Why Is My Tankless Water Heater Not Staying Hot?

A tankless water heater should give steady hot water. If the water gets hot, then cold, then hot again, something is wrong.

This can happen with Navien tankless water heaters, but it can also happen with Rinnai, Rheem, Noritz, Bosch, Takagi, and other brands.

The problem is usually one of these things:

• The unit needs cleaning

• The water flow is too low

• The gas supply is not strong enough

• The venting has a problem

• The unit is too small for the house

• A sensor or part is failing

• The recirculation system is not working right

A tankless water heater is not like a regular tank water heater. It does not store hot water. It heats water only when you turn on a faucet, shower, tub, or appliance.

That means it needs the right amount of water flow, gas, air, and clean heat transfer to work well.

How A Tankless Water Heater Works

When you turn on hot water, cold water enters the tankless unit.

The unit senses water moving through it. Then the burner turns on. The water passes through a heat exchanger, gets hot, and goes out to your fixtures.

For this to work, the unit needs several things at the same time:

• Enough water flow

• Enough gas

• Clean burner operation

• Clear venting

• Good sensors

• A clean heat exchanger

If one of those things is off, the water temperature can swing up and down.

Why Does The Water Go Hot, Then Cold?

This is one of the most common tankless water heater complaints.

It may happen during a shower. The water starts hot, then turns cold for a while, then gets hot again.

Common causes include:

1. The Unit Needs To Be Flushed

Tankless water heaters need maintenance. Hard water leaves minerals inside the heat exchanger. That buildup is called scale.

Scale acts like a layer of crust inside the unit. It makes it harder for heat to move into the water.

When scale builds up, the unit may:

• Heat poorly

• Make noise

• Show error codes

• Shut down during use

• Struggle when more than one fixture is running

This is common in homes with hard water. Many homes around Medina, Corcoran, Maple Plain, Greenfield, Independence, and the west metro have hard water, iron, or other water quality issues.

A tankless flush helps clean the inside of the heat exchanger.

2. The Inlet Filter Is Dirty

Most tankless water heaters have a small screen or filter where cold water enters the unit.

That screen catches dirt, sand, scale, and other debris.

If it gets plugged, the unit may not get enough water flow. When the flow drops too low, the heater may not fire correctly.

This can cause cold water during a shower.

3. The Flow Is Too Low

Tankless water heaters need a minimum amount of water flow to turn on.

If the faucet or shower is barely open, the unit may not sense enough flow.

This can happen with:

• Low-flow shower heads

• Partly clogged fixtures

• Dirty aerators

• Plugged cartridges

• Water softener problems

• Whole-house filter problems

Sometimes the water heater is blamed, but the real issue is a restriction somewhere else in the plumbing system.

Why Does My Tankless Water Heater Shut Off During A Shower?

If the hot water shuts off during a shower, the unit may be protecting itself.

A tankless water heater can shut down when it sees a problem.

Common reasons include:

Gas Supply Problems

Tankless water heaters use a lot of gas when running at full fire.

Many large units, including Navien and other major brands, may need close to 200,000 BTUs per hour.

If the gas pipe is too small, the gas regulator is weak, or other gas appliances are running at the same time, the tankless unit may not get enough fuel.

Signs of a gas supply issue may include:

• Flame loss

• Ignition failure

• Hot water turning cold

• Error codes

• Trouble when multiple fixtures are used

This is not something to guess at. Gas pressure should be tested with the unit running.

Venting Problems

A tankless water heater needs clean air in and exhaust out.

If the vent pipe is blocked, loose, pitched wrong, or full of debris, the unit may shut down.

In Minnesota, snow and ice can also block intake or exhaust pipes.

Venting problems can cause:

• Error codes

• Lockouts

• Poor combustion

• Short cycling

• Cold water during use

Condensate Drain Problems

High-efficiency tankless water heaters make condensate. That water has to drain away from the unit.

If the condensate drain is plugged, frozen, or installed wrong, the unit may stop working.

This is common on condensing units like many Navien models, but other brands can have this issue too.

Is My Tankless Water Heater Too Small?

Sometimes the unit is working fine, but the house is asking too much from it.

A tankless water heater has a limit. It can only heat so much water at one time.

The colder the incoming water is, the harder the unit has to work.

In Minnesota winter, the cold water coming into the house can be very cold. That means the tankless unit may produce less hot water than it does in summer.

A unit that handles two showers in July may struggle more in January.

Signs the unit may be undersized:

• Water gets cooler when a second shower starts

• A large tub takes too long to fill

• Hot water is fine at one fixture but weak with two or more

• The problem is worse in winter

This does not always mean the unit must be replaced. Sometimes the issue is maintenance, gas supply, or settings.

Why Does My Navien Tankless Water Heater Show An Error Code?

Navien units are good at giving error codes when something is wrong.

Other brands do this too, but each brand uses different numbers.

An error code may point to:

• Ignition trouble

• Flame loss

• Poor water flow

• Venting trouble

• Sensor failure

• Pump trouble

• Condensate blockage

• Gas supply issues

The code is a starting point. It is not always the final answer.

For example, a flame failure code might not mean the flame rod is bad. It could be low gas pressure, dirty combustion parts, bad venting, or another issue.

Good diagnosis matters. Replacing parts without testing can get expensive fast.

Does A Tankless Water Heater Need Yearly Service?

In most homes, yes.

A tankless water heater should be checked and cleaned on a regular schedule. For many homes, once per year is a good starting point.

Homes with hard water may need service more often.

A normal tankless service may include:

• Flushing the heat exchanger

• Cleaning inlet screens

• Checking for leaks

• Checking error history

• Looking at venting

• Checking the condensate drain

• Testing operation

• Checking water temperature

For gas units, gas pressure may also need to be checked if there are performance problems.

Can A Water Softener Help A Tankless Water Heater?

Yes, in many homes.

Hard water is hard on tankless water heaters. A softener can reduce scale buildup inside the heat exchanger.

That can help the unit last longer and heat better.

A softener does not fix every problem, though. Iron, manganese, sediment, and poor water quality can still cause issues if the system is not set up right.

For homes on wells, water testing is important. The right setup depends on what is actually in the water.

Should I Repair Or Replace My Tankless Water Heater?

A tankless water heater may be worth repairing if:

• The heat exchanger is not leaking

• Parts are still available

• The unit was installed correctly

• The gas and venting are right

• The repair cost makes sense

Replacement may make more sense if:

• The heat exchanger is leaking

• The unit has had repeated major repairs

• The installation was done wrong

• The unit is badly scaled

• The unit is too small for the home

• The repair cost is close to replacement cost

Navien, Rinnai, Rheem, Noritz, Bosch, and other tankless brands can all be good systems when they are installed and maintained correctly.

The brand matters, but the install matters more.

When Should You Call A Plumber?

Call a plumber if:

• Hot water keeps going cold

• The unit shows error codes

• The unit has not been flushed in years

• Water flow is weak

• The problem is worse when more fixtures are used

• You hear strange noises from the unit

• The unit shuts off during showers

• You smell gas

Tankless water heaters are not hard to understand, but they do need proper testing.

A good service visit should not just clear the code and leave. The goal is to find out why the code happened.

Tankless Water Heater Help In The West Metro

Predictable Plumbing works on tankless water heaters in Medina, Corcoran, Maple Plain, Greenfield, Independence, Plymouth, Delano, Minnetonka, and nearby areas.

We service Navien tankless water heaters, and we also help with other tankless brands. If your tankless water heater is not staying hot, has error codes, or has not been maintained, the right diagnosis can save time and prevent wasted repairs.