Install Instructions

IM-PR 566442 1114 (Design Manual)
86
Control strategies for radiant cooling should also
explore the thermal mass benefits of the slab.
Opportunities exist for peak-shifting in radiant
cooling, which can avoid high electricity charges
in areas with time-of-use rates. Pre-cooling a slab
in the morning during off-peak hours and then
allowing the slab to “coast” through the peak
period can not only provide cooling when electricity
rates are low but also can take advantage of
cooler ambient temperatures, which lead to higher
operating efficiencies for cooling equipment.
Studies have found that pre-cooling can reduce
energy costs by 10-50% when a building operates
under time-of-use utility tariffs, with peak demand
reductions between 10 and 35%.
27
Under time-of-
use tariffs, on-peak electricity costs can be twice
that of off-peak electricity costs.
27. ASHRAE. 2008. Handbook – HVAC Systems and Equipment. 50.14.
If pursuing a pre-cooling strategy, it is advisable
to perform building energy simulations to gauge
the dynamic response of the slab, considering
comfort, condensation potential, energy
performance and time-of-use utility rates. Typical
response times are provided in Table 3-10.
Controls should be provided to ensure that
secondary sensible cooling systems are used to
make up the difference between the instantaneous
cooling load and radiant slab cooling capacity.
Similarly, secondary sensible systems can be used
to assist in reducing the overall response time of
the primary radiant cooling system after a setback.
Room / Slab Start Temp 75°F 78°F
Slab Thickness
Estimated Time to Cool Down
(hours)
Estimated Time to Cool Down
(hours)
2" 5 6
4" 8 9
6" 12 15
8" 16 20
Table 3-10. Time required to cool down a slab to 63°F from a slab and space starting temperature of 75 or
78°F. Assumptions include: cooling equipment sized for 15 Btu/hr/ft², distribution, heat gain from sub-slab
of 20%, R-0 floor covering, ground temp/mass neglected.
3.7 Combination heating and cooling
considerations
For systems that will provide both heating and
cooling, installation of variable-speed pumps
is recommended to facilitate matching design
flow rates and loads. Sizing of equipment and
components should accommodate the largest
design load, whether heating or cooling. Insulation
should be installed based on the requirements in
heating mode.
Primary piping for heating and cooling systems
may be either two pipe or four pipe. Two-pipe
systems have one primary supply and one primary
return for both the cooling plant and heating
plant. Switching between the heating plant and the
cooling plant is accomplished by a two-position
changeover valve (Figure 3-9). Four-pipe systems
have separate supply and return primary piping for
both the heating plant and the cooling plant.
Shared piping between the cooling plant and
heating plant in a four-pipe system may be
controlled by three-way diverting valves (Figure
3-10).
A properly designed radiant heating and cooling
system can offer excellent performance, comfort
and energy efficiency in each season of the year
and across a wide range of indoor and outdoor
conditions.