Rinnai Tankless Water Heater Goes Cold? (Possible Causes & Fixes)

HK Sloan
by HK Sloan
Rinnai tankless water heaters are effective, durable, and reliable, that is until they go cold. Your Rinnai tankless water heater can go cold for several reasons, such as improper installation or a dirty water filter. Whether it be a clogged heat exchanger or a damaged water inlet, let’s explore what you can do when your Rinnai tankless water heater goes cold.

Nothing can derail your morning routine faster than a cold shower. Okay, well…there are a couple of things. A flat tire, a messy pup, a lack of coffee– any of these things are enough to send the average person into a tailspin. But a shower is sacred. A shower allows you to start fresh each morning. For some parents, a morning shower is the only personal time that they’ll get all day.

I think we can all agree that a cold shower is unpleasant. It’s uncomfortable, it’s inconvenient, and if left unaddressed, it can be dangerous. For homeowners, a shower that suddenly goes cold can indicate a bigger problem. Tankless water heaters are designed to provide an unlimited supply of hot water, yet cold water seems to be a common problem, regardless of the brand.

So why does my Rinnai Tankless Water Heater go cold? Your Rinnai Tankless Water Heater could be malfunctioning for several reasons, and most involve improper installation or maintenance. But don’t be discouraged! If you are dealing with a “cold water sandwich” or a dirty water filter, you can quickly fix the problem yourself. But if you are dealing with a faulty heating element or undersized gas line, you may need to call a plumber. For those other unknowns, check your system’s error codes.

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How a Rinnai Tankless Water Heater Works

To understand why your Rinnai Tankless Water Heater goes cold, it helps to have a basic understanding of how tankless water heaters work. You’re probably familiar with tank water heaters. Traditional water heaters heat water and save it for later use. The average residential tank water heater holds between 30-50 gallons of water. That’s why they’re so big!

As you might imagine, tank water heaters waste energy by heating water unnecessarily. These standby energy losses can deal a big blow to your energy bill each month.

Tankless water heaters, on the other hand, conserve energy and save money. Since they heat water instantly without a storage tank, tankless water heaters are much smaller. Think of it as “on-demand” hot water— it’s only there when you need it. When you turn the hot water tap, your tankless system’s flame ignites the heat exchanger. Then, cold water enters an inlet pipe at the bottom of your water heater.

As the water flows through the unit, it preheats by way of any escaping or “secondary” heat. As the water then passes through the primary stainless steel heat exchanger, your Rinnai system heats the water to the desired temperature. Finally, the hot water exits the tankless system to be distributed through the pipes.

Advantages of a Tankless Water Heater

Disadvantages of a Tankless Water Heater

  • Expensive to install 
  • Water goes cold suddenly

Rinnai Tankless Water Heater Running Cold?

Common Causes of Rinnai Tankless Going Cold

Although tankless water heaters were created to make life easier, tankless water heaters are not without their fair share of issues. One of the biggest gripes concerning tankless systems is that the water goes cold. Since tankless water heaters are pretty sophisticated computer systems, it may be intimidating to troubleshoot. However, once you identify the cause for the temperature fluctuation, you can apply the appropriate solution.

Improper Gas Line Size

Rinnai Tankless Water Heaters are only compatible with natural gas or liquid propane for maximum operating efficiency. You will have to extend the gas line to accommodate your Rinnai tankless water heater in most cases. Gas lines are typically ½” to ¾” and will work with a Rinnai system, but 1” is more fitting for a tankless water heater. After all, think about how much energy it’s using!

Your Rinnai Tankless Water Heater takes room temperature water to 120 degrees Fahrenheit within seconds. An undersized gas line can cause incomplete combustion resulting in equipment failure.

Dirty Water Filter on the Cold Water Inlet

One of the most important ways to maintain your Rinnai Tankless Water Heater is also one of the easiest. If you’re not getting enough hot water, check the cold water inlet. Located at the bottom of your water heater, the cold water inlet is a small plastic attachment that allows cold water to enter your tankless system.

The cold water inlet has a filter to stop any debris from entering your water heater. Over time, this filter can become clogged with build-up. If the filter is clogged, no water can enter the water heater, resulting in non-heated water.

“Cold Water Sandwich”

A “cold water sandwich” is not a new problem for tankless water heaters. It’s an inherent one. As you may have guessed, a cold water sandwich refers to fluctuations in your system’s water temperature. One minute you have hot water, then you don’t. Then, hot water again. And while this may be frustrating, understanding how your Rinnai Tankless Water Heater works can help you avoid this problem.

Since tankless water heaters are designed to conserve energy, your Rinnai Tankless Water Heater will shut off as soon as you turn off the hot water tap. At that moment, the water in your home’s pipes is already heated. But once you turn off the hot water tap, the water in your Rinnai system will cool. The cooled water then enters your home’s pipes. If you turn on the hot water again, your Rinnai Tankless Water Heater will distribute that hot water to your pipes within seconds.

If the hot water is turned on again before all the water in your home’s pipes has cooled, you may have inadvertently given yourself a cold water sandwich.

For example, Jeff jumps in the shower, pleasantly surprised that it’s already hot because Jan just did the dishes. But a few minutes in, Jeff gets a burst of cold water. If he knows that it’s a cold water sandwich, Jeff can step aside or ride out the cold water, knowing that hot water is following. If he doesn’t know that he’s dealing with a cold water sandwich, Jeff may turn off the shower to wait for hot water, but that will only compound the problem.

Clogged Heat Exchanger

If your water comes from a well, or if you live in a place with hard water, mineral build-up can be a serious problem. Hard water is full of minerals like calcium and magnesium. Over time, hard water can deposit these minerals onto your water heater’s components. The hard limescale can corrode your pipes and damage the heat exchanger.

The heat exchanger is another essential component of your Rinnai Tankless– perhaps the most important! The heat exchanger is a group of thin pipes that heats the water it carries inside. Just as sediment will clog the filter on the cold water inlet, the sediment that makes it through can build up inside your water heater. As hard water passes through your Rinnai Tankless Water Heater, the heat exchanger may become clogged with limescale.

Since the heat exchanger is made of narrow pipes, it can be hazardous if it becomes clogged. The heat exchanger will continue to heat without the necessary passing water to cool it down. Your Rinnai warranty does not cover damage from limescale and mineral build-up, so make sure to flush your Rinnai Tankless Water Heater regularly.

Other General Issues

Other common issues with tankless systems include bad plumbing crossovers, an improper venting system, a damaged flow sensor, or problems with the limit switch. If you suspect that your Rinnai Tankless Water Heater is malfunctioning for another reason, you can check your system for any error codes. Simply press the POWER button for a few seconds, and then press the UP arrow button. Your Rinnai Tankless Water Heater will display the nine most recent error codes.

Basic Error Codes

Error Code 10Exhaust Blocked
Error Code 11Ignition Failure
Error Code 16Over Temperature Warning
Error Code 52Modulating Solenoid Valve Signal Abnormal
Error Code 61Combustion Fan Failure

Importance of Flushing a Tankless Water Heater

Since tankless water heaters are compact, they are especially susceptible to any mineral build-up. It’s important to maintain your Rinnai Tankless Water Heater by flushing it every year.

Safety Considerations

When troubleshooting your Rinnai Tankless Water Heater, always use best safety practices. Disconnect all power before performing any maintenance and shut off all water connected to your Rinnai system. Abide by local permit laws. If you have any doubt, contact a licensed professional.

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Related Questions

What size tankless water heater does a family of four need?

For a family of four, the estimated usage fluctuates from regular to high. Your residential tankless water heater should have a capacity of approximately 50 gallons. If you use a lot of hot water, consider installing a 75-gallon water heater. If you want to be precise, you’ll need some personalized information. First, find your home’s water flow rate in GPM or gallons per minute. You will also need the average groundwater temperature in your area. With this information, you can calculate your desired temperature rise for your tankless system. When you know how much “work” your tankless system will have, you will have a better idea of what size tankless water system you will need. 

Can I still get hot water from my tankless system when the power is out?

No. During a power outage, your Rinnai Tankless Water Heater will not work. You can purchase a Hugo X-1 Battery Backup accessory, which will provide you with enough power for a week of hot water. 

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HK Sloan
HK Sloan

HK Sloan is a freelance writer currently covering DIY Home Improvement, Health, and Lifestyle. Sloan is passionate about improving situations for less, whether it be working on mind, body, or home.

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