Tankless Water Heater: No Water at the Hot Taps

The Short Answer: If cold taps run normally but hot taps give little or nothing, the blockage is almost always in the heater’s water path: a service valve left closed, a clogged cold-water inlet filter, ice in a frozen unit, or heavy scale. A tankless heater does not boost pressure, so if cold water is also weak, the problem is the house supply, not the heater.

Is it the heater, or the whole house?

Start by finding out where flow is missing. It takes a minute and tells you which problem you have.

If hot water flows but stays cold, that is a heating problem, not a flow problem; see no hot water.

Service valves left closed

Most tankless installations have isolation (service) valves on the cold inlet and hot outlet, often with drain ports for flushing. After a flush, a filter cleaning, or a repair, it is common for one of them to be left closed or partly closed. A closed hot-side valve stops everything, even though the heater itself is fine.

Navien’s NPE-2 owner manual (opens in a new tab) lists this first for “no water comes out when the hot water tap is opened”: make sure the shut-off valves on the hot and cold pipes are open. Noritz’s NRC98 owner’s guide (opens in a new tab) asks the same question. Lever handles are usually open when parallel to the pipe. If the valve drain caps were removed for flushing, make sure the small drain valves are closed before you open the main valves.

A clogged inlet filter

Every major brand has a screen on the cold-water inlet. New installations, plumbing repairs, and well water can load it with sand, pipe dope, or debris, and a packed screen can stop flow almost completely. Whether you should clean it yourself depends on the brand and model:

• Navien: The NPE-2 and NPN owner manuals include cleaning the cold-water inlet filter (and, on NPE-A2 models, the recirculation inlet filter) as an owner task. The manual’s sequence is to turn the heater off, close its gas valve and the cold-water inlet valve, open hot faucets to drain the lines, then remove, rinse, and refit the filter before refilling.
• Noritz: The NRC98 owner’s guide has the owner clean the “water drain valve (with water filter)” at the inlet: close the water supply valve, open hot fixtures, remove the drain plugs to release about 0.4 gallon, brush the filter under running water, refit it and the drain plugs, and check for leaks.
• Rinnai: The SENSEI manual (opens in a new tab) lists “clean inlet water supply filter” under its no-code troubleshooting, but its maintenance section says maintenance and repairs are to be performed by a trained and qualified professional. On a Rinnai, treat the filter as a service item unless your model’s owner manual says otherwise.

Only do this if your heater has a working cold-water inlet valve, the manual for your model assigns the job to the owner, and the heater has cooled. If the filter cap leaks afterward, close the inlet valve and call the installer.

A frozen heater or frozen pipes

Tankless heaters protect themselves from freezing with built-in electric heaters, but only while they have power, and Rinnai and Noritz both note that this does not protect piping outside the cabinet. After a power outage in freezing weather, a frozen inlet or outlet is a common cause of no flow, especially on outdoor units.

Rinnai and Noritz give nearly the same instructions: do not operate the heater while it or its piping is frozen; turn it off, close its gas and water supply valves, and wait for it to thaw, opening the water supply valve from time to time to see whether water runs. When flow returns, check the heater and piping for leaks before using it. Never use an open flame on the heater or its piping. If water appears anywhere once it thaws, close the cold inlet valve and have the heater inspected. For Rinnai specifics, see Rinnai tankless water heater frozen.

Scale in the heat exchanger

Hard water leaves scale in the heat exchanger. It usually shows up first as overheating codes, temperature swings, or falling flow, not a sudden stop, but a badly scaled unit can restrict flow severely. Noritz and Rinnai units can show scale or service reminders (Rinnai’s “SS” means it is time to flush the heat exchanger), and Navien lists E760 as a flushing/service alarm. Flushing involves isolation valves, a pump, and a descaling solution; if you have not done it before, have it done by the installer.

Brand-specific causes

Minimum flow figures explain why a heater will not fire or stays cold at a barely open tap. They do not explain a fully open tap with no water at all; that is a blockage.

What the technician will check

If valves are open and the filter is clean (or is not yours to clean), a technician will check static water pressure, the inlet and outlet for ice or debris, the heat exchanger for scale, internal flow-control and bypass valves, any mixing valve at the heater, and recirculation pumps and check valves that can push water the wrong way. On a new installation they will confirm the hot and cold connections are not reversed.

Frequently Asked Questions

Q: Can a tankless water heater cause no water pressure at all? A: Only at the hot taps, and only if something in its water path is closed or blocked. If cold taps are also weak, the cause is upstream of the heater.

Q: Will the hot water come back on its own after a freeze? A: Often it does once the ice melts, but check the heater and nearby pipes for leaks before using it. Rinnai and Noritz both say not to operate the heater while it or its piping is frozen.

Q: Hot water stopped right after the heater was flushed. Why? A: The most likely cause is an isolation valve or a flush-port drain valve left in the wrong position, or an inlet filter cap not fully reseated. Ask whoever flushed it to come back and check before anything else is replaced.