What To Consider Before Purchasing A Gas Tankless
Water Heater
Select a model that can support your water flow needs.
There are three key aspects in the installation of a gas tankless
water heater: exhaust venting, size of the gas line, and combustion
air requirements.
Venting
In general, the tankless water heater should be
installed where the vent length is as short and straight as
possible. The exceptions to the short distance would be the 38B,
125B, 1000P, 1600H, 1600P and the 125HX which have a minimum vent
length of 6 feet. If you live in a warm (non-freezing) area than you
have the option of installing the Bosch 250SXO, 125BO and 2400EO
models and the Rheem RTG53X and RTG74X models outside with no need
for venting. Otherwise the units are designed for an indoor
installation with exhaust venting.
38B, 1000P Models: Vented vertically
(through roof) only. Use 4" diameter type B (double
wall) vent - 6' minimum vertical height.
125B, 125HX, 1600H, 1600P Models: Vented
vertically (through roof) only. Use 5" diameter type B
(double wall) vent - 6' minimum vertical height.
125FX Models : Vented horizontally
(through wall) only. Use 4" dedicated, sealed, single
wall vent (stainless steel or 26+ gauge galvanized). 15'
maximum run with two elbows - must terminate horizontally.
FXHOOD required.
250SX, 2400E Models
: Can be vented vertically or horizontally. 3" diameter dedicated, sealed, single wall vent (stainless steel). 26' maximum run with one elbow - subtract 2.5' for each elbow. Please see our 3
Venting Guide
RTG-53 Models
: Can be vented vertically or horizontally. 3" diameter dedicated, sealed, single wall vent (stainless steel). 34' maximum run with one elbow - subtract 1.5' for each elbow. Please see our 3
Venting Guide
RTG-74, GT-199 Models: Can be vented
vertically or horizontally. 4" diameter dedicated,
sealed, single wall vent (stainless steel). 47'
maximum run with one elbow - subtract 5' for each elbow.
Gas Line Sizing
Check to make sure your gas line is sized properly. Tankless
water heaters draw many more BTU's than a tank water heater, so
you will want to make sure your gas line is of the proper size
and the distance the gas line is run is not too long, so the
unit will function properly. Below are tables that show
the maximum gas line run by diameter for natural gas, propane
(LP) with black iron pipe and propane (LP) with copper tubing.
If any other applications are sharing the line or for more
specific information, please consult the National Code Fuel Chart.
Maximum length of black iron pipe run for Natural Gas
The combustion air requirements only apply to indoor
installations and where air is drawn from within the room where
the water heater is installed. The 250SX, 2400E are the
only Bosch tankless water heaters that have a separate air
intake that allows for air to be drawn from the outside. The RTG-53DV GT-199DV RTG-74DV are the only Rheem tankless water heaters that allow for air to be drawn from the outside. For all other indoor
installations, the amount of combustion air should be a minimum
volume of 50 cubic feet per 1,000 Btu/hr rated input.
If the gas water heater is installed in an area smaller than what the combustion
call for, then the installer must make special provisions for combustion air in
the form of two openings, each one within 12" of the top and 12" of the bottom
of the space, having a total free area of 1 square inch per 1,000 Btu/hr of
rated input if the air is taken from inside the house. This requirement reduces
to 1 square inch per 4,000 Btu/hr if the air is taken from the outside.
Model
Combustion Air Requirements in Sealed Room
Size of openings if cannot meet combustion air
requirements in
sealed room
Combustion air requirements do not apply to
the RTG-74DV and GT-199DV when air is drawn from the outside
through the intake vent.
Note: A Gas tankless water heater cannot be installed in a mobile home, bedroom, bathroom, or any other confined living space normally kept closed.
Note: The above information is meant as guidance.
Always consult your installation manual and/or local code
requirements before any actual installation.
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