Hot water supply: temperature standards, heater types, and expansion

One-line orientation

Hot-water design has three separate decisions: delivery temperature, how the water is heated and stored, and how pipes or a closed system accommodate thermal expansion.

Key points

• Temperature standards:
  • 140°F — kitchens and laundry (sanitization requirement)
  • 120°F — showers/tubs (IPC §412.3 / ASSE 1016 limit-stop; scald prevention maximum)
  • Public lavatories / handwashing — tempered water is capped at 110°F max (ASSE 1070); ~105°F is a comfortable delivery temperature, not an ADA temperature requirement.
• How water is heated:
  • Direct: a dedicated heater transfers heat directly to the domestic water.
  • Indirect: a boiler heats domestic water through a heat exchanger.
• How water is stored and delivered:
  • Storage: keeps a tank of hot water ready but has standby heat loss.
  • Tankless: heats water on demand and needs enough fuel or electrical capacity for peak flow.
• Fuel sources:
  • Electric — suitable for small-scale applications.
  • Gas or oil — used for various sizes; higher output rate than electric in most configurations.
• Thermal expansion: hot water and hot-water piping expand as temperature rises. Use the appropriate expansion control for the system; a pipe loop is one option, not a universal rule.

Hot water — compare the three paths, then apply delivery limits

Schematic

Stacked rows keep cold supply, heating equipment, hot delivery, and the energy tradeoff in separate lanes.

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Key

• domestic hot-water path / heated position
• separate building-boiler heat loop
• cool reference position of the welded bend

Trap: indirect is not tankless — it stores domestic hot water, but a separate building-boiler loop supplies heat through a coil.

Fuel: electric suits small-scale use; gas or oil gives a higher output rate in most configurations.

Confusions / comparison

Heater type	Storage tank?	Standby loss?	Energy source	Best for
Storage (tank)	Yes	Yes	Gas / oil / electric	High simultaneous demand; low first-cost
Demand (tankless)	No	No	Gas / electric	Low–moderate demand; space savings
Indirect	Usually	Varies with tank and controls	Building boiler	Buildings with a suitable central boiler; performance depends on the whole system

Temperature	Use	Why that number
140°F	Kitchens, laundry	Kills bacteria (Legionella); sanitization
120°F	Showers / tubs	IPC §412.3 / ASSE 1016 limit-stop; scald prevention maximum
110°F max	Public lavatories / handwashing	ASSE 1070 tempered-water maximum; ~105°F is comfort, not ADA

Related

→ Plumbing: water pressure & supply (heater fuel types, building size) · piping materials (copper L most common for hot supply; insulation of hot pipes) · valves (temperature/pressure relief valve on storage heaters) .