8.3 Hot Water Supply: Delivery Distance, Temperature Control, Recirculation & Thermal Expansion

Where hot water is required (IPC 607.1)

• Residential occupancies: hot water must be supplied to fixtures and equipment for bathing, washing, culinary purposes, cleansing, laundry or building maintenance.
• Nonresidential occupancies: hot water must be supplied for culinary purposes, cleansing, laundry or building maintenance. For bathing and washing, hot or tempered water may be supplied.

Tempered water is water between 85°F and 110°F (IPC Chapter 2). Public hand-washing lavatories receive tempered water (419.5).

Temperature control (IPC 607.1.1, 607.1.2, 501 and 613)

• 607.1.1: a water heater thermostat may serve as the temperature-limiting means for fixture delivery temperatures only if the heater complies with ASSE 1082 or ASSE 1085. Otherwise, a separate limiting device is needed at or for the fixture.
• 607.1.2 tempered water is controlled by one of: an ASSE 1070 limiting device set no higher than 110°F; an ASSE 1017 thermostatic mixing valve; or an ASSE 1082 or ASSE 1084 water heater. This does not replace the protective shower valves of 412.3.
• 613.1: temperature-actuated mixing valves installed to reduce water temperatures to defined limits must meet ASSE 1017.
• 501.2: where a combination water heater and space heater needs water hotter than 140°F for heating, an ASSE 1017 valve must limit the potable hot water supply to 140°F or less.
• 501.6: water from tankless heaters for domestic use must not exceed 140°F.
• 501.8: hot water systems need automatic temperature controls adjustable across the intended operating range.

Storage temperature and Legionella. The IPC does not set a minimum storage temperature. Industry guidance often recommends storing hot water at about 140°F to limit Legionella growth, which thrives in warm water. The code handles the resulting scald risk with point-of-use limits: 120°F at showers and tubs, and 110°F at bidets and public lavatories (see Section 3.3). An ASSE 1017 master mixing valve can temper distribution water, but it does not replace the ASSE 1016 valve at each shower.

Getting hot water to fixtures quickly (IPC 607.2)

The developed length of hot or tempered water piping, from the source of hot water to the fixtures that require hot or tempered water, shall not exceed 50 feet. Recirculating system piping and heat-traced piping shall be considered to be sources of hot or tempered water.

Example: a lavatory 85 feet of piping from the water heater needs a recirculation loop or heat-traced main that comes within 50 feet of developed length of the fixture.

The energy code has its own pipe-volume rules for some commercial buildings. The IPC's own rule is the 50-foot developed length.

Circulation and heat trace systems (IPC 607.2.1)

• In Group R-2, R-3 and R-4 occupancies three stories or less above grade plane, circulation and temperature maintenance systems follow IECC R403.5.1. In other occupancies, they follow IECC C404.6.
• 607.2.1.1: pumps that circulate water between a water heater and a storage tank must run from heating cycle startup to no more than 5 minutes after the end of the cycle.
• 607.2.1.2 demand recirculation: a system whose pumps return water from the hot supply to the heater through the cold water pipe must be a demand recirculation system. The pump starts only on a signal from a user, sensing a user's presence, or sensing hot water flow to a fixture. The control must limit the temperature of water entering the cold water piping to 104°F.
• 607.2.2: where a temperature-actuated mixing valve is used with a recirculating pump, the hot or tempered return goes to the water heater's cold inlet and to the mixing valve's cold inlet or hot return connection, so the valve can keep control.

Design practice: circulation pumps on potable systems use bronze or stainless steel bodies, because cast iron rusts in oxygenated potable water. Balancing valves divide flow among several return risers. A check valve in the return keeps cold water from flowing backward into the hot system. Pumps run on timers, aquastats or demand controls as the energy code requires.

Insulation (IPC 607.5)

Piping to the water heater inlet and piping carrying heated water must be insulated per IECC R403.5.2 (R-2, R-3 and R-4 occupancies three stories or less) or IECC C404.4 (other occupancies). 308.9 requires hot water piping bundled with cold water piping in manifold systems to be insulated.

Hot water on the left (IPC 607.4)

Faucets and fittings must be installed so hot water comes from the left side of the fitting. Exception: ASSE 1016 or ASME A112.18.1 shower and tub-shower valves, where hot follows the markings on the device.

Thermal expansion (IPC 607.3)

Why it happens. Water expands when heated. From about 50°F to 140°F, its volume grows by roughly 1.7%, so a 50-gallon tank produces close to 0.8 gallon of extra water each time it heats from cold. In an open system, that water pushes back into the main. When a check valve, PRV or backflow preventer is on the supply, the system is closed. Because water cannot compress, the pressure climbs quickly. This can open the relief valve repeatedly, stress the tank, and damage fixtures and connectors.

The rule:

Where a storage water heater is supplied with cold water that passes through a check valve, pressure reducing valve or backflow preventer, a thermal expansion control device shall be connected to the water heater cold water supply pipe at a point that is downstream of all check valves, pressure reducing valves and backflow preventers. Thermal expansion tanks shall be sized in accordance with the tank manufacturer's instructions and shall be sized such that the pressure in the water distribution system shall not exceed that required by Section 604.8.

So the expansion tank must keep the system at or below 80 psi. Related rules:

• 504.4: the water heater's relief valve shall not be used as a means of controlling thermal expansion.
• 308.10: an expansion tank must be supported per the manufacturer, and not by the piping that connects to it.

Setting a diaphragm expansion tank

Manufacturers require the air precharge to be checked and set to the incoming static water pressure before the tank is connected. If the precharge is too low, water fills the tank at line pressure and leaves little room for expansion. If it is too high, the tank does nothing until the system pressure exceeds the precharge. Size the tank from the manufacturer's chart, using the heater volume, the incoming pressure, the maximum allowed pressure (80 psi by 607.3) and the temperature rise. A 40- to 50-gallon residential heater at 50 to 60 psi typically needs a tank of about 2 gallons in total volume.