Today’s tankless DHW heaters


Condensing tankless water heaters are becoming increasingly popular as the price gap with non-condensing models narrows thanks to less expensive and easier venting.

“We’re seeing a tremendous shift towards condensing tankless water heaters mainly because of efficiencies, lower cost, and ease of venting the flue products,” remarked Brian Fenske, special channel sales manager for Navien, Irvine, Calif.

The shift happened amazingly fast, added Jason Fleming, senior marketing and customer care manager for Noritz, Fountain Valley, Calif. “The sales of condensing high efficiency units has surpassed sales of the mid-efficient non-condensing units.”

“We see this as a significant change in the marketplace. The cost gap is getting a lot smaller,” remarked Kristen Metropoulos, North American product manager of domestic hot water for Bosch Thermotechnology, Londonderry, New Hampshire. And it’s not just the cost gap between condensing and non-condensing tankless, but the gap between tankless and storage tank water heaters is shrinking as code changes push up the price of the latter.

As well, the technology has matured. Manufacturers have expanded their input ranges, allowing the contractor to better tailor the tankless unit to the customer’s needs, noted James York, vice president of engineering for Rinnai North America in Peachtree City, Georgia.

Built-in DHW recirculation allows the contactor to easily boost comfort for customers. Space savings, endless hot water – albeit at a reduced flow in extreme situations – and energy efficiency continue to attract homeowners to tankless technology.

Saving space was a priority for the homeowner.

Saving space was a priority for the homeowner.

Energy efficiency

Natural Resources Canada (NRCan) reports that hot water heating is the second largest user of energy in the home. However, the actual cost to the homeowner is relatively small and, thus, not a big selling point. A standard tankless unit has an energy factor of 82; some condensing units are 95 or higher. The condensing unit can save the homeowner up to 40 percent in water heating costs.

Energy efficiency is important to homebuilders, noted Fleming. “They need to meet specific requirements to maximize any rebates they get for building efficient homes and because they need to differentiate from traditional cookie cutter home builders.”

Today’s on-demand water heaters are available in many Btu capacities. The “standard” unit has become the 199,000 Btu/h condensing model that can support four to five faucets or appliances at one time. Manufacturers offer larger units if needed and multiple units can be cascaded.

Less expensive venting gives the contractor installation
flexibility with condensing units.

A quality installation

There are three things a contractor needs to do to ensure a quality installation:

Number one is to size the unit correctly, taking into account maximum possible GPM requirements and ground water temperature.
Number two is to vent it correctly.

And number three is to ensure the gas line can provide adequate volume and pressure.

Less expensive venting gives the contractor installation flexibility with condensing units.

Less expensive venting gives the contractor installation flexibility with condensing units.

Sizing the unit

Sizing the unit correctly to meet the family’s needs is critical. “There needs to be a conversation with the homeowner as to how much, in the worst case scenario, hot water they are going to need at one time,” noted Fleming.

“As long as you know what the required or desired peak gallon per minute flow will be and you buy enough tankless water heater or water heaters to meet that demand, you will never get in trouble,” remarked Fenske.

Keep in mind that if three 2.5 GPM showerheads are operating simultaneously and the unit is rated at six GPM, in theory that will exceed the unit’s capacity but, in reality, probably not because the hot water is tempered with cold.

Gas line requirements

A ¾” gas line is ideal for most installations. However, there are units designed to work in older homes with half-inch lines. Navien, for example, advises that their units can run on a ½-inch gas line as long as it isn’t more than 24 feet. Keep in mind, though, that the gas line has to supply all gas appliances in the home.

Water temperature and water quality are critical factors. In the depths of winter in some parts of Canada the ground water temperature can be just slightly above freezing. Typically, today’s tankless units provide about 44C (80F) temperature rise. If the input water temperature is 1C, the maximum output is going to be 45C (113F), which is fine for most residential applications.

In reality, with today’s tankless heaters, the temperature remains 49C (120F), but the volume is reduced somewhat, said Fenske. The cure is more capacity, but that’s not usually necessary. “Maybe in the winter you can only run two showers instead of three showers. Most people get to know and understand that.”

Manufacturers make sizing easy with various phone “apps” or sizing software that calculates the sizing based on the location of the home and the number of faucets, appliances, etc.


Venting is where the price gap between a traditional tankless water heater and a condensing unit narrows. The traditional unit requires Category III stainless steel venting; condensing models use ULC S636 CPVC or polypropylene.

Rinnai offers concentric – pipe within a pipe – single pipe venting, which can be easily switched to a two-pipe system if that’s what’s already in the house, remarked York.

Some tankless units can also do space heating, but that’s a story for another day.

Some tankless units can also do space heating, but that’s a story for another day.

Multiple unit installation

Manufacturers have made installing two or more units easier. Piping is straightforward and, in some cases, the units can be common vented. Controls are connected with a cascade cable and the technician can then quickly program the units to “automatically operate based on load and switch back and forth and adjust their firing rate based on flow rate,” said Fleming.

Keep in mind that multiple units have to be plumbed in parallel, he added. “The biggest thing is making sure they have enough gas to supply both units, making sure the venting is done correctly to get the exhaust out and making sure there’s enough air to supply both units for combustion.”

Recirculation and comfort

Like any hot water system, the speed at which the hot water reaches the faucet depends on the distance from the tankless unit to the outlet. There can also be a lag time between when the sensor detects water flow and the unit starts generating hot water – the so-called “cold water sandwich.” Today’s tankless technology has largely eliminated this.

Manufacturers offer “recirculation ready” units that allow the technician to quickly install DHW recirculation. Rinnai’s RUR 98 model, for example, includes an on-board recirculation pump and a crossover valve that allows the contractor to add recirculation to an existing home with or without a dedicated return line, noted York. The crossover valve is installed under the deck at the farthest faucet or fixture from the heater.


Water quality and maintenance are, not surprisingly, linked. “You’ve got people that have good municipal water that will never have any service required and then you’ve got some bad areas and well systems that might need water treatment and/or regular service,” said Fenske. In rural water systems, water filtration and a water softener may be required.

“Over the years, getting people to maintain their tankless units has been a bit of a challenge,” noted Fleming. Manufacturers have addressed this in different ways. Noritz, for example, includes scale detection software that tells the homeowner or contractor when a unit needs to be cleaned. If scale buildup gets to the point where it might damage the heat exchanger, the software will shut the unit down until the full 60-minute flushing with white vinegar is done. Many contractors offer flushing as part of a service plan.

Tankless hot water heaters have come a long way since they were introduced to North America and today’s units can do pretty much anything the homeowner requires.


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