AERCO Combination Control Panel (CCP) - Manual (Page 2)

driven closed by the CCP Panel. The combination boilers
will be de-activated until needed either for domestic water
or for heating again. If domestic water is called for while
the valve is open for heating the CCP will automatically
close the motorized valve and transfer the control of the
combination modules back to domestic water production.
Combination Plant Sizing
The required size of a combination boiler plant is not
necessarily the simple sum of the heating modules and
the domestic water modules. ASHRAE has documented
and developed a method of sizing combination plants
based on the diversity of the loads. ASHRAE Systems
1984 Handbook published a researched method of estab-
lishing a relationship of domestic water load to heating
load, and a table of recommended percentage of addi-
tional BTUH to be added to the heating plant to handle
both loads.
Simply expressed,
Domestic Water Load = Diversity Factor
Heating Load
The diversity factor is then converted to a percentage
increase in the base heating load plant from Table A.
Diversity
Factor
Domestic
Heating %
Load to be
added to
Heating
Boiler
2
60%
Table A
1 .8
5 0 % 4 0 %
.6 .5 .4 .3
30% 20% 10% 0%
Example -
From conventional calculations on a typical design:
Space Heating Load = 2840 MBH
Domestic Water Load = 1705 MBH
D i v e r s i t y =
Factor 1705 MBH = .60
2 8 4 0 M B H
From Table A, a diversity factor of .60 indicates a mini-
mum of 30% of the domestic water heating load require-
ment should be added to the heating load for Total Boiler
Plant Capacity.
Total Plant Capacity =
2840 MBH + (.30X 1705 MBH) = 3352 MBH
Total Load
Output/Boiler = 3352 MBH Output = 4KC Boilers
870 MBH/Boiler
Component Design
Domestic water sizing for a combination plant should be
done in accordance with ASHRAE and ASPE standards.
The storage tank size should be adequate for small
domestic water draws without short cycling the domestic
tank aquastat. The combination of recovery and storage
should be as prescribed in the ASHRAE/ASPE methods.
The recovery of the storage tank coil exchanger should
be based on a temperature drop of 2OF, or the same
temperature drop as the heating system that the plant
supplies. When sizing the circulating pump between the
combination boiler(s) and the tank coil, the flow of the
pump must be greater than the minimum module flow of
25 gpm per module. The head of the pump must be
capable of overcoming the pressure drop of the boilers,
the tank coil, and the frictional resistance of all piping/
accessories between them. Air elimination and expansion
accessories may be required in the coil loop as well as the
main heating loop.