Sizing a Tankless Water Heater for Your Household

You bought a tankless water heater for endless hot water—but now two people can’t shower at the same time without someone getting an arctic blast. What went wrong?

In most cases: the unit wasn’t sized correctly. At H&H Heating and Air Conditioning, we see undersized tankless water heaters all the time. It’s one of the most common installation mistakes—and one of the most frustrating for homeowners.

How Tankless Sizing Works

Tank water heaters are sized by storage capacity (40, 50, 80 gallons). Tankless units are sized by flow rate—how many gallons per minute (GPM) they can heat.

The critical factor most people miss: temperature rise. A unit’s advertised GPM assumes specific conditions. In colder climates with colder incoming water, that same unit delivers significantly less hot water.

Understanding Temperature Rise

Temperature rise = desired hot water temp minus incoming water temp.

In the Richmond/Goochland area, groundwater averages 52-58°F depending on season (winter drops into the mid-40s). If you want 120°F water and your incoming water is 52°F, your unit needs to produce a 68°F temperature rise.

Here’s the problem: A tankless water heater rated for 9 GPM might only deliver 5-6 GPM at a 70°F temperature rise. Manufacturers advertise maximum GPM at ideal conditions (often 77°F incoming water)—not real-world Virginia winter conditions.

Typical Fixture Flow Rates

To calculate your needs, add up fixtures you’ll use simultaneously:

• Shower: 1.5-2.5 GPM
• Bathroom faucet: 0.5-1.5 GPM
• Kitchen faucet: 1.0-2.0 GPM
• Dishwasher: 1.0-2.5 GPM
• Washing machine: 1.5-3.0 GPM

Example: Two showers (4 GPM) + one faucet (1 GPM) = 5 GPM needed at your required temperature rise.

Sizing by Household Size

General guidelines for Richmond-area homes (accounting for our groundwater temperatures):

• 1-2 people: 6-8 GPM typically sufficient
• 2-4 people: 8-10 GPM recommended
• 4-6 people: 10+ GPM or consider multiple units
• 6+ people: Multiple units likely needed

These are starting points. Your actual usage patterns matter more than headcount.

Peak Demand Scenarios

Think about your worst-case realistic scenario:

• Morning rush: Multiple showers, running faucets
• Evening routine: Showers, dishwasher, laundry simultaneously
• Holiday gatherings: Extra guests using hot water
• Teenage children: Often longer, hotter showers

Size for your peak demand—not average use.

Gas vs. Electric: A Critical Difference

Gas tankless: 5-11 GPM capacity typical. Better for whole-house applications. Can handle multiple simultaneous demands. Our standard recommendation for most Richmond homes.

Electric tankless: 2-5 GPM capacity typical. Better for point-of-use (single bathroom, kitchen). May need multiple units for whole-house. Requires significant electrical upgrades.

For most families needing whole-house hot water, gas is the practical choice.

The Danger of Undersizing

An undersized unit means:

• Running out of hot water during simultaneous use
• Lukewarm water that never gets fully hot
• Unit constantly maxed out (premature wear)
• The “endless hot water” promise unfulfilled
• Possible costly replacement with a larger unit

Is Oversizing a Problem?

Oversizing costs more upfront and may cause short-cycling with low-demand uses, but it’s far less problematic than undersizing. If you’re between sizes, go larger.

Right-sizing is ideal—but when in doubt, size up.

When Multiple Units Make Sense

For larger homes or high-demand households, multiple tankless units may work better than one large unit:

• Point-of-use placement near distant fixtures reduces wait time
• Backup capability if one unit needs service
• Better coverage for sprawling floor plans

We often recommend a primary whole-house unit plus a smaller point-of-use unit for a distant bathroom or kitchen.

How We Size Your Tankless

At H&H, we calculate based on:

1. Richmond-area groundwater temperatures (not national averages)
2. Your specific fixtures and their flow rates
3. Realistic peak simultaneous demand
4. Your family’s actual routine
5. Future needs (growing family, planned additions)

We recommend the right-sized unit—not upselling you on more capacity than you need, and not undersizing to hit a price point.

Want to make sure your tankless water heater is sized right? Schedule a free consultation with H&H—we’ll calculate exactly what your home needs.

Frequently Asked Questions