ASHRAE Journal - March 2013 - 55

March 2013

ASHRAE Journal

communicate them to the local ﬂoor control panels. The ﬂoor
control panels will then activate the local two-position valves
to maintain the setpoint. Heating and cooling changeover and
control of ﬂoor loop supply temperature are also typically
handled by the BMS.
Control of the thermally active ﬂoor is also very different
from control of other faster responding systems.2 A basic rule
of controls theory is to avoid using a quickly varying stimulus
to control a slowly responding system because doing so can
result in “thrashing” the system by sending conﬂicting inputs
within the time frame of the system’s response. The thermally
active ﬂoor is a relatively slow-acting system, so controlling
this system with a thermostat sensing a potentially variable
medium, such as the room air, likely will have less than optimal results, especially responding to moving patches of solar
radiation.3 By the time an air thermostat reacts to rising air
temperature from a solar heated ﬂoor, the ﬂoor will have risen
to a temperature that may take a signiﬁcant time to correct.
The best control strategy would seek a more stable stimulus
and count upon the long time constant of the radiant ﬂoor to
dampen the oscillations of the more volatile room air temperature. The most important consideration is control of heating/
cooling changeover.
Heating cooling changeover should be a rare event and
should be controlled to avoid driving the ﬂoor from one mode
to another. Control sequences that recognize the thermal mass
of the ﬂoor and exploit reasonable temperature deadbands
can avoid such “thermal thrashing” of the ﬂoor.4 Systems designed by the author have used two general control schemes
for thermally active ﬂoors, both of which use embedded tem- Photo 1: Saint Meinrad Archabbey Church, thermally active floor.
perature sensors in the ﬂoor to enable it to be controlled to a
setpoint, but with two different strategies for determining the the building are glass under extensive overhangs. This project
ﬂoor temperature setpoint.
features a variable air volume system serving all closed areas,
The ﬁrst of these strategies is applicable to buildings or such as classrooms, ofﬁces, conference room, and store. The
spaces that incorporate a high thermal mass exterior envelope. exhibit/circulation/break-out area that connects these closed
In this strategy, the setpoint for the ﬂoor temperature is reset spaces is conditioned with a thermally active ﬂoor (Photo
based upon the inside surface temperature of an exterior wall. 2). Thermostats in the closed rooms control the temperature
This strategy was pursued in the implementation of a thermal- and airﬂow to those rooms and the return air ﬂows from these
ly active ﬂoor system for the renovation of the Saint Meinrad rooms through the circulation areas back to the air-handling
Archabbey Church in St. Meinrad, Ind., a late 19th century unit. The return air from the closed spaces provides dehumidiGothic Revival cathedral church (Photo 1). The church has ﬁcation and ventilation to the circulation spaces. Adequate
an air system for ventilation, dehumidiﬁcaExterior Sol-Air
Interior Air
Wall Surface
Wall Interior Floor Setpoint
tion and some sensible cooling and a ther(°F)
(°F)
(°F)
(°F)
(°F)
mally active ﬂoor system. In this project, the
16.0
70.0
65.9
35.0
80.0
ﬂoor setpoint temperature is reset based on
the temperature of the 2.5 ft (0.75 m) thick
31.0
70.0
67.0
45.0
77.0
sandstone exterior wall, outside of the 0.75
47.0
70.0
68.2
55.0
74.0
in. (19 mm) insulation and ﬁnish drywall ap62.0
70.0
69.4
65.0
Off
plied to the inside of the wall. The actual set71.0
75.0
74.7
72.5
Off
points were tuned in the ﬁeld, but the initial
79.0
75.0
75.3
77.5
72.0
ramps were as shown in Table 1.
83.0
75.0
75.6
80.0
70.0
On a later project, the Virginia Hand Call87.0
75.0
75.9
82.5
68.0
away Discovery Center, near Columbus, Ga.,
the structure is not nearly as massive as the Table 1: Radiant floor setpoint temperature as a function of wall interior temSaint Meinrad Church. Most of the walls of perature with coincident driving temperatures.

ASHRAE Journal - March 2013

Table of Contents for the Digital Edition of ASHRAE Journal - March 2013