Note: Descriptions are shown in the official language in which they were submitted.
he Invention
____ ~
This invention relates in general to heating and cooling systems for
buildings and in particular to forced-air systems in which individual room or
~one temperature can be controlledO
For the past several years, the growing energy crisis and escalating
costs have led to numerous attempts to conserve on ~uel. In addition to the
obvious measures of improving insulation to minimize heat transfer in struct-
ures which require heating or air-conditioning> other straightforward measures
such as the adjustment of room temperature to suit the activity taking place
in the room have been employed. Such individual room control has long been
available with conventional electric home heating because each room in a home
heated b~ electric baseboard resistance units is usually equipped with its o~n
thermostat which controls the temperature in the room by interrupting period-
ically the flo~ of current in the units.
Electric heating systems therefore can have the convenience of
simple individual room control, but electric heating is highly inefficient
compared to other heating methods utilizing, for example, gas or oil. I~ore-
over, electric baseboard systems lack the versatility of vil or gas-fired
central forced-air systems because they cannot be easily adapted to air cooling
or humidity control~ Also, of course, there is no capacity for filtration as
ln ~orced-air systemsO Electric systems also compare paorly to forced-air
systems in the cost of useO Some hydronic sys~ems ~hot water or steam) have
zq~e control. H~wever, these systems require separate pumps or valves for each
heating zone, and their cost is comparatively high. Moreover, hydronic heating
systems suffer rom the same disadvantages as electric systems. It is there-
fore understandable that forced-air heating s~stems are domi.nant in the United
States for residential housing~
Nevertheless~ the capabllity of some heating systems to match the
temperature of the room to the activity taking place in the room does permit
energy conservation and a reduction of heating eAepense. Most homeowners are
willing to lower overall room temperature in their homes from a typical day-
time temperature of about 74F to about 65F during the night. This provides
some fuel savings~ A far greater saving is ef$ected if those rooms where only
sedentar~ activities occur are kept at a relatively high temperature ~hile
other rooms which are less used or where more active work is done are maintain-
ed at a relatively low temperature.
For example, in a typical residence, one might have two or more
~athrooms and bedrooms, a kitchen, dining room, living room, family room and
utility roomO During the night, all of these rooms might be run at a ter,lper-
ature of about 65F without undue sacrifice of comfort. On the other hand,
during the day and evening, the kitchen, family room, and baths would probably
be kept at temperatures of about 72F to provide adequate comfort. At the same
~ime, the dining and living rooms, as well as the bedrooms and utility room,
might be left at approximately 65F, again without serious discomfort. Depend-
ing on the average outdoor te~perature, this can result in typical fuel savings
of 7 to 26%.
As previously noted~ such a system of control of temperature in dif-
ferent raoms is available with electric heating systems, but has not been pos-
sible with gas~fired, forced-air systems. One reason is that with a conven-
tional gas-fired furnace the "straightforward" solution of closing air outlets
ln rooms where the temperature has reached a desired level eventually results
in decreasing the air circulation to a point where the furnace will burn out.
Another problem is that when only one or a small number of outlets are open,
they receive the normal maximum air flow of the system. This will usually
generate a high level of noise. This approach is also hampered by the fact
that the usual forced-air furnace is oversized for the house and is relatively
inefficient. The resulting frequent heating cycles to warm one or a few rooms
~aste fuel. Other pr~vious attempts at individual room control for a central
heating system have required complicated and costly control systems.
United States Patent Nos. 2,758,791; 2,789,767; and 2,805,026
illustrate previous attempts to provide individual zone control in conjunction
with a central forced-air heating system. The furnace is fired in response to
temperature indicating signals from ~1) the furnace itself, ~2) the outside of
the building being heated, and ~3) a set of rheostats each associated ~ith a
flo~ control damper in a branch duct leading to one of the heating zones. The
positions of the dampers are in turn controlled by thermostats located in the
associated zonesO While such a system does control burnout, provides variation
in the heat supplied to a variety of zones, and compensates for the outdoor
temperature, it is nevertheless costly, complex and subject to many of the
objections discussed above such as noise, oversizing, and fuel inefficiency.
It is therefore a principal object of this invention to provide a
p~actical forced-air heating system and method ~ith individual room control.
Summary of the Invention
According to one aspect of the present invention there is provided
a forced-air heating system for a building that includes a pluralit~ of heating
zones comprising:
furnace means for generating a orced flow of warm air, said furnace
means; including a heat exchanger containing a working ~luid and a blower for
directing air into contact with said heat exchanger to extract heat from said
working fluid and produce said forced flow of warm air;
air distribution means for distributing said forced air flow includ-
ing an inlet for receiving said forced warm air flow from said furnace means
and at least one outlet located in each of said heating zones;
means for sensing the temperature in each of said heating zones and
generatlng a first signal when the temperature falls to a first preselected
value and generating a second signal when the temperature rises to a second
preselected value;
outlet air flow control means associated with at least some of said
outlets and having an open position for allowing air flow through an outlet to
its heating zone in response to an associated said first signal for that zone
and having a closed position for allowing substantiall~ no flow in response to
an associated said second signal for that zone;
furnace control means for starting said furnace means when at least
a first predetermined num~er of the sensing and generating means are generating
said first signals, for shut~ing down said furnace when less than a second
predetermined number of said sensing and generating means are generating said
first signals, and for var~ing the operating speed of said blower to modulate
said forced warm air flow in response to the number of said sensing means
generating said first signals; and
means for measuring the tempera~ure of said forced flow of warm air
at a point adjacent to said heat exchanger and for producing a warm air temper-
ature signal, and wherein said furnace control means is operable to control the
heat output rate of said furnace means in response to said warm air temperature
signal.
7~3
Accordlng to another aspect of the present invention there is
provided a forced-air heating system for a building that includes a plurality
of heating zones comprising:
furnace means for generating a forced flow of warm air;
air distribution means for distributing said forced air flow in-
cluding an inlet for receiving said forced warm air flow rom said furnace
means and at least one outlet located in each of said heating zones;
means for sensing the temperature in each of said heating zones and
generating a first signal when the temperature falls to a first preselected
~alue and generating a second signal when the temperature rises to a second
preselected value;
outlet air flow control means associated with at least some o~
said outlets and having an open position for allowing air flow through an out-
let to its heating zone in response to an associated said first signal for that
zone and having a closed positian f~r allo~lng substantially no flow in resp-
onse to an associated said second signal or that zone; and
furnace control means for starting said furnace means when at least
a first predetermined number of the sensing and generating means are generating
said first signals and for shutting down said furnace when less than a second
predetermined number of said sensing and generating means are generating said
~- t
2rs s gna sO
The predetermined numbers af the sensing and generating means can
vary according to various factors such as the number of zones, for example
rooms, being heatedO rhe preselected numbers will usually correspond to at
least 20 to 25 percent of the rooms or total heating area in the building.
In a preferred form, the forced air flo~ is modulated in response
to the number of rooms calling for heat~ Also, the control system measures
the temperature of the forced air flow leaving the furnace and uses this
measurement to control the firing rate of the furnace.
The system may utilize any of a wide variety of furnaces. In one
form, a hydronic element such as a miniature boiler or generator with a gas-
fired power burner is the heat sourceO The generator is independently thermo-
statted. Its output is circulated to a an-coil heat exchanger located at the
inlet of the distribution system. In another form, the furnace is isothermal
using a set of heat pipes to conduct heat from a combustion zone to a circul-
ation zoneO The evaporation ends of the pipes in the combusticn zone areseparated from the condensation ends in the circulation zone by a baffle. A
variable firing rate burner heats the evaporation portion of the heat pipes.
In a third form~ the furnace is a modulated warm~air furnace with the main
circulation air blower operating to modulate the air flow in response to room
demand. The furnace is again independently thermostatted.
In a hybrid form suitable for retrofitting conventional fixed rate
furnaces, there is a modified room controlO Certain "principal'l rooms are
provided with conventional registers which are always open and ~he furnace is
actuated by means of a single, standard thermostatO The remaining "secondary"
rooms are individually thermostatted, a~d the associated diffusers open and
close under the control of the room thermostats. In this hybrid system, on
demand of the central thermos~at the burner and blower are operated. The
secondar~ rooms ar0 heated only if both the standard thermostat and the indiv-
idual ther~.ostats are calling for heatO
In the accompanying drawings, which illustrate exemplary embodiments
of the present invention:
Figure 1 is a floor plan for a typical wood-frame, split-entry,
ranch style home;
Figure 2 is a simylified perspective view of a heating system
accordi.ng to the present invention ulilizing a miniature hot water boiler as a
heat source and a fan-coil unit as a heat exchanger;
Figure 3 is a perspective vie~ of a diffuser shown in Figure 2 with
a portion of the floor surrounding the diffuser broken away;
Figure 4 is a perspective view corresponding to Figure 2 with pOl'-
tions broken away showing a heat pipe furnace and an auxiliary air-condition-
ing system that operates in conjunction with the other components of the
heating system of Figure 2;
Figure 5 is a detailed perspective view of the heat pipe assembly
shown in Figure 4 but with the heat pipes oriented vertically rather than
horizontally;
Figure 6 is a simplified schematic view in side elevation of an
up$10w heat pipe furnace with a variable firing rate power combustion burner
and two banks of heat pipes suitable for use in the system shown in Figures 2
or 4;
~Q ~gure 7 ls a schematic view of a heat pipe furnace similar to that
Of Pigure 6 but wlth the combustion burner and air circulation blo~er arranged
in a parallel flo~ system rather than a counterflow system;
~ igure 8, found on the same sheet as Pigures 5 and 6, is a graph
showing typical fuel savings using a room-control heat system according to the
present invention for a house of the type shown in Figure 1 as a function of
the average degree day temperature in degrees Fahrenheit;
Figure 9 is a graph showing the percentage reduction in the heating
load of a house as a function of the average degree day temperature for
houses employing ~he present invention to achieve an average indoor room
temperature reduction of 1, ~, 3, 4 or 5 F; and
Figure 10 is a schematic diagram of the control system used in the
heating/cooling system of Pigures 2-70
Detailed Description of the Preferred Embodiments
Figure 1 shows the floor plan of a typical ranch style house of
wood frame structure. The house may have about four inches of wall insulation
1~ and six inches of ceiling insulation and has approximately 1800 sq. ft. of
living areaO Figure 1 also illustrates a schedule of temperatures in degrees
Fahrenheit which may be maintained in each of the rooms according to the
heating system and method of the present inventionO These temperatures are,
for example, held during the hours of approximately 7 A~ - 11 PM~ and the
rooms are maintained at about 65 degrees Fahrenheit during the night (11 PM -
7 AM).
Furnace efficiency with a conventional burner and blower is assumed
aS 75%O (Typical seasonal efficiency is less than 60%.) Furnace efficiency
with a miniature boiler or generator~ described in greater detail below, is
85%.
Pigure 8 shows the fuel consumption savings that can be achieved
for the house and temperature schedule shown in Figure 1 as compared to a
conventional warm air heating system that maintains the house at a generally
uniform temperature of approximately 74P during the day and evening hours and
at a temperature set back to 65P during the night hours. The heating system
and method of the present inventivn can produce this schedule of room con~rol
"3
and therefore achieve these fuel savings. Figure 8 also demonstrates that
additional fuel savings are possible using the room-varying temperature char-
acteristic of this invention together with a miniature generator or other high
efficiency (85%) heat source as the furnace rather than a conventional forced
air furnace, even one with a modulating air flow. Figure 9 shows the annual
percentage heating load reduction, again as a function of average degree day
temperature (in degrees Fahrenheit). Five plots show the variation in the
heating load reduction for average indoor temperature reductions (in F) of 1,
2, 3, 4, and 5 degreesO With the room-control s~stem of the present invention,
average indoor temperature reductions of 3~5F are typical. Assuming an
average degree day of approximately 35F, the system of the present invention
can therefore result in annual fuel savings of 30 to 60 X 106 BTU's depending
on the type of furnace used in the systemO For a typical indoor temperature
reduction of 3 to 5F, the corresponding heating load reduction for a 35F
average degree day is 7 to 12 percent.
~ igures~ 2 and 1~ show a room~controlled heating system 12 according
to the present in~ention that includes a furnace 14, a distribution system 16
and a control system 18. The furnace 14 generates a forced flow of warm air
~hich is directed from an inlet 16a of the distribution system to a set of
outlets 16h. At least one outlet is located in a separate room or heating æone
Z0 of the building being heated~ Typicall~ there is one outlet in each small
room and two or more in each large roomO The system is formed of conventional
heating components with the exception of the furnace 14 shown in Figttre 2 and
a set of registers or diffusers 22 each secured over an associated outlet 16b
and adapted to control the air flow from the heating system to the associated
room 20. The diusers 22 are controlled by an associated therntostat Z4
7~
located in the roomO The thermostats 24 are spaced from the diffusers. As
shown in Figure 2 and Figure 10, the thermostats provide input control signals
to the diffusers 22 as well as to a central control device 26 for the furnace
typically a microprocessor.
The thermostats 24 are conventional wall mounted devices which can
be set at a preselected temperature ~or temperature range). When the room
temperature falls to the set temperature ~or the lower end of the preselected
range), the thermostat in that room generates a first control signal which is
carried over wiring 28 to a linear actuator 30 of the associated diffuser ~or
diffusers~ in ~hat roomO This signal causes the actuator to move perforated
members, louvres or other flow control elements of the diffuser to an open
position which allows a maximum air flow from the distribution system to the
room. When the room temperature is at or above the set value ~or at or above
the upper end of the preselected range), the thermostat generates a second
control signal which causes the actuator 30 to move the flow control elements
of the diffuser 22 to a closed position which substantially blocks any flow of
air from the distribution system to that room. The wiring 28 also carries the
firs~ and second control signals to the furnace controller 26. One advantage
of this invention is that the control signals can be at low potential (e.g. 24
volts), whlch avoids much of the cost usuall~ associated with household wiring
that must operate at line voltage ~115 volts~.
Figure 3 shows a suitable d~f$user 22 preferably of the type
S.flJ. 3~S 29~D
~escrlbed ln ~L copending Canadian Patent Application~8889-Zt~, filed February
1, 1982, which is commonly assigned with the present application. The diffuser
22 has no exposed parts which could be readily damaged by inadver~ent external
blows to the diffuser. Its actuating system is preferably a linear actuator
-10-
or a small solenoid in combination with a damping element, either of which
acts against a spring~ The damping element or gradual action of the linear
actuator avoids the noise associated with the air flow control elements slam-
ming between the open and closed positions. The linear actuator also provides
a time delay between an actuating signal and the a~tual opening or closing.
This is particularly important during furnace shutdown since the diffuser
should be open a~ter shutdown while residual furnace heat is dissipated through
a brief continued forced air circulationO The linear actuator and spring
provide necessary force ~o open or close the diffuser against the force of the
air flow while having a relatively low operating power consumption. The
diffuser is also highly reliable in operationO
The distribution system 16 is formed of conventional duct work and
inGludes a main duct 16c and multiple branch ducts 16d that each carry a por-
tion of the forced flow of warm air from the main duct to an associated outlet
and diffuser. The size and location of the various ducts will depend upon the
location and heating needs of the room serviced by the branch duct in question
and other standard considerations such as the distance of the branch duct from
the furnace.
The furnace 14 sh~wn in ~igure 2 is a mixed hydrnnic and forccd air
furnace utilizing as a heat source a miniature boiler or generator 32 which
provides a flow of heated water or steam carried by conduits 3~ to and from
the coils of a fan-coil heat exchanger 360 A circulator 38 is connected in the
fluid path to propel the working fluid, if a liquid such as water, through the
generator and heat exchanger. The boiler is preferably fired by a power com-
bustion burner 40 with a variable firing rate controlled b~ the deviGe 26
(see ~igure 10~. The miniature boiler is preferably of the type descri.bed in
'3
United States Patent No~ 4~263~876 which is commonly assigned with the present
application. This generator has an operating efficiency of approximately 85
percent, is highly compact, reliable, and efficient in its use of fuel. In the
preferred embodiment, the miniature generator is gas-fired.
The furnace heat exchanger also includes a main circulation blower
42 which provides a modulated flow of forced air over the coils o~ the heat
exchanger 36 to the inlet 16a of the distribution system. As shown in Figure
10, the modulation of the hlower ~2 and therefore the flow of warm air ~rom
the fan coil heat exchanger is under the control of the central controller 25
in response to the number of rooms calling ~or heat, that is, the number of
thermostats 24 generating the first control signals. ~Vhile the blower in
~igure 10 is shown in a functional block form, it will be understood that this
block includes a complete blower system including speed controls~ The minia-
ture generator 32 will typically produce a supply of hot water with an upper
temperature of approximately 200Fo The water wîll lose approximately 20F
during passage through the fan coil heat exchanger. The flow will increase in
temperature from approximately 70~ to approximately 155F ater passing over
the coils of the heat exchanger 36. The system 12 of Figure 2 also includes
certain features normally associated with forced air heating systems such as
2Q cold a~r return ducts ~no~ shown), an air filter 44 and a humidifier 46. There
is also independent thermostatting for the miniature generator 32. One temper-
ature sensing device ~8 (see Figure 10) is located within the miniature boiler
and a second such device 50 is located in the air flow path immediately after
the heat exchanger. The devices 48 and 50 can be conventlonal bulb type units
ar thermocouplesO As shown in Figure 10, the devices 48 and 50 provide an in-
put signal to the controller ?60 This information is used to control the flr-
-12-
ing rate and/or the ~requency of firing of the power combustion burner ~0.
Usually the firing rate will follow the device S0 ~o maintain the output air
flow at a preselected temperatureO The device 48 acts primarily as a boiler
water temperature control.
In a t~pical operating cycle, the building will begin with all of
the rooms 20 at a temperature at or above the levels set by the thermostats
24, The temperatures in each room will typically vary according to a schedule
such as the one set forth in Figure lo When the temperature in a room falls
so that the associated thermostat for that room generates a ~irst control
signal calling for heat, the associated diffuser in that room will move from
the closed to the open position. When a preselected number of thermostats
generate a first signal calling for h0atJ the controller 26 will generate a
signal that will initiate combustion in the burner ~0. The burner ~0 is pre-
ferably gas-~ired and combustion is initîated by a spark igni~ion system
rather than a pilot to conserve fuel, The combustion begins at the maximum
input rating to quickly warm the water to desired output temperature. The cir-
culator begins circulating the wa~er ~or other liquid working ~luid~ through
the fan coil unit and the blower ~2 begins operation. The blower speed is
determined by the number of rooms calling for heat under the control of the
device 26, For example, if two rooms in an eight room house are calling for
heat, the blower will operate a low speed that will produce a normal output air
flow in the cold rooms. However, if three or more rooms are calling for heat,
the blower will operate at an increased speed.
The precise number of rooms calling for heat which will initiate
the firing of the f-urnace will vary depending on factors such as the size of
the building and the number of roomsO For exampleJ in a four or five room
-13-
house it may be desirable to fire the furnace when only one room calls for
heat whereas in an eight room house it is preferable to require two rooms to
be calling for heat before firing the furnaceO The shutdown of the furnace is
similarly under the control of the thermostats 240 In particular, the control-
ler 26 will emit a signal that shuts down the furnace when less than a presel-
ected number of the rooms are calling for heat. Again, the number will vary
depending upon factors such as the size of the building and number of rooms,
and it may be the same number as required to fire the furnace.
As a general rule, the controller 26 should fire the furnace when
at least 20 to 25 percent of the rooms call for heat in response to first sig-
nals generated by the thermostats. It should be noted that the furnace and
blower will continue to operate and supply heat ~o the distribution system
even as the diffusers 22 move from the open to the closed position under *he
control of the individual room thermostats 240 Thus, for example, in an eight
roQm house the furnace may be fired when five of the eight rooms are calling
for he~t and continue operating as three of the five associated diffusers
close until only two diffusers are openO When one more diffuser closes, the
furnace shuts down, As diffusers close, modulation of the forced air flow is
important to reduce the air flow through the remaining open diffusers to avoid
noise problems. Modulation of the air circulation and the firing rate also
conserve fuel since the furnace gradually decreases its heat output as opposed
to an abrupt termination from a maximum firing to total shutdown.
- A$'ter the furnace shuts down there is residual heat in the system
which should be dissipated, The control system 18 and the linear actuators 30
are pre$'erably constructed so that the diffusers move from the open position to
the closed position over a sufficiently long period of tim~ to substantially
7~3
dissipate the residual heat of the furnace. Modulation of the firing rate is
also a factor which improves fuel consumption efficiencyO The flring rate is
under ~he control of the device 2~ in response to inputs from the temperature
sensing devices 48 and 500 Thus, for example, as the air flow across the coil
decreases with a decreased blower speed, the firing rate can be reduced to
maintain the same temperature of the forced air stream leaving the heat exchan-
ger 360 While codes require that the outlet air temperature of the furnace be
under the control of the device 50 in the output line of the furnace, it is
also possible to place the firing rate of a furnace under the control of device
26 in response to the number of rooms calling for heat or some combination of
inputs from the thermostats 24 and the devices 48 and 50.
Fuel efficiency is also possible through on/off modulation of the
burnerO For example, the blower 42 may continue to operate during the period
between shutdown and re-firing of the furnace. With a 5 to 1 turn down ratio
on the burner, on/off modulation can provide a flexibility in the forced warm
air output of the furnace sufficient to allow the furnace in a ten room house
to fire when only one room is calling for heat. Thus the system of this in-
vention can operate with a "sensitivi~y" of 10% of the rooms calling, or less.
~lowever, for most systems the aforementioned 20-25% ~'sensitivity" is adequate. 2Q ~lth the foregoing system it is possible to reliably maintain the
various rooms on a schedule of temperatures that vary from room to room and
with the time of day. These variations can result in an average room temper-
ature reduction of typically 2 to 5P. As noted above, this temperature dif-
ferential alone results in substantial fuel savings. In addition, this sys~em
results in increased comfort to the occupants of the house since the tempera-
ture in various rooms is tailored to the use of the room and the time of day.
Also, the room control system automatically accounts for outdoor climatic
varia~ions affecting the indoor temperature and other erratic heat loss events
such as the opening of doors and drattsO The climatic conditions are princip-
ally cooling due to changes in wind velocity or direction and radiational
heating due to the sun~ With a conventional heating system employing a single
thermostat to control the temperature throughout the house, rooms which receive
radiational heating from sunlight will be typically held at a higher temper-
ature than rooms which do not receive radiational heating. In contrast, the
heating system of the present invention automatically accommodates for changes
both in climatic factors as well as changes inside the house such as a fire in
a fireplace or changes in the number of people occupying a room.
Figure 4 shows an alternative embodiment of a room-controlled heat-
lng system 12 according to the present invention where all of the components
and modes of operation are the same except that the furnace l~lisa horizontal
heat pipe furnace rather than the fan-coil furnace described with reference to
~igure 20 ~Similar elements in the ~igure 2 and ~igure 4 sys~ems are noted
with the same reference number, but the Figure 4 elements include a prime.) A
set of heat pipes 52 each have an evaporating portion 52a and condensing por-
tion 52b. The heat pipe assembly is shown in further detail in Figure 5. A
baffle 53 defines a line of demarcation between the zones 52a and 52b. The
heat pipes are each pre~erably a copper tube ~lth external ins that hav~ ~ood
thermal conductivity. Each tube contains a wcrking fluid such as water and in
some cases a wick if necessary to return condensate to the evaporation portion.
Transer of heat from a combustion duct 56 surrounding the evaporating sec~ions
52a of the pipes to the condensing portions 52b in a main air circulation duct
62 takes place essentiall~ isothermallyO
-16-
''3
The tubes are arrayed in two banks 54 and 55 with the tubes in each
bank arrayed generally across the air flow path generated by the circulation
air blower 42'. Increasing the number of banks increases the transfer of heat
from the combust;on duct to the air circulation duct, but it also increases the
resistance to the flow. Headers 57~ 57 span the ends of the tubes in each bank
and provide a fluid communication path between the tubes.
The evaporation ends of the tubes are preferably heated by a gas
burner 40' with a va~iable firing rate. The burner may have a turn down ratio
of approximately 4 to 1~ that is, the firing rate can be reduced to 25 percent
of the maximum input rating. The burner is mounted in one wall of the combus-
tion duct 56 so that the combustion products are carried into a heat exchanging
contact with the evaporating ends of the heat pipes. Figure 6 shows a similar
and preferred arrangement using the heat pipe assembly mounted at an angle in
an upflow heat pipe furnace also suitable for use in conjunction with this
invention, One advantage of the slanted pipes is that there is a natural grav-
lty return of condensate. With either heat pipe orientation the flow of com-
bustion products from the burner to an exhaust passes through a first bank 54
of the heat pipes and a second, lower temperature~ bank 55 of the heat plpes
be~ore reaching a burner exhaust 60. This ~lo~ direction is generally counter-
current to the flow of the circulating air from the circulation air blower 42'over the condensing sections of the heat pipes to a circula~ion air outlet 61
~Figure 6~ that feeds the inlet 16a of the distribution system 16.
~ ith a counterflow arrangement of the type described above i~ is
possible to recover some of the latent heat of condensation of water vapor
contained in the products of combustionO Since the latent heat of condensation
represents about ten percent of the total heat energy available in the
"3
combustion process, it is desirable to recover this energyO One drawback,
however, is the corrosive effects of the condensate and the necessity of
removing it. For flue products of natural gas combustion, condensation begins
to occur about 132F when the CO2 level is ten percent. Since the air leaving
the air circulation blower 42' is typically approximately 70 F and heat trans-
fer from the evaporating to condensing sections of each bank of heat pipes is
essentially isothermal, with a co~mterflow system the temperature of the flue
products can, if proper amounts of heat transfer surface are used in ~he heat
pipes, be reduced down to a value only slightly greater than 70F, i.e. to a
temperature sufficient to condense nearly all the water vapor in the flue
productsO Alternatively, some energy efficiency may bc sacrificed in favor of
avoiding condensation by utilizing a parallel flow system as shown in Figure 7.
In this latter system the circulation blower 42' and the circulation air outlet
61 are positioned such that the blower 42' circulates air successively ov~r
the condcnsing portions 52b of the first bank 54 and then the second bank 55
of the heat pipes 52. By utilizing this arrangement and maintaining an air
temperature of about 140F at the outlet 61, the temperature of flue products
ln the burner exhaust 60 will be held at or above 140F and condensation will
be avoided.
Ihe use of a heat pipe furnace in conju~ction ~ith the room-controll-
ed heating system of the p~esent invention offers various advantages. First,
the heat pipe furnace is highly efficient with a steady state efficiency of
over 85 percent and seasonal efficiencies approaching the steady state value.
Second, with a powered or forced combustion the flue products are character-
ized by a low level of emissions ~CO and NOX)o Third, a modulation of the
firing rate avoids problems commonly associated with oversizing of conventional
-18~
fixed firing rate furnacesO Further advantages are tha.t the heat pipe furnace
is rela.tively simple and compPct, ;t has a competitive cos~ of manufacturing
and installation, the use of a power burner reduces the si.ze of the vent
necessary for the system, a comparatively low air pressure drop in the furnace
redu.ces noise, and, since the furnace is isothermal, there is no furnace
failure due to thermal stress.
Figure ~ also illustrates the use of the present system in conjunc-
tion with a conventional air conclitioning system where an air condi.tioning
evaporator 64 is located be~.ween the furnace and the inlet 16a to the distri-
bution systemO A conventional condensing ~mit 66 can be mounted on a slaboutside the building with conduits 68 and 70 carrying the working fluid between
the condensing unit and the evaporator. If the air conditioning system is
made to operate in response to the room ~hermostats 24 and in conjunction with
the on~off diffusers 22J then the energy saving advantages of a room-controlled
forced-air system can be applied to cooling with a standard air-conditioning
system as well as heating with a furnaceO Since the system and method of the
present invention can operate to either heat or cool the rooms of a building;
the term "heating" is used herein to describe both heating and cooli.ng opera-
tions depending on whether the distribution is fed-with a forced warm air ~low
from a furnace or a forced cold air flow utilizing the circulating air blower
of ~he furnace but passing the air flow over the evaporator coils o:E an air
conditioner to produce a forced cold alr flow.
A signi.ficant advantage of the present invention is that while it
achieves its maximum fuel savings when used in conjunction wi.th a fan-ccil
furnace or heat pipe f.urnace as described above, the savings produced through
room control alone are significant i the system i.s heated by ei.ther a modulat-
-19~
7~3
ed forced air furnace or a conventional fixed rate furnace, whether gas-fired
or fired by some other fossil fuel~ With a modulated forced air furnace of
conventional design, the foregoing discussion of the operation of the system
is essentially the same except that the furnace is not as efficient as those
described above~
With a conventional fixed firing rate furnace that does not have
the capability of modulating the air flow, there must be some accommodation
made to prevent a burnout of the furnace when only one or a few diffusers are
in the open position and the air flow rate through the furnace is therefore a
small fraction of its maximum value. A modified system which offers some of
the advantages of the present invention whi.le using a conventi.onal fixed rate
forced air furnace employs conventional registers that are permanently open in
certain 'Iprincipal'' rooms such as common living areas, the kitchen and bath-
roomsO Heat to these areas is controlled by a conventional, single thermostat.
The remaining "secondary" rooms in the house~ such as the bedrooms, utility
room or other special use rooms, have air distribution outlets that are con-
trolled by on/off diffusers of the type described above with references to
Pigures 2 and 3O These diffusers are each controlled by an. associated room
thermostat in the manner described above. Since the furnace is alwa.ys able to
circulate air through the permanently open registers, there is no danger of
burnout of the furnaceO HoweverJ the "secondary" room-controlled system allows
the fuel savi.ng and other advantages attendant the present invention in at
leas~t portions of the houseO ~oth of these last mentioned systems, the one
employing a modulated forced air furnace and the one employing a conventi.onal
fixed rate forced air furnace, allow the system to retrofit existi.ng forced air
heating systems now employed in 75 to 80 percent of all residential houses in
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the United StatesO Because the existing furnace and duct work can be used,
the initial retrofitting cost is comparatively small as compared to the cost of
installing a completely new heating system. ~oreover, the fuel savings of the
version of the room-controlled system of the present invention will pay back
the initial installati.on cost usually over a period of three to ten years
depending upon the average degree day temperature of the home and on the type
of furnace firing system. The invention also provides the primary non-econo-
mic advan~age mentioned above of i.ncreased comfort to the occupants of the
~uilding.
.10 While the system and method of this invention have been described
with reference to the actuation of a single preselected number of room thermo-
stats for both s~arting and shutting down ~he furnace, start up and shut down
can be in response to the actuation of different numbers of thermostats. It is
also possible to program the controller 26 to fire and shut down the furnace
according to building areas calling for heat rather tha.n number of rooms. If
the control is r0sponsive only to room numbers, the controller can be a much
more simple device than a microprocessor. Another possible modification is
using the room control system to govern the furnace firing rate, either alone
or in combination with furnace thermostatsO
~ore generally, while the invention has been described with respect
to its preferred embodiments, various modificati.ons and altera.tions will occur
to those skilled in the art from the foregoing detai.led description ancl the
accon~anying drawings. Such modificati.ons and al.tera.tions are intended to fall
within the scope of the appended claims,
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