What is the first-hour rating of a water heater?

It is the number of gallons of hot water a storage heater can supply in the first hour starting with a full tank. It combines the stored volume and the recovery during that hour, making it the most useful number for matching a tank to a household's peak-hour demand.

Why does a tankless water heater deliver less hot water in winter?

A tankless unit has a fixed heating capacity, so the colder incoming water requires a larger temperature rise, and the maximum flow it can heat to the target temperature falls accordingly. Always size on the local winter temperature rise, not the mild-climate rating.

How do I size a water heater by occupancy?

Estimate hot-water use per person per day, typically around 12 to 20 gallons, identify the peak hour as a fraction of that daily total, and select equipment whose storage plus recovery (for a tank) or simultaneous flow (for tankless) meets that peak.

Engineering·13 min read·June 17, 2026

🌡️ Water Heater Selection and Sizing

Engineering guide for sizing storage, tankless, and heat pump water heaters for residential and commercial applications, including Legionella control requirements and energy performance metrics.

📖Plumbing Systems — Illustrated Guide (Full Access)

View →

Water Heater Types

Storage Tank Water Heaters

The most common type in residential and light commercial applications. A tank (20–120 gallons for residential; 75–500+ gallons for commercial) maintains water at setpoint temperature continuously. Heat sources include natural gas, propane, fuel oil, and electric resistance elements. The key sizing parameter is First-Hour Rating (FHR) — the volume of hot water deliverable in the first hour of operation starting with a full, heated tank — which accounts for both stored capacity and recovery rate.

Topics covered

water heater sizingtankless water heaterheat pump water heaterLegionella controlthermostatic mixing valveFHRUEFcommercial water heaterASHRAEASSE 1082