How we rate units

One listing, one model

Every product page is one exact model and carries its model number. Its figures (maximum flow, input in BTU per hour or kilowatts, voltage, amps, venting, UEF, tank size, first-hour rating, the heaters a kit fits) come from that model's own listing, and the passage each figure came from is kept on file.

When a listing doesn't give a figure, the page says “Not stated”. Gaps are never filled from a sister model.

When figures disagree

Listings sometimes print one figure in the title and another in the specification table: 9.5 GPM over an 8.0 GPM row, watts written as kilowatts, or carton dimensions in place of the unit's. Every specification is compared with the listing's own title and bullet points, and a row that contradicts them is left off the page.

Tanklessly math

The planners print their formula and every input. The flow check works like this:

• Flow you need: the fixtures running at the same time, at 2 GPM a shower, 1 GPM a bathroom sink, 1.5 GPM a kitchen sink and 2 GPM a dishwasher or washer (an equipment maker's sizing sheet). You can change the shower figure.
• Temperature rise: your set point (120°F by default) minus your winter inlet water, picked from published ground-water zones of 35 to 77°F or typed in from your own reading.
• Gas heater flow at that rise: input in BTU per hour × efficiency ÷ (500 × rise), using 0.95 for condensing units and 0.77 for non-condensing ones. These reproduce makers' own published flow tables.
• Electric heater flow at that rise: kilowatts × 3,412 ÷ (500 × rise).
• Every estimate is capped at the maker's stated maximum flow, and a unit is never listed as a fit below the flow you need.

The electrical and gas calculator turns kilowatts into amps at your voltage, shows the breakers the maker calls for, and turns BTU per hour into the cubic feet of natural gas or gallons of propane an hour the line must deliver.

Prices

Prices change daily. Product pages link to the seller, where the current price is shown; check it there before you buy.

Updated October 2026.