Abstract:
This thermostat controls to a constant ratio of temperature and humidity to give approximately constant comfort. It is especially suited for residences with air conditioning equipment but without specific humidity lowering equipment. The thermostat significantly reduces air conditioner running time without degrading comfort as humidity declines in its air-conditioned space. During winter operation the thermostat characteristically raises its dry bulb set point as humidity decreases.

Description:
BACKGROUND OF THE INVENTION 
   This invention is an automatic control mechanism for the control of a heating furnace and/or air conditioning device used to maintain a comfortable temperature in a residence. This invention does not involve any equipment directly involved with heating or cooling. 
   Thermostats for typical single family residences measure only dry bulb temperature for heating or for air conditioning. Yet it is universally known that human comfort is closely related to the combination of temperature and humidity. For example, a simple wall thermostat responds to 77 degrees F. and 20% RAH. in the same manner that it responds to 77 degrees F. and 90% R.H. in spite of the vast difference in human comfort. The thermostat of this invention recognizes this difference in comfort and adjusts dry bulb temperature to give essentially constant comfort over a range of temperature and humidity. Its usefulness is limited to residences that do not have equipment specifically for maintaining a desired relative humidity. 
   BRIEF SUMMARY OF THE INVENTION 
   This invention improves upon cited U.S. Pat. No. 5,732,879 by correcting a measurement deficiency, making the device easier to adjust, reducing its size and making it more suitable for commercial production and home owner use. In addition, this invention serves as a means to take advantage of an unexpected phenomenon that significantly reduces the energy cost of air conditioning in a typical single family residence. 
     FIG. 2  shows an operating line  48  for a conventional thermostat with respect to temperature and humidity. Operating line  48  is moved to a higher or lower temperature by moving the thermostat&#39;s set point as shown by  50 .  FIG. 3  shows an inclined operating line  49  for the thermostat of this invention. Operating line  49  may also be moved to a higher or lower temperature as indicated by moving its set point  50 . The slope of operating line  49  in  FIG. 3  shows that as humidity increases, dry bulb set point temperature decreases to maintain constant comfort. Operating line  49  has a nominal slope of −1 degree F/8% R.H., but it may be varied over a limited range to suit a personal taste. This slope is in agreement with “effective temperature” lines as described by the A.S.H.V.E. (American Society of Heating and Ventilating Engineers) Comfort Chart for Continuous Occupancies of More than Three Hours Duration—from the 1935 Guide. 

   
     DESCRIPTION OF DRAWINGS 
       FIG. 1  shows the thermostat assembly without its cover. 
       FIG. 2  shows an operating line for a conventional thermostat with respect to dry bulb temperature and relative humidity. 
       FIG. 3  shows an operating line for the thermostat of this invention with respect to dry bulb temperature and relative humidity. 
       FIG. 4  shows a schematic wiring diagram for the thermostat of this invention. 
       FIG. 5  shows an exploded view of mechanical parts used to make this thermostat. 
       FIG. 5A  shows an adjustment provided so that more or less rotation of a mercury switch  71  will result from a given change in relative humidity. This adjustment effectively alters the slope of operating line  49  in  FIG. 3 . 
       FIG. 6  shows a detail of how a humidity sensitive strip is attached to its hub. 
       FIG. 6A  shows how a temperature sensitive bimetal helix is attached to its hub. 
       FIG. 7  shows details for both hubs and for one of two identical “spanner” type wrenches used to adjust the angular relationship between the two hubs. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The thermostat of this invention incorporates improvements to the thermostat described by U.S. Pat. No. 5,732,879 (here after referred to as 879). Below are the deficiencies of thermostat 879 and improvements that are part of the present application: 
   Thermostat 879 and the present thermostat are arranged so that mercury switch  71  in  FIG. 1  is balanced atop a temperature-sensitive bimetal spiral  3  which additionally has an incidental spring characteristic. Bimetal spiral  3  is itself positioned by a humidity-sensitive strip  16  that also has an incidental spring characteristic. The humidity strip and the temperature spiral added together form a long spring with mercury in an elongated capsule balanced as a weight at its top end. When mercury in switch  71  moves from one end of its capsule to the other at a control point, the switch effectively moves from one of its bi-stable positions to its second stable position. As a result an excessively large change in temperature and/or humidity is needed to overcome this “dead zone” in the switch&#39;s rotation between bi-stable positions. Thermostat 879 has no provision for over-coming this problem, and undesired deviations from a desired temperature appear as cycling. 
   Conventional thermostats with a mercury switch and bimetal-spiral temperature-element use two electrical resistance heaters called “anticipators” to overcome the above difficulty. For the heating mode an adjustable heating resistor is mounted on an approximately 0.035 inch thick plastic support that is between the resistor and the bimetal. This spaces the heater approximately 4 millimeters from the bimetal. This heater for the heating mode uses current in series with the coil of its furnace control relay. During the air conditioning mode a fixed resistor supplies heat to the bimetal spiral through a metal heat conductor. This second non-adjustable resistor is spaced approximately 4 millimeters from the bimetal, and it is heated by thermostat voltage. In the heating mode, power is on the resistance heater when the furnace is operating. In the cooling mode, power is on the heater when the air conditioner is off. In the heating mode the setting of the adjustable resistor depends upon the resistance of the coil of the furnace control relay. The plastic support for the resistance heater acts as a time delay and as a heat sink that slows heat to the bimetal. In the cooling mode heat conduction through metal that also acts as a heat sink has a similar delay for heat intended for the bimetal. Each of these delays in the heaters slows the response of its heating or cooling cycle. It is thought that these delays are intended to offset the time delay before furnace heat or A/C cool air reaches the thermostat. 
   In the present thermostat a single electrical-resistance heater  4  is placed approximately 1 millimeter from bimetal spiral  3  in  FIG. 1  as a means to overcome the “dead zone” of mercury switch  71 . Heater  4  uses radiation to heat bimetal  3  with nothing between the two. There is no intentional delay or heat sink between heater  4  and bimetal  3 . Single resistance heater  4  is used for both heating and cooling cycles compared to the two heaters used in a conventional thermostat. A predetermined current through heater  4  is set using a variable resistor  12  in  FIGS. 1 and 4 . Current generating heat is always applied to resistor  4  when a heat contact  67  is closed in switch  71 . This means that heater  4  will warm spiral  3  when its heating furnace is on during the heat mode, and it will also warm spiral  3  when its air conditioner is off during the cooling mode. Variable resistor  12  is adjusted so that heat from resistor  4  is just short of being able to overcome switch  71 &#39;s “dead zone” when room temperature is constant. Such a setting does not anticipate heat, since final movement of switch  71  to its alternate position is made by a change in room temperature. 
   Thermostat 879 has its humidity sensitive strip permanently attached to a hub, and its temperature sensitive spiral is also permanently attached to this hub after final assembly. If an alignment adjustment is needed between the set point and measured temperature, it is necessary to bend the humidity strip. In the present thermostat, two separate hubs  28  and  29  in  FIG. 7  are used. But when the two hubs are assembled with a slotted projection  64  on hub  28  inside a matching cavity  65  in hub  29 , the hubs are held in any desired radial relative position by friction between the two hubs. Two spanner type wrenches  38  are provided as a means to rotate the two hubs radially with respect to each other to achieve alignment. Holes  45  and  41  in the two hubs are provided for the two wrenches  38 . It should be noted that alignment of a set point  13  and an indicated temperature  23  in  FIG. 5  is not a fixed condition in this thermostat, because its temperature set point varies with humidity. 
   In thermostat 879, the humidity strip is permanently fixed to its hub. To replace a humidity strip, it is necessary to also replace both hubs, a central shaft and the temperature sensitive spiral. In the present thermostat, a humidity sensitive strip  16  in  FIGS. 1 ,  5  and  6  is made separate from hub  29  as a means for its simple replacement. Strip  16  is held to hub  29  by a screw  39  as shown in  FIG. 6 . 
   For thermostat 879 it is suggested that humidity strip  16  should be “glued with an initial curvature of approximately 5″ radius”. In the present thermostat this has been changed to approximately 2″ radius as a means to improve the linearity of switch  71 &#39;s rotation with changes in humidity. 
   In thermostat 879, an adjustment to the slot angle as shown by  FIG. 5A  can only be done by loosening a screw and retightening it through an access hole in the base. This requires inconveniently removing the thermostat from a wall mounting. In  FIG. 5  the present thermostat uses a notched pin  19  as a means to attach an adjustable slot holder  14  to an operating lever  2 . An arched spring clip  20  fitted into the notch of pin  19  provides force for friction between holder  14  and lever  2 . Since a guide slots  15  attaches to slot holder  14 , this friction provides a means for adjusting the angle of slots  15  from the front of the thermostat without a tool. In  FIG. 5A  guide slots  15  in position A gives normal rotation of switch  71  for a given change is relative humidity. Position B gives more rotation of switch  71  for the same humidity change, and position C gives less rotation. 
   In thermostat 879, a humidity strip is mounted above a central shaft, and an operating lever is mounted below the same central shaft.  FIG. 1  shows the present thermostat&#39;s operating lever  2  and humidity strip  16  both below a central shaft  17  as a means to reduce overall size of the thermostat. 
   Thermostat 879 provides only a single on-off contact that can be used for heating or cooling, but it requires external circuits for both to be used. In the present thermostat mercury switch  71  in FIGS. I and  4  provides contact  67  for heat and a contact  68  for cooling. A switch  8  further provides position  62  for heat and position  63  for cooling as a means for user selection. Switch  8  also has a center-off position. 
   Thermostat 879 has no provision for control of an air-circulating fan which normally comes on automatically with central air conditioning (hereafter referred to as A/C). In  FIG. 4 , this thermostat with switch  8  in A/C position  63  and a fan switch  9  in an automatic position  60  causes activation of a fan control  55  whenever mercury switch  71  is in A/C position  68 . Fan control  55  is activated any time a manual position  61  is closed in switch  9 . 
   Thermostat 879 has only two connections to its mercury switch. Usual residence wiring for heating and A/C has at least four wires from a basement or utility area to the thermostat. In  FIG. 4  this thermostat uses a power connection  56 , a heat connection  57 , an A/C connection  58  and a fan connection  59  as a means to make this thermostat compatible with existing and future four-wire home installations. 
   Thermostat 879 has no cover. The present thermostat includes a cover  21  in  FIG. 5  as a means to protect it from transient hazards. 
   Thermostat 879 has no recessed holes for its wall mounting and leveling. The present thermostat has recessed screw holes  7  in  FIGS. 1 and 5  that allow wall mounting and leveling. 
   Construction Details 
   The thermostat is mounted on a base  1  in  FIGS. 1 and 5  that is preferably molded plastic. Base  1  is hollowed to allow a space for wiring and fittings between a wall and base  1 . Base  1  is molded with a bearing surface  72  to accept an inboard bearing  30 . Base  1  also has wall-mounting holes  7 ; two side switch holes  35 , a top switch hole  78  and hanger tabs  10  and  11  for mounting cover  21  using cover tabs  26  and  27 . Tabs  11  also act as travel limits for operating lever  2 . Base  1  includes holes for mounting a bridge  5 , variable resistor  12  and for miscellaneous wire passages. 
   Rear bearing  30  is inserted into a matching hole  79  in Operating lever  2  and welded or otherwise made a permanent part of lever  2 . Lever  2  carries position indicator  13 . Lever  2  further carries a rotatable guide-slot holder  14  that is held to lever  2  by a notched pin 19  and an arched spring clip  20 . Guide slots  15  is attached to holder  14  by a screw  37 . Friction between lever  2  and holder  14  allows an angular adjustment of guide slots  15  with respect to humidity strip  16  as shown by  FIG. 5A . In  FIG. 5A  a position A for guide slots  15  causes normal rotation of mercury switch  71  for a given change in humidity. In position A guide slots  15  aim at hub  29 . Position B causes greater rotation of switch  71  for the same change in-humidity. Position C causes less rotation. Changes from parallel position A alter the slope of operating line  49  in  FIG. 3 , and changes from parallel toward C should be limited to less than 20 degrees. Screw  37  moving in an elongated slot  76  allows lateral adjustment of guide slots  15  as a means to space strip  16  with respect to guide slots  15  to avoid any frictional contact between them. 
   Lever  2  is attached to base  1  using washers  32 , a spring  33  and a self-locking nut  34 . Nut  34  is tightened against spring  33  to give smooth operating friction to lever  2 . 
   A humidity sensitive strip  16  in  FIG. 6  is made by attaching a humidity sensitive material  16 A to a curved springy backing  16 B that is insensitive to humidity. For example,  16 A may be double weight photographic enlarging paper with emulsion side out, and backing  16 B may be 0.007 inch thick brass shim stock curved to an approximate 2-inch radius. A corrosion resistant wire guide  36  is glued or otherwise attached to backing  16 B. Wire  36  is a close fit into guide slots  15  in  FIGS. 1 and 5  with negligible friction. The opposite end of backing  16 B in  FIG. 6  is formed to partially encircle a plastic hub  29  with a tab  44  to match a slot  43  in hub  29 . A hole  42  in backing  16  B matches a threaded hole  40  in hub  29 , and a screw  39  in hole  40  fixes strip  16  to hub  29 . 
   A second plastic hub  28  in  FIGS. 5 ,  7  and  6  A carries bimetal helix  3  in a slot  47 . Helix  3  in turn carries mercury switch  71 . Helix  3  is shown in  FIG. 6A  as it is glued, for example, into hub  28 . In  FIG. 7  hub  28  has a reduced section  64  that has slots  46  crossed at right angles, and it has a raised rim at its outer end, Both hubs are drilled lengthwise for a snug fit onto common corrosion-resistant shaft  17 . Both hub  28  and  29  have raised end-bosses  66  to act as thrust bearings after assembly. Both hubs are drilled  41  and  45  to accept a spanner wrench  38 . Two spanner wrenches  38  are used later as necessary to align the thermostat&#39;s set point  13  and an indicating thermometer  23  in  FIG. 5 . In assembly both hubs are threaded onto shaft  17  as shown in  FIG. 5 . Reduced section  64  is radially compressed and inserted into cavity  65  in hub  29 . Section  64  and cavity  65  are the same size or section  64  may be slightly larger in diameter than cavity  65 . Such construction will leave the four quarters of section  64  compressed radially to produce friction between-hubs  28  and  29  as a means to hold them in radial alignment. 
   Mercury switch  71  and Bimetal spiral  3  may be purchased as an assembly from a supplier such as Precious Metals, Inc of 1704 Borns St., Reidsville, N.C. 27320, for example. The assembly may be purchased with one degree of rotation of switch  71  for a change of one-degree F. in temperature, for example. 
   A brass bridge  5  in  FIG. 5  is rotated 90 degrees from its installed position so that its shape can be better shown. Bridge  5 &#39;s installed orientation is shown in  FIG. 1 . Bridge  5  is drilled  74  for screws attaching it to base  1 . Bridge  5  is also drilled for an outboard bearing  73  for shaft  17 . In  FIG. 1  a cut out  69  in bridge  5  provides clearance for wires  18  from switch  71 . Threaded holes  75  are for mounting resistance heater support  6 . 
   A preferably molded plastic cover  21  protects the thermostat from tampering and transient hazards. Openings  31  and spacing from base  1  permit free air circulation through the thermostat. A thermometer  23  attached to cover  21  can be seen through an opening  22  and read on a scale  25 . Position indicator  13  can be viewed through an opening  24  for comparison to thermometer  23 . 
     FIG. 4  shows wiring for the thermostat when used in a typical residence with both heating and A/C and a fan that operates whenever A/C is operating. A coil  53  operates a power relay for a heating, device. A coil  54  operates a power relay for a cooling device. A coil  55  operates a power relay for a fan used in conjunction with heating and/or cooling. A power supply  51  for the thermostat is usually 24 volts, 60 hertz in the U.S. Current demand will depend upon coil resistances, but it is likely to be under 2 amps. Other voltages and hertz may be used with proper regard for insulation and wire size. Switch  8  is DPDT with a center-off position. Switch  9  is SPDT. with no center-off. The fan is either manually on in position  61 , or it will automatically come on with A/C when switch  9  is in position  60 . A switch  77  is SPST for on-off control of current to resistance heater  4  and variable resistor  12 . Switch  77  will normally be in its on position unless complete power removal is necessary for electrical maintenance. Physical form of the switches is not critical as long as they perform as indicated in  FIG. 4 . Mercury switch  71  has power-to-heat contact  67  and power-to-A/C contact  68 . Switch  77  should be combined with switch  8 , but such a switch was not readily available for the prototype. 
   Heater  4  &#39;s resistance must be high enough so that its current will be well below dropout current for either coil  54  or  55 . Typically, heater  4  will be 6000 ohms for a maximum current of 4 ma for coil  54  if switch  9  is manually on in position  61 . If switch  9  is in position  60 , the 4 ma current will divide between coils  54  and  55 . The 6000-ohm resistance of heater  4  will limit its heat to approximately 0.1 watts. Typically, variable resistor  12  is 0 to 5000 ohms. Typical heat needed for resistor  4  is 0.03 watts when resistor  4  is typically spaced 1 mm from helix  3  in  FIG. 1 . 
   Typically, four or more wires connect a heating-cooling thermostat to a basement or utility area. This thermostat requires four external connections. In  FIG. 4  these wires are power wire  56 , heat wire  57 , A/C wire  58  and fan wire  59 . 
   Comparison Test 
   This thermostat was tested against a newly purchased conventional thermostat to measure the quality of its performance in controlling A/C. The test was made in a single-family residence having gas heat and a 38,000 Btu/Hr. A/C capacity. Control wires from a basement area were connected to a multi conductor connector, and the two side-by-side thermostats were fitted with mating connectors on short extensions. This connection method was used so that the two thermostats could be interchanged easily without touching them which could jiggle a set point. By trial and error the two set points were set at 76.5 degrees F. using a laboratory type glass thermometer while relative humidity was at 72%. Set points were not touched during the month-long test. From Aug. 8, 2003 until September 2 the thermostats were alternated for control of A/C essentially each day. A/C running time was measured by a clock wired in parallel with the system&#39;s air fan which comes on automatically with A/C. Cooling degree-days (CDD) was taken from the next day&#39;s local paper for each day. Cool air return from the evaporator was typically 60 degrees F. 
   At the end of the test the conventional thermostat had controlled for 15 days and run a total of 98.4 hours. Total CDD for the 15 days was 171. The thermostat of this invention had controlled for 13 days, and it had run a total of 62.8 hours. CDD for the 13 days was 167.
 
Average conventional CDD=171/15=11.4
 
Average this thermostat CDD=167/13=12.8
 
Average hours for conventional=98.4/15=6.56 Hrs. run time/day
 
Average hours for this thermostat=62.8/13=4.83 Hrs. run time/day
 
Percent difference=(6.56−4.83)×100/6.56=26.4%
 
Conventional Hr/CDD=98.4/171=0.575 Hrs. run time/CDD
 
This thermostat Hrs./CDD 62.8/167 =0.376 Hrs. run time/CDD
 
Percent difference=(0.575−0.376)×100/0.575=34.6%
 
   Much shorter runtimes for the thermostat of this invention were unexpected and first thought to be an error. However, close observation of the two thermostats showed significantly different operating patterns. In Delaware where the test was done, a typical A/C cooling day starts at 9 to 10 am, continues through the day as needed and ends usually before midnight. Morning operations were similar, except this thermostat often started at a lower temperature than the conventional thermostat because of high humidity. Daytime operation was similar, and both thermostats called for full time A/C for periods on several days showing the A/C as undersized. However, there was a significant difference in the ways the thermostats shut down at the end of the A/C operating day. Because of a long operating period during the day, inside humidity at the end of the day was usually 10 to 15 percent lower than starting humidity. As a result this thermostat stopped A/C operation at one to two degrees higher than its morning start at the higher humidity. The conventional thermostat continued A/C operation until its starting temperature of 76.5 F. was reached although it was not needed for comfort because of lower humidity. 
   This test shows that the thermostat of this invention can be used as a means to reduce air conditioner operating time and cost at essentially constant comfort by allowing dry bulb temperature to rise appropriately as humidity decreases during a typical day&#39;s air conditioner operation. It also becomes evident that most benefit from this thermostat will be found in areas of high humidity such as east and gulf coasts of the U.S.A. 
   These observations also suggest that the difference between the two thermostats would be even greater if A/C cooling capacity were properly sized or oversized for the test building. A properly sized or oversized A/C with lower evaporator air outlet temperature causing lower humidity would result in an even greater difference in run times for the two thermostats. This thermostat should be especially suited to increase comfort where the A/C is oversized and occupants become cold from low humidity. 
   A conventional thermostat does not give credit to the air conditioner for comfort that comes from reduced humidity. 
   No definitive tests were run using the thermostat in the heat mode. However, it was noted during an extended heating period that heat control was stable and free from cycling or “under-shoot”. There was no change to variable resistor  12  or to resistor  4  from the A/C test above. As during A/C service the thermostat&#39;s set point responded to variations in humidity. Most of the observed changes were in the direction of an increased temperature set point in response to lower humidity which improves comfort, but also increases fuel cost. 
   The above test and observation show that a single adjustable resistance heater near the bimetal can be used as a means to over-come the “dead zone” of a mercury switch using a single resistance setting for both heating and air conditioning modes. The test and observation also show that a time delay and heat sink between the bimetal and its heater can be omitted from a thermostat without degradation of control. 
   The foregoing description of the preferred embodiment of the invention has been presented for the purpose of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.