1 Scope
NOTE This standard covers the storage, tempering, recirculation, and indirect-heating accessories that surround a domestic hot water source and deliver tempered potable water safely to the fixtures it serves. (1.1)
NOTE The central design tension this standard resolves is the gap between storage temperature and delivery temperature. (1.2)
1.2.1Water must be stored hot enough to suppress Legionella growth - 140°F (60°C) is the commercial floor under ASHRAE 188 - yet delivered cool enough to prevent scalding, with a 120°F (49°C) ceiling at most fixtures and lower at care-facility showers. Bridging that gap with thermostatic mixing and a properly controlled recirculation loop is the recurring engineering problem these products solve, and most of the requirements below exist to enforce one side of it without sacrificing the other.
NOTE Included equipment (1.3)
NOTE The following equipment is included within this standard: (1.3.1)
- Unfired hot water storage tanks: glass-lined (porcelain enamel), stone-lined (cement-lined), stainless steel (Type 304 or 316L), and copper-lined
- Thermostatic mixing valves: system-level (master) valves to ASSE 1017 and point-of-use or fixture valves to ASSE 1070 / ASSE 1069
- Domestic hot water recirculation pumps, their controls, and the return piping arrangement
- Potable-water thermal expansion tanks (bladder/diaphragm type)
- Heat exchangers for indirect water heating: brazed-plate and shell-and-tube
- Heat-pump water heater storage configurations (integrated and remote storage)
- Integral accessories: aquastats, temperature-pressure (T&P) relief valves, and drain valves serving the storage and distribution system
NOTE Excluded equipment (1.4)
NOTE The water heater appliance itself - the heat-generating device, whether gas, electric, oil, tankless/instantaneous, or a packaged heat-pump unit - is specified under
Water Heaters; this standard governs only the storage vessel, tempering, recirculation, and indirect-heating accessories around it.
(1.4.1) NOTE General distribution valves, strainers, unions, and piping specialties on the domestic water system are specified under
Domestic Water Piping Specialties; only the mixing valves, T&P relief valves, and drain valves integral to the storage/distribution accessories are included here.
(1.4.3) NOTE Circulating pumps specified under a general plumbing pump section are governed by
Plumbing Pumps; the dedicated domestic hot water recirculation pump and its Legionella-aware control logic are retained here.
(1.4.4) NOTE Backflow preventers protecting the potable supply are specified under
Backflow Prevention; this standard only requires the thermal expansion control that becomes necessary once a backflow preventer or check valve creates a closed system.
(1.4.5) NOTE Solar thermal collectors and solar energy heating systems, hydronic space-heating equipment and boilers, sewage and stormwater packaged pump stations (see
Packaged Pump Stations), and radiant heating or cooling panels (see
Radiant Heating And Cooling Panels) are outside this standard's boundary.
(1.4.6) 2 Referenced Standards
2.1Equipment, materials, and installation shall comply with the latest adopted edition of each of the following unless a specific edition is cited or the authority having jurisdiction has adopted a different edition.
2.2Where referenced standards conflict, the more stringent requirement shall govern unless the Engineer of Record directs otherwise in writing.
| Standard |
Title |
| ASME BPVC Section VIII, Div. 1 |
Rules for Construction of Pressure Vessels |
| ANSI/ASSE 1017 |
Performance Requirements for Temperature Actuated Mixing Valves for Hot Water Distribution Systems |
| ANSI/ASSE 1069 |
Performance Requirements for Automatic Temperature Control Mixing Valves |
| ANSI/ASSE 1070 |
Performance Requirements for Water Temperature Limiting Devices |
| ANSI/ASSE 1016 |
Performance Requirements for Automatic Compensating Valves for Individual Showers and Tub/Shower Combinations |
| NSF/ANSI 61 |
Drinking Water System Components - Health Effects |
| NSF/ANSI 372 |
Drinking Water System Components - Lead Content |
| NSF 5 |
Water Heaters, Hot Water Supply Boilers, and Heat Recovery Equipment |
| UL 174 |
Household Electric Storage Tank Water Heaters |
| UL 1453 |
Electric Booster and Commercial Storage Tank Water Heaters |
| ASHRAE 188 |
Legionellosis: Risk Management for Building Water Systems |
| ASHRAE 90.1 |
Energy Standard for Sites and Buildings Except Low-Rise Residential Buildings |
| ASHRAE Handbook - HVAC Applications |
Service Water Heating (Chapter 50) |
| IPC |
International Plumbing Code (Chapter 5; Sections 504, 607) |
| IMC |
International Mechanical Code (Chapter 10) |
| NFPA 99 |
Health Care Facilities Code |
3 Submittals
NOTE Action submittals (3.1)
3.1.1The Contractor shall submit the following action submittals for review before fabrication, ordering, or installation:
- Product data for each storage tank, mixing valve, recirculation pump, expansion tank, and heat exchanger, including capacity, pressure rating, materials of construction, and listing marks
- Shop drawings showing tank dimensions, connection sizes and locations, lifting and anchorage points, and the recirculation return connection elevation on the tank
- Storage tank capacity and recovery calculation, including the peak-hour demand basis and the associated water heater recovery rate
- Thermal expansion tank sizing calculation using the manufacturer nomograph, stating system volume, inlet pressure, pre-charge pressure, temperature rise, and maximum allowable working pressure
- Mixing valve schedule listing each valve's standard (ASSE 1017, 1069, or 1070), set outlet temperature, flow range, and pipe size
- Recirculation system schematic showing pump selection point, control method, and return loop arrangement
- NSF/ANSI 61 and NSF/ANSI 372 certification for every wetted component
- Seismic anchorage details, manufacturer-certified or stamped by a Professional Engineer, for projects in Seismic Design Category C and above
☑ Product data (tanks, valves, pumps, expansion tanks, heat exchangers)
☑ Shop drawings with connection and anchorage points
☑ Storage capacity and recovery calculation
☑ Expansion tank sizing calculation (nomograph)
☑ Mixing valve schedule with set temperatures
☑ Recirculation system schematic and control narrative
☑ NSF/ANSI 61 and 372 certification
☐ Seismic anchorage details (SDC C and above)
NOTE Informational submittals (3.2)
3.2.1The Contractor shall submit the following informational submittals:
- ASME Section VIII data report (Form U-1 or U-1A) for each ASME-stamped vessel
- Factory hydrostatic and dielectric test certificates
- Manufacturer installation, operation, and maintenance instructions
- Statement of energy performance demonstrating compliance with ASHRAE 90.1 for the service water heating equipment
☑ ASME Section VIII data report (U-1 / U-1A)
☑ Factory hydrostatic and dielectric test certificates
☑ Manufacturer installation and O&M instructions
☑ ASHRAE 90.1 energy performance statement
NOTE Closeout submittals (3.3)
3.3.1The Contractor shall submit the following closeout submittals before final acceptance:
- Operation and maintenance manuals compiling all product data, test reports, and warranty documents
- Recorded mixing valve set temperatures and recirculation control setpoints as commissioned
- Water management plan documentation interface where ASHRAE 188 applies
- Warranty certificates for each tank, valve, pump, and heat exchanger
☑ Operation and maintenance manuals
☑ As-commissioned setpoint record
☐ ASHRAE 188 water management plan interface documentation
☑ Warranty certificates
4 Quality Assurance
4.1All wetted components shall be certified to NSF/ANSI 61 for drinking water health effects.
4.2All wetted components shall comply with NSF/ANSI 372 for lead content, satisfying the federal lead-free requirement of the Safe Drinking Water Act as amended.
4.3Storage tanks and expansion tanks with an operating pressure above 30 psi shall be constructed and stamped in accordance with ASME Boiler and Pressure Vessel Code, Section VIII, Division 1.
4.4System-level (master) thermostatic mixing valves shall be listed to ANSI/ASSE 1017.
4.5Point-of-use and fixture-level tempering valves shall be listed to ANSI/ASSE 1070, and individual-fixture automatic control mixing valves shall be listed to ANSI/ASSE 1069 where applied.
4.6Commercial electric storage tank heaters integral to a storage configuration shall be listed to UL 1453; residential-scale electric storage tank units shall be listed to UL 174.
NOTE The lead-free and NSF 61 obligations are routinely overlooked on heat exchanger plate and mixing valve internal selections; both apply to every component that contacts potable water, not only the visible tank. (4.7)
NOTE Where the facility is covered by ASHRAE 188, the storage temperature, recirculation return temperature, and mixing valve setpoints specified here shall be consistent with the building water management plan. (4.8)
5 Environmental and Service Conditions
NOTE The selection of tank lining is governed first by water chemistry, and a lining mismatch is the most common cause of premature tank failure. (5.1)
5.1.1Glass-lined (porcelain enamel) tanks shall not be specified for aggressive water - low pH or high chloride content - without confirming compatibility with the local water analysis; stainless steel (Type 316L) or stone-lined construction shall be specified for aggressive water chemistry.
5.1.2The Contractor shall obtain the local potable water analysis, including pH and chloride concentration, and confirm tank lining compatibility before ordering.
Benign municipal supply (neutral pH, low chloride)
Moderately aggressive (slightly low pH or moderate chloride)
Aggressive (low pH and/or high chloride)
Softened water (elevated sodium)
5.2Storage tanks installed in Seismic Design Category C and above shall be anchored using manufacturer-certified anchorage or anchorage details stamped by a Professional Engineer.
NOTE A storage tank over 50 gallons is heavy when full and becomes a life-safety hazard if it shifts or topples in a seismic event; anchorage is coordinated with the structural engineer rather than assumed from the tank pad alone. (5.2.1)
A or B (no seismic anchorage required)
C
D
E or F
5.3Equipment shall be rated for the ambient conditions of its installed location, including mechanical-room temperature extremes and any freeze exposure on the recirculation return.
6 Storage Tanks
NOTE Storage tank type (6.1)
NOTE The tank type is selected from water chemistry, heating method, and the authority having jurisdiction, and is the primary configuration decision for the storage system. (6.1.1)
● Glass-lined (porcelain enamel)
○ Stone-lined (cement-lined)
○ Stainless steel Type 304
○ Stainless steel Type 316L
○ Copper-lined
6.1.2Glass-lined tanks shall be furnished with a sacrificial or powered anode rod, and the anode shall be accessible for inspection and replacement.
6.1.3Stainless steel tanks shall be Type 316L where chloride content is elevated, and Type 304 is acceptable only for benign water chemistry.
NOTE Heating method (6.2)
NOTE The storage tank may be heated directly by an integral source, indirectly through an internal coil, indirectly through an external heat exchanger, or by a heat-pump source; the direct-fired appliance itself is specified under
Water Heaters.
(6.2.1) ○ Direct-fired (integral gas or electric element)
○ Indirect - internal heating coil (steam or hot water)
● Indirect - external heat exchanger (brazed-plate or shell-and-tube)
○ Heat-pump indirect
NOTE Storage capacity (6.3)
6.3.1Storage capacity shall be determined by a peak-hour demand calculation per the ASHRAE Handbook - HVAC Applications or an equivalent ASPE method, and shall be coordinated with the recovery rate of the associated water heater.
NOTE A large tank paired with an undersized heater still delivers cold water during peak demand because the stored volume cannot be reheated fast enough; capacity is a function of both stored gallons and recovery, never stored gallons alone. (6.3.2)
NOTE Rule-of-thumb storage figures - on the order of 1.5 to 2.0 gallons per fixture unit for office and commercial, 8 to 12 gallons per hotel room, and 10 to 15 gallons per healthcare bed - may be used only for preliminary sizing and shall be verified by full calculation. (6.3.3)
305000
Default: 500 gallon
NOTE Tank orientation (6.4)
NOTE The tank may be vertical or horizontal; vertical orientation preserves thermal stratification and is preferred where floor area permits. (6.4.1)
NOTE Working pressure (6.5)
6.5.1The tank working pressure rating shall equal or exceed the maximum system pressure at the tank location, including any boosted-supply zones.
NOTE A tank operating above 30 psi requires the ASME Section VIII stamp; nearly all commercial storage applications fall above that threshold, so the stamp is effectively mandatory at commercial scale. (6.5.2)
NOTE Tank jacket insulation (6.6)
6.6.1The tank shall be furnished with factory jacket insulation meeting or exceeding the minimum standby-loss requirement of ASHRAE 90.1 for the equipment class.
1225
Default: 16 hr·ft²·°F/Btu
NOTE Tank connections (6.7)
6.7.1Tank connections shall be provided for cold inlet, hot outlet, recirculation return, T&P relief, drain, anode access, and temperature sensing as applicable to the heating method.
6.7.2The recirculation return connection shall enter near the bottom of the tank, not the top.
NOTE Returning cooled water to the top of the tank disrupts stratification and reduces the effective hot volume; the cooler return belongs at the bottom where it is reheated before rising. (6.7.3)
7 Thermostatic Mixing and Temperature Control
NOTE Mixing valve tiers (7.1)
NOTE Most buildings require two tiers of tempering: a master ASSE 1017 valve at the tank outlet to set distribution temperature, and point-of-use ASSE 1070 valves at fixtures to enforce the scald limit. (7.1.1)
7.1.2A single central mixing valve in a large building shall not be relied upon as the sole scald protection, because distribution and reheating can allow delivery temperatures above 120°F at remote fixtures.
○ Single master ASSE 1017 valve at tank outlet
○ Master ASSE 1017 valve plus zone valves
● Master ASSE 1017 valve plus point-of-use ASSE 1070 valves
○ Master, zone, and point-of-use valves (full three-tier)
NOTE Storage temperature setpoint (7.2)
7.2.1The storage temperature setpoint shall be not less than 140°F (60°C) for commercial and institutional systems to suppress Legionella growth per ASHRAE 188; 120°F (49°C) is permitted only for residential systems not covered by a water management plan.
NOTE Master mixing valve outlet (7.3)
7.3.1The master ASSE 1017 valve shall be set to deliver distribution water in the 120°F to 140°F band; 120°F is the common default, and the setpoint shall be raised toward 140°F where healthcare code requires maintaining an elevated distribution temperature.
7.3.2Where a facility must maintain a minimum recirculation return temperature, the master valve outlet shall be set high enough that no return leg drops below that minimum at worst-case flow.
NOTE Setting the master valve at 120°F in a building required to hold 122°F in the recirculation loop is self-defeating - the mixed supply cannot keep the return above its floor; the master setpoint and the return floor are sized together. (7.3.3)
NOTE Point-of-use temperature limit (7.4)
7.4.1Point-of-use ASSE 1070 tempering valves shall limit fixture outlet temperature to a maximum of 120°F (49°C).
7.4.2Shower valves in assisted-living, memory-care, and similar care facilities shall limit outlet temperature to 105°F (41°C) in accordance with care-facility practice and ASSE 1016 / 1070 guidance.
● 120°F (49°C) maximum - general fixtures
○ 110°F (43°C) - public lavatories
○ 105°F (41°C) - care-facility and accessible showers
NOTE Control interface (7.5)
NOTE The temperature control interface may be a standalone aquastat, an integrated electronic controller, or a connection to the Building Automation System (BAS). (7.5.1)
○ Standalone aquastat
● Integrated electronic controller
○ BAS integration
8 Recirculation System
NOTE Recirculation control method (8.1)
8.1.1A dedicated, continuously running recirculation pump controlled only by an aquastat shall not be specified, because the pump stops once setpoint is reached and the stagnant water cools into the 77°F to 113°F Legionella growth band before the next call.
8.1.2Recirculation shall be controlled by demand-based control (push-button or occupancy), a time clock, or continuous circulation with a temperature floor, in accordance with ASHRAE 188 and ASHRAE 90.1.
NOTE Aquastat-only control is the single most common Legionella pitfall in recirculation design; ASHRAE 188 effectively condemns it, and the demand or timed alternatives below keep the loop above the growth band. (8.1.3)
● Demand-controlled (push-button or occupancy sensor)
○ Time clock with temperature floor
○ Continuous circulation with temperature floor
○ Aquastat-only (not permitted)
8.2Demand-controlled or time-clock recirculation shall be provided where required by ASHRAE 90.1 for systems above the threshold capacity; constant-speed continuous pumping shall not be used where that section applies.
NOTE Recirculation return temperature (8.3)
8.3.1The recirculation system shall be designed to maintain a return temperature of not less than 122°F (50°C) at the point of return to the heat source, in accordance with IPC Section 607.2, to prevent Legionella amplification.
NOTE Recirculation pump selection (8.4)
8.4.1The recirculation pump shall be an in-line or wet-rotor type, sized for the piping heat-loss load at a design loop temperature drop of 10°F to 20°F.
NOTE Variable-speed recirculation pumps should be specified where demand-controlled operation or energy code compliance favors them. (8.4.2)
○ Fixed-speed wet-rotor
● Variable-speed wet-rotor
○ In-line with external motor
NOTE Return piping layout (8.5)
NOTE The return piping may be arranged as a single main loop or as branch returns; branch returns improve delivery time at distant fixtures at the cost of additional balancing. (8.5.1)
● Single main loop
○ Branch returns (multiple return legs)
9 Thermal Expansion Control
9.1A potable-water thermal expansion tank shall be provided wherever a pressure-reducing valve, backflow preventer, or check valve on the cold-water supply creates a closed system, in accordance with IPC Section 607.3.
NOTE Thermal expansion control is routinely missed when a backflow preventer is added late in design and the supply quietly becomes a closed system; the requirement attaches to the closed condition, not to any single device. (9.1.1)
9.2The expansion tank shall be a bladder or diaphragm type with all wetted surfaces certified to NSF/ANSI 61, and shall be factory pre-charged.
9.3The expansion tank pre-charge pressure shall be set to match the cold-water supply pressure at the tank location.
NOTE Expansion tank volume (9.4)
9.4.1The expansion tank volume shall be sized using the manufacturer nomograph or calculation accounting for actual system volume, operating pressure, pre-charge pressure, and the temperature rise from supply to storage temperature; a rule-of-thumb volume shall not be substituted for the calculation.
9.4.2The expansion tank shall be sized so that system pressure does not exceed 80% of the T&P relief valve set pressure under worst-case expansion.
NOTE A rule of thumb of roughly one gallon of expansion volume per fifty gallons of system water gives a starting point only; undersizing from rule-of-thumb alone is a frequent defect because it ignores operating and pre-charge pressure. (9.4.3)
10 Heat Exchangers for Indirect Heating
NOTE Heat exchanger type (10.1)
NOTE Where the tank is heated indirectly by an external exchanger, the choice between brazed-plate and shell-and-tube is driven by the heating medium and maintenance access. (10.1.1)
NOTE Brazed-plate exchangers are compact, efficient, and lower cost but are not cleanable, while shell-and-tube exchangers tolerate higher pressures and temperatures and can be opened for cleaning; shell-and-tube is preferred for steam service and where periodic cleaning is anticipated. (10.1.2)
● Brazed-plate (stainless plates)
○ Shell-and-tube, U-tube
○ Shell-and-tube, straight-tube
NOTE Heating medium (10.2)
NOTE The heating medium for the indirect exchanger may be low-pressure steam, high-temperature hot water, or condenser hot water; the medium determines the exchanger pressure and temperature rating. (10.2.1)
○ Low-pressure steam
○ High-temperature hot water
● Standard hydronic hot water
10.3Shell-and-tube exchangers in steam service shall be protected by a steam separator, or the specification shall state a maximum allowable particulate level, where steam quality is unknown.
NOTE Wet or dirty steam fouls a tube bundle quickly; specifying a separator or a particulate limit prevents the most common shell-and-tube maintenance failure. (10.3.1)
10.4All heat exchanger plates, tubes, and wetted surfaces in contact with potable water shall be certified to NSF/ANSI 61 and shall comply with NSF/ANSI 372.
NOTE Heat exchanger capacity (10.5)
10.5.1The heat exchanger shall be sized for the peak heating load required to maintain storage setpoint at the design recovery rate.
11 Heat-Pump Storage Configuration
NOTE Where a heat-pump water heater serves the storage system, the storage tank shall be configured for the lower temperature differential and longer recovery characteristic of heat-pump operation. (11.1)
11.1.1Heat-pump storage configurations typically require larger stored volume than equivalent direct-fired systems because recovery is slower; storage capacity shall reflect the heat-pump recovery rate rather than a direct-fired recovery assumption.
11.2Heat-pump storage tanks shall be sized in the 50 to 120 gallon range for residential and light-commercial applications, with capacity confirmed by the peak-hour calculation.
NOTE A supplemental electric resistance element should be provided for peak demand backup where the heat-pump recovery alone cannot meet the peak-hour load. (11.3)
12 Accessories and Safety Devices
12.1Each storage tank shall be furnished with a temperature-pressure (T&P) relief valve in accordance with IPC Section 504.4.
12.2The T&P relief valve discharge shall be piped full-size to an approved termination in accordance with IPC Section 504.6, without valves or reductions in the discharge line.
12.3Each tank shall be furnished with a full-port drain valve at the lowest point to permit complete draining and sediment flushing.
12.4Aquastats and temperature sensors furnished as integral accessories shall be rated for the storage temperature and located to read representative tank temperature.
13 Insulation
13.1Recirculation supply and return piping shall be insulated to the minimum thickness required by ASHRAE 90.1 for the fluid temperature and pipe size, with not less than 1 inch of insulation on 3/4 inch recirculation return pipe.
NOTE Insulating the return leg as well as the supply is what keeps the recirculation return above its temperature floor; leaving the return bare defeats both the energy and the Legionella objectives. (13.1.1)
14 Testing
14.1ASME-stamped storage and expansion tanks shall be factory hydrostatically tested at 1.5 times the maximum allowable working pressure, and the test certificate shall be submitted.
14.2Glass-lined tanks shall be factory dielectric-tested to verify lining integrity, and the test certificate shall be submitted.
14.3Each thermostatic mixing valve shall carry documentation of flow and temperature performance per its listed ASSE 1017, 1069, or 1070 test procedure.
14.4After installation, the Contractor shall verify and record the master mixing valve outlet temperature, each point-of-use valve outlet temperature, and the recirculation return temperature under design flow.
14.5The Contractor shall demonstrate that the recirculation return temperature is maintained at or above the specified minimum at the most remote return leg.
15 Installation
15.1Equipment shall be installed in accordance with the manufacturer's instructions and the adopted plumbing and mechanical codes.
15.2Storage tanks shall be set on a level housekeeping pad or stand rated for the full operating weight of the tank, and anchored as required by the Seismic Design Category.
15.3Clearances shall be maintained around each tank, valve, and exchanger for anode replacement, tube cleaning, valve servicing, and code-required access.
15.4The recirculation pump shall be installed with isolation valves and a check valve to permit servicing without draining the loop.
15.5Equipment locations, routing of recirculation mains, and the extent of the return loop shall be installed as shown on the drawings. mechanical-room equipment layout 15.6Dielectric isolation shall be provided at connections between dissimilar metals to prevent galvanic corrosion.
16 Delivery, Storage, and Handling
16.1Equipment shall be delivered in the manufacturer's original packaging with listing labels and connection protection intact.
16.2Tanks and exchangers shall be stored upright and protected from weather, dust, and physical damage until installation.
16.3Lifting shall use the manufacturer's designated lifting points; slings shall not be passed around connection nozzles or trim.
17 Warranty
17.1The storage tank shall carry a manufacturer's warranty against tank and lining failure of not less than the period scheduled for the project.
17.2Mixing valves, recirculation pumps, expansion tanks, and heat exchangers shall carry the manufacturer's standard warranty, and the warranty certificates shall be submitted at closeout.
○ 3 years
○ 5 years
● 6 years
○ 10 years
18 Spare Parts
18.1The Contractor shall furnish spare anode rods for each glass-lined tank, sized and threaded for the installed tanks.
18.2The Contractor shall furnish one spare thermostatic cartridge or repair kit for each model and size of mixing valve installed.
18.3The Contractor shall furnish manufacturer-recommended gaskets and seals for each heat exchanger to support the first scheduled servicing.
☑ Spare anode rods (one per glass-lined tank)
☑ Mixing valve cartridge / repair kit (one per model and size)
☑ Heat exchanger gasket and seal set
☐ Spare T&P relief valve (one per tank model)