Figure 1. Sizing table showing 80% thermal efficiency. (Table courtesy of A.O. Smith)
As a frequent business traveler, I can attest to the many frustrations of being on the road, including cancelled flights, long lines for rental cars, and indigestion from eating on the run. I'm in the water heater business, but as an ordinary traveler, I find nothing more frustrating than jumping into the shower for a much needed morning wake-up only to discover that the water is ice cold - no hot water to be had anywhere!

I'm sure this is as frustrating for the hotel management who has to deal with calls from angry guests on mornings when occupancy exceeds the capacity of the building's undersized domestic water heating system. The fact is, with the right water heaters, properly sized, a hotel should never run out of hot water. Standard efficiency atmospheric water heaters or high-efficiency sealed combustion water heaters can both handle the load equally well, but there is a significant difference between the two in operating and installation costs. Let's look at a "real world" example, based on a new trend in hotels.

A New Breed Of Hotel

You see it more and more often - a "hotel park," with three or more "inns" clustered together. They're usually right off an interstate, close to an industrial park or airport, with a Cracker Barrel, Denny's, or other chain restaurant nearby. These midsize "short stay" hotel chains, include LaQuinta Inns, Fairfield Inns, Comfort Inns, Wingate Inns, and Baymont Inns. All cater to the business traveler who really only needs a comfortable bed, a TV set, a telephone, a cup of coffee in the morning and, of course, a hot shower. They typically have no more than 60 guest rooms and no restaurant, since their guests can head over to Cracker Barrel if they need breakfast or a quick snack. Because they offer little in the way of frills, they also offer discount rates, so they need to keep their overhead as low as possible to make a profit.

Domestic hot water for this type of lodging can represent up to 30% of the hotel's energy operating cost. That's a significant percentage, so investing in a high-efficiency water heating system will pay big dividends down the road. Let me show you how.

Assessing Hot Water Demand

The first and most important step in commercial water heater sizing for hotels is determining the hot water demand for a specific timeframe referred to as the peak period. In a hotel, there will normally be two peak periods, one in the morning and a second in the early evening. Sizing for these periods should not include requirements for the hotel's laundry facilities. All hot water during peak periods should be devoted to serving the guest rooms. 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 between usable stored water in the tank and the recovery rate. The recovery rate represents the amount of water the system can heat (to a specific "temperature rise") in one hour. In the case of our "short stay" 60-unit hotel, we're going to base our calculations on a one-hour peak demand period in the morning and a one-hour peak demand period in the evening. We normally calculate hotel sizing based on two-hour peak demand, but the nature of these "short stay" facilities is that the guests get up and out faster than normal, 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), our sizing tables tell us we'll need to be able to provide 740 gal of hot water to satisfy our one-hour peak demand.

To achieve this objective, the sizing tables recommend a system with 240 gal of total storage. Bear in mind that the tank can only provide 70% to 80% of its stored water as useable hot water in one hour. So, we'll get a portion of the hot water demand from storage during our peak period, leaving the balance needed to come from the system's recovery capacity. Finally, after the peak period is over, we'll have to reheat 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

Figure 2. Sizing table showing 94% thermal efficiency. (Table courtesy of A.O. Smith)

Selecting For Savings

Having determined our hotel's peak demand, and 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 efficiency;
  • 100-gal storage capacity per unit, 70% tank draw efficiency; and
  • 199,000 Btuh maximum input per unit produces 193 gph recovery.
  • Option Two: Three tank-type heaters:
  • 94% thermal efficiency;
  • 100-gal storage capacity per unit, 80% tank draw efficiency; and
  • 150,000 Btuh maximum input produces 171 gph recovery.

Both of these systems produce an adequate supply of hot water, but their operating costs are significantly different. Let's compare the impact the 94%-efficient model has on the actual fuel consumption required to meet the one-hour peak demand at our 60-unit "short-stay" hotel.
80% thermal efficiency: 763,125 Btu
94% thermal efficiency: 649,468 Btu
Difference: 113,657 Btu

FIGURE 3. Yearly savings earned by using higher-efficiency hot water heaters.

The 94% Payoff

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 3 illustrates 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, there will 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 3-yr warranty period, and the comparative savings exceed $3,000. It's not unreasonable to expect this rate of savings to continue throughout the life of the heater.

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

In addition, such a design permits intake and exhaust runs up to 120 ft using 4-in. PVC pipe, or 50 ft using 3-in. PVC pipe. Use of inexpensive PVC pipe, along with a model's flexibility to use either vertical or horizontal vent runs, can produce significant savings in installation labor and materials costs compared to standard water heaters.

To summarize, the first priority in sizing a water heating system for a hotel must be making sure the system can keep up with the application's peak demand to ensure that you, our fellow travelers, and I aren't faced with cold showers 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.

Who cares how cost-effective a water heating system is if it doesn't do its job? Having no water heaters at all would be cost-effective in the short run, but long term, depriving guests of hot water will put any hotel out of business! Adequate hot water is always the most important consideration!

Once that need is met, new efficiency technologies have also created new possibilities for significant savings. Such models may have a higher initial product cost but can be expected to make up the cost difference and then some in the long run, by providing lower installation and operating costs.

I could go on, but I won't. I've got a shower to take before I run for my plane and my trip home. I just hope - well, you know. ES