A.O. Smith AWH-35, Instruction Manual

13

CLOSED WATER SYSTEMS

Water  supply  systems  may,  because  of  code  requirements 

or  such  conditions  as  high  line  pressure,  among  others,  have 

installed  devices  such  as  pressure  reducing  valves,  check 

valves, and back flow preventers. Devices such as these cause 

the water system to be a closed system.

THERMAL EXPANSION

As water is heated, it expands (thermal expansion). In a closed 

system the volume of water will grow when it is heated. As the 

volume  of  water  grows  there  will  be  a  corresponding  increase 

in water pressure due to thermal expansion. Thermal expansion 

can  cause  premature  failure  (leakage)  of  storage  tanks,  water 

heaters  and  HPWH  components  such  as  the  condenser. 

Leakage caused by thermal expansion is not covered under the 

HPWH limited warranty. 
Thermal  expansion  can  also  cause  intermittent  Temperature-

Pressure  Relief  Valve  operation:  water  discharged  due  to 

excessive pressure build up. The Temperature-Pressure Relief 

Valve is not intended for the constant relief of thermal expansion.
A  properly  sized  thermal  expansion  tank  must  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 on all closed water systems.

MIXING VALVES

Water temperature over 125°F (52°C)

can cause severe burns instantly

resulting in severe injury or death.
Children, the elderly and the

physically or mentally disabled are at

highest risk for scald injury.
Feel water before bathing or

showering.
Temperature limiting devices such as

mixing valves must be installed

when required by codes and to

ensure safe temperatures at fixtures.

Water heated to a temperature which will satisfy clothes washing, 

dish washing, and other sanitizing needs can scald and cause 

permanent injury upon contact. See Table 5.
Some people are more likely to be permanently injured by hot 

water than others. These include the elderly, children, the infirm 

and  the  physically/mentally  disabled.  The  Table  below  shows 

the approximate time-to-burn relationship for normal adult skin. 

If  anyone  using  hot  water  provided  by  the  water  heater  being 

installed fits into one of these groups or if there is a local code 

or state law requiring a certain water temperature at the point of 

use, then special precautions must be taken.
In addition to using the lowest possible temperature setting that 

satisfies the demand of the application a Mixing Valve should be 

installed upstream from the building fixtures or at the hot water 

taps to further reduce system water temperature.
Mixing valves are available at plumbing supply stores. Consult 

a Qualified Installer or Service Agency. Follow the mixing valve 

manufacturer’s instructions for installation of the valves.

WATER PIPING

Read  all  installation  requirements  in  this  manual  before 

installation begins. 
The water piping installation must conform to these instructions 

and to all local and national code authority having jurisdiction.
Costs to diagnose, perform service and repair damage caused 

by installation errors are not covered under the limited warranty.
Costs  to  correct  installation  errors  are  not  covered  under  the 

limited warranty.

MAXIMUM PIPE LENGTH

Factory installed pumps will provide the specified water flow for 

up to a total of 50 equivalent feet of water piping between the 

HPWH and the storage tank. Example: 25 equivalent feet of inlet 

(return) piping and 25 equivalent feet of outlet (supply) piping.
Exceeding  these  maximum  lengths  will  cause  the  unit  to 

malfunction and control system lock outs. 

MINIMUM PIPE SIZE

The  inlet  (return)  and  outlet  (supply)  water  piping  installed 

between the HPWH unit and the storage tank must not be smaller 

than the water connection sizes on the HPWH. See Table 4 for 

water line connection sizes and water flow rates.
Water line sizing is a critical installation requirement. Installing 

undersized  water  piping  between  the  storage  tank  and  the 

HPWH  unit  will  cause  insufficient  water  flow  and  will  have  an 

adverse impact on performance and equipment life. 

TABLE 4

WATER CONNECTIONS AND FLOW

UNIT

GPM LPM

CONNECTION SIZE (INCH)

AWH-35

7

27

1

AWH-55

11

42

1

AWH-75

15

57

1.5

AWH-100

20

76

1.5

AWH-115

23

87

1.5

AWH-140

28

106

2

AWH-170

34

129

2

PIPE SUPPORT

All  water  piping  must  be  properly  supported  per  local  code 

requirements. 

PIPE INSULATION

All piping installed between the HPWH unit and the storage tank 

must be insulated. 

COLD WATER SUPPLY

Cold water supply lines should not be connected directly to the 

HPWH inlet or T fitted into the inlet (return) water piping. The cold 

water supply lines should be connected directly to the storage 

tank  only.  See  Inlet  &  Outlet  Water  Temperature  on  page  10 

and Figure 7 and Figure 8 on page 18.

WATER PRESSURE

System water pressure should be maintained between 40 and 60 

PSI. Local code may require, and the manufacturer recommends, 

installing  a  pressure  reducing  valve  (PRV)  in  the  cold  water 

supply to the building to maintain consistent water pressure.