Hot Water Logistics: Sizing Tank vs. Tankless Water Heaters
Never take a cold shower again. We explain First Hour Rating (FHR) for tanks and Temperature Rise/GPM for tankless units. Compare the pros, cons, and payback periods of both technologies.
The water heater is the unsung workhorse of the home. When it fails, it is a crisis. Replacing it offers a choice: stick with the traditional storage tank or upgrade to on-demand tankless. The decision comes down to lifestyle and math.
The Storage Tank: First Hour Rating (FHR)
A 50-gallon tank does not give you 50 gallons of hot water. As you draw hot water, cold water enters the bottom, cooling the mix. The true metric is FHR—how many gallons can be delivered in the first hour of heavy use.
Rule of Thumb: FHR should equal (Number of Bedrooms + 1) x 12. For a 3-bedroom home: (3+1) x 12 = 48 gallons FHR.
Tankless (On-Demand): GPM and Temperature Rise
Tankless units are rated by Gallons Per Minute (GPM). But there is a catch: GPM depends on groundwater temperature. In Florida (70°F groundwater), a unit might push 8 GPM. In Minnesota (40°F groundwater), that same unit might only push 4 GPM because it has to work harder to heat the water.
You must calculate your peak demand. A shower uses 2.5 GPM. A faucet uses 1.5 GPM. If you want to shower while the dishwasher runs, you need 4-5 GPM.
Try it yourself
Find the right heater size for your climate:
Conclusion: Tankless offers endless hot water and lower bills, but installation costs are 3x higher (requires bigger gas lines). Tanks are cheap and simple but run out of water.