Patent Grant
6851948
This invention relates generally to the field of gas furnaces, and in particular to a system and method for monitoring the vent system of a furnace, so as to shut down the furnace in the event the vent becomes restricted and overheats, thereby causing a pressure greater than atmospheric pressure in the vent.
As disclosed in U.S. Pat. No. 4,401,425 to Gable et al., a manually resettable thermal switch is used to sense the temperature of flue gases within the discharge box of a forced air gas fired furnace. The thermal switch is arranged to shut down the furnace power supply and to turn off the gas valve when it senses an over-pressure condition, caused by an over-temperature condition, in the discharge box. Once the thermal switch opens, a serviceman must be called to determine the cause of the shut down and reset the switch.
Oftentimes, the thermal switch can be tripped by events other than a vent blockage, such as high wind conditions or momentary downdrafts. In the event the building being heated remains unoccupied for a long period of time during cold weather, and a trip occurs during this time, water fixtures and pipes can freeze up and burst, causing a good deal of costly damage to the structure. Service people sometimes are not readily available, and extended delays in the serviceman's arrival during cold weather can also result in broken water pipes and fixtures. Reoccurring nuisance trips where a serviceman must be called to reset the thermal switch can also be extremely annoying as well as costly.
Briefly stated, a gas furnace responsive to a thermostat includes a thermal switch wired in series between the furnace power supply and the thermostat. A microprocessor connected to the thermal switch detects when the switch opens and closes, carrying out prearranged programs in response thereto. The thermal switch is mounted so that it opens when an over-pressure in the furnace draft system is detected, as evidenced by hot flue gasses passing over the thermal switch probe. The switch is allowed to cycle at least one time before the furnace is disabled. After a certain period of the time, the combustion cycle is reinitiated and the above steps are repeated if the thermal switch again resets.
According to an embodiment of the invention, in a gas furnace that is responsive to a thermostat, and which contains a power supply, a gas valve, and an autoresettable thermal switch having a thermal switch probe that is responsive to an over-pressure condition, and a microprocessor responsive to the thermostat and the thermal switch for controlling the power supply and the gas valve, and wherein the gas furnace includes a furnace vent system, a process for detecting a blockage in the furnace vent system includes the steps of mounting the autoresettable thermal switch adjacent to an entrance to the furnace vent system so that the thermal switch opens due to a flue pressure exceeding a certain level and causing hot flue gasses to pass over the thermal switch probe; electrically connecting the thermal switch in series between the power supply and the thermostat; programming the microprocessor to carry out the following steps upon the thermostat calling for heat:
(a) instituting a combustion cycle; (b) sensing a condition of the thermal switch and detecting when the thermal switch opens; (c) determining, when the thermal switch is open, whether the number of openings of the thermal switch or the duration that the thermal switch remains open exceeds a preprogrammed criterion; (d) disabling, when the number of openings of the thermal switch or the duration that the thermal switch remains open exceeds the preprogrammed criterion, the furnace and reinstituting the combustion cycle after the furnace has been disabled for a specified period of time; and (e) allowing, when the thermal switch is open and the furnace is not in a disabled state, the thermal switch to reset and reinstituting the combustion cycle upon resetting the thermal switch.
Referring to
The inducer unit is shown in greater detail in FIG. 2 and includes an inducer box 18 that is secured to a back wall 19 in assembly. The back wall faces the heat exchanger exit and has an opening (not shown) through which flue gases are admitted into the inducer box. An inducer fan motor assembly 20 is secured by screws 21 to a front wall 23 of the inducer box adjacent to an elbow 24 that forms part of the furnace vent system. The elbow is connected to a vent pipe 25 (
Vent elbow 24 is arranged to pass over a flange 30 in the front wall of the inducer box which surrounds a flue gas discharge opening 33. As is well known, the blower wheel creates a draft within the inducer box which causes the flue gases to flow from the inducer box into the vent system. A sensor housing 40 is secured to the vent elbow and, as is explained in detail below, contains a temperature sensitive thermal switch 45 for monitoring the flue gas temperature at the entrance to the vent system.
Referring also to
Vent elbow 24 includes a linear inlet section 63 that is connected to flange 30 mounted upon the front face of the inducer box. The inlet section, in turn, is connected to a linear outlet section 64 by means of a 90 degree bend section 65. A mounting pad 67 is provided upon the elbow inlet section which surrounds a rectangular opening 68 that passes through the elbow. The sensor housing is attached to the mounting pad using a screw 69 and a tab 70 that is insertable in a slot provided in the pad so that the opening 49 in the housing is in axial alignment with opening 68 in the pad. Once attached to the elbow, the interior of the housing is in fluid flow communication with the interior of elbow 24 so that fluids such as air and flue gases can be exchanged between elbow 24 and the surrounding ambient.
Due to the flue gas temperature and velocity within the inducer box, linear inlet section 63 of vent elbow 24 is placed under a negative pressure when furnace 10 is operating normally and the vent system is not restricted. In the event the vent system becomes blocked, the pressure within the elbow region increases. Accordingly, during normal operations ambient air is drawn through the sensor housing and passed into the vent system. When the vent becomes restricted, however, the pressure in the elbow increases. The direction of flow through the housing is thus reversed, causing hot flue gases to pass over the thermal switch probe. When the normally closed thermal switch 45 reaches its preset threshold temperature, switch 45 cycles open. Switch 45 remains open until such time as the probe temperature is reduced below the threshold level whereupon switch 45 resets automatically to the normally closed position.
As illustrated in
Initially, when thermostat 72 calls for heat, a microprocessor 76 initiates a combustion cycle and monitors the condition of thermal switch 45. In the event thermal switch 45 opens, furnace 10 is programmed to shut down automatically and thermal switch 45 is allowed to automatically reset within a first preprogrammed period of time whereupon the combustion cycle is reinitiated. The preprogrammed period of time for switch 45 to reset is preferably between about one and four minutes. If the switch fails to reset within the first preprogrammed period of time, a cycle counter is incremented. The program further allows thermal switch 45 preferably to cycle three times with reset duration greater than the first preprogrammed period of time for so long as thermostat 72 continues to call for heat. If thermostat 72 is not satisfied after three such cycles, furnace 10 is shut down by microprocessor 76 for a second longer preprogrammed period, preferably about three hours. If thermostat 72 is still calling for heat at the end of the second time period, a new combustion cycle is initiated.
In the event the thermal switch cycles three times for a second time, and thermostat 72 is not satisfied, furnace 10 is again shut down for another preferable three hour period and the combustion cycle is again reinstituted.
Referring to
Block A implementation. The process begins in step 80 and initialization occurs in step 82. Thermal switch 45 is monitored in step 84, and if the safeguard circuit is open in step 86, the system checks in step 92 to see if a period of time t1 has expired, and if so, the unit is disabled in step 94 for a period of time t2. If time t1 has not expired, control reverts to step 84. If the safeguard circuit is closed in step 86, the system checks in step 96 to see if the thermal switch reset. If so, the combustion cycle is reinstituted in step 98; otherwise, control reverts to step 84.
Block B implementation. The process begins in step 80 and initialization occurs in step 82. Thermal switch 45 is monitored in step 84, and if the safeguard circuit is open in step 86, the number of switch openings (the count) that occurred within a time t3 is checked in step 88 to see if the number of switch openings exceeds the cycle limit, N1. If so, the unit is disabled in step 90 for a period of time t4. If the count does not exceed the cycle limit, control reverts to step 84. If the safeguard circuit is closed in step 86, the system checks in step 96 to see if the thermal switch reset. If so, the combustion cycle is reinstituted in step 98; otherwise, control reverts to step 84. In this implementation the cycle count is incremented when the thermal switch opens.
Block C implementation. The process begins in step 80 and initialization occurs in step 82. Thermal switch 45 is monitored in step 84, and if the safeguard circuit is open in step 86, the system checks in step 91 to see if a period of time t5 has expired, and if so, the number of switch openings (the count) within a time t6 is checked in step 93 to see if the number of switch openings exceeds the cycle limit, N2. If so, the unit is disabled in step 95 for a period of time t7. If the count does not exceed the cycle limit, control reverts to step 84. If time t5 has not expired, control reverts to step 84. If the safeguard circuit is closed in step 86, the system checks in step 96 to see if the thermal switch reset. If so, the combustion cycle is reinstituted in step 98; otherwise, control reverts to step 84. In this implementation the count is incremented when the period of time t5 expires following a switch opening. If the switch closes before time t5 expires, the count is not incremented.
This control algorithm allows a lock-out after the first trip. Preferably, for one particular furnace embodying the Block A implementation, after the draft safeguard switch has failed to reset within three minutes (t1), the furnace will lock out for three hours (t2). The draft safeguard thermal switch is selected so that it will necessarily take longer than three minutes to reset after it has opened. In another embodiment implementing Block A followed by Block B, the control algorithm allows ten (N1) switch openings within the current heating cycle (t3) should the switch close within the three minute period (t1), and upon the eleventh switch opening, disables the furnace for three hours (t4) and then reinstitutes the combustion cycle upon the resetting of the thermal switch. If upon any opening, the switch should take longer than three minutes (t1) to close, then the algorithm disables the furnace for three hours (t2) and then reinstitutes the combustion cycle upon the resetting of the thermal switch. In a Block C implementation, the algorithm counts the number of times during the current heating cycle (t6) that the switch has opened and remained open for more than three minutes (t5). If the count exceeds 2 (N2), then the algorithm disables the furnace for three hours (t7) and then reinstitutes the combustion cycle upon the resetting of the thermal switch.
The following Tables 1A and 1B provides the preferable values for the Carrier furnace product according to an embodiment of the invention:
Possible ranges (low to high) and approximate values for these parameters are contained in Tables 2-5.
Rationale: The expected switch closing time may be used to validate a switch closing. A switch reset time less than t1 or t5 may indicate an erroneous trip. The 1 min. and 10 min. values are estimates. Routine experimentation on available switches and their performance in furnace flue applications would be needed to substantiate these choices.
Rationale: The low or high value could be based on a published limit for exposure to a particular pollutant (e.g., CO) during a specified period. Computer simulations could be used to determine whether pollutant build-up exceeds an acceptable limit in the given period.
Rationale: The low value assumes that even one combustion cycle and subsequent switch opening may be sufficient to warrant disabling the unit. The high value assumes the following cycle: 1 min. to initiate combustion, 1 min. for switch to open while burners operate, 1 min. for switch to close after opening. In this case the furnace runs for 20 min. during the course of an hour. The ensuing pollutant build-up may exceed an acceptable limit.
Rationale: The low value is consistent with a transitory condition such as high wind conditions or momentary downdrafts. The high value is consistent with more persistent conditions, possibly weather related, which may dissipate in the course of a day. The preferred embodiment for the Carrier product assumes one heating cycle, which lasts as long as there is a call for heat, possibly several days.
As noted above, the particular choice of the parameters t1, t2, N1, t3, t4, t5, N2, t6, and t7 preferably are based on a strategy that balances the objective of not exceeding allowable pollutant levels with the objective of avoiding the adverse consequences of erroneous furnace shutdown.
Computer analyses and tests have been conducted on a furnace employing the above methodology which show that the restart procedure at three hour intervals results in a very low pollutant level within an average home having a tight construction and hence a relatively low air infiltration rate. The pollutant levels in this type of structure were found to be within acceptable limits after the above noted restart procedure was repeated over a relatively long period of time. The benefit of the above noted methodology lies in the fact that some heat can be provided to an unoccupied structure which will delay and, under certain conditions, prevent water in pipes and fixtures from becoming frozen during cold weather, thus causing potentially heavy and expensive damage. It should also be evident that the present methodology will allow the furnace to quickly recover in the event the thermal switch is tripped erroneously due to high winds or sudden downdrafts, particularly in older homes having marginal venting systems which are not always compatible with newer furnaces.
While the present invention has been described with reference to a particular preferred embodiment and the accompanying drawings, it will be understood by those skilled in the art that the invention is not limited to the preferred embodiment and that various modifications and the like could be made thereto without departing from the scope of the invention as defined in the following claims.
| Number | Name | Date | Kind |
|---|---|---|---|
| 4401425 | Gable et al. | Aug 1983 | A |
| 4789330 | Ballard et al. | Dec 1988 | A |
| 4887767 | Thompson et al. | Dec 1989 | A |
| 4951870 | Ballard et al. | Aug 1990 | A |
| 5488218 | Olsen et al. | Jan 1996 | A |
| 6305369 | Garloch | Oct 2001 | B1 |
| Number | Date | Country | |
|---|---|---|---|
| 20040180305 A1 | Sep 2004 | US |
