7.1 Water Heater Types, Sizing, Recovery Rate & First-Hour Rating

Water Heater Types, Sizing, Recovery Rate & First-Hour Rating

Why Chapter 5 is worth 8 percent

The Iowa content outline gives Chapter 5 — Water Heaters 8 percent, the same weight as Chapter 4, Fixtures and Fittings, and more than General Regulations, Indirect Wastes, or Traps and Interceptors. That is roughly six questions on a 75-question exam, and they split predictably between sizing arithmetic (this section), safety devices (Section 7.2), venting (Section 7.3), and location and Iowa amendments (Section 7.4).

Water heaters also concentrate more hazard per cubic foot than anything else a plumber installs: a pressure vessel, a fuel-fired burner or a high-current electrical circuit, and stored water hot enough to scald, all in a closet. The code's tone in Chapter 5 reflects that.

---

1. The four families you will meet

Two Iowa-specific notes on selection. Incoming water is cold here — main temperatures in the 40s Fahrenheit in late winter are ordinary — which punishes tankless sizing (see below). And a heat-pump water heater takes heat out of the space it sits in, so a unit in a small unconditioned basement room may fight the surrounding air rather than cooperate with it.

---

2. Recovery rate: the core formula

Recovery rate is how many gallons an hour a heater can raise through a given temperature rise.

Gas-fired:

$\text{GPH} = \frac{\text{Input (Btu/h)} \times \text{Efficiency}}{8.33 \times \Delta T}$

Electric:

$\text{GPH} = \frac{\text{Watts} \times 3.412}{8.33 \times \Delta T}$

The 8.33 is the weight of a gallon of water in pounds, and one Btu raises one pound of water one degree Fahrenheit — so the denominator is simply the Btu needed to lift one gallon through the rise. The 3.412 converts watts to Btu per hour.

Worked example — gas

A 40,000 Btu/h gas water heater at 80 percent thermal efficiency, raising water from 50°F to 140°F (a 90°F rise):

$\text{GPH} = \frac{40,000 \times 0.80}{8.33 \times 90} = \frac{32,000}{749.7} \approx 42.7 \text{ gallons per hour}$

Worked example — electric

A single 4,500-watt element at the same 90°F rise:

$\text{GPH} = \frac{4,500 \times 3.412}{8.33 \times 90} = \frac{15,354}{749.7} \approx 20.5 \text{ gallons per hour}$

That contrast is the whole story of electric versus gas storage. A gas heater recovers roughly twice as fast, which is why a 40-gallon gas heater and a 50-gallon electric heater serve similar households. Electric storage capacity is bought in the tank; gas storage capacity is bought in the burner.

---

3. First-hour rating: the number that actually matters

First-hour rating (FHR) is the gallons of hot water a heater can deliver in one hour starting from a fully heated tank. It combines what is already stored with what the burner or elements can add during that hour:

$\text{FHR} \approx (\text{Usable storage}) + (\text{Recovery in one hour})$

Usable storage is not the nameplate gallons. As hot water leaves the top of the tank, cold water enters the bottom and mixes; the outlet temperature falls below useful before the tank is empty. A common planning figure is about 70 percent of rated volume.

Worked example

A 40-gallon gas heater with the 42.7 GPH recovery calculated above:

• Usable storage: 40 × 0.70 = 28 gallons
• Recovery in one hour: 42.7 gallons
• FHR ≈ 70.7 gallons

Compare that to a peak-hour demand estimate: two 10-minute showers at 2.0 gpm (40 gallons), a dishwasher (6 gallons), and a load of laundry (20 gallons) is about 66 gallons in the morning peak. The 40-gallon gas heater fits, with little margin. Swap in the 50-gallon electric heater at 20.5 GPH recovery — usable 35 + recovery 20.5 = FHR ≈ 55.5 gallons — and the same household runs out mid-morning.

[!IMPORTANT] Size to first-hour rating against peak-hour demand, not to tank gallons against household size. "Two people, 40 gallons" rules of thumb ignore recovery entirely and are the reason so many replacement heaters disappoint the customer.

---

4. Tankless sizing: flow at a temperature rise

A tankless heater has no storage, so it is rated in gallons per minute at a stated temperature rise. Rearranging the recovery formula for a per-minute basis:

$\text{GPM} = \frac{\text{Input (Btu/h)} \times \text{Efficiency}}{500 \times \Delta T}$

The 500 is 8.33 pounds per gallon × 60 minutes per hour.

Worked example — the Iowa winter problem

A 199,000 Btu/h tankless unit at 0.95 efficiency:

• Summer, inlet 65°F, outlet 120°F, rise 55°F: GPM = (199,000 × 0.95) ÷ (500 × 55) = 189,050 ÷ 27,500 ≈ 6.9 gpm — two showers and a sink.
• Winter, inlet 42°F, outlet 120°F, rise 78°F: GPM = 189,050 ÷ (500 × 78) = 189,050 ÷ 39,000 ≈ 4.8 gpm — noticeably less.

The unit did not change; the temperature rise did. Any tankless capacity claim that does not state the rise is meaningless, and in Iowa you size on the winter rise.

---

5. Mixed-water and tempered storage

Commercial installations frequently store hot and deliver tempered, and the mixing arithmetic shows up in sizing questions:

$V_{\text{hot}} = V_{\text{mixed}} \times \frac{T_{\text{mixed}} - T_{\text{cold}}}{T_{\text{hot}} - T_{\text{cold}}}$

Example: deliver 100 gallons at 110°F from 140°F storage with 50°F cold water.

$V_{\text{hot}} = 100 \times \frac{110 - 50}{140 - 50} = 100 \times \frac{60}{90} = 66.7 \text{ gallons}$

Storing hotter buys effective capacity — 100 gallons of 140°F storage delivers 150 gallons of 110°F water — which is exactly why commercial systems store hot and temper at the point of use, and exactly why the anti-scald devices in Section 7.2 are not optional.