DHW mixing valve

Boilers can store or produce water hotter than a person should wash in. A domestic hot water mixing valve — also called a tempering valve — solves that by blending cold water into the hot supply as it leaves the boiler or tank, holding the delivered temperature near a set point. Navien's NCB-H manual describes the mixing valve on the unit's water pipe module as the part that controls the amount of cold water mixed with hot water to provide a stable domestic hot water supply, and it cautions against adding more mechanical mixing valves on the DHW supply line unless local codes require them.

Mixing valves also make combinations possible. IBC's SL G3 manual describes a thermostatic mixing valve that permits simultaneous operation with high-temperature space heating, and notes that a thermostatic tempering valve for domestic hot water may be required by local code. On tankless-coil installations, Weil-McLain's EG manual requires an automatic mixing valve to be installed following the mixing valve manufacturer's instructions, reflecting the scald risk of a hot coil.

The most common failure is scale. A mixing valve's internal element moves in response to temperature, and hard water deposits can make it stick, so the delivered temperature drifts or swings — sometimes hot enough to scald, sometimes lukewarm. A failed thermostatic element does the same thing more suddenly.

Homeowners often notice the problem in the shower: the temperature changes when another fixture runs, or the water starts hot and then cools. Debris can also block the cold or hot port, and on some valves the adjustment has simply been turned the wrong way. Because mixing valves are safety devices, a valve that cannot hold its setting should be repaired or replaced rather than ignored.

Mixing valves are also asked to do their job under changing conditions. Incoming cold water is colder in winter than in summer, and draw rates change from fixture to fixture; a healthy thermostatic valve compensates, while a worn one lets those changes show up at the tap. That is why a temperature complaint is tested at more than one time of day before a technician decides whether the valve needs adjustment, cleaning or replacement.

Measure the water temperature at a tap with a thermometer rather than by feel, and do it at different times of day. Note whether other fixtures running changes the temperature, and whether the problem is one tap or the whole house.

Do not adjust the mixing valve by trial and error on a hot system, and do not remove it to get hotter water. A scald is far worse than a lukewarm shower, and the valve's setting is what keeps the supply safe.

A technician tests the delivered water temperature, checks the valve's adjustment against the manufacturer's setting, and inspects for scale and debris. Where the valve can be cleaned and recalibrated, that is done; where the thermostatic element has failed or scale cannot be removed, the valve is replaced and the temperature re-verified at the fixtures.

The technician also looks at why the valve failed — water hardness, a missing filter, or system debris — and corrects it so the replacement lasts. If your hot water temperature swings or runs too hot, call ROMA Heating & Cooling; mixing valve service is quick work that restores safe, steady hot water.