Burner and fan
Fuel and air simultaneously enter the burner, which sets the mixture aflame at 2200 degrees F. In this Bosch GWH C 800 ES, a venturi meters the fuel intake and a variable-speed fan regulates air.
Primary heat exchanger
Water gets hotter as it passes back and forth through this copper honeycomb combustion chamber. Heat flows from the flames to the fluid through a stack of slim fins sitting straight above the burner.
Condensing heat exchanger
Maximizes efficiency by using 350 F exhaust gases to preheat cold water before it enters the primary heat exchanger. The gas exits at below 150 F, cool enough to pass through PVC.
[link href='https://seniorhelpline.info/home/how-to/a6010/tankless-water-heater-how-it-works/' link_updater_label='internal']Cost and Consumption Comparison >>>
Suck It In, Burn It Up, Blow It Out
A tankless water heater requires a fresh-air intake to feed the flame and an exhaust line to expel combustion gases. The lines vent through an exterior wall or roof either as separate intake and exhaust pipes, or as a concentric vent in which both pipes share a single conduit (see the Rinnai vent at left). Venting requirements determine where (or if) a unit can be installed. Inconveniently located gas supply lines can force vent pipes to make too long a run, inhibiting airflow. Codes can require a minimum distance from the vent openings to a home's windows, eliminating an otherwise ideal location.
Cost & Consumption Comparison
40 (GAS), 52 (ELECTRIC)
typical cost of unit
$337 (G), $312 (E)
average standby fuel loss
350-500 BTU/HOUR (G)
150-200 BTU/HOUR (E)
typical life expectancy
approximate annual operation costs
$281 (G), $492 (E)
typical size (pounds; cubic inches)
3 Ways a Tankless Pays
Massive Demand: A dormitory can use 15 showers simultaneously and never run out of hot water.
Vacation Homes: Why heat a tank of water all month if you're only going to use it one weekend?
Space-Saving: Sometimes water heaters are in the way—say, when installed in a rental apartment's kitchen. A wall-mounted tankless machine frees up wasted floor space.
3 Ways It Tanks
Hot Trickles: Some heaters "sandwich" pockets of cold water when a small hot-water draw doesn't fully fire up the tankless flame.
Tight Vents: Urban row houses present venting challenges—if you can't run a short chase out the back, forget about it.
Hard Water: Calcification plagues heat exchangers in hard-water areas. Residents must soak parts in vinegar or install a water softener.