A.O. Smith On Demand 305 Series 100 Guide

PRINTED 0209 186965-001
Design information for single and multiple unit
installations for residential, commercial, recircula-
tion and storage tank systems.
On-Demand Water Heater
System Design Manual

2
Certifications..................................................................................3
Water Quality and Scale................................................................8
Pump Sizing for Circulation..........................................................9
Tank Water Heaters in a Circulation Loop
Additional Guidelines
Pump Sizing for Storage Tank Applications
Pressure Loss Curves
Domestic Hot Water - Standard Installation
1 Water Heater
2 Water Heaters (H-
3 Water Heaters (H-
4 Water Heaters (H-
5 Water Heaters (H-
6 Water Heaters (H-
Domestic Hot Water with Optional Freeze Protection
on Outdoor Models
1 Water Heater
2 Water Heaters (H-2-
3 Water Heaters (H-3-
6 Water Heaters (H-6-
Domestic Hot Water with Circulation Systems
1 Water Heater - Preferred (H-
1 Water Heater - Optional (H-1-RO)
2 Water Heaters - Preferred (H-2-RP)
2 Water Heaters - Optional (H-
3 Water Heaters - Preferred (H-3-RP)
6 Water Heaters - Preferred (H-6-RP)
Domestic Hot Water with Backup Storage Tanks
1 Water Heater
1 Water Heater with Circulation (H-
2 Water Heaters (H-2-
2 Water Heaters with Circulation (H-
3 Water Heaters (H-3-
3 Water Heaters with Circulation (H-
6 Water Heaters (H-6-
6 Water Heaters with Circulation (H-
Maintenance Procedure
Scale Flush Procedure (MP-01)
...................................10
....................................................................10
...............................11
..................................................................12-13
(H-1).......................................................................14
2).....................................................................15
3).....................................................................16
4).....................................................................17
5).....................................................................18
6).....................................................................19
(H-1-E)...................................................................20
E).................................................................21
E).................................................................22
E).................................................................23
1-RP)...............................................24
................................................25
............................................26
2-RO)..............................................27
............................................28
.............................................29
(H-1-T)...................................................................30
1-TC)........................................31
T) ..................................................................32
2-TC)......................................33
T) .................................................................34
3-TC).....................................35
T) .................................................................36
6-TC).....................................37
.......................................................38
Table of Contents
Installing Gas Water Heater
...........................................................4-7

3
Certified to applicable U.S.
standards for appliances using
gas or other petroleum fuel.
Certified to applicable Canadian
standards for appliances using
gas or other petroleum fuel.
Certified by the Uniform
Plumbing Code (UPC)
Certified by National Sanitation
Foundation (NSF), www.nsf.org
(indoor models must use the
NSF approved top guard)
Air-Conditioning, Heating and
Refrigeration Institute
Met the California Energy
Commission (CEC) standards
Approved by the Commonwealth
of Massachusetts (G1-0208-249)
Received New York City’s Material
and Equipment Acceptance (MEA)
(MEA 327-02-E Vol. 09)
(MEA 327-02-E Vol. 10)
(MEA 327-02-E Vol. 11)
UPC
The models listed in this manual have received the following certifications except where noted:
ATI-305-N GTS-305-NI GT-305-NI
ATI-305-P GTS-305-PI GT-305-PI
ATO-305-N GTS-305-NE GT-305-NE
ATO-305-P GTS-305-PE GT-305-PE
ATI-505-N GTS-505-NI GT-505-NI
ATI-505-P GTS-505-PI GT-505-PI
ATO-505-N GTS-505-NE GT-505-NE
ATO-505-P GTS-505-PE GT-505-PE
ATI-705-N GTS-705-NI GT-705-NI
ATI-705-P GTS-705-PI GT-705-PI
ATO-705-N GTS-705-NE GT-705-NE
ATO-705-P GTS-705-PE GT-705-PE
*ATI-705A-N *GTS-705-NIA *AGT-705-NI
*ATI-705A-P *GTS-705-PIA *AGT-705-PI
*ATO-705A-N *GTS-705-NEA *AGT-705-NE
*ATO-705A-P *GTS-705-PEA *AGT-705-PE
Approvals
R
Water heaters are built in accordance with the requirements of the ASME Boiler
and Pressure Vessel Code and received the Certificate of Authorization from the
National Board. The heat exchanger has the NB and the HLW stamps.
*
The California Safe Drinking Water and Toxic Enforcement Act requires the Governor of California to
publish a list of substances known to the State of California to cause cancer, birth defects, or other
repoductive harm, and requires businesses to warn of potential exposure to such substances.
WARNING: This product contains a chemical known to the State of California to cause cancer, birth
defects, or other reproductive harm.
This appliance can cause low-level exposure to some of the substances included in the Act.

Consumer Information
The on-demand water heaters must be installed according to all local and state codes or, in the absence
of local and state codes, the “National Fuel Gas Code”, ANSI Z223.1 (NFPA 54) - current edition.
CSA American, Inc. National Fire Protection Association
8501 East Pleasat Valley Road 1 Batterymarch Park
Cleveland, OH 44131 Quincy, MA 02269
Check your phone listings for the local authorities having jurisdiction over your installation.
Consumer Responsibilities
This manual has been prepared to acquaint you (trained service technician) with the installation, oper-
ation, and maintenance of the on-demand water heater and provide important safety information in these
areas.
Read all of the instructions thoroughly before attempting the installation or operation of the water heater.
Service to the on-demand system should only be performed by a trained service technician.
Massachusetts code requires this water heater to be installed in accordance with Massachusetts Plumbing
and Fuel Gas Code 248 CMR Section 2.00 and 5.00.
Water Temperature Regulation
For systems with storage tanks, the water
temperature in certain situation may vary up
to 30°F (16.7°C) higher or lower at the point
of use such as, bathtubs,showers, sink, etc.
HOTTER WATER CAN SCALD: Water heat-
ers, are intended to produce hot water. Water
heated to a temperature which will satisfy
space heating, clothes washing, dish wash-
ing, and other sanitizing needs can scald and
permanently injure you upon contact. Some
people are more likely to be permanently in-
jured by hot water than others. These include
the elderly, children, the infirm, or physically/
mentally handicapped. If anyone using hot
water in the home fits into one of these
groups or if there is a local code or state law
requiring a certain temperature water at the
hot water tap, then you must take special
precautions.
4
Water temperature over 125°F can cause
severe burns instantly or death from scalds.
Children, disabled and elderly are at highest
risk of being scalded.
Feel water before bathing or showering.
Temperature limiting valves are available.
WARNING
Installation

In addition to using the lowest possible temperature setting that satisfies your hot water needs, a means
such as a mixing valve, should be used at hot water taps used by these people or at the water heater.
Mixing valves are available at plumbing supply or hardware stores. Follow manufacturer’s instructions for
installation of the valves. Before changing the factory setting on the thermostat see table below. Using
the lowest hot water temperature that meets your needs will also provide the most energy efficient oper-
ation of the water heater.
Never allow small children to use a hot water
tap, or to draw their own bath water. Never
leave a child or handicapped person un-
attended in a bathtub or shower.
NOTE: A water temperature range of 120°F -
140°F (49°C-60°C) is recommended by most
dishwasher manufacturers.
The thermostat of the water heater has been factory set at 120°F, to reduce the risk of scald injury. It is
adjustable and can be reset to the desired temperature setting. Some states have a requirement for a
lower setting. If you need hotter water, follow directions for temperature adjustment, but beware of the
warnings in this section.
Site Location
• Select a location near the center of the water piping system.
• Consider the exhaust vent system piping and combustion air supply requirements when selecting the
water heater location. The venting system must be able to run from the water heater to the termina-
tion with a minimal length and number of elbows.
• Locate the water heater near the existing gas piping. If installing a new gas line, locate the water
heater to minimize the pipe length and elbows.
• The water heater should be located in an area not subject to freezing temperatures. Water heaters
located in unconditioned spaces (i.e., attics, basements, etc.) may require insulation of the water piping
and drain piping to protect against freezing. The drain and controls must be easily accessible for oper-
ation and service. Maintain proper clearances as specified on the data plate.
5
Temperature
Settings Time to Produce 2nd & 3rd Degree Burns on Adult Skin
160°F (71°C) About 1/2 second
150°F (66°C) About 1-1/2 seconds
140°F (60°C) Less than 5 seconds
130°F (54°C) About 30 seconds
120°F (49°C) More than 5 minutes
80°F (27°C) - - - - - - - - - - - - - - - -
NOTE: This water heater must be installed according to all
local and state codes or, in the absence of local and state
codes, the “National Fuel Gas Code”, ANSI Z223.1(NFPA
54)-current edition.
Table “A”
Installation

Closed System/Thermal Expansion
CAUTION - PROPERTY DAMAGE HAZARD
As water is heated, it expands (thermal expan-
sion). In a closed system, the volume of water
will grow. As the volume of water grows, there
will be a corresponding increase in water pres-
sure due to thermal expansion. Thermal expan-
sion can cause premature tank failure (leakage).
This type of failure is not covered under the
limited warranty. Thermal expansion can also
cause intermittent temperature-pressure relief
valve operation: water discharged from the valve
due to excessive pressure build up. The temper-
ature / pressure relief valve is not intended for
the constant relief of thermal expansion. This
condition is not covered under the limited war-
ranty.
A properly-sized thermal expansion tank should
be installed on all closed systems to control the
harmful effects of thermal expansion. Contact a
local plumbing service agency to have a thermal
expansion tank installed.
A suitable metal drain pan
should be installed under
the storage tank.
Storage Tank
6
For Storage Tank System Installation
CAUTION - PROPERTY DAMAGE HAZARD
The storage tank should be located in an area
where leakage of the tank or connections will not
result in damage to the area adjacent to the storage
tank or to lower floors of the structure. Due to the
normal corrosive action of the water, the tank will
eventually leak after an extended period of time. Also
any external plumbing leak, including those from
improper installation, may cause early failure of the
tank due to corrosion if not repaired. If the owner/
operator is uncomfortable with making the repair a
qualified technician should be contacted. A suitable
metal drain pan should be installed under the storage
tank as shown below, to help protect the property
from damage which may occur from condensate form-
ation or leaks in the piping connections or tank. The
pan must limit the water level to a maximum depth of
1-3/4” and be two inches wider than the tank and
piped to an adequate drain. NOTE: The pan must
not restrict combustion air flow. Locate the storage
tank near a suitable indoor drain. Outside drains are
subject to freezing temperatures which can obstruct
the drain line. The piping should be at least 3/4” ID
and pitched for proper drainage.
1-3/4”
max.
At least 2” greater than
the storage tank.
Pipe to an
adequate drain
Figure “1”
Hot Water
Outlet
In a closed system use a
thermal expansion tank
Cold Water
Inlet Valve Pressure Reducing
Valve with Bypass
Main Water
Supply
Cold Water Supply
to Fixture
Temperature and
Pressure Relief Valve
Discharge line
6” Max. above drain
Drain line 3/4”
ID Min.
Drain
1“ Min.
Metal Drain
Pan 1-3/4”
Depth Max.
Massachusetts:
Install a vacuum relief
in cold water line per
section 19 MGL 142
Typical water
piping installation
Figure “2”
Storage
Tank
Hot Water
Return Line

Temperature and Pressure Relief Valve
For protection against excessive pressures and temperatures, a temperature and pressure relief valve
must be installed in the opening on the on-demand storage tank. This valve must be design certified by
a nationally recognized testing laboratory that maintains periodic inspection of the production of listed
equipment or materials as meeting the requirements for Releif Valves for Hot Water Supply Systems,
ANSI Z21.22. The function of the temperature and pressure relief valve is to discharge water in large
quantities in the event of excessive temperature or pressure developing in the water heater. The valve’s
relief pressure must not exceed the working pressure of the water heater as stated on the data plate.
IMPORTANT: Only a new temperature and pressure relief valve should be used with your water heater.
Do not use an old or existing valve as it may be damaged or not adequate for the working pressure of
the new water heater. Do not place any valve between the relief valve and the tank.
7
Explosion Hazard
If the temperature and pressure relief valve
is dripping or leaking, have a qualified
service technician replace it.
Examples of a qualified service technician
include: Licensed plumbers, authorized gas
company personnel, and authorized service
personnel.
Do not plug valve.
Do not remove valve.
Failure to follow these instructions can
result in death, or explosion.
WARNING
For Storage Tank System Installation

8
Water Quality and Scale
A complete water analysis and an understanding of system requirements are needed to protect the
on-demand water heaters and water heating systems from scale. Water analysis shows whether
water is hard or soft. Hard water, unless treated, will cause scaling or liming of the heat exchanger.
The rate of scaling increases with temperature and usage because calcium carbonate and other
scaling compounds lose solubility (fallout of solution) at higher temperatures. For example, for every
20°F over 140°F, the rate of scale increases by a factor of 2 (See figure below). Reference target
water quality levels found in the operation / installation manual and consider water treatment if these
levels are exceeded. *
210
180
150
120
90
60
30
0
3900 3300 2700
2100 1500
900 300
0
WATER USAGE, gal/day
LIME DEPOSITED, lb/yr
180°F
170°F
160°F
150°F
140°F
120°F
BASED ON 10 grains/gal HARDNESS
*Source 2003 ASHRAE Handbook HVAC Applications
*
Figure “3”

9
Pump Sizing for Circulation
1. Use the chart below or one appropriate for your conditions to determine the heat loss in the length of
the hot water supply and return piping. For example, 100 ft of 1-1/2 in bare copper tubing results in a
heat loss of 5300 Btu/h.
2. Determine the acceptable temperature drop at the last fixture in the loop. For example, if the supply
temperature from the water heater is 120 ºF (49 ºC) and an acceptable temperature at the last fixture
is 100 ºF (38 ºC) then the acceptable temperature drop is 20 ºF (7 ºC).
3. Calculate the required pump flow rate using the following formula:
FLOW RATE (gpm) = HEAT LOSS (BTU / h)
500 X ACCEPTABLE TEMPERATURE DROP (ºF )
4. Based on the above calculations select a pump for the type of circulation system you will be utilizing:
A). Preferred Method (reference drawing H-1-RP) - Reference pump manufacturers flow vs.
pressure specifications to select a pump that can provide the flow rate calculated above
while overcoming the pressure loss through:
• Tank water heater (reference manufacturer’s information)
• All building supply and return plumbing in the circulation loop (reference
local plumbling codes, standards, or practices)
B). Optional Method (H-1-RO) - Reference pump manufacturers flow vs. pressure specifications
to select a pump that can provide 3 gpm of flow or the flow rate calculated above, whichever
is greater, while overcoming the pressure loss through:
• On demand water heater (reference flow vs. pressure curve of the model
being used)
• Optional storage tank (reference manufacturer’s information)
• All building supply and return plumbing in the circulation loop (reference
local plumbling codes, standards, or practices)
NOTE: Only use pumps of brass or stainless steel construction. Do not use pumps of iron
construction as they will oxidize and clog the inlet filter on the appliance.
Nominal Size, in. Bare Copper Tubing,
Btu/h-ft 1/2 in. Glass Fiber Insulated Copper Tubing, Btu/h-ft
7.71034/3
3.02831
4.32544/1-1
4.52352/1-1
6.92662
8.3308
2/1-2
5.93493
4.840214
Approximate Heat Loss from Piping at 140 ºF Inlet, 70 ºF Ambient *
* Source: 2003 ASHRAE Handbook HVAC Applications
Table “B”

10
Tank Water Heaters in a Circulation Loop
The following applies when using a tank water heater (gas or electric) to provide heat for a circulation
loop. Drawing H-1-RP is an example.
The heat output of the tank must be equal to or greater than the calculated circulation loop heat loss.
(Reference page 5, Step 1 on calculating heat loss).
Electric Tank Water Heater
Since the input and output are the same for an electric tank water heater, this can be expressed as:
Electric Tank Input (Kw) > Circulation loop heat loss (Btu/h)
3413
(1 Kilowatt = 3,413 BTU)
Gas Tank Water Heater
When using a gas style water heater, the efficiency of the tank must be taken into account.
Available Btu output = (Btu input of tank) x (efficiency) > Circulation loop heat loss (Btu/h)
Example:
30,000 Btu input gas tank
0.62 Efficiency
30,000 x .62 = 18,600 available Btu output
Additional Guidelines
On-demand water heaters not recovering a storage tank: In applications involving a commercial
dishwasher, a hot water circulation loop feeding the dishwasher is required.
On-demand water heater recovering a storage tank: In applications involving a commercial dishwasher,
a hot water circulation loop feeding the dishwasher may be required depending on the distance between
the dishwasher and the storage tank. Refer to local codes when determining the need for circulation
loops to dishwashers.