Back to Basics_ Water Heaters

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November 10, 2003

These commercial water heaters are designed to work together in a system to provide domestic hotwater during peak period applications for short stay hotels. (Courtesy of A.O. Smith.)Issue: 11/03

Editor's Note: "Back to Basics" is a column that will run periodically in PME reviewing the basicprinciples of plumbing engineering.

As frequent business travelers know, there can be many frustrations to being on the road, includingcancelled flights, long lines for rental cars, and indigestion from constantly being on the run, attendingmeeting after meeting. There is nothing more frustrating than jumping into the shower for amuch-needed morning wake-up, only to discover that the water is ice cold--no hot water to be hadanywhere!

We're sure this is just as frustrating for the hotel manager, who has to deal with calls from angry guestson mornings when occupancy exceeds the capacity of the building's undersized domestic water heatingsystem. The fact is that with the right water heaters that are properly sized, a hotel should never run outof hot water. Standard efficiency atmospheric water heaters or high efficiency sealed combustion waterheaters can both handle the load equally well, but there is a significant difference between the two inoperating and installation costs. Let's look at a "real world" example, based on one of the trends in thehotel business.

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Commercial water heaters like this A.O. Smith Cyclone model place the down-fired burner on top ofthe unit, so condensation and resulting corrosion do not affect the burner.

You see it more and more often--a "hotel park," with three or more "inns" clustered together. They'reusually right off the interstate, close to an industrial park or airport, with a Cracker Barrel, Denny's orother chain restaurant nearby. These mid-sized "short stay" hotel chains include LaQuinta Inns,Fairfield Inns, Comfort Inns, Wingate Inns and Baymont Inns. All cater to the business traveler whoreally only needs a comfortable bed, a TV set, a telephone, a cup of coffee in the mornings, and ofcourse, a hot shower. They typically have no more than 60 guestrooms and no restaurant, since theirguests can head over to Cracker Barrel if they need breakfast or a quick snack. Because they offer littlein the way of frills, they also offer discount rates, so they need to keep their overhead as low as possibleto make a profit.

Domestic hot water for this type of lodging can represent up to 30% of the hotel's energy operatingcost. That's a significant percentage, so investing in a high efficiency water heating system will pay bigdividends down the road. Here's how it works.

The first and most important step in commercial water heater sizing for hotels is determining the hotwater demand for a specific timeframe, referred to as the peak period. In a hotel, there will normally be


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two peak periods, one in the morning and a second in the early evening. Sizing for these periods shouldnot include requirements for the hotel's laundry facilities. All hot water during peak periods should bedevoted to serving the guestrooms. Laundry should only be done during "off-peak" hours.

The key to proper sizing for the specific model water heater is to achieve the proper balance betweenusable stored water in the tank and the recovery rate. The recovery rate represents the amount of waterthe system can heat (to a specific "temperature rise") in one hour. In the case of our "short stay" 60-unithotel, we're going to base our calculations on a one-hour peak demand period in the morning and aone-hour peak demand period in the evening. We normally calculate hotel sizing based on two-hourpeak demand, but the nature of these "short stay" facilities is that the guests get up and out faster thannormal, so the peak can be compressed to one hour.

With 60 rooms, each with 3 gpm shower heads, and factoring in 100°F temperature rise requirement(water heaters must be able to provide sufficient hot water supply even in the coldest months), oursizing tables tell us we'll need to be able to provide 740 gallons of hot water to satisfy our one-hourpeak demand. (The A.O. Smith Accu-Size Guide was used for these calculations. The current defaultsare set to assure the minimum required hot water with a minimum amount of information.)

To achieve this objective, the sizing tables recommend a system with 240 gallons of total storage. Bearin mind, that the tank can only provide 70% to 80% of its stored water as useable water in one hour. So,we'll get a portion of the hot water demand from storage during our peak period, leaving the balanceneeded to come from the system's recovery capacity. Finally, after the peak period is over, we'll have toreheat the gallons drawn from storage to restore the unit to full capacity (Figures 1 and 2).

(Btu input = gph x 8.25 x temp rise x 1.0) divided by % efficiency

Having determined our hotel's peak demand, plus the storage and recovery capacity needed to meet it,here are two gas-fired options that satisfy the requirements:

Option One: Three standard tank-type heaters:80% thermal efficiency100-gallon storage capacity per unit, 70% tank draw efficiency, and199,000 Btuh maximum input per unit produces 193 gph recovery

Option Two: Three tank-type heaters:94% thermal efficiency100-gallon storage capacity per unit, 80% tank draw efficiency, and


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150,000 Btuh maximum input produces 171 gph recovery

Both of these systems produce an adequate supply of hot water, but their operating costs aresignificantly different. Let's compare the impact the 94% efficient model has on the actual fuelconsumption required to meet the one-hour peak demand at our 60-unit "short stay" hotel.

80% thermal efficiency: 763,125 Btu94% thermal efficiency: -649,468 BtuDifference: 113, 657 Btu

This burner design mixes incoming blower-driven air with incoming gas. When the mixture is ignited bythe hot surface igniter, a flame shoots downward into the submerged central combustion chamber. Theresulting hot fuel gases are then forced for high velocity through a helical heat exchanger coil. Thespiral shape of the coil keeps the hot gases swirling against the heat exchanger walls, producing a highrate of heat transfer. (Courtesy of A.O. Smith.)

As the numbers show, by using three 94% efficient water heaters instead of three 80% efficient models,we can reduce our hotel's energy consumption during that critical peak period by nearly 15%. Figure 3illustrates how that translates into dollar savings on water heating costs, as extended over a full year.

Bear in mind, the $737 estimated savings covers the hotel's two daily peak periods. Obviously, therewill be off-peak hot water use by guests, as well as hotel laundry and other maintenance requirements.We can conservatively estimate that savings during off-peak hours will increase the total by 50%,bringing total one-year savings to just over $1,100. Extend that over the three-year warranty period,


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and the comparative savings exceed $3,000. It's not unreasonable to expect this rate of savings tocontinue throughout the life of the heater.

The key to proper sizing for the specific model water heater is to achieve the proper balance betweenusable stored water in the tank and the recovery rate.

As we've shown, operating costs savings can be substantial, but there are other good reasons to choosea high efficiency commercial water heater. Certain high efficiency models offer multiple ventingoptions, including sealed power direct venting, in which all combustion air is drawn from outside thestructure. This takes indoor ventilation out of the equation, and prevents performance problems causedby negative indoor air pressure.

In addition, such a design permits intake and exhaust runs up to 120 feet using four-inch PVC pipe, or50 feet using three-inch PVC pipe. Use of inexpensive PVC pipe, along with a model's flexibility to useeither vertical or horizontal vent runs, can produce significant savings on installation labor and materialcosts compared to standard water heaters.

To summarize, the first priority in sizing a water heater system for a hotel must be making sure thesystem can keep up with the application's peak demand to ensure their guests are not met with coldshowers in the morning or at any other time.

Cost efficiency is an important consideration, but it's certainly a distant second on the priority list. Whocares how cost-effective a water heating system is if it doesn't do its job? Having no water heaters at allwould be cost-effective in the short run, but long term, depriving guests of hot water will put any hotelout of business. Adequate hot water is always the most important consideration.

Once the need is met, new efficiency technologies have also created new possibilities for significantsavings. Such models may have a higher initial product cost difference, but then provide lowerinstallation and operating costs.

We could go on and on, because commercial water heating system sizing is unique to each water heatermodel and each application. Maybe next time you spring into the hotel shower, you will give amoment's thought to the contractor or engineer who sized the application.


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