When a tankless water heater quits delivering hot water, the house notices. Showers run cold, dishes never quite rinse clean, and laundry cycles stall. Unlike a tank-style heater that gives you a buffer of stored heat, a tankless unit is all about flow and control. If anything in that chain falters, you feel it right away.
I’ve worked on enough of these systems to know that the fix can be simple, like a clogged inlet screen, or thorny, like a failing heat exchanger or a miswired recirculation pump. Below is a practical guide that focuses on diagnosis first, then sensible repair steps, along with the judgment calls that separate a quick tune-up from a necessary replacement.
Start with how tankless actually makes hot water
A tankless water heater fires only when water flows. A flow sensor tells the control board that demand is present, the gas valve opens, the fan ramps up, and the burner lights. Cold water crosses a heat exchanger, a thermistor measures outlet temperature, and the control board modulates the flame to hold your setpoint. That entire sequence happens in seconds and repeats constantly while you use hot water. Any disruption in gas, combustion air, water flow, or sensor feedback breaks the chain, and the unit either shuts itself down or struggles to hit temperature.
Understanding this order of operations helps you read symptoms. No ignition points to gas, air, or spark. Lukewarm water suggests restricted flow or scale. Temperature swings often implicate mixing valves, minimum flow thresholds, or recirculation timing.
Quick checks before you grab tools
From experience, the most common no-hot-water call with tankless is resolved in the first five minutes. Walk through these basics:
- Verify power. Tankless heaters need electricity for the board and combustion fan. Confirm the breaker and any service switch, then check the display. If the screen is blank, try the reset button and inspect fuses in the control panel if you’re qualified. Confirm gas supply. Look at the gas shutoff handle at the unit. If there was recent utility work or a gas appliance install, air may be in the line. Many units will retry ignition several times and then lock out. You might need to reset the unit after the gas line is bled by a pro. Check water isolation valves. Service valves sometimes get left partially closed after maintenance. Make sure both cold in and hot out valves are fully open. Remove faucet aerators and look for debris. If you see grit or flakes, you might have upstream sediment clogging inlet screens at the heater. Low flow equals no fire in many models. Compare multiple fixtures. If one shower is cold but a nearby sink gets hot, the problem could be a thermostatic mixing valve at that fixture, not the heater.
These quick checks rarely require tools. They narrow the field before you open the cabinet.
Reading the unit’s language: codes and patterns
Modern tankless models display error codes or blinking sequences that map to specific failures. Common ones include ignition failure, flame loss, inlet or outlet thermistor faults, and overheat. Note the code and the exact conditions when it appears. Did the unit fire and then drop out? Does it time out without ever lighting? Does it only fail on high-demand events like running two showers?
Patterns matter. A unit that works for 30 to 60 seconds, then shuts down with overheat or flame loss often has restricted water flow from scaling. A unit that never lights and throws an ignition error might have no gas, a blocked intake, or a failed igniter. A unit that only fails in the afternoon could be starved for combustion air due to negative house pressure when the dryer, range hood, and bathroom fans all run.
Lukewarm water, not cold: what that tells you
When a tankless can only raise temperature partway, focus on water-side restrictions and scaling. Minerals precipitate on the heat exchanger plate surfaces and act like insulation. The control board sees inadequate temperature rise, increases firing, and may trip over-temperature safeties if water moves too slowly. On most calls in moderate to hard water regions, I find scale after two to five years without consistent water heater maintenance.
Other lukewarm suspects:
- A cross-connection through a failed mixing valve or a stuck shower cartridge allows cold water to blend back into the hot line. An easy test is to close the cold supply valve at the heater. If hot still runs at a fixture, there’s a cross-connection. Minimum flow not met. Many tankless units need around 0.4 to 0.6 gallons per minute to fire. Low-flow faucets and clogged aerators may never trigger ignition. Try opening a second hot tap to bump total flow and see if the heater stabilizes. Recirculation loop misconfiguration. If a crossover valve or pump runs at the wrong time, the heater can short-cycle or feed itself tempered water, flattening outlet temperature.
Gas supply and combustion air, the unglamorous essentials
Tankless heaters are sensitive to gas pressure and volume. A half-inch gas line that used to feed a 40,000 BTU tank may not be enough for a 150,000 to 199,000 BTU tankless. Even if the line size looks adequate on paper, long runs, multiple tees, and other appliances can drop pressure under load. I’ve measured systems that meet static pressure but starve during operation. If your unit works when only one appliance runs but fails when the furnace or cooktop fires, suspect gas sizing or regulation.
Combustion air and venting matter just as much. Intake screens collect lint, spiders love terminations, and birds build nests at rooftop caps. Any restriction upsets the fan curve and reduces available combustion air. Condensing models produce acidic condensate that must drain freely. A clogged condensate trap can back up water into the combustion chamber, causing flame instability or lockout.
Scale, sediment, and the quiet decline
Mineral buildup changes the personality of a tankless. The burner may cycle more aggressively. The unit starts to sing under load, then short-cycles. Outlets feel inconsistent, especially with small draws. In hard water areas, I recommend descaling every 12 months for typical households, sooner for large families or high temperature setpoints. If you’ve never descaled and the unit is three to five years old, assume scale is part of your no-hot-water story.
Here is a safe, minimal toolkit I carry for routine tankless water heater maintenance and inspection:
- A pump and bucket for descaling with food-grade white vinegar or a manufacturer-approved solution. A soft brush and vacuum for intake screens and cabinet dust. A manometer for gas pressure under load, and a multimeter for basic electrical checks on thermistors and power. A small snake or tubing brush for condensate lines and trap cleaning. Replacement washers, O-rings, and inlet screen filters specific to the brand.
This is one of only two lists in this article. The goal is not gadget pride but to underscore what actually fixes common failures.
A focused walkthrough: restoring heat on a scaled unit
If the unit powers up, attempts to fire, and either trips on overheat or delivers tepid water with fluctuating temperature, try this sequence.
Shut off power and gas, then close the hot and cold isolation valves at the tankless. Attach hoses to the service ports and place both ends in a five-gallon bucket. Fill the bucket with two to three gallons of warm white vinegar or the recommended chemical descaler. Switch on your pump to circulate solution through the heat exchanger for 45 to 90 minutes. Keep the discharge hose submerged to prevent splashing and air ingestion. If the solution turns chalky or discolored early, that’s a good sign you are dissolving scale. After circulating, flush the exchanger with clean water for several minutes. While the unit drains, pull the cold inlet screen and rinse out sediment. Reopen isolation valves, restore gas and power, and run a hot tap to purge air.
This single operation clears many no-hot-water complaints and smooths unit behavior. If your heater immediately behaves, build a yearly water heater service schedule. If scale returns quickly, consider adding a sediment prefilter or a scale control system at the inlet. That’s cheaper and less invasive than jumping straight to water heater replacement.
When ignition fails: a combustion path diagnosis
A pure no-fire condition needs a structured approach. Confirm the control board is calling for heat, which you can tell from the fan spinning and the igniter clicking or glowing. If the fan never starts, you may have a control board issue, a blocked intake, a failed pressure switch, or a wiring fault. If the fan runs, you hear the igniter, but there is no flame, focus on gas. Check that the shutoff is open, then test for gas pressure under call with a manometer. Seasonal utility pressure dips and undersized or long branch lines often show up as failures at peak evening hours.
If flame lights then drops, inspect venting and condensate. I’ve found full condensate traps on attic installations that choke a unit only during long showers. Clearing the trap and rerouting the drain with proper slope solved the issue permanently.
Temperature swings and low-flow headaches
Tankless heaters control best with steady flow. A pressure-balancing shower valve combined with other fixtures opening and closing can cause seesaw temperatures. Matching a tankless to existing plumbing sometimes takes small changes. Swapping an older pressure-balance valve for a thermostatic mixing valve, adding a short buffer tank, or enabling built-in recirculation can tame swings. On the other hand, overly aggressive flow restrictors can keep the heater below its minimum firing threshold. As a test, open a second hot tap. If temperature stabilizes, you’ve likely bumped across that threshold.
Sensors, boards, and the margin for error
Thermistors, flame rods, and flow sensors do fail. So do control boards, especially after power surges. Before replacing parts, verify connections. Heat cycles loosen push connectors and ribbon cables. I’ve restored plenty of heaters by cleaning and reseating a corroded flame rod or replacing a brittle O-ring that let condensate drip onto a wiring block. When a sensor code appears, check resistance against the manufacturer’s chart at known temperatures. If the reading is way off, replace the sensor. If several sensors read erratically, suspect the harness or the board rather than the parts themselves.
The install sets the table: why sizing and piping haunt repairs
Many “repair” calls tie back to original installation choices. A unit sized at the edge of your winter groundwater temperature can deliver fine in summer and struggle in January. In the Wylie area and much of North Texas, incoming water can drop into the low 50s Fahrenheit in winter. If you want two showers and a kitchen sink at once, a 120,000 BTU unit may fall short. Correcting this isn’t a repair, it’s a capacity conversation. You either manage expectations, adjust fixtures, or consider water heater replacement or a secondary unit.
Similarly, long pipe runs and recirculation loops need careful design. Improper check valves let cold water creep into the hot line. Recirculation without a buffer can force short, repeated firings that stress the exchanger. When we handle water heater installation Wylie homeowners are often upgrading from a tank, so we trace the hot water path, check flow rates at key fixtures, and size the gas line for full load. A little design upfront saves years of nuisance calls.
Maintenance rhythm: what keeps a tankless dependable
Tankless systems reward routine care. A maintenance visit is not a ceremonial flush and a sticker. It is measured work with a result you can feel in the shower the next morning.
- Descale annually in hard water regions or every 18 to 24 months where water is softer. If you run a high setpoint or multiple back-to-back showers daily, lean toward the shorter interval. Clean or replace inlet screens and any upstream sediment filter cartridges at least twice per year. Debris at the inlet is a leading cause of low flow and ignition failure. Inspect vent terminations seasonally. Clear lint, leaves, webs, and nests. Confirm condensate drains freely and traps are primed. Test gas pressure under load, not just static. If it dips below manufacturer minimum during operation, address line sizing or regulation before winter. Verify recirculation controls, crossover valves, and check valves. Misbehaving recirculation wastes gas and creates temperature instability.
That is the second and final list. Everything else lives in normal paragraphs to preserve clarity and cadence.
When repair isn’t the best answer
A tankless that has suffered repeated overheats, has corrosion in the combustion chamber, or shows heat exchanger leaks may not be a good candidate for continued tinkering. If the unit is over 12 years old, parts are scarce, or the control board is discontinued, replacement has a strong case. I weigh three factors: total cost of parts and labor over the next two years, efficiency gains of a new model, and the pain of repeated outages for the household. If a homeowner has already invested in multiple service calls for the same persistent fault, I’ll outline a water heater replacement path with specific, measurable improvements, such as higher turndown ratio for better low-flow performance, integrated recirculation with smart controls, or corrosion-resistant heat exchangers that tolerate borderline water conditions better.
Special notes on electric tankless
While gas units dominate where https://search.google.com/local/reviews?placeid=ChIJo2SPiFAj-wERVszm7RHnqdE natural gas is available, some homes run electric tankless. Their failure modes differ. Breakers and wiring must handle substantial amperage, sometimes 120 to 160 amps for whole-house units. If a heating module fails, the unit may deliver tepid water rather than none. Scale still matters because it impedes heat transfer. Electric models depend heavily on consistent voltage. If you experience flicker or droop during heavy appliance use, have an electrician verify panel capacity and connections. Repairs on electric units often involve replacing a single heating module or triac rather than gas components, but the diagnostic logic remains: confirm flow, confirm power under load, confirm sensor feedback.
What DIY makes sense, and when to call a pro
There’s plenty a careful homeowner can do without risk: cleaning inlet screens, rinsing aerators, checking vents, descaling with the proper hoses and valves, and verifying isolation valves are open. Keep the manual handy for code lookups, and take photos before you disconnect anything. Stop short of gas work unless you’re licensed. If you smell gas, see corrosion at gas unions, or suspect a pressure problem, call for professional water heater repair immediately. Same goes for control board swaps or anything involving vent alterations. With exhaust systems, a small mistake can lead to carbon monoxide issues.
In and around Wylie, I see three patterns that benefit from professional attention: persistent ignition failures after basic checks, chronic temperature swings in multi-bath homes, and recirculation loops that never quite behave. A competent technician brings combustion analysis, manometers, and the experience to tell when a rare edge case is at play. Good water heater repair Wylie service also considers the home as a system. If your dryer, range hood, and bath fans create negative pressure, your direct vent intake might need a tweak, or a dedicated combustion air path.
Costs and timelines you can expect
For a straightforward tankless water heater repair - cleaning inlet screens, descaling, and clearing a condensate trap - the visit often runs 60 to 120 minutes. Parts are minimal, and you should see immediate improvement. Sensor replacements usually add modest parts cost and a bit of wiring time. Gas supply corrections are more involved because they may require new piping or a regulator change. Vent rerouting depends on the structure and access. When water heater maintenance has been neglected for years, budget time for multiple passes: a deep descale, then a follow-up to confirm stable performance under real household demand.
If your unit needs replacement, the timeline hinges on vent and gas upgrades. A like-for-like swap in the same footprint can be completed in a day. Up-sizing gas lines or relocating a unit may take longer. For water heater installation Wylie projects in older homes, I plan for attic access challenges, code updates like drain pans and leak sensors, and the city inspection schedule.
Long-term reliability: match equipment to water and usage
Two homes can own the same model and have completely different experiences. The difference usually comes down to water chemistry and usage patterns. If your water hardness runs above about 10 grains per gallon, scale management is not optional. Treat the water or schedule more frequent service. If your household routinely runs low-flow taps, pick a unit with a wide turndown ratio that can modulate cleanly at low demand. Families that bathe or shower in a tight window every morning may benefit from a smart recirculation schedule or a small buffer tank that smooths the start and stop cycles.
An anecdote: a family of five in a two-story Wylie home called for recurring no-hot-water events at 7 a.m. The tankless was properly sized and only three years old. Static gas pressure looked fine. Under load, with the furnace and range warming, pressure nosedived. The fix was not a new heater. We upsized the gas line from the meter to a manifold, rebalanced appliance branches, and added a buffer tank to soften cycling. The same heater has run cleanly for three winters since, with annual descaling and a quick vent check each fall.
Bringing it together
Tankless water heaters are reliable when the basics are respected: adequate gas and air, clean water paths, correctly configured controls, and thoughtful installation. No-hot-water events tend to cluster around a few themes, and an orderly diagnosis will find the root. Start simple, read what the unit tells you, and work from water to fire to control. Schedule regular water heater service so scale never gets the upper hand. If repeated repairs circle the same fundamental constraint, step back and consider a strategic upgrade. Whether you need tankless water heater repair, a tune-up, or full water heater replacement, the smartest move is to align the system with how your household actually uses hot water, and then maintain it with discipline.
For homeowners who prefer a steady hand, local water heater repair Wylie professionals see these patterns every week. The right technician won’t just clear a code; they’ll explain why it happened, what will prevent it, and where the trade-offs sit between new parts, maintenance, and replacement. That’s how hot water stops being a surprise and becomes a given.
Pipe Dreams Services
Address: 2375 St Paul Rd, Wylie, TX 75098
Phone: (214) 225-8767