US 6,283,115 B1Granted

Modulating furnace having improved low stage characteristics

AssigneeCarrier Corporation

InventorsDaniel Joseph Dempsey, Kevin Dale Thompson

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Abstract

An improved, induced-draft, gas-fired furnace which is adapted to operate at any of a high, medium and low firing rate. A pressure sensing assembly generates differential pressure signals that vary in accordance with the pressure across a heat exchanger assembly. A gas flow control assembly is responsive to the differential pressure signals, and to signals received from the furnace control circuit, to supply gas to the furnace at any of its three different firing rates, the lowest of those firing rates corresponding to a gas-air mixture which cannot be ignited. The furnace control circuit applies to the circulating air blower, and to the inducer blower, speed control signals that are so related to one another that the lowest primary wall temperature in the heat exchanger is maintained above the condensation temperature of water during steady state operation of the furnace at its low firing rate.

Claims14

Dependent claims

An improved gas-fired induced-draft furnace which has a low, a medium and a high firing rate, said furnace being of the type which includes a burner, a circulating air blower for establishing a flow of circulating air, an inducer blower for establishing a flow of combustion air, and a heat exchanger having a boundary wall, and which is adapted to operate in conjunction with temperature sensing means for sensing the temperature of a space to be heated and generating call-for-heat signals that correspond to one of said high, medium, and low firing rates;

comprising:
gas flow control means for controlling the rate at which gas is supplied to said burner, said gas flow control means serving to supply gas to said burner at one of said high, medium and low firing rates;
pressure sensing means for generating one or more differential pressure signals that vary in accordance with the pressure across said heat exchanger;
furnace control means, responsive to said differential pressure signals and to said call-for-heat signals, for:
(i) controlling the rate at which said gas flow control means supplies gas to said burner;
(ii) applying, to said circulating air blower, blower control signals which cause said circulating air blower to establish circulating airflows having magnitudes which correspond to said high, medium and low firing rates;
(iii) applying, to said inducer blower, inducer control signals which cause said inducer blower to establish combustion airflows having magnitudes which correspond to said high and low firing rates;
wherein the magnitudes of the combustion airflows which are established by said inducer blower during steady state operation of the furnace at said low and medium firing rates are so related to one another, and to the magnitudes of the circulating airflows which are established by said circulating air blower during steady state operation of said furnace at said low and medium firing rates, that all parts of said wall remain at temperatures high enough to prevent cold spot corrosion.

5 more elements

An improved gas-fired induced-draft furnace which has a low, a medium and a high firing rate, said furnace being of the type which includes a burner, a circulating air blower for establishing a flow of circulating air, an inducer blower for establishing a flow of combustion air, and a heat exchanger assembly including a primary heat exchanger and a secondary heat exchanger, said heat exchangers having respective boundary walls, and which is adapted to operate in conjunction with temperature sensing means for sensing the temperature of a space to be heated and generating call-for-heat signals which correspond to one of said high, medium, and low firing rates;

comprising:
a gas flow control assembly for controlling the rate at which gas is supplied to said burner, said gas flow control assembly serving to supply gas to said burner at one of said high, medium and low firing rates;
at least one pressure sensor for generating at least one differential pressure signal that varies in accordance with the differential pressure across said heat exchanger assembly;
a furnace control circuit, responsive to said at least one differential pressure signal and to said call-for-heat signals, for:
(i) controlling the rate at which said gas flow control assembly supplies gas to said burner;
(ii) applying, to said circulating air blower, blower control signals which cause said circulating air blower to establish circulating airflows having magnitudes which correspond to said high, medium and low firing rates;
(iii) applying, to said inducer blower, inducer control signals which cause said inducer blower to establish combustion airflows having magnitudes which correspond to said high and low firing rates;
wherein the furnace control circuit applies, to said circulating air blower and said inducer blower, control signals having magnitudes which assure that the temperatures of all parts of the wall of the primary heat exchanger remain above the condensation temperature of water during steady state operation of said furnace at said low and medium firing rates.

5 more elements

An improved gas-fired induced-draft furnace which has a low, a medium and a high firing rate, said furnace being of the type which includes a burner, a circulating air blower for establishing a flow of circulating air, an inducer blower for establishing a flow of combustion air, and a heat exchanger assembly having a boundary wall, and which is adapted to operate in conjunction with temperature sensing means for sensing the temperature of a space to be heated and generating call-for-heat signals which correspond to one of said high, medium, and low firing rates;

comprising:
a gas flow control assembly for controlling the rate at which gas is supplied to said burner, said gas flow control assembly serving to supply gas to said burner at one of said high, medium and low firing rates;
pressure responsive means for causing said gas flow control assembly to supply gas to said burner at one of said high, medium and low firing rates, depending on the differential pressure across said heat exchanger assembly;
a furnace control circuit, responsive to said call-for-heat signals, for:
(i) applying, to said circulating air blower, blower control signals which cause said circulating air blower to establish circulating airflows having magnitudes which correspond to said high, medium and low firing rates;
and
(ii) applying, to said inducer blower, inducer control signals which cause said inducer blower to establish combustion airflows having magnitudes which correspond to said high, medium, and low firing rates;
wherein the combustion airflows that correspond to said low and medium firing rates have magnitudes which are at least approximately equal to one another;
and
wherein the furnace control circuit applies, to said circulating air blower and said inducer blower, control signals having magnitudes which assure that the temperature of said wall remain above the condensation temperature of water during steady state operation of said furnace at said low and medium firing rates.

7 more elements

Dependencies are inferred from the claim text.