There are many different ways to heat hot water and depending upon the age of your heater and who specified your water heating system will determine the type and the efficiency of your water heating system.  I have been around Laundromats for most of my life and I have seen all sorts of ways to heat water.  I also know that every dollar spent in heating hot water is an expense against the profit of the Laundromat.  Personally, I always try to specify the most efficient water heating system possible.  Every dollar save on energy costs, is a dollar saved. 

Tank Type Heaters
The most common type of water heater for the home is the "Tank Type" water heater.  Tank type water heaters are used all the time in residential situations but do not work in Laundromats for several reasons.  First these heaters do not have the necessary recovery rate of commercial water heaters.  The water heaters found in the Home Depot are good for your home but will not work in the high hot water demand Laundromat environment.  Even the commercial tank type heaters are not a good choice because they start out at 80% efficiency but quickly drop to 50% efficiency in just a few years (see chart below).  This is due to scale and minerals coming out of solution and sitting on the heating surface of the tank type heater.  These minerals help insulate the heating surface so you need more and more gas to heat up the same amount of water.  These Tank type heater will typically last 5-7 years in a Laundromat environment.  I would not recommend the Tank type heater for a Laundromat.  

Natco Steam Pressure Boilers
The Natco Steam Boiler has a fire box and above it is a reservoir of water that is held under pressure.  The Natco water heater will heat the reservoir of water up to 230 degrees and about 15 psi pressure.  Hot water is produced when cold water in heated in the coil that sits in the 230 degree water reservoir.  Water coming out of the heat exchanger is so hot that you need to mix it with cold water to get it down to a reasonable temperature (125-140 degrees).  Natco steam boilers are usually in an enclosed room or back room and are characterized by giving off a lot of heat.  Even in the winter if you come into a room with a Natco Boiler, the room will feel very warm.  This extra heat is unfortunately wasted energy that is heating the room but not heating the water.  Natco boilers are typically about 50% efficient.  HK has helped reduce the energy bill of many Laundromat owners by replacing these heaters with atmospheric heaters.  Natoc Boilers generate plenty of hot water and the boilers seem to last forever.  The Natco boilers were very popular in the 1960-70's when energy costs were very in-expensive.

Atmospheric Water Heaters
Atmospheric water heaters typically are high recovery heaters with a circulating pump and a hot water storage tank.  Atmospheric type heaters are also used in instantaneous water heaters.  Atmospheric type water heaters have a copper coil (also called a heat exchanger) that is surrounded by a huge gas flame.  The water is heated in the heater and circulated to a storage tank where the hot water is held in storage or sent directly to the washers.  The atmospheric heating coil has a very small volume and the water must pass quickly through the heat exchanger to the storage tank.  The atmospheric heater uses the surrounding air for combustion and the heater is vented with insulated metal duct work (typically "B" Class vents for a gas fired heater).  The exhaust gas temperature is very high, usually in the 400 degrees F range.  These heaters are typically 80-82% thermally efficient.  The advantage of these type of heaters are that they are relativity inexpensive, they have been around for decades and are very reliable.  The disadvantage of these heaters is that they use air from the surrounding area.  Building Code requires at least 2 make-up air openings for the water heater, one high and one low to provide constant ventilation and circulation in the water heater room.  This means that there is a tremendous heat loss from exposed water pipes, circulators and storage tank due to the huge require make-up air openings.  These heaters are very reliable and typically will last from 10-15 years.