Rheem Water Heater Sizing Calculator

Fuel Type Select

Brand Filter Locked

1. Groundwater & Target Temperatures ΔT = Target − Incoming

Incoming:

Winter Groundwater Temp

°F

Desired Output Temp

°F (120°F Code Standard)

2. Peak Hot Water Fixture Demand Select fixtures operating concurrently during morning rush

Active	Fixture Type	Qty	Flow (GPM)
	Standard Shower (2.0 GPM)
	Bathroom Sink Faucet (1.0 GPM)
	Kitchen Sink Faucet (1.5 GPM)
	Bathtub Filler (4.0 GPM)
	Dishwasher (1.5 GPM)
	Clothes Washer (2.0 GPM)

Required Peak Tankless Flow Rate

6.0 GPM at 70°F Rise

Metric Equivalent: 22.7 L/min at 38.9°C Rise

Required Heating Power 210,000 BTU/hr 61.5 kW Power

Groundwater Temperature Rise 70°F 50°F in → 120°F out

Estimated Electrical Draw 256 Amps 240V Dedicated Electrical Service

Total Connected Fixture Capacity 15.0 GPM If all home fixtures opened simultaneously

Your peak simultaneous hot-water demand is 6.0 GPM (22.7 L/min) with a 70°F (38.9°C) temperature rise (incoming 50°F → desired 120°F). To deliver this flow reliably, your Gas tankless heater must produce approximately 210,000 BTU/hr (61.5 kW). Ensure the chosen model provides at least 6.0 GPM at a 70°F rise.

Dynamic Flow vs. Temperature Rise Matrix Required BTU/hr (kW)

Temperature Rise (ΔT)	2.0 GPM	3.0 GPM	4.0 GPM	5.0 GPM	6.0 GPM
40°F Rise	40k BTU (11.7 kW)	60k BTU (17.6 kW)	80k BTU (23.4 kW)	100k BTU (29.3 kW)	120k BTU (35.2 kW)
50°F Rise	50k BTU (14.7 kW)	75k BTU (22 kW)	100k BTU (29.3 kW)	125k BTU (36.6 kW)	150k BTU (44 kW)
60°F Rise	60k BTU (17.6 kW)	90k BTU (26.4 kW)	120k BTU (35.2 kW)	150k BTU (44 kW)	180k BTU (52.8 kW)
70°F Rise	70k BTU (20.5 kW)	105k BTU (30.8 kW)	140k BTU (41 kW)	175k BTU (51.3 kW)	210k BTU (61.5 kW)
80°F Rise	80k BTU (23.4 kW)	120k BTU (35.2 kW)	160k BTU (46.9 kW)	200k BTU (58.6 kW)	240k BTU (70.3 kW)

Highlighted cell indicates your exact calculated demand requirement.

Recommended Compatible Water Heater Models Matches Your Exact Load

RHEEM

Rheem RTGH-95DVLN

Performance Platinum® Condensing Gas

5.3 GPM at 70°F Rise (9.5 GPM max)

199,900 BTU/hr 0.93 UEF GAS

High-output gas unit supporting up to 3-4 simultaneous showers.

RHEEM

Rheem RTGH-84DVLN

Performance Platinum® Condensing Gas

4.7 GPM at 70°F Rise (8.4 GPM max)

180,000 BTU/hr 0.93 UEF GAS

Efficient mid-size whole-house gas tankless for 2-3 bathrooms.

RHEEM

Rheem RTEX-27 (240V)

Performance® Electric Tankless

2.6 GPM at 70°F Rise (6.6 GPM max)

27.0 kW (112.5A) 0.99 Energy Factor ELECTRIC

High-power whole-house electric unit (Requires 3x 40A double-pole breakers).

RHEEM

Rheem RTEX-18 (240V)

Performance® Electric Tankless

1.8 GPM at 70°F Rise (4.4 GPM max)

18.0 kW (75A) 0.99 Energy Factor ELECTRIC

Compact electric unit for 1-2 bathroom apartments or warm southern climates.

Occupants	Low Demand	Average Demand	High Demand
1–2 People	30 Gal (115 L)	40 Gal (150 L)	50 Gal (190 L)
3–4 People	40 Gal (150 L)	50 Gal (190 L)	65–75 Gal (245–285 L)
5+ People	50 Gal (190 L)	65–75 Gal (285 L)	80+ Gal (300+ L)

Rheem Tank vs Tankless Sizing: Platinum & Gladiator Ratings

The Rheem Water Heater Sizing Calculator maps your household's peak water demand and regional climate to the official Rheem water heating product catalog. It evaluates Rheem Performance Platinum Condensing Gas Tankless (9.5 GPM, 8.4 GPM), Rheem RTEX Electric Tankless (13 kW to 36 kW), and Rheem Gladiator Smart Storage Tank models to ensure guaranteed hot water coverage and optimal energy efficiency. For comparing with Rinnai models, see our Rinnai sizing calculator. If you only need electric tankless calculations, explore our dedicated electric tankless water heater sizing calculator, or view overall sizing rules on our water heater size guide.

Step-by-Step Rheem Unit Sizing for Storage & On-Demand Flow

Select Technology & Fuel

Choose between Rheem Gas Tankless, Rheem RTEX Electric Tankless, or Rheem Gladiator Storage Tank.

Input Fixture or Household Count

Enter simultaneous shower/faucet counts for tankless sizing or total occupants for storage tank sizing.

Specify Climate Region

Set incoming winter groundwater temperature to determine required heating lift.

View Rheem Recommendations

Inspect exact Rheem model names, flow ratings, electrical/gas requirements, and EnergyGuide ratings.

Rheem Flow Curves, First Hour Rating (FHR) & UEF Standards

Rheem units are sized according to ASHRAE flow demand curves and DOE First Hour Ratings:

Rheem Product Lineup Reference Table

Rheem Model	Type & Input	Capacity Rating	Winter Output (70°F Rise)	Efficiency (UEF)
Performance Platinum 9.5	Gas Condensing (199k BTU)	9.5 GPM Max	5.3 GPM (3 Showers)	0.96 UEF
Performance 8.4	Gas Non-Condensing (180k BTU)	8.4 GPM Max	4.2 GPM (2 Showers)	0.82 UEF
RTEX-27 Electric	Electric Tankless (27 kW)	6.0 GPM Max	2.6 GPM (1-2 Showers)	0.99 UEF
Gladiator Smart 50 Gal	Gas Storage Tank (40k BTU)	50 Gallon Volume	84 GPH First Hour	0.68 UEF

Atlanta 3-Bath Case Study: Rheem Platinum 9.5 Sizing

Scenario: Sizing a Rheem tankless unit for a 3-bath house with 4.5 GPM peak simultaneous demand in Atlanta, GA (55°F groundwater).

Peak Flow Demand 4.5 GPM

Winter Temp Rise 65°F (55°F to 120°F)

Optimal Rheem Unit Rheem Performance Platinum 9.5

Continuous Output @ 65°F Rise 5.8 GPM

• Result: The Rheem Performance Platinum 9.5 GPM Condensing unit provides a comfortable 1.3 GPM safety buffer during winter morning usage.

Expert FAQ Guide

Frequently Asked Questions

6 Questions Answered

Q1 Which Rheem water heater size is right for my home?

For a family of 3–4 with standard tank preference, the Rheem Gladiator 50-Gallon (67–75 GPH FHR) is ideal. For tankless gas preference, the Rheem Performance Platinum 9.5 GPM (199k BTU) handles 2–3 simultaneous showers in cold climates.

Q2 What is the First Hour Rating (FHR) on Rheem Gladiator tank water heaters?

Rheem Gladiator 40-Gallon gas models deliver ~67 GPH FHR, 50-Gallon gas models deliver ~84 GPH FHR, and electric 50-Gallon models deliver ~62 GPH FHR, providing ample recovery for back-to-back morning showers.

Q3 How many breakers does a Rheem RTEX-27 electric tankless require?

The Rheem RTEX-27 (27 kW) draws 112.5 Amps at 240V and requires three dedicated 40-Amp double-pole circuit breakers connected with 8 AWG copper wiring.

Q4 What is Rheem EcoNet® technology?

Rheem EcoNet® is a built-in Wi-Fi monitoring and leak detection system that sends instant smartphone alerts, allows remote temperature control, and automatically shuts off incoming water if a leak is detected.

Q5 Can a Rheem 9.5 GPM tankless unit handle a 4-bathroom house?

In warm southern climates (65°F groundwater), a 9.5 GPM Rheem unit easily powers 3–4 showers at once. In cold northern climates (40°F water), it delivers ~4.9 GPM, supporting 2 simultaneous high-flow showers.

Q6 What warranty does Rheem offer on Performance Platinum and Gladiator models?

Rheem Gladiator tank models and Performance Platinum tankless models typically include a 12-year limited warranty on the tank/heat exchanger and parts, with 1-year in-home labor.

Rheem RTGH-95DVLN

Rheem RTGH-84DVLN

Rheem RTEX-27 (240V)

Rheem RTEX-18 (240V)

Rheem Tank vs Tankless Sizing: Platinum & Gladiator Ratings

Step-by-Step Rheem Unit Sizing for Storage & On-Demand Flow

Select Technology & Fuel

Input Fixture or Household Count

Specify Climate Region

View Rheem Recommendations

Rheem Flow Curves, First Hour Rating (FHR) & UEF Standards

Rheem Product Lineup Reference Table

Atlanta 3-Bath Case Study: Rheem Platinum 9.5 Sizing

Frequently Asked Questions

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