ASHRAE Journal - February 2013 - 40

Radiant Heating and
Cooling System Layout
Floor Slab
Heat Exchanger
The thermally active floor
Pump
Expansion Tank
system is configured as an
isolated loop connected by
heat exchangers to the heating and cooling sources and
thermally coupled to the
Manifold
space through tubing embedded in the floor slab. Loop
isolation avoids introducing
fouling and debris from the
primary heating and cooling
loops for the building into
Boiler
the small diameter tubing
that will be embedded in the
Chiller
floor for the life of the building. A typical flow diagram is
shown in Figure 4.
The author’s typical de- Figure 4: Radiant heating and cooling floor flow diagram.
sign for a multi-zone thermally active slab is a constant temperature (in each mode), The shorter loop length is more appropriate to more varied
variable flow approach. Depending upon the magnitude and floor plates. Manifolds should be located as close as posuse of the space, modulating valves will be on either each sible to the served floor area. If 300 ft (91.6 m) loops are
loop or on each manifold. If there is a modulating valve used, a floor area located 75 ft (22.9 m) from the manifold
on each loop, the typical design would incorporate a slow- would expend half of the loop length as “home run” through
acting two position valve. For larger spaces, with less small an uncontrolled floor space. Location of manifolds early in
scale architectural articulation and with a transient occu- the design process resolves the architectural accommodapancy, entire manifolds, incorporating up to as many as 10 tion of these elements. A wall recess of approximately 5 in.
loops would have a single slow-acting two-position valve (127 mm) deep by about 20 in. (510 mm) high by about 40
for capacity control. Zoned capacity control is important in in. (1.02 m) wide, is required for a 10 loop manifold, and
the cooling mode to modulate the floor in response to mov- provision of access panels can be controversial in a high
ing patches of solar radiation. A portion of the floor under finish space. Zone configuration, with temperature sensor
full sun will require full flow, while a shaded portion of the in the middle, should respond to perimeter adjacency and
floor will require significantly reduced flow to maintain the solar “patch” patterns through the course of the summer
minimum floor temperature of 68°F (20°C). The circulation day.
pump for this system has a variable frequency drive and a
Sizing circulation pumps for the radiant loop requires
pressure controlling bypass, to accommodate very low part comparison of maximum system flow rates for both heating
loads. The floor loop will use either two heat exchangers, one and cooling. Maintenance of the floor at a maximum temfor heating and one for cooling, or a single four-pipe change- perature of 80°F (26.7°C) in heating mode can be achieved
over heat exchanger. Control of supply temperature on the moving no more than 0.8 gpm (0.05 L/s) of hot water at 90°F
secondary side of the heat exchanger is easily achieved with (32.2°C). The heating requirement, however, is likely to have
a two-way valve on the primary side, controlled by a tem- little diversity with most perimeter zones simultaneously at
perature sensor on the discharge of the secondary loop of the the above flow rate. Maximum cooling is likely to be highly
heat exchanger.
diversified with insolated zones requiring full flow through
The author has used 5/8 in. (15.88 mm) high density the loop, while shaded zones require only 0.3 gpm (0.02 L/s)
cross linked polyethylene tubing on almost all radiant heat- to maintain the minimum floor temperature of 68°F (20°C).
ing/cooling projects to date. A common delivery method Depending on the orientation of the glazed walls of the space
for this tubing is a 300 ft (91.4 m) roll. Maximum water and depth of the floor plate, diversity for cooling can easily
flow through a loop of this tubing is between 2.0 and 2.5 be below 50%. The preferred method for sizing these pumps
gpm (0.12 to 0.16 L/s) (glycol solution maximum flow will is to examine the insolation patterns on a number of design
be less). Extension of the loop length to 600 ft (182.8 m) days to determine the maximum percentage of the floor area
results in a loss in capacity per unit area of between 8 and in sun over the course of the year, calculate the diversity for
10% under high solar radiation conditions. This measure full cooling flow and compare that to undiversified full heatmay be cost effective for large undifferentiated floor areas. ing flow.

ASHRAE Journal

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February 2013

ASHRAE Journal - February 2013

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