Gas Tankless Water Heater Sizing Calculator

Fuel Type Locked

Brand Filter Brand

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

RINNAI

Rinnai RX199iN / RXP199iN

SENSEI® RX Series (Condensing)

5.5 GPM at 70°F Rise (11.1 GPM max)

199,000 BTU/hr 0.98 UEF GAS

Best for large households (3-4 simultaneous showers in cold climates).

RINNAI

Rinnai RX160iN

SENSEI® RX Series (Condensing)

4.4 GPM at 70°F Rise (9 GPM max)

160,000 BTU/hr 0.97 UEF GAS

Ideal for standard 2-3 bathroom homes with moderate to cold groundwater.

RINNAI

Rinnai RX130iN

SENSEI® RX Series (Condensing)

3.6 GPM at 70°F Rise (7 GPM max)

130,000 BTU/hr 0.96 UEF GAS

Ideal for 1-2 bathroom homes or warmer climate zones.

RINNAI

Rinnai V65iN

High Efficiency Plus (Non-Condensing)

3.8 GPM at 70°F Rise (6.5 GPM max)

150,000 BTU/hr 0.82 UEF GAS

Cost-effective non-condensing gas solution for small-to-medium demand.

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)

Natural Gas & Propane Tankless Sizing: Thermal BTU Demands

The Gas Tankless Water Heater Sizing Calculator computes the necessary natural gas or liquid propane (LP) burner input (in BTU/hr) to provide continuous whole-house hot water. It accounts for temperature differentials between incoming groundwater and desired tap temperature, evaluating high-efficiency condensing ($95\%+$ UEF) vs standard non-condensing units and verifying fuel supply line capacity. If evaluating electric options instead, compare power requirements with our electric tankless water heater sizing calculator, or see baseline flow sizing on our universal tankless water heater calculator. For fuel line purging and volume calculations, use our gas pipe volume calculator.

Step-by-Step Gas Supply Sizing & Temperature Rise Selection

Select Fixture Demand

Choose simultaneous bathroom showers, kitchen faucets, and high-demand appliances to determine peak GPM.

Set Winter Groundwater Temperature

Enter coldest incoming municipal or well water temperature (e.g. 40°F North, 50°F Central, 65°F South).

Pick Efficiency Class (Condensing vs Non-Condensing)

Compare 95% condensing (PVC vented) against 82% non-condensing (stainless steel Category III vented).

Review Output & Gas Line Sizing

Inspect total BTU/hr demand, CFH gas meter volume, and recommended gas supply pipe diameter.

Burner Thermodynamic Formulas & Gas Pipe Line Diameters

Gas burner demand is calculated by converting fluid thermal load divided by burner efficiency:

BTU/hr = (GPM × ΔT × 500.4) / Thermal Efficiency

CFH (Natural Gas) = (BTU/hr) / 1030

Where 1 ft³ of natural gas delivers approximately 1,030 BTU of energy.

Gas Tankless Capacity by BTU/hr & Climate Zone

Burner Rating	Gas Line Req	Cold Climate (75°F Rise)	Moderate Climate (60°F Rise)	Warm Climate (45°F Rise)
120,000 BTU/hr	1/2" or 3/4"	3.0 GPM	3.8 GPM	5.1 GPM
160,000 BTU/hr	3/4"	4.1 GPM	5.1 GPM	6.8 GPM
180,000 BTU/hr	3/4"	4.6 GPM	5.7 GPM	7.6 GPM
199,000 BTU/hr	3/4" (or 1")	5.1 GPM	6.3 GPM	8.5 GPM

Cold Climate Whole-House 199k BTU Sizing Case Study

Scenario: Sizing a gas tankless water heater for a 4-bedroom house with 5.5 GPM simultaneous demand in Minneapolis (winter water temperature 42°F).

Peak Flow Demand 5.5 GPM

Temperature Lift (ΔT) 78°F (42°F to 120°F)

Condensing Gas Demand 199,000 BTU/hr

Gas Volume Consumption 193 CFH (3/4" Gas Line)

• Heat Load: 5.5 × 78 × 500.4 / 0.96 = 223,800 BTU/hr

• Result: Max residential 199,000 BTU/hr unit will deliver 4.9 GPM in winter, sufficient for two full high-pressure showers and a faucet simultaneously.

Expert FAQ Guide

Frequently Asked Questions

6 Questions Answered

Q1 How many BTU do I need for a gas tankless water heater?

Required BTU/hr is calculated as: BTU = (GPM × Temperature Rise × 500.4) / Thermal Efficiency. For 5.0 GPM at a 70°F rise with a 95% condensing unit, you need approximately 184,000 BTU/hr (standard 199,000 BTU/hr residential maximum).

Q2 What is the difference between condensing and non-condensing gas tankless?

Condensing units feature secondary heat exchangers achieving 93%–97% efficiency, allowing cool exhaust to be vented through inexpensive PVC/polypropylene pipe. Non-condensing units operate at 80%–84% efficiency and require expensive, specialized Category III stainless steel venting.

Q3 What size gas line does a gas tankless water heater require?

Most whole-home units (160k–199k BTU) require a dedicated 3/4-inch gas line. If the line run is under 10–20 feet and gas meter supply pressure is adequate (up to 2 psi), some newer models can use 1/2-inch lines, but 3/4-inch is strongly recommended.

Q4 Can a gas tankless run on propane (LP)?

Yes. Most gas tankless water heaters come in both Natural Gas (NG) and Liquid Propane (LP) configurations, or offer manufacturer conversion kits. Propane delivers approximately 2,500 BTU/cu ft compared to ~1,030 BTU/cu ft for natural gas.

Q5 Why is 199,000 BTU/hr the common maximum for residential gas tankless?

Under US plumbing and mechanical codes, units rated at 200,000 BTU/hr or greater fall under commercial boiler standards, which carry much stricter inspection, ASME certification, and permitting requirements.

Q6 Do gas tankless water heaters work during power outages?

No, unless connected to a battery backup or generator. While gas heats the water, electricity is required to power the electronic ignition, exhaust vent blower fan, flow sensors, and digital controls.

Rinnai RX199iN / RXP199iN

Rinnai RX160iN

Rinnai RX130iN

Rinnai V65iN

Rheem RTGH-95DVLN

Rheem RTGH-84DVLN

Rheem RTEX-27 (240V)

Rheem RTEX-18 (240V)

Natural Gas & Propane Tankless Sizing: Thermal BTU Demands

Step-by-Step Gas Supply Sizing & Temperature Rise Selection

Select Fixture Demand

Set Winter Groundwater Temperature

Pick Efficiency Class (Condensing vs Non-Condensing)

Review Output & Gas Line Sizing

Burner Thermodynamic Formulas & Gas Pipe Line Diameters

Gas Tankless Capacity by BTU/hr & Climate Zone

Cold Climate Whole-House 199k BTU Sizing Case Study

Frequently Asked Questions

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Water Heater Size Guide

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Tankless Sizing for 4 Bathroom House

Tankless Water Heater Sizing Chart