Inducer speed control method for combustion furnace
In a multistage combustion furnace having a motor driven inducer blower and a pressure sensing device or set of switches for sensing pressure in the combustion gas flowpath through the furnace, a high firing rate blower speed and low firing rate blower speed are set based on the blower speed setting required for a medium firing rate. Each particular furnace, having its own pressures and combustion gas resistance to flow characteristics, may be provided with a database of inducer blower motor speeds required to achieve predetermined pressures in the combustion gas flowpath for a variety of combustion gas venting systems generating such resistance.
1. A method for operating a multistage combustion furnace, said combustion furnace including a heat exchanger including at least one combustion gas flowpath, an inducer blower for inducing the flow of air and combustion gases through said flowpath, for discharging combustion gases to a vent conduit, a blower motor drivably connected to said blower, pressure sensing means for sensing the pressure within the combustion gas flowpath at a predetermined location, a combustion fuel control valve and a control system including a microprocessor operably connected to said control valve, said blower motor, said pressure sensing means and to a thermostat for receiving signals for a call for heat for a space serviced by said furnace, said method comprising:
starting said combustion furnace at a first firing rate in response to a call for heat by said thermostat;
starting said blower motor at a predetermined speed for said first firing rate;
learning a blower speed for said first firing rate, wherein the blower speed at which said pressure sensing means is actuated is the learned blower speed for said first firing rate; and
determining a blower speed for said blower for a different firing rate based on the learned blower speed for said first firing rate.
2. The method set forth in claim 1 wherein:
said blower speed at said different firing rate is determined as a multiple of said learned blower speed for said first firing rate.
3. The method set forth in claim 1 wherein:
said learned blower speed for said first firing rate is learned by selectively incrementing and decrementing said blower from said predetermined blower speed.
4. The method set forth in claim 1 including the step of:
setting a blower speed at second and third speeds for respective second and third firing rates, each of the second and third speeds being based on the learned blower speed for said first firing rate.
5. The method set forth in claim 4 wherein:
said first firing rate is a medium firing rate of said furnace and said second blower speed corresponds to a low firing rate of said furnace.
6. The method set forth in claim 4 including the step of:
said first firing rate is a medium firing rate and said third blower speed corresponds to a high firing rate of said furnace.
7. The method set forth in claim 1 including the step of:
providing values of blower speed required to overcome combustion gas flow resistance of a venting system for said furnace to generate predetermined pressures in said furnace.
8. The method set forth in claim 1 wherein:
said pressure sensing means comprises plural pressure sensing switches including at least a low firing rate pressure switch and a medium firing rate pressure switch; and
generating a fault signal for said furnace, if at furnace startup, said low firing rate pressure switch does not close.
9. A method for operating a multistage combustion furnace, said combustion furnace including a heat exchanger including at least one combustion gas flowpath, an inducer blower for inducing the flow of air and combustion gases through said flowpath, for discharging combustion gases to a vent conduit, a blower motor drivably connected to said blower, pressure sensing means for sensing the pressure within the combustion gas flowpath at a predetermined location, a combustion fuel control valve and a control system including a microprocessor operably connected to said control valve, said blower motor, said pressure sensing means and to a thermostat for receiving signals for a call for heat for a space serviced by said furnace, said method comprising:
starting said combustion furnace at an intermediate firing rate in response to a call for heat by said thermostat;
starting said blower at a predetermined speed for said intermediate firing rate;
learning a blower speed for said intermediate firing rate, wherein the blower speed at which said pressure sensing means is actuated is the learned blower speed for said intermediate firing rate; and
determining a blower speed for said blower for a different firing rate based on a multiplier applied to the blower speed learned for said intermediate firing rate.
10. The method set forth in claim 9 including the steps of:
selectively incrementing and decrementing said predetermined blower speed to learn the learned blower speed for said intermediate firing rate.
11. The method set forth in claim 10 including the step of:
setting second and third blower speeds for respective low and high firing rates based on the learned blower speed for said intermediate firing rate.
12. The method set forth in claim 9 including the step of:
providing values of blower speed required to overcome combustion gas flow resistance of a venting system for said furnace to generate predetermined pressures in said furnace and basing said blower speed for said intermediate firing rate and said multipliers on said values of blower speeds required to overcome said flow resistance.
13. The method set forth in claim 9 wherein:
said pressure sensing means comprises plural pressure sensing switches including at least a low firing rate pressure switch and an intermediate firing rate pressure switch; and
generating a fault signal for said furnace, if at furnace startup, said low firing rate pressure switch does not close.
14. A method for operating a three stage combustion furnace, said combustion furnace including a heat exchanger including at least one combustion gas flowpath, an inducer blower for inducing the flow of air and combustion gases through said flowpath, for discharging combustion gases to a vent conduit, a blower motor drivably connected to said blower, pressure sensing means for sensing the pressure within the combustion gas flowpath at a predetermined location, a combustion fuel control valve and a control system including a microprocessor operably connected to said control valve, said blower motor, said pressure sensing means and to a thermostat for receiving signals for a call for heat for a space serviced by said furnace, said method comprising:
starting said combustion furnace at a first firing rate in response to a call for heat by said thermostat;
operating said blower motor at a first blower speed for said first firing rate;
learning a blower speed for said first firing rate, wherein the blower speed at which said pressure sensing means is actuated is the learned blower speed for said first firing rate; and
determining second and third blower speeds for said blower for second and third firing rates based on said learned blower speed.
15. The method set forth in claim 14 including the steps of:
selectively incrementing and decrementing said first blower speed to learn the learned blower speed for said first firing rate prior to determining said second and third blower speeds.
16. The method set forth in claim 14 including the step of:
providing values of blower speeds required to overcome combustion gas flow resistance of a venting system for said furnace to generate predetermined pressures in said furnace for said respective firing rates.