Draft control arrangement for venting of combustion apparatus
Vented, gas-fired appliance relying on natural draft for the removal of products of combustion are equipped with a draft hood which isolates the combustion chamber from excessive updraft or backdraft in the vent. The conduit between the draft hood relief opening and the outside is permanently open and therefore causes heat loss, especially during cold and windy weather. 
     For energy conservation it is desirable that the airflow in the vent always be restricted to the safe minimum. During the standby phase of the appliance the safe minimum is a sufficient air flow to remove the vent gases of the pilot flame, if so equipped, and to provide some air change in the vent to prevent excess condensation. During the operating phase the safe minimum is a sufficient air flow to remove the products of combustion while retaining heated room air, regardless of the varying draft in the vent. 
     Thermally actuated vent dampers, designed with the above ideal performance in mind, are known. When the main burner of the applicance is turned on products of combustion rise to the damper while it is still essentially closed and a part of the products of combustion escape temporarily through the draft hood relief opening. Since this spillage of products of combustion is undesirable, one main objective of thermal vent damper design is a quick opening upon the start of main burner operation. 
     In recent years the efficiency of gas-fired appliances has been raised. More heat is being transferred from the burner to the medium to be heated resulting in lower vent gas temperatures which in turn requires new thermal vent damper designs aimed at being closed at temperatures normally prevailing during standby (e.g. up to 50° C.) for energy conservation, but quickly opening at a temperature of e.g. 65° C. 
     During continued operation of the appliance the vent gas temperature at the damper may rise to e.g. 90° C. at low flame (in variable-output appliances) and e.g. 150° C. at full flame. In addition, a safety margin must be provided for temperatures of e.g. 300° C. which may occur as a result of malfunction in, or maladjustment of, the appliance. The temperature-sensitive actuating element of the vent damper, in addition to opening the damper in a narrow temperature range of e.g. between 50° C. and 65° C. must be designed to withstand the stress of the exposure to the additional rise of temperature of e.g. up to 300° C. 
     Thermally actuated vent dampers manufactured according to prior designs cannot meet the stringent requirements to be closed at 50° C., to be essentially open at 65° C., to withstand repeated temperature overloads of up to 300° C. and to maintain their operating characteristics during their lifetime. The vent damper described in this invention is able to perform within the above-listed specifications through a combination of a damper element having a knife-edge support and a bimetal strip shaped in form of a loop with one of its ends fastened to a fixed bracket and its other end movably connected to the damper element. 
    
    
     The foregoing and other features and objects of the invention will be described in more detail in the following specification and drawings in which: 
     FIG. 1 is a plan view of the damper according to a preferred embodiment of the invention. 
     FIG. 2 is a section along line II--II of FIG. 1 with the damper in the cold state (closed position). 
     FIG. 3 is a section similar to FIG. 2 with the damper in fully open position. 
     FIG. 4 shows a partial section along line IV--IV of FIG. 2, and 
     FIG. 5 shows a perspective view of a component part in a damper according to another embodiment of the invention. 
    
    
     In FIGS. 1 and 2 the cylindrical housing 10 forms a part of the vent through which the vent gases flow. Arrow 12 in FIG. 2 indicates the direction of the gas flow which is parallel to the central axis of housing 10. As shown in FIG. 1, housing 10 is divided by a partition 14 into two ducts 16a and 16b. Semicircular, essentially flat damper elements 20a and 20b are positioned in a respective one of the ducts 16a and 16b. In FIG. 2 the damper element 20 is shown at its fully closed position in which it assumes an angle α of approximately 15° from a plane perpendicular to the housing central axis. The partition is crossed by a support bracket 22, and damper elements 20a, 20b are mounted on bracket 22 to swing around axis A--A. As shown in FIG. 2 the damper element, in the cold state, rests on stop 14a formed by a lug of partition 14. 
     A bimetal element, indicated generally as 24, supports damper element 20b and provides for its temperaturedependent actuation. The bimetal element comprises a half-base 26 and two sides 30 and 32 of the triangle. Between half-base 26 and side 30 of the triangle is a rounded corner 34. An additional rounded corner 38 is located between sides 30 and 32 of the triangle. The half-base 26 is riveted 42 via a U-loop 40 to bracket 22. One leg 40a of the U-loop 40 continues to half-base 26, while the other leg 40b is riveted to support bracket 22. The second side 32 of the triangle is pivotally connected with damper element 20 at a knife-edge connection 11. 
     The damper elements 20a and 20b and their associated parts are similar in construction and operation and it will suffice to explain in detail the element 20b. Damper element 20b has a rectangular opening 46 through which a part of bimetal element 24 protrudes so that half-base 26 and corner 34 lie above damper element 20b. A slot 48 is provided through which leg 40b of the U-loop protrudes in such a manner that a pivot line 21 is formed at the upper edge of support bracket 22 that secures damper element 20b against shifting lengthwise. The width of slot 48 is commensurate with the width of bimetal element 24 so that element 24 holds damper element 20b in position against lateral displacement. Slot 48 in the longitudinal direction of damper element 20b is sufficiently wide to allow a swiveling movement of 60° to 70°. A bend 13 of the damper element 20b forms a stop which can move against support bracket 22. 
     FIG. 2 shows the bimetal element 24 in a cold state in which the element is under internal prestress which is maintained by damper element 20b pressing against stop 14a. 
     Bimetal element 24 is formed by a bimetal strip, approximately 1.5 cm wide, with the layer having the higher coefficient of expansion (the active layer) at the outer side of the triangle. 
     As shown in FIG. 2 partition 14 protrudes below the lower edge of housing 10 so that the partition protects bimetal element 24 in areas where it extends below housing 10. The lower contour of partition 14 is shaped in such a manner that - regardless how the damper is placed on a table - the extending parts of the bimetal elements do not touch the table. Damper element 20 also lies, in all its possible positions, within the protective area formed by partition 14. When the damper is installed in a vent containing any obstacle the partition will touch such obstacles first, thereby preventing damage to, and assuring free movement of, bimetal elements 24 and damper elements 20. 
     Partition 14 also performs a closing function that is necessary due to the opposite inclination of the damper elements 20 lying adjacent along axis A--A, according to FIG. 1. Without partition 14, large triangular gaps would remain between the two damper elements 20 through which room air would escape unimpeded during the cold state. 
     When the appliance is started up and bimetal element 24 warms up the damper assumes the position shown in FIG. 3 whereby angle α is increased to α&#39;. The position according to FIG. 3 corresponds to a temperature increase of, e.g., 65° C. The damper is then essentially open. During a further increase in temperature, side 30 of the triangle passes from a bow toward the interior of the triangle, as shown in FIG. 2, to a nearly straight shape, while side 32 changes from a very slight bow toward the inside of the triangle to a slight bow toward the outside of the triangle. 
     In the embodiment of FIGS. 1-4 connecting point 11 is formed by notches 15 near the end 32a of side 32 of bimetal element 24. Slots 17b and 17c are formed at opposite sides of opening 17 at positions which are rotationally offset from the bimetal strip. During assembly, end 32a of bimetal element 24 is twisted so that it can be passed through slots 17b and c in opening 17. As the twisting force is released the notches 15 engage rim 17a of the opening 17. 
     In FIG. 5 an embodiment is shown which provides a modified connecting means which prevents unintended disengagement of the bimetal element from the damper element. In this embodiment, instead of punching out the slots shown in FIGS. 1-4, tabs 19a and 19b are formed from the material of the slots. After assembling damper element 20b&#39; and the bimetal element, these tabs are then bent back to their original position in the plane of damper element 20b. Thereby slots 17b&#39; and 17c&#39; are closed and an accidental disengagement between damper element 20b&#39; and the bimetal element is prevented.