The present invention relates to furnace ignition systems in general; and in particular, it relates to an ignition system for a fuel-burning appliance which eliminates the need of a conventional draft hood. Not only does the present invention perform the functions of a conventional draft hood, but it also provides a stack damper when the appliance is off, controls the value of draft while the appliance is operating, and provides loss-of-draft safety by shutting off the source of fuel if draft is lost and cannot be re-established within a predetermined time after the damper has been driven to the full open position.
Draft hoods have been used in combination with gas-fired appliances such as furnaces, water heaters, and the like for many years. A draft hood is a fitting which is normally located in the flue pipe leading from the appliance to the vent stack, or in the stack itself, which vent the combustion products from the fire box of the appliance to a chimney.
A draft hood serves a number of purposes in an installation of this type. In the case of a down draft or stoppage anywhere in the flue or stack beyond the draft hood, it permits the escape of combustion products into the ambient. A draft hood also neutralizes the effect of stack action of the chimney on the operation of the appliance by drawing air from the ambient and exhausting it through the chimney when the appliance is first fired.
Even though draft hoods have long been known to constitute a major heat loss to the outside, both when the furnace is being fired and when it is not, nevertheless, they have been widely used because of the relatively low cost of fuel and because any alternative, particularly in residential usage, did not provide the degree of safety and reliability for the various conditions encountered in operation for a typical residential furnace.
Even with the advent of substantial increases in fuel cost, draft hoods have continued in use because of the difficulties in providing a system which accomplishes all of the purposes of a draft hood in a safe and reliable manner, and which does not require an expenditure on the part of the home owner for purchase and installation, which would not be offset by the savings in the cost of fuel.
The present invention provides an ignition and control system for a furnace which eliminates the need for a conventional draft hood while, at the same time, providing a stack damper when the furnace is off, controlling the value of draft when the furnace is fired, and shutting the furnace down in the event that draft is lost during any portion of the operating cycle. All of these functions and features are accomplished in a system which is economical to purchase and install, and which has shown to be safe for residential use after analyzing all of the possible failure modes. Briefly, the present invention includes a damper plate which is movable under control between a closed and an open position. When a call for heat signal is generated, as by the closing of thermostat contacts in the room being heated by the appliance, a holding relay is actuated to energize a damper motor to open the damper, provided the damper plate was closed when the signal was generated. That is to say, the system requires that the damper plate be in the fully closed position before an ignition cycle can commence. This insures that all major system components will have been checked for operability during the previous ignition cycle, prior to initiating a new ignition cycle.
Assuming that all components are operative, the damper is run to the full open position, and when this is proved, the fuel supply and ignition circuit is actuated to supply fuel to the appliance. At the same time, a trial-for-draft period is commenced, and a draft controller is enabled for controlling the position of the damper plate. Draft is proved through a stack transducer sensing pressure (draft) and communicating with the controller.
The draft controller actuates a first switch if draft in the flue is less than a first predetermined value, and it actuates a second switch if draft is greater than a second predetermined value higher than the first value. If draft greater than the second value is sensed within the trial-for-draft period, the draft controller moves the damper off the full open position to thereby satisfy the conditions for the trial-for-draft period and the damper is thereafter positioned by the controller. If at the end of the trial-for-draft period, the damper remains in the open position, indicating draft has not been established beyond the second value, the system shuts off the fuel and locks out.
Once draft is established during the trial-for-draft period, the predetermined draft values define a desired draft range in which the system considers draft to be normal. If draft is less than the first value, a damper motor is energized to open the damper to increase flow; and if draft is greater than the second value, the second switch energizes a damper motor to close the damper until a draft value is established within the desired draft range. Either a reversible motor can be used to actuate the damper, or, as in the case of the illustrated embodiment, separate motors can be used, each being actuated to rotate in a different direction to open and close the damper.
If neither of the draft controller switches is closed, the damper motor is de-energized, and this indicates that the draft value is within the desired range. Hence, the damper remains at rest as long as draft is within the desired range. If draft is lost, the system moves the damper to the full open position, and a loss-of-draft safety timing period commences. If draft is not re-established during this period, the system shuts off the fuel and locks itself out.
If the call for heat is satisfied, the system closes the damper and is prepared for another operating cycle, having tested the operativeness of all major components without failure. Any failure would have locked the system out. The various safety features of the system can only be understood in light of the particular circuitry of the system; and they will be described after the system is disclosed in detail. Of particular interest is the fact that no single component failure will permit a condition in which the damper plate is closed and the fuel and ignition circuit is energized.
Other features and advantages of the present invention will be apparent from the following detailed description accompanied by the attached drawing wherein identical reference numerals will refer to like parts in the various views.