Demand Breakdown
Full Calculation Table
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How to Use the Domestic Hot Water Calculator
Enter the quantity of each fixture type in the building. Each fixture has a published fixture unit value reflecting its typical hot water draw relative to a standard lavatory faucet, similar in spirit to how the fitting equivalent length calculator weights different fitting types by their effect.
If the building has a hot water recirculation loop, enable it and enter the loop length and pipe heat loss rate from the pipe insulation heat loss calculator. Recirculation sizing is separate from peak demand sizing, since it runs continuously at a much lower flow rate.
Click Calculate DHW Demand to see total fixture units, estimated peak demand flow rate, and recirculation flow rate if enabled. Use the peak demand result on the pipe sizing calculator to size the main supply line.
Sizing Domestic Hot Water Systems by Fixture Demand
Domestic hot water demand does not scale linearly with fixture count, because not every fixture in a building draws hot water at the same moment. A house with five bathrooms rarely has all five showers running simultaneously, and a commercial building with forty lavatories almost never sees every one in use at once. The fixture unit method captures this by weighting each fixture type by its typical draw, then converting total fixture units into a realistic peak demand using a published curve rather than simply summing every fixture's maximum flow rate.
How Fixture Units Work
Each fixture type gets a fixture unit value based on its typical hot water flow relative to a baseline fixture, usually a private lavatory faucet set at a value of 1. A bathtub or shower might be worth 1.5 to 2 fixture units, while a kitchen sink or dishwasher in a commercial kitchen could be worth more, reflecting their higher typical draw. Adding up the fixture units for every fixture in a building gives a total that this calculator converts to an estimated peak demand flow rate using a published diversity curve, the same general approach the National Institute of Standards and Technology's original 1940 fixture unit study by Roy B. Hunter established and that plumbing codes still use today, applied here specifically to hot water.
From Peak Demand to Pipe Sizing
Once you have peak demand flow rate, it becomes the design flow rate for sizing the main hot water supply piping, the same way a heating load becomes a design flow rate on the GPM calculator. Take this number to the pipe sizing calculator to find a recommended pipe diameter, since the main DHW supply needs to handle this peak flow without excessive velocity or pressure drop, even though that peak only happens for short periods during the day. The same peak figure is also a useful starting input for water heater selection, alongside recovery rate and storage volume; the solar water heater calculator walks through that sizing process for a solar-assisted system specifically.
Why Recirculation Is a Separate Calculation
A recirculation loop's job is keeping hot water close to every fixture so users do not wait for cold water to clear the line, and it does this by continuously circulating a small flow rate, not by matching the building's peak demand. Recirculation flow rate is sized from the loop's heat loss rather than fixture count: enough flow needs to move through the loop to replace the heat lost through the insulated pipe wall, a rate you can pull from the pipe insulation heat loss calculator, without the water cooling below an acceptable temperature by the time it returns. That acceptable temperature is not just a comfort question; Health Canada's guidance on water temperature and scald prevention recommends storing water at 60°C to control Legionella growth while limiting delivered temperature at the fixture to around 49°C, which is exactly the kind of balance a recirculation loop needs to maintain across its whole length. This is why recirculation piping is almost always much smaller than the main supply line, and why sizing it using peak demand flow would produce an oversized, underused pipe. Once sized, the recirculation pump itself can be selected on the pump sizing calculator using this flow rate and the loop's total head.
A Note on Accuracy
This calculator uses commonly published fixture unit values and a simplified demand curve for estimating purposes. Plumbing codes in different Canadian provinces may specify their own fixture unit tables or demand curves for code compliance, so always confirm final sizing against the applicable provincial plumbing code and CSA B125 fixture standards for permit-stamped work. Commercial buildings pursuing energy code compliance can cross-check overall system efficiency assumptions against the ASHRAE 90.1 calculator, since DHW distribution losses factor into that broader energy picture. Backflow prevention on any DHW makeup connection should also be checked against the backflow preventer sizing calculator.
Frequently Asked Questions
The fixture unit method assigns each plumbing fixture a weighted value representing its typical hot water draw, then converts the total fixture units for a building into an estimated peak demand using a published conversion curve. This accounts for the fact that not every fixture runs at the same time, so peak demand is far less than the sum of every fixture's maximum flow rate running simultaneously.
A DHW recirculation loop runs continuously or on a timer to keep hot water available at fixtures without a long wait, which means it operates at a much lower flow rate than the building's peak demand piping. Sizing recirculation piping using peak demand flow would produce an oversized pipe that runs at very low velocity most of the time, so recirculation loops are sized separately based on heat loss from the loop rather than fixture demand.
This calculator estimates peak flow demand for pipe sizing, which is related to but not the same as water heater sizing. Water heater sizing also needs to account for recovery rate, storage volume, and the duration of peak demand periods, since a smaller heater with fast recovery can sometimes serve the same peak demand as a larger heater with slower recovery. Check the manufacturer's sizing guide using this calculator's peak demand figure as one input among several.
Commercial fixtures, especially in restaurants, gyms, and healthcare facilities, often have higher hot water draw than equivalent residential fixtures and may warrant different fixture unit values or additional load for kitchen equipment, laundry, or other process loads outside the typical fixture list. For commercial buildings with non-standard hot water uses, supplement this calculator's fixture count with the manufacturer's specific equipment hot water demand data.
Related HVAC Tools
Size from peak demand flow
Convert flow units
Recirc loop heat loss rate
Size the recirc pump
DHW makeup connections
Check supply line velocity
Larger DHW systems
CSA B125 fixture standards