How to Size Tankless Water Heaters for a Laundromat

Why laundromat water-heater sizing is different

Sizing water heaters for a laundromat is not the same as sizing water heaters for apartments, showers, restaurants, or a small commercial bathroom group. A laundromat has a large connected water load, but only a fraction of that load is hot water, and an even smaller fraction is simultaneous hot-water demand.

That distinction matters.

If you size a tankless water-heating system by adding up every washer’s maximum hot inlet flow, you will almost certainly oversize the system. If you size it by guessing too low because “most people use cold,” you risk slow fills, customer complaints, and nuisance machine errors. The right approach is to size for realistic peak hot-water demand, then verify that design against plumbing code, fuel-gas code, manufacturer requirements, and local authority-having-jurisdiction requirements.

Modern laundromats also change the math because customer behavior has changed. In many vended stores, fewer than 20% of customers choose true hot-water cycles. When the store offers ozone sanitation, hot-water usage can fall even further because ozone laundry systems are built around ambient or cold-water performance; the International Ozone Association notes that ozone is effective in laundry systems in ambient-temperature water and decomposes rapidly at higher conventional wash temperatures.

That means tankless sizing should be based on actual use, not fear-based connected load.

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The core sizing rule

A tankless water heater must be sized by two variables:

1. Flow rate, in gallons per minute.
2. Temperature rise, in degrees Fahrenheit.

The standard heat formula is:

BTU/hr output = GPM × 500 × Temperature Rise

The “500” factor comes from the weight of water and the number of minutes in an hour.

For example, if winter incoming water is 50°F and the laundromat needs 120°F water at the washer header, the system needs a 70°F rise.

At a 70°F rise:

1 GPM requires about 35,000 BTU/hr of heat output

So a heater or heater rack delivering 20 GPM at a 70°F rise needs:

20 × 500 × 70 = 700,000 BTU/hr output

The Building America Solution Center gives the same basic selection method: determine the practical maximum flow rate needed at one time, determine the required temperature rise from winter cold-water temperature to the desired hot-water setpoint, and then select equipment that can deliver that flow at that rise.

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Example laundromat: 22 Wascomat WUD washers

For this example, assume a laundromat with:

The WUD specification sheet lists the relevant washer capacities, approximate water use per full-load cycle, inlet flow rates at 45 psi, and recommended water pressure of 30–90 psi; it also notes that WUD machines use Automatic Savings to adjust water to load weight, so the full-load gallons shown above are a conservative sizing reference, not necessarily every-cycle consumption.

The important number here is not 597 gallons. That is total washer water per full-load turnover. It includes cold fills, rinses, and program-dependent water.

The dangerous number is also not 257 GPM. That is the theoretical connected hot-fill capacity if every washer’s hot valve were open at once. Designing a tankless system for 257 GPM at a 70°F rise would require roughly:

257 × 500 × 70 = 8,995,000 BTU/hr output

That would be an extreme oversize for a self-service laundromat where fewer than 20% of customers are choosing hot-water loads.

The correct design question is:

How much of that connected hot-fill capacity is realistically active during peak demand?

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The demand-factor method

For laundromats, a practical sizing formula is:

Design hot GPM = Connected hot-fill GPM × Hot-cycle selection factor × Fill coincidence factor

Where:

Connected hot-fill GPM is the total potential hot inlet flow from the washers.

Hot-cycle selection factor is the percentage of customers using true hot-water cycles. For this article, we are using the operating assumption that fewer than 20% of customers select hot water.

Fill coincidence factor accounts for the fact that washers do not fill continuously for the entire cycle. They fill in bursts, and not every hot-water customer starts at exactly the same second.

For a conservative laundromat calculation, use a fill coincidence factor between 0.35 and 0.50 unless you have store data showing otherwise.

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Sizing calculation for the example store

The example laundromat has 257 GPM of connected hot-fill capacity.

Scenario 1: No ozone, conservative hot-water usage

Assume:

• 20% of customers select hot water.
• 50% fill coincidence during a busy period.
• 70°F temperature rise.

Calculation:

257 GPM × 0.20 × 0.50 = 25.7 GPM design hot flow

BTU/hr output required:

25.7 × 500 × 70 = 899,500 BTU/hr output

A common commercial condensing tankless unit rated around 199,000 BTU/hr input at roughly 95% efficiency delivers about 189,000 BTU/hr output. At a 70°F rise, that equals roughly:

189,000 ÷ 35,000 = 5.4 GPM per unit

So:

25.7 ÷ 5.4 = 4.8 units

That points to five 199,000 BTU/hr tankless units for a strong no-storage, all-tankless design.

A slightly less conservative but still reasonable calculation uses 35% fill coincidence:

257 × 0.20 × 0.35 = 18.0 GPM

BTU/hr output:

18.0 × 500 × 70 = 630,000 BTU/hr output

That points to roughly four 199,000 BTU/hr tankless units.

Practical recommendation without ozone

For this exact mix of WUD730, WUD745, WUD760, and WUD775 machines, a defensible non-ozone tankless design is typically:

Four to five commercial condensing tankless units, depending on whether the design includes storage/buffer capacity and how conservative the owner wants to be.

Use five units when the system is expected to handle peak fill events with little or no storage.

Use four units plus properly sized storage or buffer capacity when the system is staged well, the store has verified low hot-water selection, and the owner accepts slightly slower recovery during rare peaks.

For redundancy, add an N+1 strategy where the store can still operate acceptably if one heater is down.

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Scenario 2: Ozone sanitation store

Ozone changes the demand profile. It does not eliminate the need for hot water, but it can significantly reduce hot-water usage because ozone laundry is commonly designed around cold or ambient-temperature washing. EPRI describes ozone laundering as using primarily cold water for light to moderately soiled laundry, because cold water holds ozone better and heat destroys ozone; it also notes that heavier-soil applications may still use 100–110°F water depending on site needs.

For an ozone laundromat, the hot-cycle selection factor may be closer to 5–10%, depending on pricing, signage, machine programming, customer education, and whether the store still actively promotes “hot” as a premium wash option.

Ozone sizing at 10% hot-water usage

257 × 0.10 × 0.50 = 12.85 GPM

BTU/hr output at 70°F rise:

12.85 × 500 × 70 = 449,750 BTU/hr output

That points to roughly:

449,750 ÷ 189,000 = 2.4 units

So a practical design would be three 199,000 BTU/hr units.

Ozone sizing at 5% hot-water usage

257 × 0.05 × 0.50 = 6.43 GPM

BTU/hr output at 70°F rise:

6.43 × 500 × 70 = 225,000 BTU/hr output

That points to roughly:

225,000 ÷ 189,000 = 1.2 units

So the practical design range becomes two units, especially if a small storage/buffer tank is included and actual usage is monitored.

Practical recommendation with ozone

For this example store, assuming the ozone system is properly installed, promoted, and programmed:

Three commercial condensing tankless units is the safer ozone-based design. Two units may work when hot-water usage is documented below roughly 5–7% and the system includes buffering, good staging, and owner acceptance of reduced redundancy.

Do not size an ozone store as if hot water is gone forever. Customer behavior changes. Winter inlet water is colder. New customers may still select hot. Specialty loads, maintenance cycles, and owner preferences may still require hot water.

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Summary sizing table

This table assumes 50°F winter incoming water, 120°F delivered hot water, a 70°F rise, and approximately 189,000 BTU/hr output per 199,000 BTU/hr input condensing tankless unit. Always use the selected manufacturer’s certified flow chart at the actual design temperature rise.

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Why tankless brochures can mislead laundromat owners

Many tankless heaters advertise high GPM ratings, but those ratings are often based on a mild temperature rise. A unit that appears to deliver 10–11 GPM at a 35°F rise may only deliver about half that flow at a 70°F rise.

For laundromats, winter incoming water temperature is the deciding condition. A store in a cold climate may need a 75–85°F rise. A store in a warmer climate may need only a 55–65°F rise. That difference can change the heater count.

The right process is:

1. Confirm winter incoming water temperature.
2. Choose the required washer hot-water delivery temperature.
3. Calculate temperature rise.
4. Determine realistic simultaneous hot-fill flow.
5. Select tankless equipment based on the manufacturer’s flow chart at that exact rise.

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Tankless-only vs. tankless with storage

A pure tankless rack must satisfy peak instantaneous flow. That works well when the heater count is high enough and the system has adequate gas, venting, water pressure, and staging.

A tankless-plus-storage design can be better for laundromats because washer fills are short bursts, not steady showers. A small commercial storage or buffer tank can absorb brief peaks, reduce burner short-cycling, and help maintain stable header temperature.

For the example store:

• Five tankless units, no storage is a strong conservative non-ozone design.
• Four tankless units with storage/buffer may be a better cost-performance design.
• Three tankless units with ozone is a strong practical design.
• Two tankless units with ozone and storage may be acceptable only with verified low hot-water selection.

The more aggressive the sizing, the more important it becomes to monitor hot-water selection, tankless run time, inlet temperature, and customer complaints.

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Building code and installation requirements

Final code compliance depends on the local authority having jurisdiction. Model codes become enforceable only when adopted into law by a jurisdiction, often with local amendments. The International Fuel Gas Code introduction explains that model codes are incorporated by reference by the AHJ, and local law includes the amendments enacted by that jurisdiction.

A laundromat tankless installation normally touches plumbing, fuel gas, mechanical, electrical, and sometimes energy code requirements.

1. Plumbing code

Most U.S. jurisdictions use some version of the International Plumbing Code, Uniform Plumbing Code, state plumbing code, or local amendment. IAPMO describes the Uniform Codes as setting standards so plumbing assemblies, materials, and technologies are safe and effective, and makes the 2024 Uniform Plumbing Code available for online reading.

Key plumbing-code issues include:

• Water heater listing and installation per manufacturer instructions.
• Maximum domestic hot-water temperature limits.
• Temperature and pressure relief protection where required.
• Relief-valve discharge piping.
• Thermal expansion control.
• Drain pans where leakage could cause damage.
• Service access.
• Backflow protection where chemical injection or cross-connection risk exists.
• Proper pipe sizing and pressure maintenance to the washers.

The IPC water-heater chapter governs the design and installation of water heaters and related safety devices, and IPC language for tankless domestic water heaters includes a 140°F maximum for domestic-use water from tankless heaters.

2. Relief valves and discharge piping

Relief-valve discharge piping is not a casual detail. ICC’s CodeNotes on water-heater safety explains that IPC Section 504.6 and IRC Section P2804.6.1 address discharge piping for pressure relief, temperature relief, or combination relief valves, including air-gap termination, full-size discharge, gravity drainage, visible termination, no trapped sections, and no valves or threaded caps at the outlet.

Even when a tankless model uses a pressure relief valve instead of a storage-type T&P arrangement, the manufacturer’s installation instructions and local code determine the required safety-valve arrangement.

3. Thermal expansion

Closed plumbing systems need thermal expansion control. Watts notes that both IPC and UPC require thermal expansion to be addressed, and that a T&P relief valve is not considered a thermal expansion device.

This matters in laundromats because backflow preventers, pressure-reducing valves, and check valves can create a closed system. Do not rely on dripping relief valves as pressure control.

4. Fuel-gas code

Gas-fired tankless systems can create large input loads quickly. Five 199,000 BTU/hr tankless units represent roughly 995,000 BTU/hr of gas input before dryers are counted.

That is a major point in laundromats, because the dryers often dominate total gas demand.

The International Fuel Gas Code regulates fuel-gas distribution piping, appliances, appliance venting, and combustion-air provisions; it is intended to protect life and property from hazards associated with fuel gas and combustion byproducts.

The IFGC also states that gas piping systems are sized to supply maximum demand while maintaining the supply pressure needed for safe appliance operation.

Before selecting the final tankless count, confirm:

• Gas meter capacity.
• Gas service pressure.
• Regulator capacity.
• Header sizing.
• Longest developed pipe length.
• Dryer gas load.
• Water-heater gas load.
• Future expansion allowance.

In some jurisdictions, the National Fuel Gas Code, ANSI Z223.1/NFPA 54, is the governing fuel-gas standard; the 2024 edition was ANSI-approved and became available in January 2024.

5. Venting and combustion air

Tankless heaters must be vented exactly as listed and as approved by the manufacturer. The IFGC’s venting chapter covers chimneys, vents, venting systems, direct-vent appliances, side-wall mechanically vented appliances, and the hazards associated with hot, toxic, or corrosive combustion gases.

For condensing tankless systems, also plan for:

• Approved vent material.
• Maximum vent length.
• Intake-air location.
• Exhaust termination clearance.
• Common-vent approval, if used.
• Condensate drainage.
• Condensate neutralization where required.
• Freeze protection.
• Maintenance access.

Building America’s guidance also emphasizes confirming sufficient gas supply, required clearances, venting, condensate removal for condensing units, local-code compliance, manufacturer instructions, required valves, expansion tanks, and piping.

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The biggest sizing mistakes

Mistake 1: Sizing from total washer gallons

The example store can use about 597 gallons of water per full-load turnover, but that is not the tankless flow requirement. Much of that water is cold, and the washers do not draw it all at once.

Mistake 2: Sizing from total connected hot-fill flow

The connected hot-fill flow is 257 GPM, but designing for every hot valve open at once would create a massively oversized system.

Mistake 3: Ignoring actual customer behavior

The most important operating fact is that fewer than 20% of customers use hot-water loads. In an ozone store, that number can drop significantly. Treat hot selection as a measurable variable, not a guess.

Mistake 4: Ignoring winter inlet temperature

A tankless system that works in summer may underperform in winter if it was sized using average incoming water temperature.

Mistake 5: Forgetting the gas service

A tankless rack may be correctly sized hydraulically but impossible to fire properly if the gas meter, regulator, or pipe sizing is inadequate.

Mistake 6: No storage, no redundancy, no service plan

Commercial laundromats need uptime. Multiple staged units, isolation valves, unions, service valves, water treatment, and redundancy should be designed from the beginning.

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Recommended design approach for this laundromat

For the example laundromat with:

• 5 WUD730 washers
• 5 WUD745 washers
• 6 WUD760 washers
• 6 WUD775 washers

The practical sizing conclusion is:

Without ozone

Use a design range of 18–26 GPM of hot-water production at winter design temperature rise.

That typically means:

Four to five 199,000 BTU/hr commercial condensing tankless units

Use the lower end only when the store has storage/buffer capacity, good staging, and documented low hot-water selection.

With ozone sanitation

Use a design range of roughly 6–13 GPM of hot-water production at winter design temperature rise, depending on actual customer selection.

That typically means:

Two to three 199,000 BTU/hr commercial condensing tankless units

For a new ozone installation without long-term operating data, three units is the safer recommendation. For a proven store where hot selections are consistently under 5–7%, two units with storage/buffer capacity may be defensible.

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Final rule of thumb

For laundromats, tankless water-heater sizing should be based on:

Peak realistic hot-fill demand, not total washer capacity and not total connected inlet flow.

The example store has 257 GPM of connected hot-fill potential, but the actual tankless design target is more likely:

• 18–26 GPM without ozone
• 6–13 GPM with ozone and documented low hot-water use

The best design is not simply “more BTUs.” It is a balanced system with the right number of staged heaters, adequate gas supply, proper venting, code-compliant safety devices, sufficient water pressure, scale control, and enough redundancy to keep the laundromat operating during service events.

Bottom line: For this WUD mix, a non-ozone store should generally plan around four to five commercial tankless units, while an ozone-equipped store can often justify two to three units—provided the owner documents hot-water usage and the installer designs the system to local code, manufacturer specifications, and winter incoming-water conditions.