Water Heater Sizing + Selection Reference
Why this matters
Customer's #1 hot water complaint: "runs out". The plumber who sizes correctly delivers consistent hot water + sells the appropriate product. Wrong size = unhappy customer + repeat service call. Right size + right type + correct install = satisfied customer + reviews + referrals. This is the field card.
Tank vs tankless overview
| Feature | Tank | Tankless |
|---|---|---|
| Capacity | 30 - 80 gal stored | Continuous flow |
| Energy | Standby heat loss | Heat only when running |
| Lifetime | 8 - 12 yr | 15 - 25 yr |
| Install cost | Lower | Roughly 2-3x tank |
| Recovery rate | Limited | Continuous |
| Space | Larger footprint | Wall-mount; small |
| Cold-water sandwich | None | Brief lag possible |
Tank sizing
Person-based: 1 - 2 people 30 - 40 gal; 3 - 4 people 50 gal; 5+ people 65 - 80 gal. By peak demand (worst-case 1-hour usage): shower (10 min × 2.5 GPM) 25 gal; bath fill 30 - 40 gal; dishwasher 7 gal; washing machine 12 - 20 gal (hot). Recovery rate (GPH heated): 40-gal electric (4500W) 22 GPH; 40-gal gas 38 - 42 GPH; 50-gal high-efficiency gas 50+ GPH. First-hour rating (FHR): tank gallons + GPH first hour from cold (typical 50 - 80 gal).
Tankless sizing + heat sources
Sized by simultaneous fixture demand + temperature rise. Rise = incoming → desired output: northern winter 40°F → 120°F = 80°F rise; southern winter 60°F → 120°F = 60°F rise; summer 65 - 75°F → 120°F = 45 - 55°F rise. GPM capacity rated per rise (Rinnai V94iN: 9.4 GPM at 35°F rise; 6.6 GPM at 60°F; 4.4 GPM at 80°F). Fixture demand: shower 2.0 - 2.5 GPM; low-flow shower 1.5 GPM; bath fill 4 - 6 GPM; lav 1.5 - 2.0 GPM; kitchen 1.5 - 2.5 GPM. Two showers + dishwasher = 6.5 GPM; need tankless rated 6.5+ at expected rise.
Heat sources: natural gas (most common, 150K - 199K BTU/hr residential); propane (same, different jets); electric whole-house (28 kW = 117A on 240V; rare due to capacity); point-of-use electric (5 - 15 kW; under-sink).
Gas + electric tank types
Atmospheric (natural draft): B-vent/chimney; lowest efficiency EF 0.60 - 0.67. Power vent: blower-vented horizontal/vertical PVC; EF 0.65 - 0.70. Power direct vent: sealed combustion, outside air; EF 0.70 - 0.80. Condensing: PVC vent + condensate drain; EF 0.85 - 0.93. Heat pump (hybrid electric): pulls ambient heat; 2 - 3x efficient vs standard electric; EF 2.50 - 3.50. Electric tank: 4500W single/dual-element; slow recovery (1/3 of gas); no venting required.
Tankless install considerations
Gas line sizing: tankless requires larger gas line than tank (often 3/4" OR 1" vs 1/2" tank). Upsize if needed.
Venting: PVC OR stainless; some models specific.
Combustion air: sealed combustion = outdoor air; non-sealed = indoor air.
Mounting location: indoor (basement / utility room) OR outdoor (sheltered).
Electrical: most tankless need 120V for control + ignition.
Tank install considerations
Drip pan + drain: required by most codes; especially upstairs / attic installs.
Pressure relief valve: T&P (temperature + pressure) at top OR side.
Anode rod: sacrificial; should be inspected + replaced 4 - 6 yr.
Earthquake straps (seismic areas): California requires 2; other quake-prone states 1.
Combustion air: gas atmospheric needs 1 sq in per 1,000 BTU.
Vent termination: from chimney top OR side wall per code + manufacturer.
Expansion tank
Required by code when closed system (PRV OR check valve on supply).
Sizing:
- 40 - 50 gal tank: 2-gal expansion tank
- 65 - 80 gal: 4-gal
- 100+ gal: 6 - 8-gal
Pre-charge to supply pressure (typically 50 PSI).
Recirculation pump
Provides instant hot water at fixture (no waiting).
Types:
- Dedicated return line (best; needs piping during construction)
- Crossover (uses cold-water line; sensor-controlled)
Common brands: Watts, Grundfos, Taco, Bell + Gossett.
Add 10 - 15% to water heater capacity when recirc installed (continuous demand).
Energy efficiency standards
Federal minimum (2025 standards):
- Standard gas tank: EF 0.62 - 0.67 (varies by size)
- Electric tank: EF 0.92 - 0.95
- Tankless gas: EF 0.82+
- Heat pump electric: EF 2.50+
Energy Star certified: higher than federal min.
Customer rebates: utility OR federal rebates for ENERGY STAR + heat pump + high-efficiency tankless. Varies by region.
Customer questions to ask
Number of occupants: sizing baseline.
Bathroom count: simultaneous use potential.
Peak demand timing: morning rush?
Existing equipment age: replacement OR add to existing?
Fuel availability: gas, propane, electric, geo, solar?
Space constraints: tank vs tankless real-estate.
Budget: short-term install vs long-term operation costs.
Climate: cold water temp; freeze risk for tankless outdoor.
Common water heater problems
Tank: no hot water (pilot out, element fail, thermostat); insufficient hot water (undersized, recovery slow, sediment buildup); leaking (tank rusted = replace; fitting = repair); banging/popping (sediment; flush annually); discolored water (anode depleted; replace).
Tankless: no hot water (gas low, ignition fail, flow sensor stuck); cold-water sandwich (brief cold burst when fixture cycles); scale buildup (hard water; flush vinegar annually); won't fire (minimum GPM not reached; faucet not open enough).
Lifespan + maintenance
Annual flush: drain 2 - 3 gallons to flush sediment.
Inspect anode rod: every 4 yr; replace at 50% consumption.
Inspect T&P valve: lift + reset annually.
Flush tankless: vinegar OR descaler annually (hard water) OR biannually (soft).
Replace at end of life: tanks 8 - 12 yr; tankless 15 - 25 yr.
The single most-useful water-heater habit is the ASK ABOUT THE COMPLAINT before sizing. "Need bigger water heater" - but the actual problem may be sediment buildup in current tank, leaking T&P valve dropping water, undersized supply line bottle-necking flow, OR family changes (now has 3 teenagers showering at same time). Diagnose the real problem first; oversize-replace is sometimes the wrong answer. Customer + your wallet both come out better with the actual fix.
References
- ANSI / CSA (Z21.10) water heater standards
- IPC + UPC water heater install codes
- ENERGY STAR water heater specifications
- Manufacturer data (Rinnai, A.O. Smith, Bradford White, Rheem, Bosch)
- Manuall internal: Water Pressure + Flow, Pipe Sizing Materials