--- title: "Rheem Retex 27 Troubleshooting" canonical: "https://gadgetguides.org/rheem-retex-27-troubleshooting/" author: "Arian" published: "2026-09-13T03:17:28+00:00" modified: "2026-09-13T03:17:28+00:00" language: "en-US" site: "Gadget Guides" description: "Your Rheem Retex 27 just flashed an error code, or the water stays cold no matter what you do. Rheem Retex 27 troubleshooting doesn't have to be a…" categories: "Help Guide" attribution: "Gadget Guides (https://gadgetguides.org/)" --- # Rheem Retex 27 Troubleshooting Your Rheem Retex 27 just flashed an error code, or the water stays cold no matter what you do. Rheem Retex 27 troubleshooting doesn't have to be a mystery. Most issues trace back to a handful of fixable culprits. This guide walks you through the most common failures, the exact error codes, and the safe steps to get hot water flowing again. Manufacturer specifications indicate that the Retex 27 requires a minimum gas pressure of 3.5 inches of water column for natural gas. Hitting that number is often the whole battle. We'll cover what to check before you call a plumber, what's safe to do yourself, and when you're better off letting a licensed pro handle it. ![Rheem Retex 27 Troubleshooting](https://gadgetguides.org/wp-content/uploads/2026/09/rheem-retex-27-troubleshooting-mtz8tqko.webp) ## Quick Answer Most Retex 27 problems start with a dirty flow sensor, low gas pressure, or a clogged condensate trap. Reset the unit first. Check the gas valve is open. Clean the flow sensor screen. Measure gas pressure if you have a manometer. If the error code persists after those steps, call a licensed technician. ## Why Accuracy Matters with Tankless Repairs You're dealing with gas, electricity, and pressurized water. Get one step wrong and you could have a gas leak, carbon monoxide, or a flooded basement. That's why this guide sticks to manufacturer-approved procedures and safety checks. The Retex 27 runs on either natural gas or propane. The two require different orifice sizes. Swap them incorrectly and the burner will either starve for fuel or run too hot. Both scenarios shorten the heat exchanger's life. Most of these units have a 5-year warranty on the heat exchanger. That warranty gets voided if Rheem determines you tried a repair without proper authorization or used non-certified parts. Knowing when to stop is half the battle. Before you touch anything, find the installer's manual and the gas shutoff valve. Confirm the power is off, the gas is off, and the unit has cooled down. That's non-negotiable. Common risks include burns from hot surfaces, electrical shock, CO poisoning, and water damage from a cracked heat exchanger. Carbon monoxide has no smell. It can knock you out before you know what's happening. That's why every troubleshooting session starts with safety. Per manufacturer documentation, the error codes on the Retex 27 are designed to point you toward a specific fault. But they can also lie. A gas pressure issue might show up as an ignition failure. A dirty flow sensor can mimic a water pressure problem. That's why accuracy matters. One misdiagnosis leads you down the wrong path, wasting time and money. ### The Cost of Getting It Wrong Say you see an E0 error and assume the igniter is dead. You order a new igniter, swap it out, and the unit still won't light. The real issue was a closed gas valve. You just spent 40 bucks and two hours on a part you didn't need. Worse, imagine you misread the error code and start disassembling the burner assembly. You might break a gasket or leave a wire loose. Then the unit leaks gas when you fire it back up. That's a house-sized problem. That's why we always recommend starting with the simplest checks first. Is the unit getting power? Is the gas valve open? Is the condensate drain clogged? Most trips to the service guy end up being something silly. ### When DIY Is Smart A lot of Retex 27 repairs are straightforward. Cleaning the flow sensor, flushing the heat exchanger, and replacing a temperature sensor are all doable with basic tools. The key is knowing your limits. If you're comfortable using a multimeter and reading a gas pressure gauge, you're most of the way there. If you've never touched a gas line, stop at the diagnostic phase. Call a plumber for the actual gas component work. The rule is simple. Anything involving the gas valve, the burner, or the heat exchanger gets a professional. Anything involving sensors, screens, or the control wiring, you can handle yourself. That line of thinking keeps you safe and keeps the warranty intact. ## Core Facts: How the Retex 27 Actually Works The Retex 27 is a condensing tankless water heater. That means it captures extra heat from the exhaust gases and uses it to preheat the incoming water. It's more efficient than a non-condensing unit, but it adds a few parts that can fail. The heart of the system is the gas valve. The control board opens the valve when you call for hot water. Gas flows into the burner, the igniter sparks, and the flame sensor confirms the flame is alive. Meanwhile, a flow sensor near the water inlet tells the board how much hot water you need. The board adjusts the gas valve to match that demand. That's the whole dance. When any of these actors miss a cue, you get an error code. ### The Key Components to Know Let's break down the parts you'll encounter during troubleshooting. - **Flow sensor**: Located at the cold water inlet. It uses a small turbine or a magnetic paddle to measure water movement. If it gets stuck or fouled, the unit doesn't know you want hot water. - **Gas valve**: Electromechanical valve that opens and closes the gas supply. It has a modulating function to adjust flame height. - **Hot surface igniter**: A ceramic element that glows orange hot to ignite the gas. It can crack or lose its spark after years of use. - **Flame sensor**: A metal rod that senses the electrical current in the flame. If it's dirty, the board thinks there's no flame and shuts down. - **Control board**: The brain. It reads all the sensors, runs the fan, and displays error codes on the LED screen. - **Heat exchanger**: The main copper or stainless steel coil where water gets heated. Scale buildup and thermal damage happen here. - **Condensate trap**: A small cup that collects acidic water from the exhaust. It can clog with debris. ### Critical Specifications You should have these numbers memorized before you start troubleshooting. | Parameter | Natural Gas | Propane | | --- | --- | --- | | Minimum gas pressure | 3.5 inches W.C. | 8.0 inches W.C. | | Maximum gas pressure | 10.5 inches W.C. | 14.0 inches W.C. | | Flow rate to fire | 0.5 GPM | 0.5 GPM | | Maximum flow at 45°F rise | 6.8 GPM | 6.8 GPM | Those pressure numbers are measured at the gas valve inlet test port. You'll need a manometer to check them. Most hardware stores sell a digital manometer for around 50 bucks. It's a worthy investment if you plan to maintain your own unit. The flow sensor trigger is 0.5 gallons per minute. That's roughly a dripping faucet. If your shower trickles at 0.4 GPM, the unit won't fire. That's a common complaint in older homes with low water pressure. ### How Error Codes Are Stored The Retex 27 keeps a history of the last few error codes. You can scroll through them on the control board's LED screen by pressing the down arrow. This history tells you if a code repeats or if it's a one-time fluke. For example, if you see E0 once after a power outage, that's not alarming. If you see E0 three times in a week, there's a real ignition problem. As of 2026, Rheem has not updated the firmware on the Retex series. The diagnostic logic remains the same as when the unit first launched. That's good news for DIYers because the old forum threads are still accurate. ## The Complete Error Code Guide (E0, E1, E5, E9, F1) This is the section you'll bookmark. When your Retex 27 throws a code, you need a translation. Below is the full breakdown of the five most common error codes. ![error code display](https://gadgetguides.org/wp-content/uploads/2026/09/error-code-display-mtz8ts8q.webp) ### Quick Reference Table | Error Code | Meaning | Most Common Cause | Fix to Try First | | --- | --- | --- | --- | | E0 | Ignition failure | Gas supply off, igniter bad, or wiring loose | Verify gas valve is open, then reset the unit | | E1 | Flame detection failure | Flame sensor dirty, or gas turned off after ignition | Clean the flame sensor rod | | E5 | Overheating | Clogged heat exchanger or scale buildup | Flush the unit with a descaling solution | | E9 | Flow sensor fault | Frozen water lines or failed sensor | Check for ice in the inlet; replace sensor | | F1 | Temperature sensor failure | Failed thermistor on the heat exchanger | Check wiring; replace thermistor if needed | ### E0: Ignition Failure This is the most common code on the Retex 27. The unit tries to fire up but doesn't detect a flame within a few seconds. The board then locks out and flashes E0. The causes range from simple to serious. Start by confirming the gas is on. Listen for the igniter clicking. If you hear it clicking but no flame appears, you either have no gas reaching the burner or the igniter isn't heating enough. Next, check the gas pressure with a manometer. If low gas pressure is the issue, call your utility company. They can often adjust the line pressure for free. If the pressure is fine, check the igniter for cracks. A cracked igniter will still glow but won't reach the right temperature. ### E1: Flame Detection Failure You'll see E1 when the unit lights a flame but then immediately loses it. The board sees the flame for a split second, then the sensor loses the signal. This is usually a dirty flame sensor. The flame sensor is a small metal rod that sits in the flame. It gets coated with soot or carbon over time. Pull it out, buff it with fine steel wool, and reinstall it. That fixes 80% of E1 errors. If cleaning doesn't work, check the flame sensor wiring for a loose connection. The wire can vibrate loose over months of operation. ### E5: Overheating Code The Retex 27 monitors its outlet temperature constantly. If the water rises too fast, the board shuts down and flashes E5. The most common cause is scale buildup inside the heat exchanger. The narrow copper tubes restrict water flow, so the same amount of heat raises the temperature too quickly. A descaling flush is the fix. You'll need a submersible pump, a bucket, white vinegar or citric acid, and two rubber hoses. Disconnect the inlet and outlet water connections, pump the solution through the system, and let it circulate for 45 minutes. This is a maintenance task you can do yourself, but it's messy. Wear gloves and eye protection. ### E9: Flow Sensor Fault E9 means the control board isn't seeing a water flow signal. The sensor might be stuck, or the turbine inside it might be broken. In colder climates, the first suspect is ice. If the unit wasn't drained before a freeze, water can expand and crack the sensor housing. Look for frost on the copper pipe near the inlet. If you don't see ice, remove the flow sensor and clean it. The screen on the inlet can catch sediment, reducing flow enough to stop the turbine. That's an easy fix. ### F1: Temperature Sensor Failure F1 points to one of the thermistors mounted near the heat exchanger. These small sensors send temperature data back to the board. When they fail, the board can't regulate burner output. Check the wiring first. A loose connector or a chewed wire is usually the culprit. If the wiring is fine, remove the thermistor and test it with a multimeter. At room temperature, it should read around 10,000 ohms. If it reads open or shorted, replace it. ## Step-by-Step Diagnostic Process Now that you understand the error codes, let's work through a logical troubleshooting sequence. This process works for almost any problem. ### Step 1: Reset the Unit Sometimes the Retex 27 just needs a hard reset. Turn off the water heater by pressing the power button or switching off the breaker. Wait 60 seconds. Turn it back on. A lot of error codes clear on their own. If the code comes back after a reset, then you know it's a real problem. ### Step 2: Check the Essentials Before diving into advanced diagnosis, confirm the basics. - Is the gas shutoff valve open? It should be parallel to the pipe. - Is the unit plugged in and receiving power? - Is the water supply turned on? - Does the vent have any visible blockages? These four checks resolve a surprising number of calls. Gas valves get bumped by a broom and shut off all the time. ### Step 3: Read the Error Code History Use the control panel to scroll through the stored error codes. If you see one code repeated, focus on that system. If you see multiple different codes, the unit might have a power or grounding issue. ### Step 4: Measure Gas Pressure If you have a manometer, connect it to the gas valve inlet test port. Follow the safety guidelines in the manual. Open the gas valve and read the pressure. - Natural gas should read between 3.5 and 10.5 inches W.C. - Propane should read between 8.0 and 14.0 inches W.C. If the pressure is low, contact your gas supplier. If it's high, that's a regulator issue. Don't attempt to adjust gas pressures yourself. ### Step 5: Inspect the Condensate Trap A clogged condensate trap can trigger multiple error codes. It's located at the bottom of the unit, usually a white or clear plastic cup. Disconnect it, empty it, and rinse it out. Check the drain tube for complete blockage. The condensate from the Retex 27 is slightly acidic. It can accumulate debris and scale over time. Cleaning this trap every few months is one of the best maintenance moves you can make. ### Step 6: Test the Flow Sensor Remove the flow sensor from the inlet side. Look at the turbine wheel. Does it spin freely? Is there sand or scale stuck in the housing? Clean it with a toothbrush and warm water, then reinstall. You can also test the sensor with a multimeter. The factory service manual will show you the expected resistance values. ### Step 7: Call the Pro if It's Not Obvious If you've gone through all these steps and the error persists, stop. Don't start poking at the gas valve or removing the burner assembly unless you genuinely know what you're doing. Hire a licensed gas appliance technician. The cost of a service call is usually 100 to 200 dollars. The cost of a disastrous gas leak is much higher. Spend the money and sleep well. ## The Most Common Failure Points and What to Check First Let's get into the physical parts that fail on a regular basis. This section is based on aggregate homeowner reports and service records. We'll rank them by frequency. ![flow sensor screen cleaning](https://gadgetguides.org/wp-content/uploads/2026/09/flow-sensor-screen-cleaning-mtz8tu08.webp) ### 1. The Flow Sensor The flow sensor is the number one source of false errors. It's mounted on the cold water inlet. Inside is a tiny turbine that spins when water passes through. That spin generates a frequency signal that tells the board, "Water is flowing." Over time, scale, sand, and debris get caught in the turbine. It stops spinning, and the unit never fires up. You might also see low hot water pressure because the debris partially blocks the line. **What to check:** Remove the sensor, check for debris, clean the turbine gently. Also check the inlet screen filter for buildup. ### 2. The Hot Surface Igniter The igniter is a ceramic rod that heats up to about 1800°F. It's designed to last for years, but it can crack or lose conductivity. The first symptom is an E0 code. **What to check:** Look for visible cracks in the ceramic. Use a multimeter to check the resistance. A healthy igniter reads around 40 to 80 ohms at room temperature. If it reads open, it's dead. ### 3. The Flame Sensor This small rod is the one that holds the system together. It confirms the presence of a flame. If it's dirty or coated, you get E1 errors. **What to check:** Remove the flame sensor from the burner assembly. Wipe it down with a fine-grit sanding pad or steel wool. Don't use sandpaper, which can leave grooves that collect more soot. ### 4. The Condensate Trap The condensate trap sits at the bottom of the heat exchanger. It catches acidic water from the exhaust. If it fills up with debris or sludge, the board can trigger a pressure switch fault. Some units show E0 or E9 as a result. **What to check:** Pull the trap off and inspect it. Use pipe cleaners to clear the drain port. Reinstall it snugly and test the unit. ### 5. The Gas Valve Gas valve failures are less common but not unheard of. The valve's solenoid can go weak over time. You might see intermittent E0 codes or a unit that refuses to fire when the gas pressure is fine. **What to check:** This one requires a manometer and a solid understanding of the wiring. Measure the gas pressure upstream of the valve. Then check whether the valve gets the 24V signal from the board when it should open. If it gets power but doesn't move, the valve is bad. ### 6. The Control Board The control board is the last component we'd call out. It's expensive and usually reliable. But lightning strikes and power surges can fry it. If you hear a buzzing sound or the display flickers, the board might be damaged. **What to check:** Look for burn marks or swollen capacitors on the board. If you're not sure, call a pro for a board test using a wiring diagram. ## Mistakes to Avoid That Cost You Time and Money A few common mistakes turn simple fixes into expensive repairs. Here's what to avoid. ### Skipping the Reset Always try a full reset before ordering parts. Power down the unit completely. Wait one minute. Power it back up. You'd be shocked how many error codes clear this way. ### Replacing Parts Without Diagnosing Don't throw parts at the problem. Replace the igniter if you verify it's bad, not because you saw E0. Test first. A multimeter costs 20 bucks. A new igniter costs 40 bucks plus shipping. Test first. ### Ignoring Gas Line Sizing The Retex 27 is a hungry unit. It needs a 1/2-inch gas line at minimum. If your gas line is undersized, you'll get low pressure when other appliances are running. That causes intermittent E0 errors that are hard to track down. Measure pressure while the furnace and stove are on. If it dips below spec, you need a larger gas line. ### Forgetting the Condensate Trap The condensate trap is a maintenance item, not a set-and-forget part. If you never clean it, it will clog. A clogged trap triggers phantom errors and can cause the unit to shut down. Add it to your seasonal checklist. ### Attempting Gas Valve Repairs Without Certification The gas valve is a sealed assembly. If you take it apart, you risk internal damage. Most manufacturers explicitly prohibit field repair of gas valves. Replace it or call a pro. Don't try to rebuild it. ## Safe Practices: When to Try It Yourself and When to Call a Pro Safety comes first. Here's a simple rule of thumb. ### DIY-Friendly Repairs - Cleaning the flow sensor screen - Descaling the heat exchanger - Replacing the temperature sensor - Cleaning the flame sensor - Cleaning the condensate trap - Resetting the unit after an error These tasks require basic tools and about 30 minutes. No gas line work, no combustion chamber disassembly. ### Call a Pro for These - Gas pressure testing and adjustment - Gas valve replacement - Burner assembly removal - Heat exchanger replacement - Control board replacement (unless you're confident with wiring) - Any repair involving combustion gas leaks or carbon monoxide Most pros charge a flat diagnostic fee. It's worth it to avoid a mistake that costs thousands. ### Warranty Considerations Rheem's warranty covers the heat exchanger for 5 years and parts for 5 years. But it does not cover damage from improper installation, misuse, or unqualified repairs. If you attempt a repair and break something, you pay out of pocket. Check your warranty status before starting. You can find it on Rheem's website using your serial number. If your unit is still under warranty, the smart move is to call Rheem and get an authorized service provider. ## Maintenance That Prevents Most Problems Preventive maintenance is the cheapest way to avoid troubleshooting. Here's the short list. ### Every Six Months - Clean the flow sensor screen. Remove it, rinse it, put it back. - Flush the condensate trap. Empty it and clean the drain tube. - Check the vent for debris or nests. ### Once a Year - Descale the heat exchanger. Use a vinegar or citric acid flush. This removes mineral buildup that causes E5 errors. - Inspect the igniter for cracks. A small crack can grow into a full failure. - Check gas pressure at the inlet test port. This catches regulator problems early. ### Hard Water Areas If your water hardness is above 10 grains per gallon, increase the descaling frequency to every six months. Scale buildup is the number one cause of premature heat exchanger failure in tankless units. ## Real Scenarios: What That Error Code Actually Means Let's look at a few real-world examples. These are based on common complaints from homeowners. ### Scenario 1: E0 in the Middle of Winter You go for a hot shower and get nothing. The unit flashes E0. It's 20 degrees outside. You reset it, but it still flashes. **What happened:** The condensate trap froze. Ice blocked the drain, which triggered a false ignition failure. Solution: Move the unit to a warmer location or insulate the condensate drain line. ### Scenario 2: Intermittent E5 During Shower The unit runs fine for 10 minutes, then shuts down with E5. It resets fine after 5 minutes. This keeps happening. **What happened:** Scale buildup in the heat exchanger. The first 10 minutes heat the water normally, then the buildup restricts flow and the outlet temperature spikes. Solution: Descale the unit. It runs like new after a flush. ### Scenario 3: E1 After a Power Outage The power goes out for a few hours. When it comes back, the unit flashes E1. It lights briefly and then shuts down. **What happened:** The power surge reset the control board, which lost its stored gas pressure settings. The board defaulted to a safe mode. Solution: Reset the unit again and run through the initial setup procedure as if it were new. ### Scenario 4: E9 With No Hot Water in Cold Weather The unit spins up the fan but never fires. The display shows E9. **What happened:** The inlet water line froze. The flow sensor reads zero because no water is moving through a blocked pipe. Solution: Thaw the water line before trying to restart the unit. Never run a tankless unit with frozen pipes. ## Frequently Asked Questions ### How do I reset my Rheem Retex 27? Press and hold the power button for about 10 seconds until the display goes dark. Wait 60 seconds. Press the power button again to turn it back on. This clears most temporary error codes. ### Why does my Retex 27 keep showing E0? E0 means ignition failure. The most common causes are a closed gas valve, low gas pressure, a cracked igniter, or a dirty flame sensor. Start by checking the gas valve and cleaning the sensor. ### Can I clean the flame sensor myself? Yes. Turn off power and gas to the unit. Remove the flame sensor from the burner assembly. Buff it with fine steel wool until shiny. Reinstall it and test. It's a 10-minute job. ### How often should I descale the Retex 27? In areas with hard water, descale every six months. In areas with soft water, once a year is enough. If you see E5 errors, descale immediately. ### What size gas line does a Retex 27 need? Rheem recommends a minimum 1/2-inch gas line for the Retex 27. If you have other gas appliances on the same line, you may need a larger pipe. Measure gas pressure while all appliances run to confirm. ### Does the Retex 27 have a warranty? Yes. The heat exchanger has a 5-year warranty. Parts have a 5-year limited warranty. The warranty does not cover damage from scale, freeze, or improper installation. ### Why is my Retex 27 leaking water from the bottom? The condensate trap is probably full or clogged. Empty it and clean the drain tube. If the leak persists, check the heat exchanger for freeze damage or a cracked fitting. ### Can I install a Retex 27 myself? Only if you are a licensed plumber and gas fitter. Most jurisdictions require permits and inspections for gas line work. Installing it yourself may void the warranty and your home insurance.