Fixed No Hot Water, Now a Mixing Valve Fault Shows: Two-Fault Decision Tree

Why this matters

You restored hot water (relit a pilot, swapped an element, reset a high-limit), and now a fixture runs scalding, never reaches temperature, or surges hot and cold. This is a two-fault job: while the heater was down, the thermostatic mixing valve (TMV) or anti-scald device was never challenged with real hot supply, so its fault stayed hidden. Now that hot water arrives, a stuck, scaled, or cross-connected mixing valve reveals itself. The safety stakes are real. A TMV that fails to limit temperature can deliver scald-temperature water to a tub or shower. Diagnosing the second fault correctly protects the occupant and finishes the job.

Symptom presentation

After hot water is restored:

• A tub or shower runs scalding even at a moderate handle position, or will not get warm no matter the setting.
• Output temperature surges and drops (hunting) at a TMV-protected fixture.
• A point-of-use or whole-house mixing valve set to a target temperature delivers far above or below it.
• Hot shows up at a cold-only fixture, or cold bleeds into hot (cross-connection through a failed mixing-valve check or a single-handle cartridge).

The common cause is a TMV element fouled with scale, a stuck or installed-backward valve, a failed internal check allowing cross-flow, or a mis-set anti-scald limit stop that was never tested because there was no hot water to test it.

Quick checks

1. Measure delivered temperature at the affected fixture with a thermometer in the stream after a 60 to 90 second stabilization. Compare to the intended set point (110 to 120 F typical for fixtures; verify local scald limits). A delivered reading at or above 130 F at a tub or shower is a scald exposure that must be corrected before the call closes.
2. Confirm tank or source temperature at the heater outlet. The TMV cannot deliver what the source does not supply.
3. Verify hot and cold are connected to the correct ports on the mixing valve (inlets not reversed).
4. Check the cold supply to the valve is open and full pressure; a starved cold side makes a TMV run hot.

Isolation tree

Branch A: fixture runs scalding.

• Cold side starved or closed: open/restore cold; a TMV with no cold cannot temper. Confirm cold pressure at the valve.
• TMV element stuck or scaled: the wax/thermostatic element no longer modulates. Service or replace the cartridge.
• Anti-scald limit stop set too high or not set: adjust the limit stop to the target maximum and verify.

Branch B: fixture will not get hot.

• Hot side starved (partially closed stop, clogged inlet screen, or undersized supply): restore hot flow to the valve.
• Inlets reversed: hot and cold swapped at the valve ports defeats the element. Re-pipe correctly.
• Source temperature too low: the heater is not actually delivering set temperature; return to the heater diagnosis.

Branch C: temperature hunts hot and cold.

• Pressure imbalance between hot and cold at the valve, or a worn pressure-balance/thermostatic element. Confirm balanced inlet pressures, then service the element.

Branch D: hot at a cold fixture or cold bleeding into hot.

• Internal check valve failed or a single-handle cartridge worn, creating cross-connection. Replace the cartridge or the integral checks.

Confirming diagnosis

• Stabilized stream temperature at the fixture is the definitive reading. Run for at least 60 seconds and read mid-stream, not at the surface or the edge of the flow.
• Isolate the valve: dead-head cold and read hot-only delivery, then reverse. A TMV that cannot blend down with full cold present has a failed element. A working element should drive delivered temperature toward the cold inlet temperature when the hot is restricted.
• Pressure-gauge both inlets at the valve. A TMV needs reasonably balanced inlet pressures; a 10-plus psi imbalance drives hunting and uneven delivery.
• Check the inlet check valves. With the valve isolated, cross-flow between hot and cold inlets (felt as the cold line warming when only hot is drawn) confirms a failed integral check and a cross-connection path.
• After any adjustment, re-verify the maximum delivered temperature cannot exceed the scald limit at any handle position, sweeping the handle from full hot through the range while monitoring the thermometer.

Remediation

• Set source water heater to a temperature that controls Legionella risk at storage (commonly 140 F storage) and rely on the TMV to temper distribution down to a safe fixture maximum. Verify against local code, which often caps tub/shower delivery at 120 F and may require lower at specific fixtures.
• Service or replace the thermostatic element/cartridge if scaled or stuck. On hard-water systems, recommend scale mitigation upstream.
• Correct reversed inlets, restore full cold and hot supply, and set/verify the anti-scald limit stop.
• Replace failed internal checks to eliminate cross-connection. Re-test delivered temperature at every protected fixture before leaving.

References

• ASSE 1017, Performance Requirements for Temperature Actuated Mixing Valves for Hot Water Distribution Systems.
• ASSE 1016 / ASME A112.18.1, Performance Requirements for Automatic Compensating Valves for Individual Showers and Tub/Shower Combinations.
• ASSE 1070, Performance Requirements for Water Temperature Limiting Devices (point-of-use scald protection).
• International Plumbing Code (IPC) Section 424, individual shower and bath/shower valve and temperature limit requirements.
• Uniform Plumbing Code (UPC) Section 413 / 418, shower and bathtub control-valve temperature limiting provisions.