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									 B   Y      K   E   I   T   H      W   H   I   T   W   O   R   T H


                                                 arge radiant heat systems place      low a system to meet ever-changing load

                                       L         unique demands on a boiler or a
                                                 series of boilers. These systems
                                       are typically characterized by cold starts
                                                                                      requirements for optimal system effi-
                                                                                      ciency. Another option is to add mass to
                                                                                      the piping system to increase boiler run
                                       with long boiler run-times, high water vol-    times during periods of low demand. For
                                       ume, high mass, cooler required supply         this, water tanks can easily add mass to a
                                       water temperatures, and short boiler cycle-    piping system.
                                       times when the mass is at temperature.             If you have a large boiler for a high-vol-
                                          Of course, large radiant systems require    ume system, the boiler will short-cycle
                                       a boiler or boilers with high output. A key    when the mass is at temperature. The
                                       advantage of radiant systems, however, is      short-cycling is the enemy. To get around
                                       that when the thermal mass of a floor or       the large boiler for high-thermal mass is-
                                       heated surface has reached temperature,        sue, but short cycle times when the mass is
                                       shorter and less frequent boiler cycle-times   at temperature, mass must be added to the
                                       are required. Better yet, a boiler system      boiler system.
                                       with modulation permits the heating, and          Snow melting systems pose a different
                                       later heat-maintenance of the heated sur-      challenge: high demand and high mass
PHOTOA. Installation of 29
                                       face. Either a fully modulating burner, or     with extremely cold water/glycol temper-
miles of radiant tubing
underneath the field at Gillette       the lead-lag staging of boilers, would al-     atures. Here, the challenge is not short-
Stadium in Boston.                                                                    cycling of the boiler. Typically, a boiler
                                                                                        will not short cycle because the load is so
                                                                                        high with snow melting (melting snow
                                                                                        and atmospheric losses). The real en-
                                                                                        emy in snow melting, as far as a boiler is
                                                                                        concerned, is thermal shock and flue-gas
                                                                                             Thermal shock happens when freez-
                                                                                        ing return-water temperatures come
                                                                                        crashing into the heat exchanger in a
                                                                                        long, hard, cold start. The new genera-
                                                                                        tion of condensing boilers take this bru-
                                                                                        tal jab in stride: See “Most Wanted
                                                                                        Characteristics of Radiant System Boil-
                                                                                            To prevent thermal shock, boiler by-
                                                                                        pass piping, the mixing primary and sec-
                                                                                        ondary returns, is one of the more popu-

 2 January 2004
lar methods used to prevent cold           w
system fluids from returning directly
into the boiler, avoiding the key risk      Most Wanted Characteristics for a Radiant-System Boiler
of condensing the boiler flue-gas,            Sure, you can use a non-condensing boiler with boiler bypass piping and a
which can deliver a lethal blow to          heat exchanger to protect the heat source from thermal shock as cold, incoming
the sturdiest of non-condensing             fluids from the radian system rush into the unit at start-up. Or, you could
boilers. Thermal shock and flue gas         specify a boiler designed to handle the rigors of large radiant system duty.
condensation are two entirely differ-       There are some new products out there than can meet these challenges. Aside
ent problems, but they both occur           from ease of maintenance, high efficiency, and affordability, here are some
in a snowmelt system due to the             ideal qualities for a boiler intended to feed large radiant or snowmelt systems:
cold return water temperature                 • Condensing boiler with sealed combustion. The ultimate tool for low, and
                                            super-low liquid temperatures at start-up. Ideally, this boiler’s secondary heat
Trying not to bring the outside in
                                            exchanger transfers exhaust heat to warm liquids before they reach the primary
   The goal of any heating system is
                                            HEAT EXCHANGER. At peak efficiency, water vapor produced in the
to keep us warm. However, a heat-
                                            combustion process condenses back into liquid form to release latent heat.
ing system does this not by heating
                                            Also, its sealed combustion, positive-pressure design assures that the boiler
us; we don’t need the heat. A heat-
                                            can be installed in many environments, even outdoors. Without the need of
ing system keeps us warm by con-
                                            room air for combustion, the boiler is not affected by limited air from within the
trolling the heat loss from our bod-
                                            building, or by negative pressures created by other equipment. However, not all
                                            condensing boilers have sealed combustion.
   High energy heat concentrations
                                              • Fully modulating.These units provide not just one or two stages of firing
go to low energy cold concentra-
                                            capacity, but a wide range of modulation between 100 and 25 percent of the
tions—always hot to cold. Because
of this, our bodies try to heat the         input rate. Fully modulating tries to (and comes closest to) exactly matching the
objects around us. Comfort is de-           required output with the energy input.However, this feature is expensive may
termined by the amount of heat we           not justify the added expense.
lose from our bodies. If we lose too          • Durability. Units that have stainless steel secondary heat exchangers and
much heat, we feel uncomfortably            accept water with up to 20 gpg hardness are good examples. Also, these
cold. If we lose too little heat, the       systems should be built for direct contact with a 50-50 glycol mix, with no need
opposite happens. Naturally at ob-          for an external HEAT EXCHANGER. Keep in mind that inhibited glycol has
ject temperatures of above 85 F, ob-        corrosion inhibitors, whereas uninhibited glycol will “eat” cast iron boilers
jects will start to heat us.                rather quickly.
                                              • No risk of thermal shock. A radiant heating system’s boiler should have
Convective vs radiant                       heat exchangers that go with the flow, at any temperature. For example, while
   A heating system heats the objects       a boiler that requires a minimum of 70 F return water temperature on an
around us to control the heat loss          ongoing basis, initial start-up temperatures should be far lower. Peak operating
from our bodies. Heating systems            efficiency occurs when the return water temperature reaches 70 degrees, and a
do this in different ways and to dif-       fully automatic internal mixing system protects the primary heat exchanger
ferent degrees of effectiveness. A          from condensation. An ideal system is one in which when temperatures begin
forced air convective system heats          to rise, the boiler's control system responds accordingly, maintaining the
air and then uses a fan to blow the         desired output temperatures and obtaining maximum efficiency from the
air within interior spaces. However,        boiler. Therefore, whether the radiant floor system includes placement of
air is a great insulator and a poor         tubing in cold outside areas (snow melting), or traditional low temperature
conductor of energy. We get insula-         indoor systems, the boiler will not be damaged by short periods of cold return
tion from still air spaces.So why use       water, even as low as 30 degrees.
an insulator to conduct energy?
   A baseboard system is another
type of convective system. The air        floor, setting up a convective loop        duction (items in contact with the
around the baseboard unit is heated.      within a space.                            floor radiation – hot going to cold)
Hot air rises along outside walls to         So, does a radiant floor provide        and some convection, with the air
the ceiling, transfers its energy along   radiate heat? Yes and no. Radiant          being heated near the floor, and ris-
the way, cools and falls back to the      floor heat transfers energy by con-        ing. Using the equations in the

                                                                                                         BSE January 2004 3
R A D I A N T      S Y S T E M S

ASHRAE 1999 Handbook (HVAC                                                          thin slab, under or over a wood sub-
Applications, Chapter 52, Radiant           Low Mass vs. High Mass                  floor—and lastly, the spacing. This
Heating and Cooling), for an 80 F             With Radiant Floor heating, mass      issue could encompass an article all
panel temperature, 70 F room tem-           refers to the thermal mass of the       on its own.
perature, and 68 F average unheated         heated space. The thermal mass is          The piping material can be any
surface temperature, total heat out-        comprised of the objects in the         number of substances, but the two
put is 17 Btu per square foot, of           space that the heating system must      most popular are PEX (cross-linked
which 37 percent is convective. The         warm to control the heat loss from      polyethylene) or EPDM-ethylene
remainder is radiative and conduc-          our bodies. The goal of any             propylene diene monomer, a syn-
tive. When compared to a convec-            heating system is to warm the           thetic rubber. Both of these materi-
tive system, 37 percent is actually         objects around us to control our        als are flexible, durable and have
small.                                      heat loss.                              good high-temperature properties.
                                              High thermal mass systems             Further, and maybe most impor-
What do you need all this pipe              consist of large, thick concrete        tantly, both of these materials have a
for?                                                                                proven record for long life and dura-
                                            slabs. There can also be a lot of
   On one memorable project, the            mass in the space, like granite or
                                                                                    bility. That’s an important attribute
owner of a general contracting firm         more concrete. Another key
                                                                                    when you consider that the material
summoned me to a site. As he stood                                                  is inaccessible when buried in con-
                                            consideration is insulation.
among an impressive gridwork of                                                     crete.
                                            Without insulation below the slab
rebar and thousands of linear feet of                                                  Both materials work equally well.
                                            (but most importantly, always at
exposed tubing, neatly arranged for                                                 It depends upon the application as
                                            the perimeter), the soil below the
the next day’s cement pour, he                                                      to which material will work best—
                                            slab becomes part of the high
asked pessimistically: “Does this                                                   EPDM has superior flexibility, but
look right to you?” I scanned the                                                   costs more. PEX is not as flexible, is
                                              Low thermal mass systems have
plans and the tubing and saw no                                                     less expensive, but works just fine
                                            less concrete or less mass in the
crossed circuits and everything was                                                 for concrete slabs.
tied down properly. Pressure                space. And what may also be a key          Because the tubes warm the ther-
gauges showed a consistent 50-psi           contributing factor will be the         mal mass, another important differ-
on all loops.                               presence of insulation below and        ence between radiant floor heating
   “Yes,” I told him. “It looks fine.”      around the slab, helping to isolate     and other forms of heat has to be
   “Well, then,” he demanded,               and reduce the heated mass.             mentioned. Radiant floor heating
“What in the world do you need all            Of course, high mass systems          stores energy in the mass of the
this pipe for?”                             take a long time to heat from initial   structure. Unlike forced-air sys-
   He soon learned that all those           system start-up. Consequently,          tems, which store heat in the air,
pipes are there to perform an impor-        they also take a long time to cool      and try to use the air to transfer
tant function: heat transfer. The           down, or loose their energy.            heat, radiant floor heating stores the
pipes carry warm water, or a mixture          Low thermal mass systems don’t        heat in objects in the room. When a
of water and glycol, to transfer heat       take nearly as long to heat up.         door is opened on a cold winter day,
to the material that surrounds them.        They respond to changes in heat         the warm air – and all the energy in
Radiant floors are often concrete,          demand much faster. However,            it – escapes. Remember, the air does
but can be wood, gravel, sand, or           low thermal mass systems cool           a very poor job of transferring the
even dirt. What are the implications        down much more readily.                 heat to the actual mass of the space
of the type of flooring material for                                                   Similarly, with radiant floor heat-
the radiant system design? The                                                      ing, when the door is opened, the
flooring material encompass items                                                   warm air escapes. But the objects
such as supply water temperature,         walls of the pipe into the floor. The     still have stored energy, so the space
tubing spacing, and the actual abil-      floor warms, and then heats the rest      returns to temperature quickly
ity to deliver the required energy.       of the space as hot moves to cold.        when the door is closed (See
The thicker and less conductive the          The amount of pipe is dependent        ASHRAE 1999 Applications Hand-
floor covering, the less the ability to   upon the application—commercial,          book, A52.6 Radiant Heating and
deliver energy into the space.The         residential, or snow melting—and          Cooling, Design for Total Space
water transfers heat through the          where the pipe is installed—slab,         Heating. “In addition, the peak load

4 Month Year
                                                                          R   A   D   I   A   N   T   S   Y   S   T   E   M   S

may be decreased due to                                                              Clearly, to heat large spaces,
heat (cool) stored in the                                                          we must move enough water to
structure (Kilkis, 1990,                                                           accommodate the heat loss of a
1 9 9 2 ) ”For this reason,                                                        space. One gallon per minute
many large aircraft hangers,                                                       of water equals 10,000 Btuh.
storage facilities and ware-                                                       For radiant floor heating, we
houses, and trucking distri-                                                       almost always design for a 20 F
bution centers, to name a                                                          delta T. One of the only sys-
few, are heated with radiant                                                       tems that has a smaller delta T
floor heating. In a hanger,                                                        are perma-frost protection sys-
when large bay doors are                                                           tems, which keep groundwater
closed, the space reaches                                                          beneath large freezer floors
the setpoint temperature in                                                        from freezing. So if we had a
just 15 or 20 minutes, an                                                          heat loss of 50,000 Btuh, we
impossibility with a forced-                                                       would have to pump 5 gpm;
air system. Of course, the                                                         and for 500,000 Btuh, we’ll
number of warehouses and FIGURE 1.Standard manifold installation.                  need 50 gpm. For the larger of
storage facilities that are                                                        these, a 50 gpm volume doesn’t
heated with radiant floors                                                        care if it’s traveling though one
compared to the number                                                            pipe or ten pipes—it still needs
heated with unit heaters or                                                       to be 50 gpm.
floor mounted furnaces is
small, but that number is                                                         Pipes In, Pipes Out
growing.                                                                             Another part of the pipe puz-
   Pipes in the floor, how-                                                       zle is where manifolds can be lo-
ever, require some special                                                        cated. The manifold is a piece of
precautions. If heavy ma-                                                         copper or stainless steel pipe, or
chinery is to be mounted to                                                       extruded or cast brass or ther-
the floor, the setting of an-                                                     moplastic with barbs. The job of
chor points must be located                                                       the manifold is to transition
long before people begin                                                          from all those pipes in the floor
popping holes in the con-                                                         to a common supply and com-
crete. If there are trench-                                                       mon return pipe to go back to
drains or floor drains, the                                                       the boiler and pump location. A
piping must go around                                                             single manifold location with a
those. Basically, anything FIGURE 2. Reverse return manifold installation.        supply and a return manifold in a
that goes through the floor,                                                      large area such as a warehouse
is bolted to the floor, or protrudes 8-in. for wood, 12 in. for the more      would have a lot of pipes protruding
from the floor, the pipe should go conductive concrete.                       from it. The amount of linear
around, or special provisions must         A volume of water moving           footage of piping is the same, but
be made.                               through one relatively long pipe has   the amount protruding the slab at
   But again, why all those pipes? a lot more friction loss than the          any given location changes .
One key reason is comfort. If piping same volume moving through 10               However, if this same manifold
in a concrete slab is spaced wider pipes of much shorter length. In a         pair is split into several manifold
than 12-in. apart, or wider than 8- small system with, say, 5 gpm, fric-      pairs in several locations, there
in. when installed under a wood tion loss isn’t a big deal. But in-           would obviously be fewer pipes en-
subfloor, “thermal stripping” which crease the system by a factor of 10 or    tering and leaving each location.
is when the difference betw een 20 or more, and piping losses start           You may say that you need the same
heated and unheated areas is appar- playing a much larger role. In large      number of pipes either way, so what
ent to the touch of the foot. The systems, there is a constant juggling       difference does the number of man-
difference reflects the actual con- act between the amount of pipe and        ifolds make? The difference is the
ductivity of the material involved. the type of pump required.                total cost of the project. For exam-

                                                                                                      BSE Month Year 5
R   A   D   I   A   N   T   S   Y   S   T   E   M   S

    Case Study: Gillette Stadium                                                                  Conclusion
                                                                                                     At first, it may seem that radiant
       Among the more interesting applications of radiant heat systems are outdoor                floor heating or snowmelting sys-
    athletic fields. The goal is not to provide snowmelt, but to lengthen the growing
                                                                                                  tems are detrimental to boilers--
    season of the grass. During winter months, the grass wants to go dormant. By
                                                                                                  variable energy requirements with
    applying just the right amount of heat, we can keep the grass growing and the
                                                                                                  potentially cold return water crash-
    groundskeeper happy. This should also start the growing season earlier than would
                                                                                                  ing into a heat exhanger. However,
    otherwise be possible.
                                                                                                  with proper primary-secondary sys-
       In these applications, much depends upon the time of year. In early spring or late
    fall, more heat is required due to the ambient temperature and surrounding ground
                                                                                                  tem piping employed with a mixing
    temperature. Water temperature can range from 110 F to 160 F depending upon                   method to protect both the radiant
    depth of tube, tubing material. Ground temperature depends upon the type of grass             floor heating system and the boiler,
    used.                                                                                         boilers can be ideally matched to ra-
       One such job is an installation done last year at the New England Patriots field at        diant floor heating systems.
    Gillette Stadium. The Patriots new radiant heating/turf warming system consists of               Radiant floor heating systems re-
    153,000 linear ft, (29 miles) of PEX tubing that feed warmth to the soil.                     quire large energy inputs at startup
       Even with new developments in irrigation and soil management, the ability to               and greatly reduced energy input
    grow turf is directly related to the condition and temperature of the root zone.              once the thermalo mass reached
    Constant root zone temperatures help to accelerate turf growth, allowing for faster           temperature. Ideally, the inpute en-
    repair of damaged areas, and also help to maintain a more pliable soil condition, the         ergy of heated “mass” of the boiler
    perfect environment for roots to grow.                                                        system would be variable. In other
       Radiant turf warming systems are rapidly being integrated into the design of new           words, multiple boilers could be
    and reconditioned professional field projects. There’s more to a radiant turf system          staged to meet the ever-changing
    than just installing tubing under the soil. See the cross section above for details.          loads. Conversely, thermal mass
                                                                                                  could be added to the boiler system
                                                                                                  to act as a capacitator, or “shock ab-
ple, if an area needed 20 circuits,                     manifold location, you’re going to        sorber.” The best way to add ther-
each circuit with a supply side and a                   get a hot-spot there. More manifold       mal mass to a boiler system is to add
return side, that would be 20 supply                    locations translate into more even        a hot-water storage tank. The boiler
ends and 20 return ends, or 40 ends                     floor temperatures, especially near       can feed the storage tank and the
of pipe coming to a single location.                    the manifold .                            “loads” of the system can draw off
A manifold can typically have only                         But wouldn’t those extra mani-         the storage tank.
15 circuits, due to the flow issues                     fold locations require some sort of          It may also seem that radiant floor
and the ability to balance flow on a                    balancing to ensure equal flow to         heating uses a lot of piping. How-
per circuit basis—even when using                       each manifold pair? Perhaps. Bal-         ever, we learned some fundementals
reverse return systems. Greater                         ancing valves can be employed. A          of radiant floor heating--the pipe is
number of circuits equals a larger                      simpler approach is “ return” pip-        required to heat the thermal mass of
manifold.                                               ing, using a first-in, last-out tech-     the space, not just the air. The struc-
   But, if we took those same 20 cir-                   nique, as seen in photo B. In this        ture has a lot more thermal mass ,
cuits and split them into 4 groups of                   way, the first manifold fed off the       thus requiring more heat-exhange
5 circuits each, we could use four                      supply trunk has to be the last mani-     surface and more pipe.
separate manifold locations. Each                       fold feeding into the return trunk.
manifold location would then only                       This concept can even be extended         About the author
have eight ends each. We would still                    down to the circuit level.The first          Keith Whitworth is a systems de-
have the same 20 circuits to heat the                   circuit closest to the supply pipe on     sign engineer with Watts Radiant
space, and we would perhaps need                        the supply manifold should be the         Inc. Please provide more on Keith,
some more copper pipe for supply                        last circuit farthest away from the re-   his credentials, years in the business,
and return piping, but we would get                     turn pipe on the return manifold.         and email address.
a system that is easier to install and                  Reverse return always ensures a self-
provides better heat distribution.                      balancing system, because the water
This is important because when you                      “sees” the same pressure drop
have a huge mass of pipes going to                      through each circuit, or manifold.
one location in the floor, for one

6 Month Year

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