The installation drawings in this bulletin are typical.
Check local code requirements for vacuum breaker devices and
cold-water inlet check valves.
If you install a check valve in any of these systems, you
must install a thermal expansion tank.
There are two methods of manifold installation: series and
parallel. Manifold means to pipe together with several
apertures that make multiple connections. That is what man
folding is all about. Think about the intake and exhaust
manifolds on a car. The carburetor sprays gasoline into the
intake manifold (think of this as the cold water supply
line). After the fuel is ignited by the spark plugs, the hot
combustion gases are pushed out of the engine through the
exhaust manifold (think of this as the hot water supply
line). The intake and exhaust manifold on a car is an
example of a parallel design.
Parallel installation uses heaters that are identical in
both BTU input and storage capacity. Normally, parallel
installation is used when there is a demand for large
quantities of hot water over a short period of time, such as
filling a large garden tub or back to back showers.
Equipment being equal, two or more heaters connected in
parallel will deliver more hot water than the same heaters
connected in series. In a parallel configuration, the hot
water demand is taken equally from each unit. For every
gallon of hot water drawn from each unit, one gallon of cold
water is introduced into the water heater.
Each heater will then perform the same amount of work to
heat the cold water. Another important point of parallel
installation is the length of the supply piping and delivery
piping – they must all be the same length. As shown in the
drawing below, section
must be the same length as section
must be the same length as
must be the same length as
The same holds true for the hot water outlet side. Because
the water pressure is constant along the cold inlet piping
and hot water supply piping, the heater with the closest
'run' will do the majority of the work. To prevent this we
‘balance’ the unit with equal pipe lengths. When installing
heaters (and storage tanks) in parallel, it is important to
accurately plan and measure the distances from the cold
water supply pipe to the heaters and from the hot water
outlet on the heater to the hot water supply line. This will
equalize the work between the two water heaters.
Series installation uses heaters that are not identical in
both BTU input and storage capacity. An example might be
when an expansion or addition is made in an application that
will require additional hot water. For example, you add a
new bathroom to your home. The existing system will not
handle the demand and a new water heater must be installed.
The new heater is not identical to the existing heater. In
this example, series installation may be appropriate.
When installing water heaters in series, the heater with the
largest input (BTU or KW) should be the first heater in the
series, at the cold-water inlet side of the system. Series
installation draws hot water from one tank at a time. As hot
water is drawn, it is taken from the last heater in the
series. For every gallon of hot water drawn, preheated water
is introduced into the last heater in the series and cold
water is introduced into the first heater in the series. In
a series configuration, the first heater, piped to the
cold-water inlet, will do the majority of the work. The
second (or remaining) heater will not work as hard because
it receives preheated water, not cold water. The last heater
in the series will do very little work. In the example below
the installation shows a series system with a gas water
heater and an electric water heater. The larger gas heater
is supplying the hot water into the cold inlet side of the
When there is a demand for hot water it is drawn from the
hot water side of the electric heater.