Water heater equipment

First hour rating (FHR) is the number that actually matters on a storage tank. It is the gallons of hot water the unit can deliver in the first hour starting fully heated, and it combines tank volume with burner or element power. A 40-gallon tank with a strong burner can out-deliver a weak 50-gallon tank. To use it, estimate your busiest hour — say two showers plus a dishwasher — and buy an FHR that covers it. ENERGY STAR's own criteria require a minimum FHR of 51 gallons for certified gas storage units and 45 gallons for certified heat pump water heaters.

Tankless is sized in gallons per minute (GPM) at a temperature rise, not in gallons of storage. A tankless unit does not run out of hot water, but it does run out of flow. Two showers at once in a cold-groundwater climate is a real constraint. ENERGY STAR's criteria for certified gas tankless units require a maximum flow of at least 2.8 GPM over a 67°F rise — useful as a floor, not as a target for a whole house. Ask your installer for the GPM at your winter incoming water temperature.

UEF (Uniform Energy Factor) is the efficiency figure. Higher is better within the same category. ENERGY STAR 2026 criteria require UEF of at least 0.81-0.86 for gas storage, at least 0.95 for gas tankless, and at least 3.30 for integrated heat pump water heaters. That heat pump figure is above 1.0 because the unit moves heat rather than making it, which is why heat pump units have by far the lowest running cost of the electric options.

One equipment note with real safety weight: gas units burn fuel and must vent combustion gases outside. Atmospheric, power-vent, and direct-vent are different systems with different requirements, and they are not interchangeable. If you change unit type you may be changing the vent, and that is licensed work — never DIY.

How to choose

Ask for the FHR, not the gallons

Have the installer write the first hour rating on the quote and explain how it covers your busiest hour. If they cannot, they are guessing at your sizing.

For tankless, ask for GPM at your winter temperature rise

A unit rated 2.8 GPM over a 67°F rise delivers much less in Minnesota in January than in Florida. The rating is only meaningful with a temperature rise attached.

Heat pump units need space and warmth

They pull heat from the surrounding air, so they want a reasonably sized, reasonably warm room, and they produce condensate that needs a drain. A tight interior closet is often the wrong location.

Check the ENERGY STAR warranty floor

ENERGY STAR certification requires a manufacturer warranty of at least 6 years on the system or sealed system for gas storage, tankless heat exchangers, and heat pump units. Confirm the actual warranty on the exact model you are buying — do not rely on a salesperson's summary.

Do not change fuel type casually

Electric to gas, or a natural-draft unit to a high-efficiency condensing one, can mean new gas piping, new venting, and a condensate drain. Price the whole change, not the box.