Water Heater Sizing Calculator
3 Bedroom House
Cold Water Temperature: 69.0
Fixtures and Appliances:
Showers: 3
Bath Faucets: 4
Kitchen Faucet: 1
Dishwasher: 1
Washer 0
Bathtub: 2
Roman Tub 1
Multihead Body Spa Shower GPM: 5.0 1
Peak GPM (99%): 5.0
Heater KW Requirement: 21.0 - 26.0
(Method 1:)
Heater KW Requirement: 22.0 - 26.0
(2.5-3.0 GPM@130°F)
(Method 2:)
This interactive online calculator is designed to assist trade professionals in creating a sizing estimate for
SEISCO tankless water heaters. The calculations herein are based on industry flow rate standards and
normal environmental usage factors. The data generated by the calculator is for illustrative use only. All
sizing should be reviewed by a trained professional and all applicable code and licensing requirements
must be considered. In using this calculator, the user agrees to take full responsibility for the output
created and to independently verify its results for a particular application. Please contact SEISCO
customer service with any questions regarding proper sizing for an application.
Instructions:
1. Enter Average Groundwater temperature from map on tab 2
2. Use the calculator for each hot water zone individually. Enter the number of fixtures/appliances in each zone
(For most homes we recommend a single zone. For larger homes, we recommend grouping
nearby fixtures and appliances to shorten distances and reduce wait for hot water or alternatively
use a demand recirculation loop)
(Note: Assumes 1/2 restriction on Roman Tub Fill)
(Note: Estimates Peak GPM for 99% of all hot water draws. 1% will exceed this flow rate)
3. Method 1: Calculated Recommendation
4. Method 2: 2.5-3.0 GPM of 130 °F Water
Midwestern Climate
Souttern Climate
Ground Water Temperature: 80 75 70 65 60
Temperature Rise to 105°F: 25 30 35 40 45
Model BTU KW GPM GPM GPM GPM GPM
RA14 47,782 14.0 3.8 3.2 2.7 2.4 2.1
RA16 54,608 16.0 4.4 3.6 3.1 2.7 2.4
RA18 61,434 18.0 4.9 4.1 3.5 3.1 2.7
RA22 75,086 22.0 6.0 5.0 4.3 3.8 3.3
RA28 95,564 28.0 7.6 6.4 5.5 4.8 4.2
RA32 109,216 32.0 8.7 7.3 6.2 5.5 4.9
2XRA22 150,172 44.0 12.0 10.0 8.6 7.5 6.7
2XRA28 191,128 56.0 15.3 12.7 10.9 9.6 8.5
2XRA32 218,432 64.0 17.5 14.6 12.5 10.9 9.7
Heat and Boost Options:
RA22+Tank¹¹ 90,445 26.5 7.2 6.0 5.2 4.5 4.0
RA28+Tank¹ 110,923 32.5 8.9 7.4 6.3 5.5 4.9
RA32+Tank¹ 124,575 36.5 10.0 8.3 7.1 6.2 5.5
¹ GPM Rate unlimited, listed rate is after tank is exhausted
Northern Climate HUD
Midwestern Climate
55 50 48 45 40
50 55 57 60 65 77
GPM GPM GPM GPM GPM GPM
1.9 1.7 1.7 1.6 1.5 1.2
2.2 2.0 1.9 1.8 1.7 1.4
2.5 2.2 2.2 2.0 1.9 1.6
3.0 2.7 2.6 2.5 2.3 2.0
3.8 3.5 3.4 3.2 2.9 2.5
4.4 4.0 3.8 3.6 3.4 2.8
6.0 5.5 5.3 5.0 4.6 3.9
7.6 7.0 6.7 6.4 5.9 5.0
8.7 7.9 7.7 7.3 6.7 5.7
3.6 3.3 3.2 3.0 2.8
4.4 4.0 3.9 3.7 3.4
5.0 4.5 4.4 4.2 3.8