Tankless Water Heater Pressure Drop

The Short Answer: Some pressure loss through a tankless heater is normal because water passes through a heat exchanger, a filter, and valves. A noticeable drop that only affects hot water usually comes from a clogged inlet filter, isolation valves not fully open, scale in the heat exchanger, or a heater that limits flow to reach the set temperature. If hot and cold are both weak, the cause is elsewhere in the house.

Start with a hot-versus-cold comparison

Open one faucet to full cold, then full hot, and compare.

Common causes in the heater

Inlet filter. Most tankless heaters have a screen on the cold-water inlet that catches sediment. After plumbing work, a water main repair, or on well water, it can clog quickly. Your manual says whether you may clean it yourself and how; on many models it is reached after closing the cold isolation valve.

Isolation valves. Service valves under the heater are used for flushing. If one was left partly closed, or a service port was left open, flow drops.

Scale. In hard water, mineral buildup narrows the heat exchanger’s passages and reduces flow and output. Descaling is routine maintenance; how often depends on hardness.

Flow limiting. Many gas tankless heaters reduce flow when they cannot reach the set temperature, especially in winter when incoming water is colder. This feels like a pressure drop but is the heater protecting the outlet temperature. If it happens only at peak demand, the heater may be undersized for the household. Stiebel Eltron’s Tempra specifications (opens in a new tab) show how flow and temperature rise trade off on electric models; gas models follow the same principle.

Causes outside the heater

• A pressure-reducing valve that is failing or set low reduces both hot and cold.
• A water softener or whole-house filter that is clogged or partly bypassed affects both.
• Long runs of small pipe to distant bathrooms reduce flow more noticeably than with a tank heater.
• A shower mixing valve or recirculation check valve that lets cold into hot can make hot pressure seem low and temperature unstable.

High static pressure (above about 80 psi, a common code limit) is a separate issue: it does not cause weak flow, but it stresses fixtures and relief valves. A plumber can check it with a gauge at a hose bib. See the thermal expansion (opens in a new tab) background for how pressure rises in closed systems.

What you can check

• Run the hot-versus-cold test above at two or three fixtures.
• Clean aerators on any single weak fixture.
• Look at the isolation valves under the heater; handles should be in line with the pipe.
• Note whether the problem changes with the season or with how many taps are open.

What the plumber will check

A plumber will measure static and flowing pressure, inspect and clean the inlet filter, descale the heater if needed, and check the pressure-reducing valve and any softener or filter. If the heater is limiting flow because demand exceeds its capacity, options include using fewer fixtures at once, a larger unit, or a second heater; the plumber can calculate what your household needs.

For low pressure right after a new heater goes in, see low water pressure after installing a water heater. Brand examples: Rinnai low pressure and Navien low pressure.

Frequently Asked Questions

Q: Does a tankless heater always reduce water pressure? A: It adds some restriction compared with an open tank, but on a correctly sized and piped system most people do not notice it at normal flows.

Q: Will flushing fix low pressure? A: Only if scale is the cause. A clogged inlet filter, a closed valve, or an undersized heater needs a different fix.