The Boiler Room Blog

Advice on commercial water heater selection, maintenance and safety issues for businesses that rely on a steady supply of hot water.

Correct Commercial Water Heater Sizing: It’s No Guessing Game

how to choose commercial water heater options

When you’re in the market for a new or replacement commercial water heater, correct sizing is crucial to the success of your business. With so many commercial water heater options out there, here’s what you need to know to get it right.

Depending on your commercial hot water usage requirements, there may be multiple water heater tank sizing options to meet your needs. With so many options available, how do you know what type and size water heater to choose? We can help point you in the right direction.

What You Need: A steady supply of hot water, when and where you need it.

The Bottom Line: Size too small and you won’t have enough hot water to run your business, especially during peak operating times. But size too large and you’ll end up spending excess money in both upfront capital and long-term operating costs. Whether you run a restaurant, hotel, apartment complex, senior living facility, salon or fitness center—any business that depends on hot water needs the right water heater sizing and setup.

Our Best Advice: Only work with experienced and knowledgeable commercial water heater professionals: you’ll not only keep your customers, you’ll also save money across the board.

Note: This post was initially published on 10/08/11. Because we often receive requests for information on commercial water heater sizing, we’ve updated this article with comprehensive and current information. Last updated: 11/14/16.

Most major water heater manufacturers have a large selection of products available, so they break their product lines down into groups and subgroups by certain defining factors (commercial vs. residential, gas vs. electric, chimney vented vs. direct vented, etc.), narrowing the selections down to absolute specifics by series and model. Each series of water heater models has a set of features and spec sheets, usually laid out in tables, indicating mechanics, measurements, storage capacities, inputs and outputs. Convenient, yes. But these specs are written for contractors who install water heaters, so if the Average Joe doesn’t know and understand the sizing requirements for a specific facility, the specs won’t mean anything. While you can rest at ease trusting the advice of a water heater professional, knowing what questions to ask BEFORE you purchase or rent a commercial water heater will allow you to make an informed decision that fits your business and your budget.

Commercial Water Heaters vs. Residential Water Heaters

There’s a BIG difference between sizing a water heater for a residence and sizing a water heater for a commercial application. There isn’t a very wide range of size options for residential water heaters because the majority of homes can get by with the common sizes – a household of four people can easily work around peak usage times and simply not run hot water appliances while people are showering.

Commercial applications operate under very different circumstances than homes – and each other – so there are a variety of additional factors to consider when accurately sizing a water heater for a business, especially one that requires hot water to stay open. Apartment complexes, assisted living facilities, and hotels all have numerous households under one roof, sharing hot water. Those people can’t work around peak times, so commercial water heaters must be sized to accommodate all.

That’s why commercial water heaters come in a wide variety of sizes: sizing isn’t a guessing game. Manufacturers design different types of commercial water heaters and equipment for specific applications—so only an experienced, knowledgeable professional can accurately calculate your hot water needs to determine the correct piece of equipment for your business.

What Factors Go Into Correct Water Heater Sizing?

When we talk about correct sizing, we’re not just talking the physical size of the unit—that’s just one contributing factor. There are two main identifying characteristics of commercial water heater models: storage capacity and input. So according to model numbers, it boils down to how many gallons of hot water the tank stores and how much power the burner uses to raise the temperature of that water. But to know which combination of tank size and power input is the right combination, you need to identify the hot water requirements of your business.

We’re looking for the exact water heater (or boiler or series of water heaters) that will provide your business enough hot water whenever you need it.

An experienced commercial water heater sizing professional takes into account various important characteristics of your business operations, including, but not limited to:

  • The number of fixtures (dishwashers, sinks, showers, pools and more) that require hot water
  • Your hours of operation (24/7, lunch hours only, etc.)
  • Your peak business hours (when your business uses the MOST hot water)
  • Your geographic location (for seasonal impacts on base water temperature)
  • Venting requirements and/or restrictions
  • Physical footprint limitations

Using these qualifiers, your commercial water heater sizing professional will accurately calculate the type, necessary BTUs**, gallons, and physical size of the specific water heater you need to provide your facility with consistent hot water—and that means consistent business.

Different business applications use hot water differently. Some businesses require a lot of hot water at peak operating hours (for example, a restaurant during dinner rush) while others need a steady amount of hot water throughout the day (like salons, for example). That’s why there’s such a wide variety of water heater models with different gallon capacities and energy inputs – because different combinations of these factors produce different results. Let’s break this down.

Hot Water: Right Now (Capacity) + How Fast (Recovery Rate)

When you’re at that peak demand for your business, you need enough hot water and NOW. So let’s say ALL your hot water sources are turned on simultaneously at full blast and set to HOT. (That’s all your dishwashers, showers, sinks, etc. – you name it.) This is often called dump load and it reflects the gallons of hot water your restaurant, apartment building, hotel, or other commercial facility requires at peak demand. But in reality, hot water isn’t pulled like a dunk tank at a fair – it’s still drawn over some time, so peak demand usage is typically measured in gallons per hour (GPH).

That’s why the second important sizing factor is the hourly input, aka BTU. (British Thermal Unit, literally the amount of energy required to raise the temperature of one pound of pure water by one degree Fahrenheit.) The BTU input of a commercial water heater determines the recovery rate – aka how many gallons of hot water it can make per hour, based on the starting and ending water temperatures. (For sizing, recovery rates are usually calculated assuming a 100°F rise in water temperature, based on heating 40°F ground water to the manufacturers’ default set point of 140°F.)

Ironically, an input of 199,000 BTU will produce 193 gallons of hot water per hour (100°F hotter than ground), whether the storage capacity is 35 gallons or 75 gallons or 100 gallons. Smaller tank capacities will simply deplete and refill faster than larger tanks, and they’ll do so more frequently than larger tanks. That’s why finding the right balance between these factors is so important:

  • Gallons (Capacity: Hot Water Available Right Now): If all of your primary fixtures are operating at the same time, how much hot water is immediately available? Correctly sized equipment has the appropriate hot water storage (number of gallons) available to ensure that when you run your commercial dishwasher during the dinner rush you still have enough hot water available for employees and guests to wash their hands.
  • BTU/hr (Input Recovery Rate: Hot Water Replenished): Put simply, BTU input refers to the burner capacity – or the amount of heating power the burner is capable of generating hourly. In other words: how quickly can your water heater create and replenish your hot water supply over time? The lower the BTUs, the longer it will take the heater to get back up to set temp; the higher the BTUs, the faster the recovery rate.

Without sizing to correct dump load requirements, you run the risk of running out of hot water during your busiest business hours. And if your tank sizing is sub-par, when you’re running at full capacity (so your dump load is used up), your heater is working overtime to reheat that supply of hot water you desperately need. This situation is not only hard on your water heating equipment, it’s hard on you as a business owner because you’re always running out of hot water when you need it most – and who needs hot water issues on top of a hectic rush period??

Correct water heater sizing requires taking into account the specific water use characteristics of your unique business operation and then choosing the piece of equipment that best suits those needs. When your heater goes down (and it will), don’t let your plumber talk you into a like-for-like replacement of your current model without analyzing your evolving hot water needs. You won’t regret it.

A Note on Gas vs. Electric Water Heater Sizing

We’re talking about gas commercial water heaters in this post. Calculating electric water heater requirements works similarly, as you’ll need to factor in capacity, fixtures, etc. However, the BTU aspect applied to gas water heaters only. Electric heaters use kilowatts instead of BTUs. The same basic rules apply: the higher the voltage, the faster the recovery—but it’s to a much lesser scale. If you double the BTUs of a gas water heater, you get double the recovery. If you double the kilowatts of an electric water heater, you only get about 1.75 the amount of hot water recovery. We typically recommend gas water heaters for commercial applications for this and other reasons.

Why is Correct Water Heater Sizing So Important?

Own a restaurant?

What happens when your tap starts to run cold in the middle of your dinner rush? What if your dishwasher stops blasting hot water over lunch? You need hot water (now!) or you risk health code violations and inadequate service. Too many problems and you’ll see your patronage dwindle, and fast. Sizing here really depends on various things, namely hours of operation, average cover counts and dishwashing machines. For example, if you’re only open for dinner hours, then often smaller storage capacities combined with higher BTU inputs may be adequate, but if you’re open for 2-3 meal services daily a larger tank is usually more appropriate. Sanitizing dishwashers require 180°F water to properly sanitize, so one primary heater combined with a second booster heater is common in restaurant applications.how to choose a commercial water heater

Own a hotel?

It’s 6:30am on a Tuesday and half of your guests hop in the shower (at the exact same time) to get ready for the long business day ahead. Cold shower? Unacceptable. And if you have a restaurant or kitchen onsite, that’s a whole other consideration (see above). That’s why hotels nearly always end up with multiple heaters (plus extra storage tanks too) – and the bigger the hotel, the bigger the heaters likely needed.

Own an apartment building?

Let’s say most of your tenants arrive home from work at 6pm. Twenty people start making dinner, ten others jump in the shower, and another six start running the laundry machines. Do you have enough hot water to keep up with spurts of high usage? Larger apartment complexes need larger storage capacities with average to high BTU inputs and frequently require more than one water heater.

Own a salon?

No hot water for hairdresser sinks, spa baths and hot towel treatments? Keep your customers (and your employees) relaxed and happy, confident there will always be a steady supply of hot water to meet their needs. Sizing really depends on the fixtures counts and footprints here. Smaller storage capacities and higher BTU inputs are frequently seen because of the need for some hot water throughout an entire day. But additional hot water services like pedicure baths increase dump loads and then larger storage capacities are needed.

Own a fitness center?

Pre-work and post-work rushes can get pretty hectic. What happens when your showers go down because your water heater cut out during these high-demand times? Your members might become ex-members. Are there steam rooms in your facility? Pools or hot tubs? All these services impact hot water needs, therefore sizing for fitness centers is too varied to even offer a general example here – it must be catered to individual facilities’ needs.

The above examples should help illustrate the necessity of having a professional evaluate your commercial facility’s needs – the possibilities are complex and shouldn’t be handled lightly, especially when it’s your business at stake.

Now let’s take a further look at the options you’ll likely be facing once your hot water requirements have been established.

Pros & Cons: One Large Commercial Water Heater vs. Multiple Smaller Heaters

Once a trained water heater professional analyzes your hot water requirements, it’s time to look at how many commercial water heaters you need down in that boiler room. Like we said, usually you have options. It’s important to know the potential benefits of installing one, large commercial water heater versus the potential benefits of installing several smaller water heaters.

Picking Your Piping

When a trained water heater technician installs one big commercial water heater, the main thing to remember is that ALL the hot water in your facility is generated by that large unit. When several smaller water heaters are installed, this is called a redundant system. A redundant system is staged—meaning each smaller water heater fires up to meet demand, but only as needed.

A redundant system can be piped two ways. Those smaller water heaters can be piped in series, or piped using a balanced system.

  • In Series 101: Let’s say you have three smaller water heaters in a row: # 1, # 2, & # 3. When those three smaller units are piped in series, it means they’re piped in a row and used one at a time, in order, firing to meet your current hot water demand. So water heater # 1 is always in use; # 2 is sometimes in use; and # 3 may not be used much at all. As you might expect, water heater # 1 is always running, and hard—so it’s going to break down faster than water heaters # 2 and # 3.
  • Balanced System 101: With a balanced system, those three water heaters are parallel piped, meaning all units are running, but at a much lower rate. All your hot water needs are drawn from all three units equally. As you might suspect, when you have multiple water heaters, a balanced system really is the way to go. All of the units are powered evenly, instead of constantly beating on that # 1 water heater.

So why choose one large water heater over several smaller water heaters, or vice versa?

Let’s look at some details…
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When You’re Down, You’re DOWN

One major disadvantage to one large water heater? When that large heater breaks down (and it will break down, because it’s mechanical equipment), you’re left with NO hot water. With one big water heater, your hot water is either ON or OFF. Just like a light bulb, there’s no in-between setting. With several small water heaters, if one unit goes down, the other units are piped to keep you in hot water; so while you’ll have less hot water, your business operations will be much less affected.

Also, keep in mind your one large water heater regularly fires at a high BTU, meaning the fire burns hotter than it would in a smaller unit. Though built with more durable components, higher hotter heat means more wear, causing both the components and the commercial water heater to fail more quickly. Lower heat on those smaller water heaters means longer equipment life.

Additional Commercial Water Heating Options

A benefit of going with multiple smaller water heaters is that you have a lot more options in terms of water heater models. Those smaller commercial units have a wider variety of BTU (think horsepower) options and gallons (think size) options you can mix and match to create the best hot water scenario for your commercial application. Plus, a system with multiple smaller water heaters can be piped a variety of ways, as we mentioned earlier.

Wiggle Room

One obvious drawback to piping in multiple smaller water heaters is the larger physical footprint. If you want more water heaters, even if they’re smaller, you’re going to need more space and more chimney access. Limited boiler room space or a narrow chimney width can limit your water heater options, so you’ll likely be required to choose one big commercial water heater if that’s the case.

You’re probably also concerned with the wiggle room (or lack thereof) in your wallet. From an economic standpoint, installation costs look similar. One big water heater weighs a LOT, often requiring more men and possibly even special equipment to move it—so you’re looking at more labor costs. However, the install with multiple smaller water heaters requires at least twice as much piping, so that extra work time and extra materials are going to cost you, too.

A licensed commercial water heater installer should be able to price out your commercial water heater options based on your particular building layout and hot water needs.


There are many different water heater sizes and setups because each commercial application is unique. The best water heater system for your restaurant, hotel, apartment complex, or other commercial application depends on your layout, your capacity, your fixtures, and your hot water requirements. Depending on the size of your facility, how much hot water is needed immediately vs. over time, and how many fixtures you have, risking the wrong size water heater just isn’t worth it!

Facility Fixtures & Equipment: Hot Water Worksheet

Pick your business type, then use the list to count your fixtures. This will help your commercial water heater specialist determine which commercial water heater setup and installation will ensure your business always has hot water.

Restaurant:

  • Average cover counts for rush times
  • Bar sinks
  • Prep sinks
  • Scrub/mop sinks
  • Dishwasher sinks
  • Dishwashers
    • Rack style dishwashers
    • Conveyor style dishwashers
  • Customer restrooms
  • OTHER HOT WATER FIXTURES

Hotel:

  • Number of guestrooms
  • Hot tubs
  • Regular showerheads
  • Restricted-flow showerheads
  • On-site laundry machines
  • Fitness centers
  • Pools
  • Lobby restrooms
  • (For kitchen, restaurant and bar facilities –see above)
  • OTHER HOT WATER FIXTURES

Apartment Building:

  • Number & type of units and/or max tenant capacity
  • Number of bathrooms
  • How is building heated? (Do you have an existing boiler?)
  • Fitness centers
  • Pools
  • Hot tubs/Jacuzzis
  • Washing machines
  • OTHER HOT WATER FIXTURES

Salon:

  • Customers/day
  • Shampoo station sinks
  • Restrooms
  • Pedicure baths (big hot water users)
  • OTHER HOT WATER FIXTURES

Fitness Centers:

  • Customers/day
  • Showers
  • Steam room
  • Pools
  • Hot tubs
  • OTHER HOT WATER FIXTURES

Questions about your hot water needs? Leave a comment below!

Water heater emergency? Not enough hot water? Call us for a FREE consultation: 1-800-356-1444.


Featured image courtesy of Flickr user William Warby; post image courtesy of Pixabay user Tante Tati.

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Correct Commercial Water Heater Sizing: It’s No Guessing Game
Article Name
Correct Commercial Water Heater Sizing: It’s No Guessing Game
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These water heater tips will help take the guesswork out of commercial water heater sizing. Don't take chances when it comes to hot water needs for your business! Get it right the first time.
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Reliable Water Services
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6 Responses to Correct Commercial Water Heater Sizing: It’s No Guessing Game
  1. Great article. We’re architects in Santa Barbara CA helping out friends opening a new salon. The facility will have six stylist stations (w/no plumbing fixtures) and two shampoo chairs. I assume the chairs will be used frequently throughout the work day so the dump load will be high for the two fixtures. Space is limited to locate one a single water heater at floor level. They do have a 12′ high suspended ceiling so I wonder if there might be an opportunity to locate one or more smaller water heaters above the ceiling in the staff room. Can you provide me some direction on what size water heaters we should be considering? This might be a case where smaller multiple w.h. might be appropriate. Can you tell me if there are special enclosure issues in placing water heaters in an area above a dropped ceiling?

    Many thanks

    Reply
    • Hi Tom, we’re glad you found this article useful. And the situation you’ve described perfectly illustrates how challenging sizing can be! To start, installing smaller water heaters on upper levels is certainly doable – as long as they can still be vented safely and there’s proper drainage available, so if there’s any type of leak (from T&P valve/overflow pipe, from heater when it dies, or other) it doesn’t flood the room and cause damage below. That being said, if the salon facility you’re sizing will indeed only have 2 shampoo sinks, your best bet might be installing 2 commercial grade tankless water heaters in parallel, instead of a traditional tank-type water heater. Tankless units are great for situations that call for small amounts of hot water throughout the day and have limited space available. Installing two of them as a balanced system would provide 6 gallons per min, and if one goes down the other can still provide 3 gallons per minute by itself. However, if there will be a pedicure bath or any onsite laundry machines, this may not work as well because those appliances would likely require more hot water at one time than the tankless units could handle. Hopefully this points you in the right direction for a good solution – we’d love to hear how it all works out!

      Reply
  2. I’m going to have to look for a tankless water heater for our home. We’ve had some issues with our old water heater system and I heard about a tankless option, and I was really looking to upgrade. I like the things that I’ve heard about tankless water heaters and I would love to look some more into them! Thanks for the ideas!

    Reply
  3. I like how you mentioned that too small of a boiler will produce too little water and a boiler that is too large will produce too much and could cost extra money in the long run. I think sometimes a temporary water heating need comes up and the best thing to do would be to talk to a professional about what size of a boiler you would need to rent in order to provide for your needs. Not all boilers are going to fit your needs so having a professional opinion would be valuable so you don’t end up renting a boiler that is larger than necessary or smaller than necessary.

    Reply

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