There is a big push to provide the most energy efficient design in all aspects of the construction industry. MEP Engineering incorporates this approach into every project we are involved with. Unfortunately with all the resources and tools available to people, it makes performing their own research in some of the new technologies confusing.

One item we are routinely asked to consider during our design process is the use of a tankless water heater system.

Pros:

Let’s first take a look at where we think a tankless water heater can provide the greatest benefits. Tankless heaters are a good choice in commercial buildings or in residential buildings which have minimal hot water demand, in other words, with a minimum number of plumbing fixtures. The load required in these applications can easily be accommodated by this type of system. There are many advantages to these systems, including; less energy use by not continually heating a large volume of water; less energy use by eliminating heat loss in tanks and through pipes. They also provide continuous hot water, and have a longer life-span. Tankless water heaters comes in a variety of configurations. The most common is the direct vented unit that eliminates the requirement for ducted combustion air system, this certainly provide an upfront cost savings during installation. A typical tankless water heater’s life expectancy is typically 20 years, versus a typical 13 years for a standard tank heater. If used in the right environment they can be very compact in size and can be placed in locations that tank type water heaters can not be located for maintenance and removal.

Applications where we recommend tankless water heaters be used:

• Residential water heating systems
• Residential snow melt systems
• Commercial buildings with minimal hot water demand

Cons:

Let’s next look at what are some of the limitations of these systems. The biggest misconception about this type of water heating system is that it can be used in any application. While this is generally true, applications that have a high consumption of hot water usually require a more complex system and greater infrastructure to support the demands. In situations where hot water demand is high, multiple heaters may be required. Obviously the addition of multiple heaters greatly increases the overall installed cost.

Tankless heaters also require more gas or electric demand since they’re heating water instantaneously upon a demand request. The increased gas demand means they require larger gas piping. Additionally, plumbing codes require a circulating system where the hot water piping exceeds 100 feet in length. When a circulating system is required, the manufacturer requires a small hot water storage tank to be installed or the warranty will be voided. In this scenario the tankless water heater system is no longer tankless and to be the most efficient, multiple tankless heaters may be required.

Applications where we recommend they should be avoided:

• Restaurants
• Commercial buildings with shower facilities or large numbers plumbing fixtures

Project Example: Typical installation in a commercial application

Douglas County School District
Summit View Elementary School, Highlands Ranch, Colorado

In the pictures below you can see that the total demand required the use of four Rinnai gas fired water heaters (three of which are pictured below) running in parallel. The total distance of system hot water piping required the use of a circulating system. Therefore, the system required the use of a storage tank.        

Conclusion:

Tankless water heaters can be a great way to provide an overall energy savings in electricity or gas when used in an appropriate application. You need to carefully analyze what your requirements are and weigh all the costs when considering which direction to go.