gpm (Building Science Education, gas-fired tankless).

Graphic: made by our team from the Building America Solution Center ASHRAE flow table and DOE Building Science Education.

Those figures are mixed flows, hot plus cold. Only the hot share passes through the heater; the Solution Center's own sample home assumes 1.5 gpm of roughly 120°F water per shower, mixed down with cold. That is why one shower on its own usually sits inside a whole-house unit's range, while a shower plus a dishwasher fill plus a tub filling can.

A quick test tells you whether this is your problem. Run one shower alone for ten minutes. If it stays hot, then start the dishwasher or open a second shower and watch what happens. Hot alone and lukewarm together points at capacity, not a fault. Fixes, from free to costly: run the dishwasher and laundry outside shower time, fit lower-flow showerheads, add a small point-of-use heater near a far-flung fixture (ENERGY STAR's bulletin describes these), or have the unit resized.

Clean the cold water inlet filter

Gas tankless units such as Rinnai and Navien models have a filter screen where the cold water enters, and sand or scale flakes collect there. Navien's NPE-2 user manual lists a dirty cold water inlet filter as the first check when the water "turns cold and stays cold," next to making sure the tap draws at least 0.5 gallons per minute (Navien NPE-2 manual). Rinnai's manual puts the same filter at the top of its list when nothing happens with water flowing, and again under code 63, recirculation low flow.

Navien's procedure is short: turn the unit off, close the water supply valve on its inlet, open the hot taps and drain the heater into a bucket, then remove the filter, rinse it under clean cold running water and scrub it with a brush if needed. Follow the steps for your own model, because filter locations and valves vary. If the screen is packed with white grit, that grit came from somewhere, which leads to the next cause.

Scale in the heat exchanger

Hard water leaves mineral scale on the hot surfaces inside the heat exchanger. The Solution Center says scale reduces capacity and efficiency and in extreme cases can cause complete water heater failure, and that many manufacturers recommend yearly professional cleaning with a mild acid. Rinnai's manual adds two details owners rarely hear: scale build-up is accelerated when the heater is set at a high temperature, and damage caused by lime build-up is not covered by its warranty.

Watch the display. On Rinnai units, LC means scale in the heat exchanger that needs flushing to prevent damage, and SS (Service Soon) is a reminder the installer can set to come up when it is time to flush and service the unit. Rinnai's flushing procedure pumps four gallons of undiluted food-grade white vinegar through the heat exchanger for at least an hour, connected through isolation valves on the hot and cold lines, and Rinnai strongly recommends having those valves installed. If your unit has no service valves, a plumber should fit them first.

A gas supply that cannot keep up

Gas tankless units burn a lot of fuel in short bursts. The Solution Center compares a tankless model that might need 199,900 Btu with a typical tank heater at 40,000 Btu, and says a tankless unit might need 3/4-inch branch lines where many tank heaters get by on 1/2-inch piping. The DOE measure guideline says the in-house gas line serving the unit will need to be 3/4 inch in most cases, and the gas line to the meter may also need upsizing.

Signs of a starved burner: hot water drops off when the furnace, a gas dryer or the range turns on, or the unit throws ignition or flame codes. Rinnai's troubleshooting for code 11 (no ignition) and code 12 (no flame) includes making sure the gas line, meter and regulator are sized properly and, on propane, that there is gas in the tank. Gas piping is a licensed trade job, so this is a call for a plumber or gas fitter with a manometer, not a DIY fix.

Hot and cold crossing over

If one tap never gets fully hot even when nothing else is running, cold water may be leaking into the hot side through a faulty single-handle mixing valve or a misplumbed fixture. Navien lists cross plumbing between the cold and hot lines as a cause of water that is not hot enough. Rinnai describes a crossover test: with the cold supply to the heater shut off, open the pressure relief valve, and if water keeps flowing, there is bleed-over in the plumbing. The same manual warns that testing the relief valve should only be done by a trained professional because the discharge can scald, so leave this test to a plumber.

Recirculation systems are a related trap. Navien's checklist for water that turns cold asks whether an external recirculation system has a check valve on its return line.

Too little flow, or a brief cold burst

Sometimes the problem is not too much demand but too little. Each tankless unit has a minimum flow that starts the burner, 0.4 to 0.9 gpm according to the DOE measure guideline, and a trickle from a faucet may not fire it at all. Rinnai lists 0.4 gpm for most RE models and 0.26 gpm for the RE199 and RE180. If the water goes cold when you turn the shower down, our guide to a shower that goes cold after 5 minutes covers low-flow showerheads and mixing valves in detail.

A short cold slug in the middle of hot water is different again. DOE calls it the cold water sandwich effect: during frequent on and off use, occupants may notice a sudden temperature drop for a few seconds, because safety standards require a delay in the ignition sequence. Most units stay ready to fire for about the first minute after flow stops and relight within 1 to 2 seconds. It is a comfort issue, not a sign the unit is too small.

Check the setpoint last

A low setpoint can feel like running out, especially in winter. Navien ships NPE units at 120°F and lists "is the set temperature too low?" as the first question when the water is not hot enough. Turning the dial far higher is not a free fix, though: a higher setpoint means a bigger temperature rise, which lowers the flow the unit can deliver, and Rinnai notes that high settings speed up scale. If you have a recirculation pump, ENERGY STAR advises against continuous circulating systems because they keep the heater firing, and recommends a push-button demand pump with a temperature-sensing shutoff instead.

When the unit is simply too small

If the inlet filter is clean, there is no scale code, the gas supply checks out and the unit still fades with your normal morning routine, it is undersized for your house at your winter inlet temperature. The options are a larger unit, a second unit, or a point-of-use heater for the farthest bathroom. Before you buy, it is worth knowing how to tell if you have a tankless water heater and what kind, and running your fixture list through the calculator so the replacement is sized on the real peak, not a guess.