generate three gallons of domestic hot water
per minute.
Rather than operating an oversized boiler to
deliver adequate domestic hot water, a better
alternative is to add a storage tank—often
called an “aqua-booster”— in conjunction with
the coil. Domestic hot water is generated by the
coil and the tank maintains sufficient volume to
satisfy larger needs. Figure 4.
38 National Oilheat Research Alliance
External tankless coils
Figure 5 below, shows an
older style external tankless
(AKA “sidearm”) coil. It
is a small tank with a heat
transfer coil inside. As the
boiler water circulates by
gravity over the coil it heats
the domestic water inside.
The most popular external
coil today is the highly efficient
plate heat exchanger
(See Figure 6 on following
page). It features a series of
plates containing two separate
water passages sandwiched
together. Boiler water flows through
every other plate and domestic water flows
through the adjoining plates allowing heat
transfer while maintaining complete separation
of boiler water and potable water. The domestic
hot water produced is typically stored in an
aqua-booster.
Flat plate heat exchangers are also used for
radiant heating and snow melting applications.
Indirect water heaters
An indirect-fired water heater
is a storage tank with an internal
heat exchanger. See Figure 7 on
following page.
Compared to a direct-fired water
heater, in which the aquastat
controls the oil burner operation,
in an indirect the aquastat
controls a circulator that allows
boiler water to flow through the
heat exchanger.
In contrast to tankless heaters,
indirect water heaters typically
enable a smaller boiler to be used
because of the amount of domestic
hot water stored in the tank.
Figure 4
Figure 5