How to Choose a Water Heater

Choosing a water heater is easier when the decision is split into separate questions: storage or on-demand delivery, fuel path, sizing method, efficiency rating, and installation constraints. The right answer is not the same for every home. It is the system whose hot-water delivery, energy profile, and installation requirements match the way the household actually uses hot water.

Start With Tank or Tankless

Begin by deciding whether the home needs stored hot water or on-demand hot water. A tank system stores heated water, so the buying question is capacity and first hour delivery. Bob Vila defines First Hour Rating as the hot water a full tank delivers in one hour, and its media-sourced tank sizing ranges are based on household size. Those tank ranges are useful for storage systems, but they should not be treated as tankless flow rules.

Tankless systems work differently. They heat water as it moves through the unit, which makes flow and temperature rise the central sizing questions. ENERGY STAR states that tankless gas water heaters can be 24%-34% more energy efficient for homes using 41 gallons or less of hot water daily. Rheem says tankless systems commonly last 15 to 20 years or more, with well-maintained units reaching 25-30 years, while tank systems are listed at 8-12 years.

Those numbers make tankless attractive when efficiency, space, and long service life matter. They do not eliminate sizing work. A tankless model that cannot meet simultaneous demand will disappoint even if its efficiency story is strong. A tank model can still be the simpler fit when the household wants stored capacity and familiar replacement work.

Choose the Fuel Path Before the Model

The fuel decision shapes both performance and installation. This Old House reports that natural gas tankless units cost $750-$2,000 and can reach up to 11 GPM. It also reports electric units at $500-$1,500, calling them the lowest equipment cost option in that fact set. Those equipment numbers are not a complete installed price, but they show why the fuel question should come before brand shopping.

Gas tankless systems often enter the conversation when higher simultaneous flow is required. This Old House reports that gas models achieve the highest flow rates, up to 11 GPM, and can handle simultaneous demands. That is a strong reason to investigate gas when overlapping hot-water uses may occur.

Electric tankless systems have a different constraint profile. HeaterForWater lists full-size electric tankless units at 240V, with some point-of-use units running on 120V. The same fact set says amperage draw can range from 40 to 170 amps depending on size and output, and that whole-house models run from 12.2 kW to 36 kW. Eccotemp notes there is no universal breaker or wire size and that whole-home models may need multiple 240V circuits. The practical takeaway is simple: do not choose electric tankless by equipment price alone. Confirm the electrical path before treating it as the easy option.

Size Demand With Flow and Temperature Rise

For tankless sizing, do not start with a generic household label. Start with hot-water flow and temperature rise. Navien defines temperature rise as desired setting minus inlet temperature, using an example of 120F minus 55F equaling a 65F rise. That example explains why the same model can perform differently when inlet water is colder.

Navien also gives simultaneous fixture estimates: about 3.5 GPM for 1-2 fixtures, 5 GPM for 2-3 fixtures, and 7 GPM for 3-4 fixtures running at the same time. The same source says to choose a unit with flow equal to or greater than maximum simultaneous hot-water demand, using the coldest-day rise.

A good quote should connect those pieces. It should identify likely overlapping uses, the expected flow, and the rise assumption. If the proposal only names a model and a headline GPM, the sizing argument is incomplete. For tank systems, the comparable sizing conversation is different. The Bob Vila media-sourced ranges tie storage volume to household size, while First Hour Rating explains delivery from a full tank over one hour.

Read UEF Beside the Installation Scope

Uniform Energy Factor is the efficiency checkpoint. ENERGY STAR defines UEF as the DOE industry standard for measuring water heater efficiency, and says a higher UEF means the product is more efficient and has lower operating costs. That makes UEF the right number to ask for when comparing specific models.

The range depends on system type. A. O. Smith's hotwater.com efficiency article lists gas tankless at 0.82 UEF for non-condensing models up to 0.95 UEF for condensing models. The same article lists standard gas tank heaters at 0.60-0.70 UEF. Heat pump water heaters occupy a different category: an A. O. Smith comparison article says a heat pump water heater can be up to four times more efficient than a tankless heater, with HPWH UEF up to 3.88 versus tankless around 0.95.

UEF is not the whole decision. A heat pump water heater still stores water and has placement requirements. ENERGY STAR says heat pump units should be in interior spaces kept 40-90F year-round with 1,000 cubic feet of air space. The efficient model still has to fit the actual room and electrical situation.

Separate Condensing From Non-Condensing

Condensing and non-condensing tankless systems are not just marketing tiers. Navien says condensing units use dual heat exchangers with a secondary stainless steel exchanger, while non-condensing units use a single heat exchanger. Its comparison page lists condensing UEF up to .96 and non-condensing UEF up to .80.

That extra heat recovery changes the installation conversation. Navien says condensing units can vent with PVC, CPVC, or polypropylene, while non-condensing units need metal CAT.III venting. It also says condensing units require a dedicated drain line and condensate neutralizer kit, while non-condensing units have no drainage requirement.

Use that split as a final check. If the goal is higher efficiency and the project can handle drainage, a condensing model may fit. If the replacement location makes drainage hard to access, Navien lists non-condensing as the better use case. The best water heater is the one that clears both the performance test and the jobsite test.