The Secret Formula for Heating Your Water

By Jeff Deal | Jan 31, 2011

There are no “rules of thumb” or guesswork required to properly size a water heating system to your specific laundry – just simple facts, along with a few engineering formulas, are all that is required.

So, what exactly are those basic facts and how do you find them?

• Quantity. How many gallons per hour do you need to heat?
This is determined by the quantity of washers and the gallons of hot water used per cycle on a hot setting. Multiply the quantity of each size washer by the gallons used on a hot setting. This is gallons per cycle for the laundry.

For example, let’s look at a sample generic laundry:

• 10 small washers x 10 gallons per cycle = 100 gallons
• 10 medium washers x 25 gallons per cycle = 250 gallons
• 10 large washers x 40 gallons per cycle =    400 gallons
• Total gallons per wash cycle = 750 gallons

A word of caution: The water usage of various generations of washers by the same manufacturer vary greatly, with many manufacturers recently revamping their entire washer design to drastically reduce the amount of water used. A quick look at 30-pound washers by a single manufacturer provides the following facts:

• Pre-1996 – 15 gallons of hot water per cycle
• 2000-2008 – 12 gallons of hot water per cycle
• 2009 – 8 gallons of hot water per cycle

So, to properly size your coin laundry, you must know the brand and model of each washer.

• Time. Over what period of time do we need to heat this water?
What is the possible number of cycles per hour per washer? For most current models, this number is two; be sure to check your cycle time and do the math. For example, a 23-minute cycle time would equal two loads per hour, including loading and unloading. Since one hour is the unit of time used in utility measurement, you will want to determine the gallons per hour (gph) required:

• 100 gallons in small washers at 23 minutes per cycle (2) = 200 gph
• 250 gallons in medium washers at 23 minutes per cycle (2) = 500 gph
• 400 gallons in large washers at 35 minutes per cycle (1.75) = 700 gph
• Total gallons possible per wash cycle = 1,400 gph

• Temperature rise. How much do we need to heat the water? What is the coldest water temperature coming into your laundry in the dead of winter? This temperature will vary from 35 degrees to 75 degrees, depending on your geographic location. How hot do you want the water to be in your washers? The national average is 120 degrees, while 110 degrees is the minimum you can safely operate most water heaters.

The higher you keep the delivered temperature, the more it costs to operate the store and the greater the probability of water-chemistry-related problems. The required temperature rise is calculated by subtracting the minimum incoming temperature from the desired wash temperature:

• 120 degrees (wash) – 40 degrees (winter incoming) = 80 degrees of temperature rise

The Formulas

We have now gathered the basic data needed, and it’s time to apply the engineering formulas:

• Gallons per hour (1,400) x temperature rise (80) x 8.33 = BTU/hour required (932,960)
 
Wait a minute! We never calculated any number like 8.33. Where did that number come from? What the heck is a “BTU/hour required?”

One BTU will heat one pound of water 1 degree. BTU/hour is the quantity of BTUs required in one hour to do the work described.

Where did we get 8.33? You will notice in the above example we indicated that BTUs heat pounds of water. However, water is typically measured in gallons, so we need to convert gallons of water to pounds of water in order to utilize this engineering formula. Since there are approximately 8.33 pounds of water in each gallon, we add the multiplier 8.33 into the equation so that our final result is based on the pounds of water contained in the gallons.

However, we’re not done yet. At this point, we have calculated how many BTUs are required to be put into the water to heat it. Water heaters are not 100 percent efficient, so we must divide the BTUs required by the efficiency of the water heater we intend to use. (Today’s water heating equipment ranges from 75 percent to 99 percent efficient.) This will equal the BTU/hour input required from your water heater:

• BTU/hour output required (932,960) divided by the heater efficiency (.80) = BTU/hour input required (1,166,200)

We have three more steps left to properly size the water heating system for your coin laundry. First, size the storage tank to go with the water heater.

This is just as important as sizing your water heater. Too much stored water will cost more than necessary to operate; however, with too little, you will run out of hot water. And running out of hot water due to under-sizing the heater or under-sizing the storage tank can cause premature failure of the water heater system. (It also might cause premature loss of customers!)

The water heater is sized for the demand equally over one hour, but the actual operation of a coin laundry is such that as much as 25 percent of the hour's load could occur in a very short period of time; this is called the “dump load.” This is when 50 percent or more of the washers are filling at the same time, which is not uncommon during the weekend. And this is when the storage tank comes into play. Without the storage tank, water temperatures will drop or fill times of the washers will increase waiting for the water to pass through the water heater.

Here are the typical hot water fill rates for various size washers:

• Topload or frontload, up to 20 pounds – 5 gallons per minute
• Frontload, 25-30 pounds – 10 gallons per minute
• Frontload, 35-50 pounds – 15 gallons per minute
• Frontload, 60-80 pounds – 20 gallons per minute
    
To size the storage tank for our example store, multiply the required gallons per hour by .30 (30 percent), and we will have sufficient water to handle the dump load. This applies only to coin laundries, not to commercial laundries. Our example store requires a 420-gallon storage tank when sized at 100 percent.

Because the physical possibility of all of the washers being cycled the maximum possible times per hour diminishes with the quantity of washers in the store, we need to make an allowance for quantity. With the maximum gallons per hour calculated, you take the total number of machines and use this list:

• 0-30 machines – 100 percent of total BTU requirements
• 31-50 machines – 90 percent of total BTU requirements
• 51-65 machines – 85 percent of total BTU requirements
• 66-80 machines – 76 percent of total BTU requirements
• 81-100 machines – 70 percent of total BTU requirements

If our store above had 51 washers, we would use a factor of .85: 1,400 x .85 = 1,190 gallons per hour possible and 357 gallons required in storage.

The final step involves usage data to properly size the water heating system for your particular store. No one else knows your customer base. A second adjustment to the maximum gallons per hour possible should be made, based on your customers’ use of hot, warm and cold temperature selections. Surveying your existing laundry on a busy Saturday or Sunday, you might find that 50 percent of water temperature selection is hot, 30 percent is warm and 20 percent is cold. We would then use a 65 percent usage factor.

In our volume-based calculation, the maximum hot water usage was 1,190 gallons. Therefore, our adjusted gallon usage would be 65 percent of that, or 773.5 gallons. We would then size the water heating system to provide 515,460 BTUs per hour, and store 232 gallons of hot water.