Condensing Tankless Continued from page 33
How does a condensing tankless water heater work? Like any tankless water heater, it is an on-demand sys-
tem with no hot-water storage and no pilot light burning 24/7. When a hot-water appliance, faucet or fixture is turned on, the tankless system senses flow, and a spark igniter activates the gas burner. As cold water enters the system, the heat exchanger rapidly raises the temperature of the flow to the designated set point before the water proceeds to its destination. However long the fixture or
densing unit, the secondary heat exchanger will lower this temperature to around 105 F to 130 F — a major dif- ference without sacrificing hot-water output or water pressure. This reduction is why venting with PVC is per- mitted.
What happens to the condensate? The process of reducing combustion-gas temperatures
necessarily creates condensate in the flue, an acid-water mixture that drips back into the water heater and through the secondary heat exchanger. If that device is made of aluminum or copper, the unit will likely corrode over time, undermining both performance and longevity. A secondary heat exchanger made entirely of stainless steel will not have this corrosion problem, extending the life of the water heater, albeit at a higher unit cost.
Is condensing tankless any more difficult to install than a conventional model? No, and with PVC venting, installation should go more
quickly and easily. The installer can build the vent run using four-inch PVC throughout. Or, if the termination point in the wall is three inches and the homeowner does not wish to enlarge it, the installer can reduce the run to that smaller size. The biggest installation difference is the need to install
a condensate line, but this requirement should not add much time or cost to the installation. A condensing tank- less model typically has a half-inch port at its base. Following local codes, the installer simply runs a pipe from this port to a nearby drain. Finally, a condensing tankless unit will be slightly
A condensing unit (right) uses the same sequence of steps to generate hot water as a conventional tankless unit (left), but the former also incorporates a secondary heat exchanger to capture more heat from the combustion gases before they enter the venting at the top of the flue. This secondary heat exchanger then preheats the incom- ing water on its way to the primary heat exchanger, increasing the unit’s efficiency.
appliance operates, the tankless unit will meet the demand for water at the set temperature; i.e., “endless hot water.” A condensing unit uses the same sequence of steps to
generate hot water but also incorporates a secondary heat exchanger to capture more heat from the combustion gases before they enter the venting at the top of the flue. This secondary heat exchanger then preheats the incom- ing water on its way to the primary heat exchanger, increasing the unit’s efficiency.
How much heat does the secondary exchanger absorb? In a conventional tankless unit, the gas temperature at the top of the flue is between 330 F and 360 F. In a con-
Page 34/Plumbing Engineer
larger than a conventional unit with comparable specifi- cations — but not by much. The larger dimensions should not impact handling on the job site nor will they lessen the inherent space-savings advantage of tankless. A great example is the ability to install a tankless water heater in a crawl space — not exactly an easy proposition for a tank-type water heater.
Will a condensing tankless water heater require a larger gas line? That is a possibility. The typical gas storage water
heater offers inputs of 40,000 to 50,000 Btu, while the most popular condensing units are modulating devices that max out at 199,000 Btu. So the installer must be aware of the possible need to upsize the gas line from half-inch to three-quarter inch. That shouldn’t be a nega- tive, because the size of the gas main coming into a dwelling is usually three-quarter inch as well. We tell our trainees that they must be mindful of proper pipe sizing for both the gas supply and the gas draw.
How does a contractor sell his consumer customer on using condensing tankless? The more difficult selling job will likely be switching
the customer from tank to tankless. As I tell contractors who attend my training sessions, the first question to ask a homeowner customer is, “Have you ever run out of hot water?” If the answer is ‘yes,’ you have probably a better
February 2011
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