Thermo Products COMBOMAX Models from 8 kW to 24 kW : 240 Volts ( single phase ) - Manual (Page 7)

COMBOMAX MC Electric boiler Set-up, use and care guide (revised January 2006) , Page 7.
Pump flow rate = Boiler output ÷ BWTD ÷ 500
• Pump flow rate is expressed in U.S. gallons
per minute or GPM.
• The Boiler output ( in net BTU per hour) is
the maximum amount of heat to be
transferred through the heating circuit to
meet the hot water demand.
• BWTD is the boiler water temperature drop
through the heating circuit.
For example, a 24 Kw COMBOMAX
MC boiler has
a rated output of 81,964 BTU/hour. The system
is designed for a boiler water temperature drop
(BWTD) of 20°F.
Pump flow rate = 81,964
ч 20 ч 500 = 8,2 GPM.
The following table shows the required flow rate
as a function of boiler water temperature drop
(BWTD) and rated power output.
Table 5 : BWTD vs. flow (GPM)
BWTD
Model Kw 10 o
F
20 o
F
30 o
F
40 o
F
COMBOMAX XX-8 8 5,5 2,7 1,8 1,4
COMBOMAX XX-10 10 6,8 3,4 2,3 1,7
COMBOMAX XX-15 15 10,2 5,1 3,4 2,6
COMBOMAX XX-18 18 12,3 6,1 4,1 3,1
COMBOMAX XX-20 20 13,7 6,8 4,6 3,4
COMBOMAX XX-24 24 16,4 8,2 5,5 4,1
3.5.9.2 Selecting a circulator
Consult the manufacturer’s operating
characteristic curves to select the proper model.
These curves plot flow versus pressure, together
with other information such as efficiency and
power. Ask your pump dealer or HVAC
wholesaler to recommend the circulator which
best fits your needs.
3.6 DOMESTIC HOT WATER CIRCUIT
3.6.1 Connecting the domestic hot water
piping
The COMBOMAX
MC
domestic hot water coil may
be set up alone or in parallel combination with
other COMBOMAX
MC
units or storage tanks.
Parallel connections should be made using the
reverse-piping method to ensure equal flow
through each boiler.
The HOT WATER OUTLET and the COLD
WATER INLET connections are clearly marked.
Inlet water connections (COLD WATER INLET)
are to be made to the copper pipe (sweat
connection) at the bottom of the heater. Outlet
water connections (HOT WATER OUTLET) are
to be made to the three-way thermostatically
controlled valve (sweat connection) at the top of
the heater.
The installation of copper unions or copper alloy
unions is recommended on the HOT and COLD
water lines, so that the water heater may be
easily disconnected for servicing if necessary.
These unions must be dielectric (insulating) if
you are making steel-copper connections.
A shutoff (ball) valve is present on the
COMBOMAX
MC
domestic hot water outlet for
servicing convenience.
Use only clean copper or approved plastic pipe
for water connections. Local codes or
regulations shall govern the exact type of
material to be used.
Insulate all pipes containing hot water,
especially in unheated areas.
Thermometer(s) should be installed to indicate
the temperature of the water at or near the outlet
of the water heater and storage tank(s), if
provided.
3.6.2 Expansion tank on the cold water
supply line
Determine if a flow check valve, a back flow
preventer, a pressure-reducing valve, a water
meter or a water softener is present on the cold
water supply line.
A flow check valve creates a closed system and
prevents the water, as it is being heated, from
expanding back into the cold water supply line.
Pressure can build up within the water heater,
causing the pressure relief valve to operate
during a heating cycle. This excessive operation
can cause premature failure of the relief valve
and possibly of the water heater itself.
Replacing the relief valve will not correct the
problem. One method of preventing pressure
build-up is to install an expansion tank on the