Why does my tankless water heater go cold mid-shower?

A brief one is. Rinnai's FAQ says a one to two second temperature drop is normal without recirculation, and Rinnai's diagnostic guide explains why: when hot water is cycled on and off within a short period, some cold water can enter the hot line during the ignition sequence. Navien calls the pattern a cold water sandwich, the undesired introduction of cold water into the hot supply line when the unit turns on and off frequently.

What is not normal is cold water that lasts more than a couple of seconds, or that happens when nobody touched the faucet. That points to one of the causes below.

Rinnai's diagnostic guide lists the causes of a cold water sandwich. The ones a homeowner can check are near the top.

### Flow that is too low

Rinnai's guide says to open the fixture and read the flow rate, then to shut off the water feed lines to the heater, remove the inlet water filter and check for debris. It says to clean the filter if needed, put it back, and recheck the flow. It says you should have at least 1 gallon per minute at every fixture and that if you have less, to remove the restrictor and clean the strainer in that fixture. It explains that low flow, from a clogged filter or improperly sized plumbing, can drop the flow below the water heater's minimum, which makes the unit cycle on and off and creates a cold water sandwich.

The same guide lists low-flow showerheads, especially in areas with high ground water temperatures, and says to remove the flow restrictor if possible. If not, the fixture may need to be replaced. Rinnai's FAQ says the unit needs a minimum flow to ignite, and lists a lower flow to stay on, so turning the shower way down can shut the burner off.

What can you do without tools?

1. 1Turn the shower valve up. If you turned it down to save water, the flow may have fallen below the minimum.
2. 2Unscrew the showerhead and clean the screen. Mineral debris clogs it.
3. 3Check whether someone opened a cold tap. A toilet flush or a washing machine cycle can change pressure and flow at the shower.
4. 4Clean the inlet filter. Shut off the water valves to the heater first. Navien's manual tells installers to instruct owners that the filter must be cleaned periodically to maintain proper water flow.

If you're planning a low-flow showerhead with a tankless heater, check the heater's minimum flow first; our page on are low-flow showerheads worth it explains why. If you have a tank heater, or the swings come from the valve rather than the heater, see why your shower temperature keeps changing.

### Cold water bleeding into the hot line

Rinnai's list includes a faulty valve diaphragm or check valve in the plumbing that lets cold water bleed into the hot side, and it says to check the entire system, upstream and downstream of the fixture. Its test for it is simple to follow, though you should stop and call a plumber if it shows a problem.

1. 1Turn off the hot water ball valve at the heater. This should shut the hot water off to every fixture.
2. 2Open a hot water fixture. After roughly two minutes, the residual water should bleed down to no flow.
3. 3If water keeps flowing at any rate, Rinnai says there is a faulty fixture, a check valve or a crossover in the plumbing.

Rinnai adds two follow-ups. If the home has a recirculating system, isolate the return loop, since water can bypass the heater through a faulty check valve. And on single handle fixtures, a bad diaphragm in the fixture can cause back feeding, and you can either replace the fixture insert or install a check valve in the hot feed. Our page on why hot water only comes with the recirculation pump running covers the same kind of check valve failure.

### Causes for a technician

Rinnai's list also includes causes that are not do-it-yourself:

• Inlet gas pressure changing. Rinnai says undersized meters, regulators, gas lines or tanks can cause pressure to surge, and that it must be checked while the unit is running.
• Fluctuating inlet water pressure. It can be caused by improperly sized plumbing or fluctuating utility supplies.
• Debris in the fan motor blower wheel.
• A loose or bad electrical connection. Power dropping out momentarily can cycle the heater.
• Demand systems. Rinnai says that if the water is turned back on within a few minutes of a demand system cycling, a cold water sandwich is created.

Do not open the cabinet on a gas heater unless you are qualified. If you smell gas at any time, leave, and call 911 and Southwest Gas at 877-860-6020.

### What should you tell a technician?

A few details make the visit faster. Say whether the cold burst happens at one fixture or all of them, whether it happens at the same point in every shower, and what else was running. Rinnai's guide says to record the flow rate at the fixture using the water heater's temperature control pad, so write down that number if your controller shows it, before and after you clean the filter. Mention any error code on the display, and whether the home has a recirculating system, since Rinnai says to isolate that loop when testing for bleed over.

Not as a first fix. Rinnai's technical bulletin warns that water temperature above 120 degrees F can cause serious injury, and says the mixing valve setting should be done by a licensed contractor per local code. It also says the heater has a dip switch setting that optimizes the flow activation rate when the set point is above 135 degrees F, and that it applies to certain models and applications, such as high inlet water temperature or recirculation. That is a technician's adjustment, and a hotter set point is a scald risk. Our page on what temperature to set a water heater covers the trade-offs.

Often, yes, and manufacturers offer it. Navien's manual says the recirculation feature on its "A" models eliminates any minimum flow requirement and any hot, cold, hot stacking, which it calls the cold water sandwich, and that it delivers hot water to fixtures more quickly. It also says energy consumption increases because the heater operates to maintain the water temperature within the circulation loop, and it points to Intelligent Preheating, a timer or the HotButton kit to limit that. Our page on how to get instant hot water at every faucet compares the options.

Grundfos says application of its COMFORT pumps with tankless heaters is limited, and our page on whether a Grundfos recirculation pump needs a return line explains why.

Call if cleaning the inlet filter and shower screen did not help, if the bleed over test shows flow, if there is an error code, or if the unit sits on a gas line you cannot inspect. A plumber can measure flow and gas pressure, test the plumbing for bleed over and flush the unit. If the heater is scaled up, our page on how often to descale a tankless water heater in Phoenix explains why, and common tankless water heater problems covers the rest.

HQ Plumbing and Air is licensed and insured, takes emergency calls 24/7, and gives free estimates with an upfront price before any work starts. See our water heater services.

If the heater is a Navien, its own manual has a checklist for cold water that we cover on our page on why a Navien tankless gives cold or no hot water.