steam to hot water heat exchanger.
2.7.4.4.1 SYSTEM CAPACITY
(a) For any hydronic heating system with pumping capacity equal to or smaller
than 3.7 kW (5.0 HP), provide two heat exchangers and circulating pumps of
100 percent capacity each. One heat exchanger and pump shall act as stand-by.
(b) For any hydronic system with pumping capacity larger than 3.7 kW (5.0
HP), provide two heat exchangers and circulating pumps of 50 percent capacity
each. No stand-by equipment is required.
2.7.4.4.2 For reheat and miscellaneous heating applications, such as duct-
mounted hot water coils, fan coil units, unit heaters, radiant panels,
cabinet unit heaters, heat exchangers, etc., provide one common heating
system comprising of two heat exchangers, two circulating pumps, and
associated piping. The use of two totally separate and independent heating
systems (one for reheat and other for perimeter terminal units) shall be
considered only if the use of two systems is proven to be cost effective by
the life cycle cost analysis. With a single common heating system care shall
be taken to select the lowest hot water temperature and optimum flow rates to
compensate for the effect of the hot water temperature reset schedule and to
offset the generally constant reheat load of the interior spaces.
2.7.4.4.3 When the heating hot water system is selected for the preheat
application, where ambient or mixed air is below freezing, the hot water
shall be mixed with glycol to prevent freezing of the coil. This system
shall be independent of heating/reheating and shall be designed with two heat
exchangers and two pumps similar to the configuration outlined in paragraph
2.7.4.4.2 above.
2.7.4.4.4 For heating hot water systems with pumping capacities of 5.6 kW
(7.5 HP) and smaller and with tight space conditions, use in-line centrifugal
pumps. For all other systems, use floor mounted, open drive centrifugal
pumps. The pump speed, in either case, shall not exceed 1750 RPM.
2.7.4.4.5 Provide a by-pass type cartridge water filter, 1 to 2 percent of
total flow capacity. See VA Standard Detail 15705-16 for piping connection.
2.7.4.5 CONTROLS
2.7.4.5.1 Provide a hot water temperature reset schedule to be inversely
proportional to the outdoor air temperature. The heating hot water or glycol
solution temperature shall not be less than 58 degrees C (140 degrees F). The
reset schedule shall be adjustable.
2.7.4.5.2 For a system involving multiple terminal units, the use of variable
flow heating system with two-way, straight thru, modulating valves is
strongly preferred over a constant flow system involving 3-way, modulating,
bypass valves. On systems using two-way valves, care shall be taken to
prevent "dead heading" of pumps or no flow conditions by:
(a) Providing at least 15 percent constant flow. This can be achieved by
providing three-way, constant flow valves at the farthest end of each
circuit.
HVAC DESIGN MANUAL
2-14
HVAC SYSTEMS AND EQUIPMENT
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