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Water Heater Sizing Guidelines

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How to determine the size or capacity of hot water heating equipment needed at a building.

The peak hot water demand rate and number of hot water users as well as a number of other factors determine the necessary water heating capacity or hot water storage for buildings.

This article includes tables of typical hot water daily usage volume and hot water peak use rates by building type for residential and commercial buildings, drawing on several expert sources and a range of hot water use research articles.

This article series explains how to improve the total quantity of hot water available from your water heating system. Here we discuss How much hot water do we get out of a hot water heater tank? How to improve hot water water quantity from any water heater.

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Water Heater Size / Capacity Requirements

Rheem tankless water heater (C) D FriedmanHow to determine the size of water heating equipment needed for a residential or commercial building.

Quantity of hot water means the total volume of hot water available. Flow rate of hot water, is the gallons per minute of hot water delivered somewhere (out of the heater or at a plumbing fixture).

Hot water pressure, as most folks mean it, refers to the flow rate at the fixture.

Question: what is the number of water heaters required in 3 story commercial building

2017/09/14 anil said:

Can anyone give comments on number of water heaters required in 3 story commercial building?

This question was posted originally at HOT WATER CIRCULATOR PUMP SPEEDS DELIVERY



Interesting question but not one that I could answer in a useful way as I have not a shred of information about your specific building, its intended use, number of occupants, number of bathrooms. You would work in a different sequence of than just an arbitrary number. Below we provide two tables that will help you out:



The National Renewable Energy Laboratory cited below has provided the following table showing the estimated gallons of hot water usage per person per day for U.S. buildings. Hot water usage requirements for your building could be very different as you'll see by the hot water requirements questions I pose below.

Hot Water Usage by Building Type1

Building Type Estimated Hot Water Use Per Person Per Day Comments
Hospital 52 43 197  
Hotel / Motel 20 16.7 76  
House 15.8 13.1 60 Typical one-family residence
Office 1.1 0.9 4.1  
Restaurant 2.4 2.0 9  
School 0.5 0.4 2  
School with Showers 1.9 1.6 7.2  

Notes: adapted from the NREL source given below.

Source: DOMESTIC HOT WATER ASSESSMENT GUILDELINES [PDF] National Renewable Energy Lab (NREL), 1617 Cole Blvd., Golden CO 80401 USA Tel: 303-275-3000 Website:, retrieved 2017/09/15, original source:

Watch out: the data above describes estimates of total hot water consumption per person per day but it does not reflect the peak hot water demand rate. In hospitals, hotels, motels, restaurants, schools and other buildings, or special areas within those buildings (such as a kitchen or laundry), satisfying the peak hot water demand usage rate can require greater water heating capacity.

See the next table below to find examples of peak hot water demand rates in gallons or litres per hour.

Table of Peak Hot Water Service Demand in Buildings1

Space Type Hot Water
Usage Rate per Hour
Point of Use2
Data Sources
Gal/H L/H °F °C
Guest room, small hotel 1.75 6.6 110 43 Jiang et al. 2008, ASHRAE 2007
Guest room, large hotel 1.25 4.7 Jiang et al. 2008, ASHRAE 2007
Laundry, small hotel 67.5 255.5 140 60 Jiang et al. 2008, ASHRAE 2007
Laundry, large hotel 156.6 592.8 Jiang et al. 2008, ASHRAE 2007
Restrooms (primary school) 56.5 214.0 110 43 ASHRAE 2007
Restrooms (secondary school) 104.4 395.0 ASHRAE 2007
Gym (secondary school) 189.5 717.2 ASHRAE 2007
Small office 3.0 11.4 Jarnagin et al. 2006, ASHRAE 2007
Medium office (per floor) 9.9 37.5 Jarnagin et al. 2006, ASHRAE 2007
Large office (per floor) 21.3 80.6 Jarnagin et al. 2006, ASHRAE 2007
Apartment 3.5 13.2 Gowri et al. 2007
Outpatient healthcare 30.0 113.5 Doebber et al. 2009
Hospital Hot Water Requirements  
ER waiting room 1.0 3.8 120 49 Engineering judgment1
Operating / surgical cytoscopic 2.0 7.6 Engineering judgment1
Laboratory 2.0 7.6 Engineering judgment1
Patient room 1.0 3.8 Engineering judgment1


1. This InspectApedia table was excerpted / adapted from: Deru, Michael, Kristin Field, Daniel Studer, Kyle Benne, Brent Griffith, Paul Torcellini, Bing Liu, Mark Halverson, Dave Winiarski, Michael Rosenberg, Mehry Yazdanian, Joe Huang, Drury Crawley COMMERCIAL REFERENCE BUILDING MODELS OF THE NATIONAL BUILDING STOCK [PDF], Technical Report NREL/TP-5500-46861 February 2011, (Page 17),

National Renewable Energy Lab (NREL), 1617 Cole Blvd., Golden CO 80401 USA Tel: 303-275-3000 Website:, retrieved 2017/09/15, original source:

The authors provide the underlying assumptions for the peak hot water demand rates in the table above. They also provide data describing the total water usage per occupant or visitor that differs from our previous table, and the authors give their assumptions about the building size in floor area, number of floors and other factors.


The peak service hot water (SHW) demand for each reference building space type is shown in [the table above], along with the temperatures at the fixture and the sources of data.

These numbers are used in conjunction with the operating schedules to estimate the total hot water consumption.

Hot water use in kitchens is included in the Commercial Kitchen section. Natural gas water heaters are used in all cases; storage tanks are kept at 140ºF (60ºC).


Hot Water Requirements Basic Design Questions

Questions that need to be addressed when estimating the total hot water capacity usage and then how that hot water is produced, stored, and distributed in a building include at the very least the following:

You need to find both the total hot water volume required over a given period of time and how it needs to be distributed: to how many locations?

Then you'd decide between a single large central water heater or multiple individual water heaters and their locations.

In the United States the US DOE offers some advice on "sizing a water heater" that might help you out. Below we provide several references and guides for choosing the water heating size or capacity or flow rate for both commercial and residential buildings.

Water Heater Sizing Guides & Tables for Water Heating Equipment, Calorifiers, Hot Water Cylinders, Geysers


Continue reading at HOT WATER QUANTITY IMPROVEMENT - how to get more hot water, or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see PLUMBING DRAIN FIXTURE UNITS DFUs - measure of the wastewater volume or loading of a drain waste or vent system

Or see SEWAGE FLOW & DESIGN FLOW ESTIMATES - plumbing fixture flow rates


Or see WATER QUANTITY USAGE TABLES - daily water usage & peak water usage per person

Or see these

Hot Water Quantity & Flow Improvement Articles

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WATER HEATER SIZE SPECIFICATION at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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