HYDRONICS A: The technology will work
equally well everywhere, regardless of weather conditions. But the fact is that, in warmer climates, tankless water heaters are often mounted outdoors, eliminating the need for venting. In these cases, the PVC venting advantage is not relevant. If the end user or homeowner is not interested in the additional efficiency, the less expensive conventional tankless will be the likely choice. After all, tankless is still much more efficient than a traditional tank-type water heater, not to mention all of the space savings because of its wall- mount capability. All of which is why condensing
tankless water heaters are more popular in the Midwest and the Northeast, as well as in Canada,
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 incoming water on its way to the primary heat exchanger, increasing the unit’s efficiency. Q: How much heat does the
secondary exchanger absorb? A: In a conventional tankless unit,
the gas temperature at the top of the flue is between 330°F and 360°F. In a condensing unit, the secondary heat exchanger will lower this temperature to around 105°F to 130°F — a major difference without sacrificing hot-water output or water pressure. This reduction is why
venting with PVC is permitted. Q: What happens to the
condensate? A: 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. Q: Is condensing tankless any more
difficult to install than a conventional model? A: No, and with PVC venting,
installation should go more quickly and easily.
e Turn to PVC on p 59 49
The author shows a condensing tankless water heater with the front cover removed and with the secondary heat exchanger at the top of the unit.
where water heaters must be installed indoors and where the PVC venting advantage is critical. In the Midwest, storage tank water heaters are often located in the middle of the basement, so the installer will usually place the tankless replacement in the very same spot, running PVC vent lines up to 25 feet. Being able to extend a vent line that far without worrying about material costs is a huge attraction. Q: How does a condensing
tankless water heater work? A: Like any tankless water heater,
it is an on-demand system 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
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phc february 2011
www.phcnews.com
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