Tankless units are hardy and efficient, but they are not magic. They have filters that clog, heat exchangers that scale, sensors that drift, and vent systems that demand respect. I have seen brand-new installations fail on day three because the gas line was undersized, and I have seen 12-year-old units purr along because the homeowner flushed them every spring. If your tankless water heater is acting up, a calm, methodical approach usually gets you back to steady hot water without drama.
This guide walks through how I diagnose the most common problems in the field: no hot water, inconsistent temperature, error codes, ignition failures, and odd noises or smells. I’ll explain what you can safely check, what to leave for a pro, and the patterns that point to root causes. Along the way, I’ll highlight where water heater service, maintenance, and even water heater replacement may be the smarter move. For homeowners around Wylie and the North Texas suburbs, I’ll call out details specific to our hard water and typical gas infrastructure, since those matter for water heater installation Wylie projects and long-term reliability.
How tankless systems think
Understanding a few basics helps you troubleshoot faster. A tankless water heater does not store hot water. It senses flow, confirms safe venting and combustion conditions, ignites the burner or engages electric elements, and modulates output to maintain a set temperature as water passes through a heat exchanger. Three subsystems must agree:
- Water side: adequate flow, clean inlet screens, non-restrictive fixtures, and reasonable temperature rise. Fuel and combustion side: correct gas pressure and volume or electrical supply, good ignition, clean burners, clear venting for air intake and exhaust. Control side: sensors (flow, temperature, flame, pressure), safety limits, and a control board that can interpret and respond.
When symptoms appear, think in these lanes. For example, lukewarm water under high demand often points to capacity and temperature rise limits on the water side, while rapid cycling or ignition failure leans toward gas supply or venting issues.
Safety first, always
Even seasoned techs slow down around combustion appliances. Turn off power before opening covers. If you smell gas, stop and call your gas utility or a licensed pro. Don’t defeat safety switches or jump wires to “force” operation. On electric tankless units, treat every conductor with respect and confirm the circuit is dead with a meter, not just a breaker position.
No hot water at all
You turn the tap and nothing warms up. Start simple and avoid tearing into the unit prematurely.
Check power and basic settings. Many service calls come down to a tripped GFCI, a toggled service switch, or the temperature set too low. A sudden “cold shock” after a power outage can also result from factory resets, so confirm the controller shows your usual setpoint, typically 120 to 125 F for homes.
Confirm water flow. Tankless heaters need a minimum flow rate to fire. If a single faucet barely dribbles, the flow sensor may never engage. Debris in aerators and showerheads is common after plumbing work, and in Wylie the hard water can leave grit when scale breaks loose. Open a second hot fixture fully to test. If that triggers heat, clean the aerator screens and shower cartridges.
Inspect inlet screens and the cold-water isolation valve. Most tankless models have a small inlet filter. Shut off the cold inlet, relieve pressure, and gently remove and rinse the screen. If the filter is packed with sediment, you found the culprit. I see this after main line repairs or new construction.
Move to fuel supply. For gas units, check that the gas valve is fully open and the unit’s gas service line is properly sized. A common mistake during water heater installation Wylie wide is running a long 1/2 inch gas line to a high-BTU condensing tankless unit. It may pass a static pressure test but starve under load. When someone turns on the furnace, the water heater drops out. If you suspect undersizing, a pro can clock the meter or test pressure under flow. For electric units, verify all required breakers are on. Many whole-house units use multiple dedicated 40 to 60 amp breakers, and losing one leg will disable heating.
Listen for ignition attempts. If the fan spins, you hear clicks, then a fault, the unit is trying to light. That narrows it to ignition or flame detection. If there is silence, the flow sensor or control board may not be calling for heat.
Consider the vent and air intake. Birds, leaves, wasps, and even a plastic bag stuck to a concentric cap can block airflow. Condensing units with long horizontal runs are especially sensitive. A blocked intake prevents the pressure switch from closing, so the unit never proceeds to ignition.
Intermittent hot water or temperature swings
Few things irritate a household more than a shower that cycles from hot to cold to hot. This behavior can come from the fixture, the plumbing system, or the water heater’s modulation.
Start at the fixture. Modern pressure-balanced and thermostatic shower valves are great, until they aren’t. A failing cartridge can cause “hunting” as it tries to control temperature. If only one shower misbehaves while sinks run hot and stable, you have a valve issue, not a tankless problem.
Check for low-flow fixtures. Many tankless models struggle below a threshold, often around 0.4 to 0.6 gallons per minute. If you crack the hot tap just a little, the unit may cycle on and off. Raise the flow to test. If stabilizing requires more flow than you want, enable any available “minimum flow” settings in the controller or consider a unit with a better low-flow modulation range during your next water heater replacement.
Look for crossover. A faulty single-handle faucet or a recirculation system with the wrong check valve allows cold to push into the hot line, blending away heat. Turn off the cold isolation valve at the heater. Open a hot tap. If water still flows for more than a second or two, cold is crossing into the hot lines somewhere. Isolate by closing fixture supply stops one at a time until the flow stops, then replace the offending cartridge.
Think about ground water temperature and capacity. In winter, North Texas incoming water can drop into the 50s. If your unit is sized to raise 60 F water by 55 degrees, it may struggle to raise 50 F water to the same setpoint at peak flow. Two showers and a dishwasher at once can overwhelm a single mid-size unit. If you moved from a tank to a tankless and kept the same gas pipe, you might be limited by fuel delivery. This is where professional load calculations and, sometimes, a second unit or a buffer tank solve chronic complaints.
Scale plays a role too. A scaled heat exchanger transfers heat poorly. The control board sees outlet temperature lagging, raises burner output, overshoots, then throttles back. The result feels like pulsing hot and cold. If you have not flushed the heater in the last year, especially with hard water, put descaling near the top of the list.
Meaningful error codes and what they point to
Manufacturers map their own codes, but they cluster around similar faults: ignition failure, flame loss, temperature limit, inlet thermistor out of range, exhaust or intake blockage, condensate drain fault, and flow sensor error. Never clear an error repeatedly without addressing the cause. Two or three repeats is my soft limit before I pause and reassess.
Ignition failure or flame loss after ignition. Often fuel or air related. I start at the vent terminations, then the condensate trap on condensing units. A full trap blocks the pressure switch circuit. If venting checks out, I measure gas pressure during ignition. A drop below spec when valves open points to supply issues or a clogged gas filter at the unit. On older units, igniters crack or develop carbon buildup, and flame sensors get oxidized. Gentle cleaning with a fine abrasive pad on a flame rod can restore signal, though replacement is cheap insurance.
Exhaust temperature limit or overheat. Scale, again, tops the list. A partially blocked heat exchanger forces hot spots. Inlet filters clogged with debris starve flow, raising outlet temperatures fast. Damaged or pinched recirculation check valves can cause water to short-circuit and overheat. I also check for improperly reduced vent sizes and long runs with excessive elbows that restrict flow, common in retrofits.
Inlet or outlet thermistor errors. Sensors fail with age and heat, and wiring connectors corrode in humid garages. A quick resistance check against the service manual’s temperature-to-ohms chart will confirm if a sensor is reading correctly. Replace both sensors as a matched pair if the price is modest. If both read fine, the harness or control board may be at fault.
Flow sensor error. Debris can stick the turbine. A careful cleaning often revives it, but if the plastic is worn or the magnet weak, replace the sensor. On well systems, install a finer prefilter to protect the turbine.
Condensate faults on condensing models. Acidic condensate must drain freely. A sagging hose, missing air gap, or a long horizontal run without slope will back up into the combustion chamber and shut you down. I clear the line, confirm a 1/4 inch per foot fall, and add a neutralizer cartridge if one was never installed. Code requires proper routing to avoid sewer gas and corrosion, and that’s an easy inspection point during water heater maintenance.
Descaling the heat exchanger the right way
People ask how often to flush a tankless heater. The honest answer is, it depends on your water hardness and usage. In Wylie and much of North Texas, unsoftened water can be 12 to 18 grains per gallon. Without a softener or anti-scale system, annual descaling is a good rhythm, and high-use homes may benefit from every six to nine months.
A proper flush uses isolation valves and a small pump to circulate a descaling solution through the heat exchanger. I prefer food-grade citric acid or a commercial descaler approved by the manufacturer. Vinegar works in a pinch, but it is slower and leaves a smell that lingers. I warm the solution for better performance, run 30 to 45 minutes, then rinse thoroughly until the discharge runs clear and pH normalizes. While the system is open, I pull and clean the inlet screen, inspect the flow sensor, and check gaskets for signs of weeping.
Beware shortcuts. Pouring acid into the inlet without flow, or using harsh acids like muriatic, can pit copper and eat seals. If your unit is still under warranty, stick to the manufacturer’s procedure and chemicals. After flushing, many units show smoother modulation and regain a few degrees of available temperature rise at the same flow.
Gas supply and venting, the two quiet saboteurs
Walk into half the misbehaving tankless water heater repair jobs I see, and you can trace the root cause to one of two things: not enough gas, or poor venting. Both are easy to get almost right, which is worse than clearly wrong.
Gas first. A 199,000 BTU condensing unit at full tilt wants roughly 200 cubic feet of gas per hour. Run that through 35 feet of 1/2 inch pipe with four elbows, and you starve it every time the furnace joins the party. I still find installations where the plumber reused the old tank’s gas stub-out and thought it would be fine. It may light, then flake out under demand with intermittent ignition codes. The fix is not a magic regulator tweak. It is a correctly sized gas line from the manifold, or sometimes a separate dedicated run.
Then venting. Condensing models use PVC, CPVC, or polypropylene, and non-condensing models use stainless steel Category III. Mixing materials or sizes because “that’s what the supply house had” invites problems. Too many elbows, insufficient pitch on horizontal runs, and no bird screen lead to seasonal misery. I always check for proper clearances to doors and soffits and a secure termination cap. Negative pressure in tight homes can also affect intake air if the mechanical room relies on passive combustion air. Laundry rooms with dryers often trip this issue.
If you are planning water heater installation Wylie or a retrofit, do not treat vent design as an afterthought. A few extra minutes of planning saves hours of callbacks.
Recirculation loops: convenience with caveats
A recirculation pump gives near-instant hot water at distant fixtures. Many newer tankless models support internal or external pumps and several control strategies, like timers, motion sensors, or demand buttons. When recirculation is set up correctly, it is delightful. When it is not, it creates strange behavior.
A missing or failed check valve lets hot water drift into the cold line, which masks a crossover. A poorly insulated return line bleeds heat constantly and makes the unit cycle to maintain temperature, shortening life. On condensing units, constant short burns with cold returns can lead to heavy condensate production, so the drain system needs to be robust. When troubleshooting, I temporarily disable the pump and cap the return to see how the heater behaves in standard on-demand mode. If problems vanish, fix the recirculation configuration before accusing the heater.
Funny noises, smells, and other signals
A healthy tankless unit is not silent, but you should hear a consistent fan, a soft ignition, then a steady burn. Clicking or loud whooshing suggests delayed ignition. That can come from improper gas-air mix, dirty burners, or incorrect vent length. A high-pitched whine often points to a failing fan motor. Rattling sometimes means a loose condensate line vibrating against the housing.
Odors matter. A sour smell at the vent is often condensate sitting in a trap or drain without a proper neutralizer. A metallic hot smell might be dust burning off after a period of disuse. Any hint of gas, stop. Use a combustible gas detector if you have one, or back out and call a pro.
Water around the base can be harmless or serious. A teaspoon is probably condensate splash. A cup or more demands attention. Look for drips at unions, isolation valves, and the pressure relief valve discharge. On older copper heat exchangers, pinhole leaks can appear where scale trapped heat. These are often terminal for the core section, and replacement makes more sense than repair.
Controls, sensors, and honest electronics
Control boards are robust, but they do fail. Before condemning a board, rule out its inputs. A thermistor reading 30 F high will make a good board think it is overheating. A floating ground or reversed polarity on the supply can cause nuisance trips. I carry a simple line tester and confirm hot-neutral-ground every time. If a home has frequent power flickers, adding a small surge protector or a UPS with proper grounding can extend board life. Keep in mind that some manufacturers will deny warranty claims if surge damage is evident.
On electric tankless units, heating elements burn out more frequently in hard water. A megohmmeter test across the element to ground will confirm insulation health. Replacing elements is straightforward, but be prepared for seized screws or brittle wires on older units. Use new gaskets and torque evenly to avoid seepage.
Maintenance that actually matters
The services that give the best return are simple, cheap, and regular. Here is a compact annual routine I recommend to homeowners, or that we handle during scheduled water heater service:
- Inspect and clean inlet water screen, flow sensor turbine, and aerators at fixtures. Flush and descale the heat exchanger, verify condensate drain function and neutralizer media if applicable. Check gas pressure under operation, verify vent and intake are clear and correctly sloped, and test combustion air availability in enclosed spaces. Verify temperature setpoint and recalibrate mixing valves at fixtures if needed, especially after a remodel or cartridge replacement. Review error history in the controller and address any recurring codes before they become shutdowns.
That’s one of the two allowed lists. It captures the essentials without fluff. If you stick to that checklist, you sidestep most emergencies, and your tankless runs closer to factory efficiency for more years.
When repair crosses into replacement
No system lasts forever. Heat exchangers on quality units often carry 10 to 15 year warranties for residential use, but sensors, pumps, fans, and boards are wear parts. Here is how I weigh water heater repair against water heater replacement:
Age and part availability. If the unit is 12 to 15 years old and needs a control board plus a fan, you are putting expensive parts into a platform that may have other components near end of life. Newer models often add better modulation ranges, improved condensate handling, and smarter recirculation controls. You also get a fresh warranty. Replacement may cost more today but save aggravation over the next decade.
Water quality and history. A unit that ran for years on very hard water without maintenance may be deeply scaled. Flushing can help, but efficiency and peak output rarely return to new. If the heat exchanger shows repeated overheat faults even after descaling, it may be permanently compromised.
Capacity mismatch. Households change. If two teenagers joined the family or you remodeled with multiple body sprays, the original unit may simply be undersized. Rather than chasing symptoms, upsize or add a second unit in parallel. A good water heater installation Wylie contractor will calculate peak simultaneous demand https://www.pipedreamsservices.com/plumbing-services/plumbing-installation-wylie-tx and select a model, or a pair, that meets it with headroom for winter inlet temperatures.
Structural issues. Recurring vent code violations, impossible gas line upgrades in tight spaces, or corroded flue paths in old masonry can all push the decision. Sometimes relocating the unit to an exterior wall with short, direct vent runs fixes three problems at once and lowers future maintenance.
Noise tolerance and placement. Garage installs are forgiving. A unit in a closet next to a nursery or home office is less so. If noise has always been a complaint, look at newer models with quieter fans and refined combustion controls when planning water heater replacement.
Local realities that shape outcomes
Working around Wylie, Plano, and the Lake Lavon area, two realities color every tankless water heater repair or installation. First, water hardness is high. Without a softener or anti-scale system, plan for regular flushing and keep spare inlet screens. Second, gas systems in older homes tend to be pieced together. If your home added a gas range or patio heater over the years, the manifold may be at its limit. Before a tankless upgrade, a thorough load calculation ensures you don’t create a hidden bottleneck. Reputable companies that handle water heater repair Wylie wide will check these basics even on a quick service call, because otherwise you risk treating symptoms instead of causes.
Electrical supply matters for electric tankless models. They demand heavy breakers and fat copper. Retrofitting into a panel that is already near capacity can trigger a whole-house electrical upgrade. Decide that upfront rather than after the drywall is patched.
Finally, code and permitting are not paperwork for paperwork’s sake. Proper vent clearances, seismic strapping where required, condensate neutralization, and combustion air sizing are all about safety and reliability. Skipping steps costs you later.
A realistic path to steady hot water
If your tankless is misbehaving, start with the obvious: power, gas, flow, and simple screens. Move to venting and condensate. Pay attention to the pattern of faults and when they happen. Early morning only, during high wind, or when multiple appliances run together points toward supply and venting constraints. Random shutdowns in long showers often tie back to scale or failing sensors.
When a repair makes sense, do it well. Use OEM gaskets and sensors, clean what you touch, and verify performance under real demand, not just a sink test. When a replacement makes more sense, lean on a proper sizing exercise and an honest discussion of your home’s plumbing and usage. If you are planning a fresh water heater installation Wylie or a major remodel, consider future fixtures and the last 10 feet of distribution. Sometimes a small recirculation loop or a strategically placed buffer tank takes the edge off a marginal scenario.
Most importantly, put maintenance on a calendar. Tankless heaters are thoroughbreds. They love clean water, steady fuel, and a little attention. In return, they deliver compact, efficient, non-stop hot water for years. With a clear eye and a few well-chosen tools, you can keep yours on track, avoid the emergency calls, and reserve the heavy wrenches for the jobs that truly need them.
Pipe Dreams Services
Address: 2375 St Paul Rd, Wylie, TX 75098
Phone: (214) 225-8767