Tank size, first hour rating, and tankless flow rates explained. Plus which Bradford White and Navien models fit your household.
Get a Free EstimateFor a standard tank water heater, household size is the simplest starting point:
| Household Size | Recommended Tank Size |
|---|---|
| 1-2 people | 30 – 40 gallons |
| 3-4 people | 40 – 50 gallons |
| 5+ people | 50 – 75 gallons |
These are general guidelines. Your actual needs depend on usage patterns. A household of 3 where everyone showers in the morning within an hour needs more capacity than a household of 4 that spreads hot water use throughout the day.
The first hour rating (FHR) tells you how many gallons of hot water a tank unit can deliver in one hour, starting with a full tank of hot water. It accounts for both stored hot water and the recovery rate (how fast the unit heats new water coming in).
A 40-gallon tank with a high FHR can deliver more hot water in peak demand than a 50-gallon tank with a low FHR. When comparing models, check the yellow EnergyGuide label for the FHR number.
Add up the hot water used during your busiest hour. Typical usage:
If three people shower in the morning and you run the dishwasher, your peak hour demand is roughly 40-50 gallons. Choose a water heater with an FHR that meets or exceeds that number.
| Model | Tank Size | Best For |
|---|---|---|
| Bradford White RE340S6 | 40 gallon | 1-3 people, moderate use |
| Bradford White RE350S6 | 50 gallon | 3-4 people, standard use |
| Bradford White RG275H6N | 75 gallon | 5+ people or high demand |
Bradford White water heaters are sold exclusively through professional installers. You will not find them at big box stores, which means the unit you get has been properly sized and installed by a licensed plumber.
Tankless units are sized differently. Instead of gallons stored, you need to know two things: the flow rate you need (GPM) and the temperature rise required.
Philadelphia's winter inlet water temperature is approximately 50°F. To deliver 110°F water at the tap, the unit needs to raise the temperature by 60°F. In summer, inlet water warms to around 70°F, requiring only a 40°F rise. Winter performance is the limiting factor.
| Fixture | Typical Flow Rate (GPM) |
|---|---|
| Kitchen faucet | 1.0 – 1.5 GPM |
| Bathroom faucet | 0.5 – 1.0 GPM |
| Shower | 1.5 – 2.5 GPM |
| Dishwasher | 1.0 – 1.5 GPM |
| Clothes washer | 1.5 – 2.0 GPM |
Two showers running at the same time = 4-5 GPM. At a 60°F rise (Philadelphia winter), you need a tankless unit rated for at least 5 GPM at 60°F rise. The Navien NPE-240A delivers 5.4 GPM at 60°F rise, which handles most Philadelphia homes with 2-3 simultaneous fixtures.
| Model | Max GPM (60°F Rise) | Best For |
|---|---|---|
| Navien NPE-180A | 4.0 GPM | 1-2 people, 1-2 fixtures |
| Navien NPE-210A | 4.7 GPM | 2-3 people, 2 fixtures |
| Navien NPE-240A | 5.4 GPM | 3-5 people, 2-3 fixtures |
| Factor | Tank (Bradford White) | Tankless (Navien) |
|---|---|---|
| Installed cost | $1,550 – $3,200 | $3,200 – $5,800 |
| Lifespan | 8-12 years | 20+ years |
| Energy savings | Baseline | 20-30% less energy |
| Space | Floor space required | Wall-mounted, compact |
| Hot water supply | Limited by tank size | Continuous (within GPM rating) |
For tank water heaters: 1-2 people need 30-40 gallons, 3-4 people need 40-50 gallons, and 5 or more people need 50-75 gallons. But the first hour rating (FHR) matters more than tank size. FHR tells you how many gallons of hot water the unit can deliver in the first hour of use.
First hour rating (FHR) measures how many gallons of hot water a tank water heater can deliver in one hour starting with a full tank. A 40-gallon tank with a high FHR can outperform a 50-gallon tank with a low FHR. Check the yellow EnergyGuide label for the FHR number.
Tankless sizing is based on flow rate (GPM) and temperature rise. In the Philadelphia area, winter inlet water temperature is around 50°F, requiring a 60°F rise to reach 110°F at the tap. For 2 simultaneous fixtures, you need roughly 4-5 GPM. For 3+ fixtures, 6-8 GPM. A Navien NPE-240A handles most Philadelphia homes.
Tankless water heaters cost more upfront ($3,200-$5,800 installed vs $1,550-$3,200 for tank) but last 20+ years compared to 8-12 for a tank unit. They also save 20-30% on water heating energy by eliminating standby heat loss. Best for: high hot water demand, limited space, or long-term ownership.
We install Bradford White tank water heaters and Navien tankless units. Bradford White is made in the USA, sold exclusively through professional installers, and has the best warranty support in the industry. Navien is the top-selling tankless brand in North America with excellent cold-climate performance.
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