plumbing troubleshooting
Fix no hot water
Common symptoms: no hot water; cold water only; lukewarm water; water heater not heating; pilot light out; tankless error code
Stop and call a pro if:
- gas leak (smell)
- live electrical at water heater
- scalding risk
- TPR valve discharging
Step-by-step diagnostic flow
Step 1
Before anything else — do you smell gas (rotten egg / sulfur) near the water heater or anywhere in the house?
Do not flip switches, do not relight a pilot, do not use a phone in the area. Get out first.
Step 2
Is the temperature & pressure relief (TPR) valve actively discharging water from its drain pipe?
The TPR valve sits on the top or upper side of the tank with a copper or PEX pipe running down toward the floor.
Step 3
What kind of water heater do you have?
Gas units have a vent pipe out the top and a gas line. Electric units have a thick electrical conduit and no vent. Tankless units are wall-mounted and much smaller.
Step 4
Look at the sight glass or control window near the gas valve. Is the pilot light or burner status indicator lit?
Many newer units have a blinking LED status code instead of a visible pilot — note the blink pattern if so.
Step 5
Go to the electrical panel. Is the double-pole breaker labeled for the water heater fully ON (not tripped or in the middle)?
Step 6
Look at the tankless unit's display. Is there an error code shown?
Possible outcomes
Evacuate now and call the gas utility
high confidenceGas odor is a life-safety emergency. Ignition sources (switches, phones, flames) can trigger an explosion.
- Leave the building immediately — do not flip switches, do not relight anything
- From a neighbor's phone or outside, call your gas utility's emergency line (and 911 if needed)
- Do not re-enter until the utility clears the building
Shut the heater down and call a licensed plumber now
high confidenceAn actively discharging TPR valve indicates dangerous overpressure or overheat — possibly a failed thermostat or stuck gas valve. This can rupture the tank.
- Turn the gas control to OFF (or flip the electric breaker for the heater)
- Close the cold-water shutoff valve above the tank
- Keep people away from the heater
- Call a licensed plumber immediately — do not attempt to cap, plug, or replace the TPR valve yourself
Identify the unit before troubleshooting
low confidenceDiagnostic steps differ for gas, electric, and tankless. Working blind risks shock or gas mishandling.
- Photograph the data plate (silver/white sticker on the side of the unit)
- Note brand, model, fuel type, and year
- Re-run this guide once fuel type is known, or contact a plumber
- Photo of the data plate
- Photos of the top of the unit showing any vent / gas line / electrical conduit
Relight the pilot following the unit's instructions
medium confidencePilot outage is a common cause of no hot water on standing-pilot gas heaters. Safe to attempt only after confirming there is no gas smell.
- Confirm one more time there is no gas odor anywhere near the unit
- Read the relight instructions printed on the gas control or side of the tank
- Turn gas control to OFF, wait the full 5 minutes listed on the label before relighting
- Follow the unit's exact pilot-relight sequence
- If the pilot will not stay lit after 2–3 attempts, stop — likely a failed thermocouple or gas valve; call a plumber
- Brand/model
- Whether the pilot lights but won't stay lit, or won't light at all
Gas control or burner fault — call a licensed plumber
medium confidenceStatus codes, failed ignition, or no heat with a lit pilot point at the gas valve, thermopile, or burner — these are not safe DIY repairs.
- Photograph the status LED blink pattern (or note the sequence in seconds)
- Photograph the data plate
- Leave the gas control alone and contact a licensed plumber
- LED blink pattern / error code
- Data plate photo
- Age of unit
Reset the breaker once — then escalate if it trips again
medium confidenceA tripped breaker can be a one-off, but a repeat trip means a shorted element, failed thermostat, or wiring fault — that requires a pro.
- Push the breaker fully to OFF, then firmly back to ON
- Wait 30–60 minutes for the tank to begin heating
- If the breaker trips again, leave it OFF and call a licensed plumber or electrician — do not keep resetting
- Breaker amperage rating (printed on the handle)
- Whether the trip is immediate or delayed
Likely failed element or thermostat — call a plumber unless you're qualified
medium confidenceWith breaker on and no hot water, the most common causes are a burned-out upper element, failed upper thermostat, or tripped high-limit reset. Testing requires turning the breaker OFF, verifying dead with a non-contact tester, and using a multimeter.
- If not comfortable with breaker lockout + multimeter testing inside a live panel, call a licensed plumber
- If qualified: shut breaker OFF, verify dead with a non-contact voltage tester at the heater junction box, then test elements for continuity and check the high-limit reset button under the upper access cover
- Data plate photo (wattage of elements)
- Age of unit
- Whether water is fully cold or just lukewarm
Look up the error code in the unit's manual
medium confidenceTankless error codes are brand-specific and usually map directly to a cause (ignition failure, flame loss, vent blockage, scale buildup, flow sensor).
- Write down the exact error code shown
- Photograph the data plate and find the model-specific code chart (manufacturer website or the manual)
- Codes pointing to scale, flame, or venting issues are typically pro repairs — gas, combustion, and condensate work are not DIY
- Error code
- Brand and model
- Whether the code clears and returns, or stays on
No power to the tankless — check the dedicated circuit
medium confidenceEven gas tankless units need 120V for the control board and fan. A tripped breaker or unplugged unit will leave the display blank.
- Check the breaker labeled for the tankless and reset it once if tripped
- If the unit is cord-and-plug, verify it's plugged in
- If power restores but the unit still shows nothing, the control board may have failed — call a plumber
- Brand/model
- Whether breaker holds or re-trips
Flow or gas-supply issue — call a plumber
low confidenceA tankless with normal display but no heat usually has insufficient flow (clogged inlet filter, undersized fixture), insufficient gas pressure, or a scaled heat exchanger. Diagnosis needs gauges.
- Verify you're running a fixture above the unit's minimum flow rate (most need 0.5–0.75 gpm to fire)
- If the unit hasn't been descaled in 12+ months and you're on hard water, schedule descaling
- Call a licensed plumber for gas-pressure and heat-exchanger diagnosis
- Brand/model
- Water hardness if known
- Last descale date
- Whether all fixtures or just some fail to get hot
Pro launch updates
Get notified when Pro launches, plus a monthly code-change digest. Email only — no account required.