New to water heaters? Shopping for one means running into unfamiliar words — GPM, condensing, temperature rise, heat exchanger — and most sound more complicated than they are. This guide breaks down every term you'll actually encounter, in plain language. Use the search bar to jump straight to a term, or browse by letter below.
The things most people want answered before they buy, in plain language.
Cold water flows through the unit; a sensor detects the flow and fires the burner or activates the heating elements, which transfer heat to the water through a heat exchanger as it passes. Hot water is delivered on demand — there's no tank storing pre-heated water.
They eliminate standby loss (the energy a tank wastes keeping water hot while idle) and typically last longer than tanks. Upfront cost is higher. Savings depend on usage, fuel rates, and climate — heavy and steady users often see the biggest long-term benefit.
Not from "using it up" the way a tank empties — output is continuous. The limit is flow: if simultaneous demand exceeds the unit's GPM at your required temperature rise, the temperature will drop. Proper sizing prevents this.
Add up the GPM of fixtures you expect to run at once, then determine your required temperature rise (desired temp minus your incoming water temp). Match a unit that delivers that GPM at that ΔT. Colder incoming water reduces a unit's usable flow.
A short burst of cold water mid-use, caused by leftover hot water in the pipes running out before freshly heated water arrives. Buffer tanks and recirculation systems minimize it.
Tankless: endless hot water, space savings, no standby loss, longer lifespan, higher upfront cost. Tank: lower upfront cost, simpler, but finite capacity, standby losses, and shorter lifespan. The best choice depends on demand pattern, space, budget, and fuel.
Condensing units add a second heat exchanger to reclaim heat from exhaust gases, reaching ~90%+ efficiency and using inexpensive plastic venting (but producing acidic condensate to drain). Non-condensing units are ~80–85% efficient, cost less upfront, and need heat-rated venting.
Gas units generally deliver higher flow and suit whole-home demand but require venting and gas supply. Electric units need no venting but demand substantial electrical capacity and are more limited by incoming water temperature — often better for point-of-use or warm climates.
Roughly annually, more often in hard-water areas. Descaling dissolves mineral scale on the heat exchanger to protect efficiency and lifespan. Isolation valves make this much easier — a strong reason to install them from day one.
In hard-water regions it's strongly worth considering. Softening reduces scale buildup, cuts descaling frequency, and protects the heat exchanger over the long term.
It keeps hot water circulating so it's available instantly at the tap, eliminating the wait and water waste. Worth it for long pipe runs or convenience-focused households; it can be timer- or demand-based to limit energy use.
Often 15–20 years with proper maintenance — typically longer than storage tanks (which average ~10–12) — partly because serviceable components can be replaced rather than the whole unit.
Yes, but colder incoming water increases the required temperature rise, which lowers usable GPM. Size accordingly and ensure freeze protection for outdoor or unconditioned installs.
An HPWH moves heat from surrounding air into a storage tank using a refrigerant cycle, making it far more efficient than standard electric (it moves heat rather than making it). It heats more slowly, performs best in warm, ventilated spaces, and is a tank-based technology — a different category from tankless.
NOx (nitrogen oxides) are regulated combustion emissions. Air-quality regions (such as parts of California) require Low-NOx or Ultra-Low-NOx units. It's an important spec when selecting equipment by location.
Commercial applications often demand far higher and more variable flow, requiring cascade/rack systems of multiple linked units, common venting, master mixing valves, buffer tanks, and redundancy (N+1) so service or a single failure never interrupts supply.
Multiple tankless units controlled as one, firing in sequence (lead/lag) to meet large commercial loads. It provides modular capacity, easier maintenance, and built-in redundancy.