Interface circuit for adding solar heat to a building heating and cooling system

An interface circuit for adding a solar heat system to a building having a building heating and cooling system and having a building thermostat system, the interface circuit being especially wired to be compatibly interposed between the building thermostat system and the building heating and cooling system without rewiring the latter and being also connected to the solar heat system, the interface circuit thereby adding the solar system as a heat source, and the interface circuit automatically substituting the solar source for the building heating system when heat is called for at a level within the capability of the solar system to supply.

FIELD OF INVENTION 
This invention relates to an interface control circuit to be interposed 
between existing building thermostats and an existing building heating and 
cooling system, and to be connected to an auxiliary solar heating system 
to make the latter compatible in operation with the building system and 
automatically controlled by its existing thermostats. 
BACKGROUND AND PRIOR ART 
It is well known that great effort is being made to conserve power, and 
part of the conservation is accomplished by using more solar energy. 
Accordingly, many buildings which have been previously heated entirely by 
consumable fuels are now being converted so that solar sources may be used 
whenever their energy is adequate for the purpose. Efficient use of this 
energy requires achieving a degree of automatic control, and also 
achieving compatible operation between, on the one hand the main heating 
and cooling system of a building and its already existing thermostats, and 
on the other hand an auxiliary solar heating source newly added to the 
building. 
The prior art shows various circuits for controlling operation of an 
auxiliary solar heating system together with a conventional building 
heating and/or cooling system. 
U.S. Pat. No. 4,034,912 to Thomas Edward Hayes teaches a method and 
apparatus for combining a solar heating system with a fuel fired heating 
system for the purpose of substituting the solar source for the fuel 
burning source whenever the energy in the solar source appears adequate to 
supply the required heat. This system provides a microprocessor control 
circuit which integrates the two systems together, as distinguished from 
an interface according to the present invention which introduces an 
auxiliary solar source into an existing system without requiring extensive 
rewiring of the latter. 
U.S. Pat. No. 4,034,738 to Everett M. Barber, Jr. provides a heating system 
in which a control system seeks first to heat the building with solar 
energy, but failing that effort changes over to a fuel burning system if 
the thermostat continues to fall. No interface system is provided for 
adapting an auxiliary solar system to an existing building heating and air 
conditioning system without rewiring the latter. U.S. Pat. No. 3,977,601 
to Vittorio Bearzi is of a similar type showing, however, the use of a 
fuel burning system to supplement the solar source. 
U.S. Pat. No. 3,028,093 to Richard D. Miller shows a system which controls 
several separate fuel burning heating systems from a common thermostat 
while at the same time compensating the current through the anticipator of 
the thermostat so that the thermostat sees substantially the same current 
regardless of whether one, or several of the heating systems, is at the 
moment being operated. The patent does not show an auxiliary solar system 
in combination. 
U.S. Pat. No. 4,052,000 to Terrence C. Honikman shows interfacing of a 
solar water heater with an existing electrical water heater by interposing 
a control logic circuit which is responsive to the temperature in the 
solar system as one criterion for substitution. 
THE INVENTION 
The present invention provides an interface control circuit having 
terminals which are appropriate for interposing of the interface circuit 
between an existing building thermostat system and the control relays of 
an existing building heating and cooling system. Since there are several 
common types of thermostats which are normally used in buildings, the 
interface circuit is provided with terminals suitable for receiving 
connections from different conventional systems, and the circuitry within 
the interface is such as to make it operable with most common thermostat 
systems. Likewise, the interface has output terminals which can be 
connected directly to the relays controlling the building heating and 
cooling system, including relays for controlling a compressor in a 
reversible heat pump system, a relay for controlling a reversing valve for 
such a compressor so that it can perform either heating or cooling 
functions for the building, a relay for controlling a building duct air 
circulating fan, a relay for controlling a building electrical heat strip, 
and terminals for receiving control power from an existing control 
transformer of the 24 volt type also comprising a part of the conventional 
building system. In addition, the interface circuit has terminals for 
connection to a relay for controlling the circulation of a solar heating 
medium, and also has terminals for connection to an aquastat immersed in 
the solar medium for determining whether the medium has stored sufficient 
energy to make its use efficient. 
As pointed out above, various different circuits appear in conventional 
systems. Therefore the interface has switch means which can be selectively 
moved to one of several positions so as to alter the circuitry within the 
interface and make it compatible with several different types of common 
heating and cooling systems and common thermostat systems, the switch 
being properly set at the time of installation of the interface, depending 
on the type of pre-existing building equipment with which the auxiliary 
solar system must be compatible. 
OBJECTS AND ADVANTAGES 
It is an important object of the present invention to provide an interface 
circuit having terminals designed to correspond with, and labelled to 
match, the terminals of conventional existing thermostat systems and 
existing heating and cooling systems used in buildings, whereby existing 
thermostats can supply all functions needed to control a heating and 
cooling system to which an auxiliary solar heating source has been added. 
It is a corollary object to provide an interface control circuit which 
uses the existing 24 volt control transformer in the building to supply 
the power needed to operate the control relays for the interface unit and 
for the auxiliary solar source. 
It is another important object of the invention to provide an interface 
control unit which adds a very small load to the existing 24 volt control 
power transformer, the additional load being under 6% of the normal 
transformer load, which is low enough so that the existing system 
transformer is not likely to be overloaded. 
It is a further object of the invention to provide an interface system 
wherein the heating and cooling anticipators in the existing thermostat 
will function in their normal manner whether the system is operating the 
conventional heating and cooling system of the building or whether it is 
operating the auxiliary solar energy source. In either event, the amount 
of current drawn through the heating and cooling anticipators is such that 
it is within normal adjustment tolerances for proper setting of the 
anticipators, whereby their calibration is not upset by the additional 
auxiliary solar source. 
A major object of the invention is to provide an interface control system 
operative to substitute an auxiliary solar energy source for the fuel 
consuming source of an existing conventional system, which conventional 
source is disabled when the solar source is substituted. This substitution 
is made only if the temperature of the storage medium for the auxiliary 
solar source is sufficient to provide adequate heat. If the temperature of 
the solar medium falls below a preset level, the auxiliary solar source is 
automatically disabled and operation of the conventional heating system is 
restored. It is a feature of this invention that only one or the other of 
the heat sources can be operated at one time, because simultaneous 
operation of both systems would be likely to cause damage. For instance, 
in the case of a building having a hot air duct system circulated by a 
circulation fan, the solar source heat exchanger would probably be 
operated in the same ductwork in series with the conventional heating 
system, usually ahead of the conventional heating system. Care must be 
taken to ensure that the conventional heating source is not operating when 
the solar heating source is putting heat into the duct upstream from it, 
since the conventional heating system is designed to receive air entering 
the duct at room temperature. If the entering air were preheated by an 
upstream solar source the air entering the conventional system would then 
be well above 80.degree. F., which would cause the conventional heating 
system to overheat. It is also necessary to be sure to disable the solar 
heating system before operating the air cooling mode of the conventional 
system, since the compressor would operate at a much higher head pressure 
and heat up if the air coming into the duct were solar preheated and 
therefore excessively hot, rather than at room temperature. The interface 
system is therefore provided with relays that are mutually interlocked in 
such a manner that it is impossible for the conventional heating system or 
the conventional cooling system of the building to be run if the auxiliary 
solar heat is operative, and vice versa. 
A further important object of the invention is to provide an interface 
operative with a conventional building heating system having both a 
primary and a secondary heating stage, usually operated by separate heat 
thermostats. The primary heating stage is usually adequate for heating the 
building, but the secondary heating stage becomes energized during periods 
of severe weather to provide an additional amount of heat to supplement 
the primary heating system. The present interface operates the solar 
system only when heat demands on it are within its capability to supply, 
i.e. when the temperature thereof is adequate, and only when the primary 
heating system is operating by itself, meaning non-severe weather 
conditions. If conditions become more severe and the second thermostat 
closes for the secondary heating system in the building, the interface has 
a relay whose contacts operate to disable the solar heating source and 
restore the conventional building primary heating source to operation. 
Thus the solar heating system is used only during relatively non-severe 
conditions, but if the temperature in the building begins to fall despite 
adequate temperature in the solar heating medium, then the system 
automatically adds the secondary heat source and switches back from solar 
to primary heat source at the same time. 
The conventional heating equipment can be either a fuel burning system, an 
electrically operated system such as a reversible heat pump and/or 
resistance element heat strips, or a combination thereof in the case of a 
building heating system having primary and secondary heat sources. It is 
an advantage of the present system that substitution of the solar source 
for the conventional heating source can be done to provide peak-hour 
heating, thereby reducing the cost of energy drawn from utility lines 
during hours of peak use demand. If desired, additional circuitry can be 
installed to favor use of solar power during peak utility demand hours. 
Still another major object of the invention is to provide an interface 
circuit in which a switch is provided which is adjustable at the time of 
installation of the interface to accomodate its circuitry to several 
different types of conventional building equipment which is already 
installed. For example, where a reversible heat pump constitutes the 
conventional source of heat and cooling, such heat pumps commonly come 
with two different modes of control. In one type of heat pump system, the 
reversing valve for the compressor must be energized during cooling mode, 
whereas in the other conventional type of heat pump system the reversible 
compressor valve must be energized during the heating mode. The switch in 
the interface allows the interface to be matched to the existing building 
heating and cooling system without requiring any rewiring of the latter. 
Other objects and advantages of the invention will become apparent during 
the following discussion of the drawings.

FIGS. 1 and 2 of the present drawings show the same interface circuit unit 
made according to the invention and connected between a building 
thermostat unit and a building heating and cooling system, wherein the 
heating and cooling systems in the two figures and the thermostats in the 
two figures are somewhat different but of usual and standard types. The 
interface is provided with a slide switch S1, S2, S3 which can be moved 
between two different positions at the time the interface is installed in 
order to accomodate the different types of heating systems and the 
different types of thermostats normally encountered. 
The building heating and cooling system shown to the right in each of the 
figures, includes the normal 24 volt transformer TR supplying control 
power to the various switches and relays in the system. In addition, the 
building is equipped with a fan DF which is controlled by a fan relay FR 
which when energized causes the fan to circulate heat through the ducts D 
within the building. The heat is generated by a reversible heat pump 
having a compressor CM which is controlled by a compressor relay CR, and a 
reversing valve relay RVR controls the position of a reversing valve RV of 
the compressor to select between a cooling mode in which cool air is 
pumped into the house ducts D and heat is dissipated outside of the house, 
and a heating mode in which heat is delivered into the house ducts D and 
is taken from the ambient air outside of the house, all in a manner well 
known in the prior art. The building heating system may also include a 
secondary source of heat such as electric heater strips HTR which are 
controlled by a heat strip relay HR. The compressor CM, the fan DF, and 
the reversing valve RV, and the heat strips HTR themselves are only 
schematically shown in the drawings. 
In addition, the building includes a thermostat system which is assumed to 
be already installed and is shown to the left in the drawings. FIG. 1 
includes a cooling thermostat C1 having an anticipator resistor CA 
connected across it, and the thermostat system further includes a primary 
heating thermostat H1 having an anticipator resistor HA connected in 
series with it, the thermostat H1 controlling the compressor when in the 
heat mode, and the thermostat C1 controlling the compressor when in the 
cooling mode. The secondary heat strip source HTR is controlled 
bye*second thermostat H2 which likewise has an anticipator HB connected 
in series with it. 
Ordinarily, in the absence of the present interface the terminals R, G, Y, 
W1, 0 and W2 are connected to similarly labelled terminals of the building 
heating and cooling system. However, the present interface is installed 
for the purpose of adding an auxiliary solar heating system to the 
building. This solar heating system is not shown in detail, but includes 
suitable solar collectors providing heat to a heat storage device which is 
not shown, and this heat storage device has a heated liquid medium which 
is then pumped to a heat exchanger HE which is installed in the building 
duct system D and has heat circulated from it when the duct fan DF is 
turned ON. The solar storage medium pump SP is controlled by a relay SR, 
and the solar system further includes an aquastat T which is immersed in 
the liquid medium for the solar heat system and includes a switch ST which 
closes when the medium has a temperature sufficient so that it can serve 
efficiently as a source of heat for the building. 
It is the purpose of the interface system which is shown in the dashed line 
box in the center of FIG. 1 and FIG. 2 to permit the solar source of 
building heat to be substituted for the existing building heating system 
and to be controlled by the existing thermostat system of the building 
without requiring any changes in the existing wiring of either of those 
systems. Accordingly, the interface is inserted between the terminals R, 
G, Y, W1 and W2 as shown in FIG. 1, or between the terminals R, G, Y, 0 
and W2 as shown in FIG. 2, these thermostat terminals having been 
disconnected and separated from the heating and cooling system terminals 
as shown in the drawings. 
Referring to FIG. 1, the particular type of thermostat system shown in that 
figure includes a manual fan switch FS which can be moved either to a 
continuously "ON" or to an "AUTOMATIC" position, the fan switch FS being 
shown in the "AUTOMATIC" position in the present drawings. In this 
position, the fan runs only when the system is supplying either heating or 
cooling to the building, whereas in the "ON" position, the fan runs 
continuously in the building ducts D. 
The interface which comprises the subject of the present invention has been 
carefully designed to be compatible with most building thermostat systems 
and with most building heating and cooling systems, and it functions to 
add auxiliary solar heat to the building system. This solar heat can be 
selected instead of the primary heating system of the building whenever 
the solar medium temperature as measured by the aquastat T is sufficiently 
high. The secondary building heating system which can be called into 
operation when the thermostat H2 is closed supplements the primary heating 
system controlled by the thermostat H1, but is added to it only during 
periods when maximum heat is required in the building, i.e. during severe 
weather conditions. The auxiliary solar heat is substituted for the 
primary heating system only during milder weather conditions. Therefore, 
if the thermostat H2 closes, indicating severe conditions, the relay 
system within the interface operates in such a manner as to disable the 
auxiliary solar heat source and cause the building heating system to 
operate using the compressor and the electrical heat strips instead of the 
solar system. 
The interface shown within the dashed line box in the centers of both 
figures includes a first relay K1 having four contacts, which relay 
controls the heat pump compressor CM and air circulation fan DF during 
heating functions in which the compressor is running as a heat pump. The 
interface further includes a relay K2 having a single contact which 
disables the solar heat pump and thereby eliminates solar heat as a source 
whenever severe conditions are encountered, during which time the 
compressor CM and electrical heat strip HTR system of the main building 
system are used. Whenever the such conditions are less severe as indicated 
by opening of the thermostat H2, the relay K2 enables the auxiliary solar 
heating system again as a possible source of heat, assuming that its 
storage medium has an adequate temperature. The relay K3 is used only in 
heat pump systems of the type shown in FIG. 2 which activate a reversing 
valve during the cooling mode but which require no reversal of the valve 
during the heating mode. The relay K3 prevents simultaneous solar heating 
and cooling which would be a highly undesirable combination. Finally, the 
relay K4 is controlled by the aquastat and serves to enable solar heating 
or to disable it as a heat source if its temperature is inadequate. 
The functioning of the system can be best understood by describing the 
system in terms of its various functional modes. 
With reference to FIG. 1, if the solar storage medium temperature is below 
a temperature adequate to close the aquastat switch ST, and if primary 
heating is called for by closing of the thermostat H1, the 24 volt source 
will supply power through the terminal R at the top of FIG. 1 to the left 
side of the thermostat H1, which will then deliver power through the 
terminals W1 to the slide switch S2 within the interface, which has three 
different switch segments S1, S2 and S3. The purpose of the switch S is to 
provide easy changeover between the type of reversible heat pump 
compressor system which is normally in the cooling mode as shown in FIG. 
1, and the type of heat pump which is normally in the heating mode as 
shown in FIG. 2. 
Referring to FIG. 1, the power supplied through the input terminal W1 from 
the hot side of the power transformer TR is delivered to the switch 
segment S2 and energizes the winding of the relay K1, whose other side is 
connected to a common terminal C leading directly to the common side of 
the control power transformer TR. The relay terminals K1A and K1C are 
normally open and are therefore closed when the winding is energized, 
whereas the terminals K1B and K1D are normally closed and therefore open 
when the winding of the relay K1 is energized. Since the relay K4 is 
deenergized because the aquastat T has been assumed to encounter too low a 
solar medium temperature, the relay contact K4B will be closed, and 
therefore power will be supplied through the switch segment S2 to the 
output terminal W1, thereby energizing the compressor reversing valve 
relay RVR to put the compressor into the heating mode. The contact RV1 is 
operated by the relay RVR and turns on the compressor relay CR, thereby 
starting the compressor CM. Thus, when the thermostat H1 closes, the fan 
relay FR is energized through the contact K1C, the reversing valve RVR is 
reversed to the heating mode, and the compressor relay CR is turned on, 
thereby supplying normal primary stage heating to the building and 
operating its duct circulating fan DF. The presence of power on the input 
line W1 from the thermostat H1 also energizes the terminal A1 of the solar 
medium aquastat, but since the temperature in the solar medium is assumed 
to be inadequate, no power is delivered to the terminal A2 through the 
aquastat switch ST which remains in open condition, whereby the auxiliary 
solar heat relay SR remains open. 
If secondary stage heat is also called for, the relay H2 will be closed and 
will be supplying power from the hot terminal of the control power 
transformer TR to the terminal W2, and this power will directly energize 
the heat strip relay HR, thereby turning on the resistant heat strips HTR 
to add to the heat supplied by the compressor of the building heating and 
cooling system. 
However, in the event that adequate solar source medium temperature exists, 
the aquastat switch ST will be closed, and therefore power from the hot 
side of the control power transformer TR will be supplied through the 
thermostat H1, through the input terminal W1, and through the terminal A1 
to the terminal A2, whereby to energize the relay K4, assuming that the 
relay contacts K3A and K2A remain closed. Thus, the solar heat pump will 
be energized through the relay contact K4A when the relay K4 closes, 
thereby running the solar heat pump SP to supply heat to the exchanger HE 
in the building ducts D, which heat is then circulated by the fan under 
the control of the relay FR. 
However, if secondary stage heat is also called for by closing of the relay 
H2, the appearance of power at the terminal W2 will energize the relay K2, 
thereby opening the contacts K2A. As a result, whenever the thermostat H2 
is closed, the relay K4 will be opened since the contact K2A is open, 
thereby de-energizing the solar heat pump relay SR and eliminating 
auxiliary solar heat as a source. 
On the other hand, if demand for secondary stage heat ceases as indicated 
by opening of the relay H2, then the relay K4 will close, assuming that 
the contacts K3A and K2A remain closed. As a result power will be supplied 
to the auxiliary solar relay SR through the now closed contacts K4A. The 
relay K4, which is still closed, causes the contacts K4B to be open, 
thereby eliminating power flow to the compressor CM and to the reverse 
valve relay RVR, thereby opening the contacts RV1 and disabling the 
compressor relay CR. As a result, since the relays K1 and K4 are closed, 
power is delivered to the circulating fan relay FR through the contacts 
K1C, and power is delivered to the solar source relay SR through the 
contacts K4A, whereby the primary stage heat from the reversible heat pump 
is eliminated and heat from the auxiliary solar source is substituted in 
its place for circulation by the duct circulation fan DF under the control 
of the relay FR. 
As stated above, it does not matter whether the solar source is available 
or not when the secondary stage thermostat H2 is closed, because the 
closure of the secondary source thermostat H2 energizes the relay K2 
thereby de-energizing the relay K4. 
In the cooling mode as shown in FIG. 1, the relay K1 is open since the 
contacts H1 of the thermostat are open. Instead, the cooling thermostat C1 
will be closed, thereby supplying power from the hot side of the power 
transformer TR, through the terminals Y to the switch segment S1. This 
power will flow through the normally closed contact K1D, thereby directly 
energizing the compressor relay CR. Since the compressor reverse valve RVR 
is not energized when the contacts C1 are closed, the reversing valve 
relay contacts RV1 will be open, whereby the reversing valve will be in 
the normal cooling mode position so that the compressor will supply cool 
air into the ducts D of the building to be circulated by the circulation 
fan DF. Moreover, power will be supplied from the cooling thermostat C1 
through the fan switch FS and the terminals G and the normally closed 
contacts K1B, so that the circulation fan DF will be turned on by its 
relay FR. Since the thermostat H1 is open, no power will be supplied to 
the aquastat terminals A1, whereby no power can be supplied to the solar 
source relay SR. In this way, it is impossible for the solar medium to be 
circulated through the heat exchanger HE in the ducts D of the building 
when the system is operating in the cooling mode. 
Referring now to FIG. 2, this figure is essentially the same as FIG. 1 as 
far as the interface located within the dashed box is concerned. The only 
difference within the box is that the switch S has been moved downwardly 
into its lowermost position which is the position required for use with a 
building heating system using a heat pump whose compressor CM is normally 
in the heating mode instead of the cooling mode as shown in FIG. 1. Also, 
there is a connection to the terminal O, but no connection to the terminal 
W1 since a different building thermostat is used. 
The operation of the circuit of FIG. 2 will be described with reference to 
the various possible types of heating and cooling demanded of the system. 
First, assuming that inadequate solar heat is available because the 
temperature of the medium as measured by the aquastat T is too low, when 
heat is demanded by closing the primary thermostat H1, the closing of this 
thermostat puts 24 volt power from the hot side of the control power 
transformer TR arriving through the terminals R at the top of the figure, 
through the primary thermostat H1 which delivers power through the input Y 
terminals to the switch segment S1 which then energizes the winding of the 
relay K1 whose other side is connected to the common power terminal C. 
Energizing of the relay K1 thereby closes the terminals K1A and energizes 
the compressor relay CR to start the compressor CM. Since the compressor 
is in the normal heating mode already, it is not necessary to energize the 
reverse valve relay RVR. The closing of the relay K1 opens the terminals 
K1B, but closes the terminals K1C to deliver power through the terminals G 
to the circulation fan relay FR. Thus, the compressor generates heat for 
circulation in the building duct D by the circulation fan DF. 
Conversely if adequate solar heat is available as indicated by closing of 
the aquastat T which measures the temperature of the solar storage medium, 
then power will be supplied through the terminals A1, the aquastat switch 
ST and the terminals A2, and this power will close the relay K4 whose 
other side is connected through the normally closed contacts K3A and K2A 
to the common terminal. Energizing of the winding of the relay K4 closes 
the contacts K4A and thereby energizes through the terminals P the 
auxiliary solar heat pump relay SR. Moreover, closing of the relay K4 
opens the contacts K4C and thereby disrupts the flow of power through the 
terminals Y to the compressor relay CR. However, since the relay K1 
remains energized, the contacts K1C continue supplying power to close the 
circulating fan relay FR. Thus, when adequate temperature exists in the 
solar storage medium, the fan DF continues to run in the building ducts D 
and the auxiliary solar heat pump SP continues to operate through the 
relay SR, but the compressor relay CR is de-energized by opening of the 
contacts K4C. 
When a higher degree of heating is required, due for instance to severe 
temperature conditions, the thermostat H2 also closes, and under these 
circumstances, solar heat is never substituted for heat generated by the 
compressor of the reversible heat pump. The closing of the thermostat H2 
energizes the contacts W2, thereby closing the heat strip relay HR and 
also energizing the relay K2 which opens the normally closed contacts K2A, 
thereby de-energizing the relay K4 to open the contacts K4A and disable 
the auxiliary solar heat pump relay SR. In this manner the relay K4 is 
de-energized regardless of whether or not the aquastat T shows adequate 
temperature of the solar storage medium, i.e, whenever the secondry heat 
thermostat H2 has been closed. 
In the cooling mode of the thermostat system of FIG. 2, both the thermostat 
contacts C0 and C1 are closed. Closing of the thermostat contact C0 
applies a control voltage from the hot side of the control power 
transformer TR to the O terminal and to the switch segment S3. It also 
closes the relay K3, thereby opening the contacts K3A, completely 
disabling the auxiliary solar heat system to ensure that no heat can be 
supplied when cooling is demanded. Power through the switch segment S3 is 
also applied to the W1 terminals thereby reversing the reverse valve relay 
RVR and changing the compressor to the cooling mode. Current through the 
other closed cooling thermostat contact C1 also applies power through the 
terminals Y to actuate the relay K1. Power on the terminal Y also passes 
through the closed contacts K4C and through the newly closed contacts K1A 
so as to energize the compressor relay CR and start the compressor CM 
running, the compressor now being in the cooling mode because of 
energizing of the RVR relay. Since the relay K1 is closed, the contacts 
K1C are also closed, thereby energizing the duct circulating fan relay FR. 
Thus, the system runs in the cooling mode while ensuring that the solar 
heat system is disabled by opening of the contacts K3A. 
This invention is not to be limited to the exact forms shown in the 
drawings, for obviously changes may be made within the scope of the 
following claims.