Reliance Water Heater No Hot Water: Diagnosis and Fix Guide

Common Symptoms

• Hot water runs out much faster than normal — recovery time seems very long
• Water is warm but never reaches the temperature set on the dial
• Reliance 6-Series gas: pilot stays lit but main burner fires sporadically or briefly
• Cold plastic debris or flakes appearing in hot water faucet strainers (dip tube failure)
• Reliance electric 12-Series: only the top portion of the tank is hot — bottom stays cold
• Reliance electric: circuit breaker trips when unit has been heating for 5–10 minutes
• Sediment noise (rumbling, popping) from bottom of tank for past several months
• Reliance 9-Series power vent: unit cycles briefly and shuts off before tank fully heats

Most Likely Causes

1. 1

   Reliance Gas: Borderline Thermocouple — Sporadic Main Burner Firing

   A thermocouple or thermopile that is degraded but not completely failed produces enough millivolts to hold the pilot intermittently, but drops below the gas valve's minimum threshold during normal operation — especially when the burner fires and ambient temperature near the thermocouple rises. The Honeywell/White-Rodgers gas valve on Reliance models requires a sustained minimum of approximately 17mV (thermocouple) or 325mV (thermopile). A borderline thermocouple in the 17–25mV range will hold the pilot while the burner is off, but when the main burner fires and the thermocouple becomes slightly hotter or slightly cooler due to flame dynamics, voltage fluctuates and the valve intermittently drops out. The result: sporadic heating, lukewarm water, or the burner firing briefly then cutting off. Replace when millivolt reading is below 25mV on a thermocouple model or below 400mV on a thermopile model.

2. 2

   Reliance Dip Tube Failure — Broken Cold-Water Inlet Tube

   The dip tube is a plastic (polypropylene or polyethylene) pipe inside the tank that routes incoming cold water to the bottom, where it is heated before rising to the hot water outlet at the top. Reliance water heaters manufactured before approximately 2005 used an early-generation plastic dip tube material that becomes brittle over time and fragments into small chips. When the dip tube breaks, incoming cold water mixes with hot water near the top of the tank — the net result is a tank full of warm water that never gets fully hot. Telltale sign: small white or gray plastic chips or flakes accumulating in hot-side faucet aerators and strainer screens throughout the house. Confirm by removing the cold-inlet nipple at the top of the tank and inspecting the dip tube for cracks, brittleness, or missing sections.

3. 3

   Reliance Gas: Sediment Buildup — Insulated Lower Tank

   Calcium and magnesium carbonates from hard water accumulate on the tank bottom over time, eventually forming a thick sediment layer. On Reliance gas models, sediment insulates the tank bottom from the burner heat, dramatically reducing heating efficiency. The burner runs for longer periods to achieve the same heat transfer — and in severe cases, the burner overheats the tank bottom (producing rumbling/popping sounds), which trips the TCO thermal switch. A tank with 1–2 inches of sediment takes 2–3× longer to recover hot water and may never reach the full setpoint. Flushing is only effective in early-stage sediment buildup — heavy sediment (5+ years without flushing) typically cannot be completely removed.

4. 4

   Reliance Electric 12-Series: Failed Lower Heating Element

   The lower heating element on a Reliance electric water heater handles approximately 60–70% of the tank's recovery work. When the lower element fails (open circuit — most common failure mode), the upper element continues to work, keeping the top one-third of the tank hot. You get short bursts of hot water followed by rapid cold when the small upper-heated zone is depleted. Lower element failures are often caused by sediment buildup that causes the element to overheat (the element wire reaches the center of the sediment layer rather than the water), leading to burnout. Test the lower element with the 240V breaker off: disconnect leads and measure terminal-to-terminal resistance — should be 12–16 ohms for 4500W. Open circuit (OL) = failed element.

5. 5

   Reliance Electric 12-Series: Upper Thermostat Sequencing Failure

   Reliance electric water heaters use a sequential thermostat control: the upper thermostat controls power to the upper element and also gates power to the lower thermostat. When the upper thermostat reaches setpoint, it switches power to the lower circuit so the lower element can finish heating the lower tank section. If the upper thermostat fails in the 'open' position (stuck contacts), the lower circuit never receives power — the lower element never fires regardless of how cold the lower tank is. The result is identical to a failed lower element: short hot water runs followed by cold. Diagnose by testing the upper thermostat's switching behavior with the 240V breaker off — the thermostat contacts should be closed when the tank is cold and open when the tank is at or above setpoint.

6. 6

   Reliance Gas: Gas Supply Pressure or Orifice Restriction

   If a Reliance gas water heater's main burner fires but the flame appears blue and weak, or if the burner shuts off before the tank reaches setpoint, the gas supply pressure may be insufficient or the burner orifice may be partially clogged. Reliance natural gas models require a manifold gas pressure of approximately 3.5 inches water column (WC) for natural gas. A partially closed gas shutoff valve upstream, a failing gas pressure regulator, or multiple high-demand gas appliances running simultaneously can drop pressure below minimum. Additionally, spider webs or debris inside the burner orifice — a known issue on Reliance units stored or idle for extended periods — restrict gas flow and produce a weak, unstable flame.

Quick DIY Checks

1. 1Step 1 — Reliance gas: test thermocouple millivolts with main burner firing: the most reliable thermocouple test on a Reliance gas unit is done while the main burner is firing (not just the pilot). Relight the pilot per the label instructions, then turn the thermostat dial up high enough to fire the main burner. With the burner running, connect a DC millivolt meter to the thermocouple lead at the gas valve body (TH and TP/TH terminals). Healthy thermocouple with burner running: 25–35mV. Below 17mV = replace. Thermopile: 400–750mV healthy, below 325mV = replace. A reading that starts healthy and drops after 2–3 minutes of burner operation indicates a borderline thermocouple (heat soak effect) — replace even if it initially reads 20–22mV.
2. 2Step 2 — Check for dip tube failure (plastic debris in hot water): run hot water from several faucets and check the aerator screens. Remove the aerator by unscrewing it from the faucet spout and look for small white, gray, or tan plastic chips or flakes. Even a few chips confirm dip tube failure. To definitively inspect: turn off the cold water supply to the water heater, drain 2–3 gallons from the drain valve, then loosen and remove the cold-water inlet nipple at the top of the tank. Look into the tank opening with a flashlight — the dip tube should extend 12–18 inches down into the tank. If it is short, cracked, broken, or missing, replace it. Replacement dip tubes are inexpensive (under $15) and install without draining the full tank.
3. 3Step 3 — Flush sediment from Reliance gas model (annual maintenance): connect a garden hose to the drain valve at the bottom of the tank. Run the hose to a floor drain or outside. Turn off the gas supply (turn the knob to PILOT, not OFF — keeps the thermocouple warm). Close the cold-water inlet valve at the top. Open a hot-side faucet in the house (prevents vacuum). Open the drain valve and allow 3–5 gallons to drain — the initial water will be rust-colored and may contain sediment particles. Close the drain valve, restore cold water briefly to stir remaining sediment, then drain again. Repeat until water runs clear. On tanks with heavy sediment, the drain valve itself may clog — clear with a straightened coat hanger or replace the valve.

1. 4Step 4 — Reliance electric 12-Series: isolate upper vs lower element failure: fill the tank fully and run hot water at a faucet until it goes cold — note how many minutes until cold. If you get 10–12 minutes on a 50-gallon tank (the upper element's zone), but no recovery hot water after 60–90 minutes of waiting, the lower element has failed. To confirm: turn off the 240V breaker, remove the lower access panel, fold back insulation, disconnect the lower element leads. Measure terminal-to-terminal resistance: 12–16 ohms = good; OL (overload/infinite) = open circuit failure, replace the element. If the lower element tests good but no heat is reaching the lower zone, test the upper thermostat switching contacts (as described in Step 5) — a failed upper thermostat prevents the lower circuit from being powered.
2. 5Step 5 — Reliance electric 12-Series: test upper thermostat for sequencing failure: with the tank cold, turn off the 240V breaker and open the upper access panel. Disconnect the wires from the upper thermostat's switch terminals (typically labeled L1, L2, T1, T2 or similar). With the tank COLD, the upper thermostat's main contacts should be closed — measure continuity (or ~0 ohms) between L1 and T1 terminals (or equivalent main power-in to element-out terminals). If those contacts are open (OL) when the tank is cold, the thermostat is stuck open and must be replaced — the upper element and lower circuit are both dead regardless of element condition. Use Reliance/AO Smith thermostat kit 100110321 or equivalent.
3. 6Step 6 — Reliance gas: inspect and clean the burner orifice and assembly: turn the gas valve to OFF and disconnect the gas supply at the union fitting below the valve. Remove the burner access door at the bottom of the tank (typically 2–4 screws). Carefully pull out the burner and manifold assembly — it slides out on a bracket. Inspect the main burner orifice (the small brass fitting at the manifold end) with a flashlight: look for spider webs, debris, or carbonized buildup partially blocking the orifice opening. Clean with compressed air — never use a wire or drill bit that could enlarge or deform the orifice. Also inspect the pilot assembly and thermocouple positioning — the thermocouple tip should sit 3/8 to 1/2 inch into the pilot flame. Reinstall in reverse order, restore gas, check for leaks at the union with soapy water, then relight.

Repair vs Replace

Recommended Tools & Parts

• Reliance/AO Smith Thermocouple 9003972

  OEM replacement thermocouple for Reliance 6-Series atmospheric gas water heaters. 36-inch lead. Replace when millivolt test shows below 17mV or below 25mV on a borderline-reading unit that causes sporadic main burner firing.

  $12–$20

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• Reliance Water Heater Dip Tube Replacement

  Replacement dip tube for Reliance tank water heaters. Replace when plastic debris appears in hot-water aerators or when cold-inlet dip tube inspection reveals breakage. Universal cross-compatible 3/4-inch NPT dip tube, trim to tank height.

  $8–$15

  Buy on Amazon →

• Camco 02142 4500W Screw-In Heating Element

  Universal screw-in 4500W 240V element for Reliance electric 12-Series models. Replace lower element when short hot-water runs followed by extended cold confirm lower element failure. Includes new rubber gasket. Full tank drain required before removal.

  $18–$35

  Buy on Amazon →

• Reliance/AO Smith Thermostat Kit 100110321

  Replacement thermostat for Reliance electric 12-Series models with ECO safety cutout. Replace upper thermostat when sequencing failure test shows open contacts with cold tank, or when ECO trips repeatedly. Upper thermostat also gates power to lower circuit.

  $22–$40

  Buy on Amazon →

Links are Amazon affiliate links (tag: fixitfastai-20). Prices are estimates.

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