Water heater glossary

Short, direct answers to the terms that come up when choosing a water heater.

120 V (plug-in) heat pump water heater: A 120 V heat pump water heater plugs into a standard household outlet — often a shared 15 A circuit — so it can replace a gas water heater without new wiring or panel work. The trade-off is slower recovery and usually no resistance backup.
Compressor cutoff temperature: The compressor cutoff is the ambient temperature below which a heat pump water heater stops using its heat pump (typically around 37–45 °F) and falls back to electric resistance heating — a key spec for garage and basement installs in cold climates.
Condensing vs non-condensing (gas): A condensing gas water heater extracts extra heat from its exhaust until water vapor condenses, reaching UEF ratings around 0.9+; a non-condensing unit vents that heat outdoors. Condensing units need a condensate drain and can use PVC venting.
Connected (smart) water heater: A connected water heater has Wi-Fi and app control: scheduling, vacation mode, leak alerts, and — the financial hook — eligibility for utility demand-response programs that pay you for letting the utility shift heating to off-peak hours.
Direct vent: A direct-vent water heater draws combustion air from outdoors and exhausts outdoors through a sealed two-pipe or concentric arrangement — no indoor air is used, eliminating backdraft risk and depressurization problems.
Draw pattern: A draw pattern is the standardized daily hot-water usage profile (Very Small, Low, Medium, or High) a water heater is tested under for its UEF rating. It indicates the household size the unit is designed to serve.
First-hour rating (FHR): The first-hour rating is how many gallons of hot water a storage water heater can deliver in one hour starting from a full tank — the single most important sizing number for tank-type heaters, more informative than tank size.
GPM (gallons per minute): GPM measures how much hot water a tankless water heater can deliver continuously. A shower uses roughly 2 GPM, a bathroom sink 0.5–1 GPM, and a tub filler 4 GPM — a tankless must sustain the sum of everything running at once.
Heat pump water heater (HPWH): A heat pump water heater uses a refrigeration circuit to move heat from surrounding air into the tank, delivering 3–4 units of heat per unit of electricity — the most efficient water heater type sold, with UEF ratings of 3.0–4.5.
Hybrid mode: Hybrid mode is a heat pump water heater setting that combines the efficient heat pump with fast electric resistance elements: the heat pump does routine heating, and the elements jump in when demand outruns it.
Minimum activation flow: Minimum activation flow is the smallest water flow (typically 0.25–0.6 GPM) that triggers a tankless water heater to fire. Below it, the unit stays off and the tap runs cold.
Power vent: A power-vent water heater uses a fan to push exhaust through plastic pipe out a sidewall, instead of relying on a natural-draft chimney. It frees placement from the chimney but needs an electrical outlet and adds fan noise.
Rated vs nominal volume: The nominal size on the sticker ("50-gallon") is a marketing class; the rated storage volume in the certification record is the actual measured capacity, typically 5–10% less (a "50-gallon" tank often holds ~45–47 gallons).
Recovery efficiency: Recovery efficiency is the share of the fuel or electricity input that actually ends up in the water while the unit is heating. Gas units run about 80–99%; heat pump models exceed 100% several times over because they move heat rather than make it.
Refrigerant GWP: GWP (global warming potential) measures how strongly a refrigerant traps heat if leaked, relative to CO₂. Older HPWHs use R-134a (GWP 1430); newer models use R-513A (~630), R-32 (~675), or CO₂ itself (GWP 1) as regulations phase high-GWP refrigerants out.
Temperature rise: Temperature rise is the difference between incoming groundwater temperature and your hot-water setpoint. It determines how much flow a tankless water heater can actually sustain: the colder the groundwater, the fewer GPM you get.
Therm: A therm is 100,000 BTU of natural gas energy — the billing unit on US gas bills. Certified gas water heaters list annual use in therms per year, so multiplying by your local rate gives the yearly running cost.
Uniform Energy Factor (UEF): UEF is the standard efficiency rating for US water heaters: hot-water energy delivered per unit of energy consumed under a standardized DOE test. Higher is better — gas units score around 0.8–0.98, heat pump models 3.0–4.5.