With an Integrated Combo System, the hot water
temperature is approx. 130˚F (54˚C) which is
60˚F (15.5˚C) above the return air temperature.
These units typically have a temperature rise of
35˚F (2˚C) to 40˚F (4˚C) and therefore would
deliver air at the diffuser at approximately 105˚F
(40.5˚C) to 110˚F (43˚C).
Design vs. Field Conditions
The factors discussed above become very
important to consumer comfort. Even small
differences between design parameters and
actual field conditions can impact greatly on
output capacity. Therefore, it is important to do a
thorough and complete commissioning of the
integrated combo system to ensure the design
parameters are met.
Water System Pressures
Within the water system of an Integrated Combo
system, there are three terms that the
designer/installer must understand. These are
head pressure, water flow rate, and pressure drop.
Head Pressure
Head pressure is the pressure created by the
circulation pump to push water through the
piping system. It is this pressure which is used
to overcome the resistance to water flow
(friction) caused by the water pipe and fittings. It
is similar in concept to the external static
pressure in an air duct system. Head pressure
is measured in feet or water (millimeters
of water).
Note: Although the water in the combo system is
pressurized by the domestic water system the pump
is required to create water flow in the heating loop.
The domestic water system applies the same
pressure to the supply and return sides of
heating loop.
Note: The vertical height of the heating loop does
not impact on the head pressure as the pressure
required to push the water up the vertical height is
offset by the weight of the water in the vertical drop
on the other side of the heating loop.
Flow Rate
Flow rate is the amount of water flowing in
the system. It is directly related to the head
pressure and the resistance to flow. Flow rate
is measured in gallons per minute (liters
per minute).
Pressure Drop (PD)
Pressure drop (PD) is the reduction in total
pressure caused by components added to a
piping system such as coils, valves, and fittings.
The measurement of pressure drop is the
difference in pressure on the inlet side of the
component and the outlet side. Pressure drop is
measured in feet of water (millimeters of water).
When connecting the water lines for heating loop
(air handler) to the domestic water system, the
pipes should be connected with a “tee” to the
side of a vertical domestic water pipe or the
bottom of a horizontal domestic water pipe. This
is to help prevent air from entering the heating
loop. The connections should be as near as
practical to the water heater.
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