CA1286388C - Capacity control for integrated furnace - Google Patents

Abstract

CAPACITY CONTROL FOR INTEGRATED FURNACE

ABSTRACT OF THE DISCLOSURE

A method and control system are disclosed for operating an integrated heating system for space heating and tankless domestic hot water heating utilizing an infrared burner module and heat exchanger coil. When the system is below 100% capacity, the burner is pulsed using a constant pulse period and varying the on-pulse width to vary capacity, or where a minimum on-pulse width is maintained the off-pulse width is varied to vary capacity.

Description

~Z8638~1 CAPACITY CONTROL FOR INTEGRATED FUR~ACE

Back~round of the Invention This invention relates generally to a control for an inte-grated heating system and more particularly, to a modulatedcontrol for an integrated heating system for space heating and tankless domestic hot water heating which utilizes an infrared burner module and a heat exchanger coil.

In heating systems for homes and commercial buildings, central furnaces to heat a space all operate on the same general principle. Air for a space to be heated circulates through a closed system generally comprising sheet metal ductwork, and is heated either as it passes through a heat exchanger in contact with a burning fuel, or as it passes in contact with a secondary fluid which has been heated by a --burning fuel. Since burning the fuel results in the produc-tion of noxious combustion gases having exhaust temperatures which can exceed 500F, it is necessary to exhaust the combustion gases through a chimney or flue to the atmosphere.
These systems are relatively inefficient as evidenced by the high exhaust temperatures of the flue gases, and costly due to the construction of the necessary flue or chimney.

Indirect fired furnaces, ones in which the air being heated is not contacted dlrectly by the combustion gases generated, are generally used in both forced air systems and hydronic systems.

A forced air system consists primarily of a heat exchanger having combustion chambers arranged in relation to the flow of air to be heated such that fuel is introduced at one end of a chamber where a flame causes heat to be generated. The heat passes through a series of internal baffles before exiting through the other end of the combustion chamber into the flue or chimney. Simultaneously, circulated space air .

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passes around the outside of the heat exchangers to absorb heat through conduction and convection.

A hydronic system consists primarily of a firebox having a heat exchanger therein. The heat exchanger is in a closed loop for continuously circulating water, a water glycol solution or other suitable heat exchange medium from the heat exchanger to a remote radiator in the space to be heated.
However, this system is also relatively inefficient and expensive due to the combustion gas temperatures at the outlet of the firebox and the cost of the chimney.

Thus, the inefficient home heating system is generally the largest consumer of energy with the domestic hot water system being the second largest consumer of energy. In supplying - domestic hot water for homes and commercial buildings, potable hot water systems with ordinary glass-lined, hot water storage tanks are generally used. It is common for ~ these systems to have an enclosed water tank in which are ; 20 spiralet coils of tubing through which flows the water to be heated. At the lowermost portion of the tank there is normally a burner whose heat is allowet to pass over the coils, thereby heating the water in the tank for use within the home or building. Again, as in the space heating systems for homes and buildings, the heat which is not transferred to the heat exchanger during demand "on-time" and also during standby "off-time", iB exhausted at the top of the tank into a flue or chimney to the atmosphere as well as being lost - through the tank jacket. Thu6, a domestic hot water system is also inefficient because a great portion of the heat is lost directly up the chimney to the atmosphere.

Because of the rising costs of energy, the incentives to conserve energy are increasing. Consequently, there is currently considerable interest in recovering energy, such as '' ,', ,, - ~ , : ', .' ' ~ -,: , , ,. ~ , -.

lZ86388 waste heat from combustion heaters which is usually injected into the atmosphere without recovery.

In an attempt to reclaim reject heat, heat exchanger coils have been installed in the flue of a furnace to transfer some of the waste heat to domestic hot water heaters, thus recov-ering some usually wasted heat.

However, a drawback to conserving energy by reclaiming reiect heat from a furnace for use by domestic hot water heaters is that both systems are controlled independently, and the energy saved is limited by the temperature of the water in the hot water tank for potable use and typically maintained between 120F and 160F, the average being at or above the flue gas condensing temperature therefore limiting the efficiency of recovery at or up to a maximum threshold of the -product of 88% to 90%. The necessity for dual control schemes for semi-integrated furnaces and hot water heaters is due to the blue flame burners used by both systems. In semi-integrated appliances dual controls are necessary because there is not true integration of a common heating loop that provides capacity at different required tempera-tures for both heating and hot water. This requires a rapid on-off response with modulation of input and flow controls and blue-flame burners by nature are not capable of control-ling modulation this way effectively and therefore are limited to operation at some fraction of full input during continuous operation. Capacity of these burners cannot be reduced as demand for hot water is reduced but are fired at full capacity under all operating conditions.

Thus, there is a clear need for an integral liquid-backed gas-fired heating and hot water system having a modular design and a capacity control scheme for the integrated system.

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~ , 12~3638~3 Summary of the Invention It is an Gbject of the present invention to provide a control system for an integrated space heating and tankless hot water heating system.

It i~ another ob;ect of the present invention to provide a control system for an integrated heating system having a liquid-backed gas-fired heating module with a radiant burner which will control the heat output of the infrared burner to match the rate at which energy is required for either space heating or domestic water heating, or both.

A further object of the present invention is to provide a control system for an integrated heating system having a radiant burner which more efficiently controls the capacity of the heating system.

These and other objects of the present invention are attained by providing a capacity modulated control for a heating system for heating a space in a building and domestic hot water. The heating system having a liquid-backed heating module with a quick response and 8 tankless domestic hot water system, permits maximum radiant heat transfer capacity to be reached quickly, thus allowing pulsing of the burner to maintain heating module liquid temperatures within desired limlts.

The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, its operating advan-tages and specific ob~ects attained by its use, reference should be had to the accompanying drawings and descriptive matter in which there is illustrated and described a pre-ferred embodiment of the invention.

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lZ86388 Brief Description of_the Drawings In the accompanying drawings forming a part of the specifi-cation, and in which reference numerals shown in the drawings designate like or corresponding parts throughout the same, s Figure 1 is a schematic diagram of an integrated space heating and hot water system embodying the control of the present invention;

Figure 2 is a graph of the transient domestic hot water temperature response to a call for domestic hot water in an integrated space heating and domestic hot water system;

Figure 3 is a graph of the percentage output capacity versus the percentage of the on time of the pulse period of an integrated space heating and hot water system embodying the control of the present invention; and Figure 4 A-E i6 a comparison of full capacity control with the pulsed control of the pre~ent invention.

Description of the Preferred Emhodiment Referring now to Figure 1 there may be seen a schematic view of residential heating 6ystem 10 using a liquid-backed heating module 12 for supplying energy to a series fluid loop including a tube-in-tube heat exchanger 50 and a fan coil 14.
The fluid loop further includes a liquid pump 16 for circu-lating fluid therethrough and an expansion tank 28 to provide ' for the volume increase of the heated fluid ant for dampening any pressure surges in the fluid loop. The fluid loop arrangement consists of tischarge pipe 52 which extracts hot fluid from heating module 12 on demand. The heated fluid flows through the tube-in-tube heat exchanger 50 of conven-tional construction. The fluid then flows through pipe 54 - 35 and through a three-way diverting valve 56. In a first position the three-way valve 56 allows the fluid to flow ... . . .
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. -directly to the liquid pump 16 through pipe 55 and back to the heating module 12 through pipe 57. In a second position the three-way valve 56 allows the fluid in the loop to flow through pipe 58 into fan coil 14 and through pipe 59 back to the suction of liquid pump 16.

Further, as shown, the domestic hot water loop includes cold water inlet pipe 62 connected to the inlet of tube-in-tube heat exchanger 50 and outlet pipe 64 which discharges hot domestic water to tap 43 after passing through flow switch 66. A mixing valve 60 connects pipe 64 to bypass pipe 65.
Mixing valve 60 is preferably a temperature responsive valve which mixes the hot water flowing through the heat exchanger 50 and the cold water flowing through the bypass pipe 65 to ensure that the hot water flowing from the tap 43 i8 at a desired set temperature. --As further illustrated, the heating module 12 includes a gas line 30 having a regulator 32 for supplying fuel to the module. Further, air is supplied to the module through line 34. The air/fuel mixture i8 ignited and burned on the infrared burner 18 locatet centrally within housing 20. The air/fuel i6 lOOZ premixed, thus, no secondary combustion occurs. The heat exchange means 19 is located in spaced relation to the infrared burner 18 to receive heat from the infrared burner. The heat exchange means is generally in the form of a helical coil and has the fluid flowing therethrough which absorbs heat from the infrared burner, which in turn transfers this heat to the domestic hot water and the space to be heated.

Further, Figure 1 illustrates the integrated domestic hot water/space heating system having a control system in accor-; da~ce with the principles of the present invention. This control system comprises a microcomputer system 80, a systeminterface board 82, and a power supply 83. The microcomputer : : . . : . '' ' - ' ' '' .
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system 80 may be any device, or combination of devices, suitable for receiving input signals, for processing ~he received input signals accordin~ to preprogrammed procedures, and for generating control signals in response to the pro-cessed input signals. The control signals generated by themicrocomputer ~ystem 80 are supplied to control devices which control operation of the integrated heating system in re-sponse to control signals provided to the control devices from the microcomputer system 80.
As shown in Figure 1, the system interface board 82 is connected by ribbon cable 89 to the microcomputer system 80.
The system interface board 82 includes switching devices for controlling electrical power flow from the main power supply 83 to three-way valve 56, liquid pump 16, inducer blower 38, gas valve 32, and ignition device 40. Preferably, the switching devices are electronic components, such a6 relays, which are controlled in response to control signals from the microcomputer system 80 which are supplied through the ribbon cable 89 to the electronic components on the system interface board 82.

According to the present invention, the control sy6tem determines when to operate the integrated heating system to 6atisfy the need for space heat and/or domestic hot water.
For the purpose of this tisclosure "pulsing" shall mean turning the infrared burner on and off repeatedly while the inducer fan runs continuously during the pulse period.
Further, "pulse period" shall mean the sum of one "on" and one "off" pulse. The infrared burner 18 of the module 12 has the unique feature that it has a quick response time which allows the maximum radiant heat transfer capacity to be reached quickly, e.g. in about one second, thus transferring its entire output energy to the liquid loop in a short period. More specifically, according to the present inven-tion the temperature of the space to be heated i~ sensed by a .... -, :
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', thermostat 85 and a 6ignal indioatlve of this temperature i8 provided by way of electriaal line 91 to the microaomputer system 80. Further, the flow rate of domestic hot water flowing through tap 43 i~ sensed by flow ~ensor 66 and a ~ignal lndicative of this flow is provided by way of electrical line 29 to the microcomputer system 80. Still further, the temperature of the series fluid loop is 6ensed by temperature sensor 68 at the outlet of the heat exchange means 19 and a signal indicative of this temperature is provided by way of an electrical line 26 to the microcomputer system 80.
Also, the temperature of the domestic hot water loop is sensed by temperature sen60r 67 and a signal indicative of this temperature i8 provided by way of electrical line 27 to the mloro¢omputer system 80. Turning now to Figures 2 and 3, there is exemplified the quiok response tlme whioh allows oontinuou~ use of dome6tio hot water and the output capaoity of the infrared burner as a peroentage of "on" time.

In Figure 2, ourve 70 indicate~ the water temperature with re#peot to time for a one GPM flow through tap 43 while ourve 70' indioates the temperature per time for a 2 GPM flow.
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In Figure 3, the output oapaoity of the infrared burner 18 is shown as a percentage of the burner on time.

Turning now to Figures 4 A-E, there is shown the output capacity of a burner from a first time (T1) at whioh ~demand was initiated and a second time (T2) at which time demand was terminated. Thus, Figure 4A shows a burner at 100% oapacity, Q, from inltiation time to termination time, without modulation. Specifically, Figure 4B shows a 50% capacity, 0.5Q, made up of three normally equal capacities Q', Q" and Q" where Q~ + Q" + Q"'= O.5Q. In Figure 4C, there is shown a burner at capacity and 25%
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121~ 8 8a capacity, respectively, using pulse width capacity modulation of the present invention. Accordingly, the pul~e period is maintained constant while the "on" pulse of the burner is modulated to vary the capacity.
Moreover, as shown in Figures 4 D-E the frequency of the pulse period is varied to obtain a 25% capacity and 17%
capacity respectively. Thus, under the frequency modulation scheme a minimum on-pulse width i 8 maintained and the off-pulse width is changed to vary the capacity.
The frequency modulation of Figures 4 D-E may be necessary in those " .

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lZ863~8 circumstances where a minimum on-pulse width is required by a code agency.

According to the present invention, each time it is desire~
to energize the heating module, for exampIe, when the flow sensor 66 detects flow through tap 43, the microcomputer system 80 provides another control signal by way of the ribbon cable 89 to the appropriate switching device on the system interface board 82 to supply power from the power s~pply 83 through the system interface board 82 to the ignition device 40. The microcomputer system determines the domestic hot water demand as a function of the temperature of the clo~ed loop liquid leaving the module 12 and adjusts the pulse period of the infrared burner so that the domestic hot : -water i8 maintained at a desired temperature. Moreover, if the demant at the tap 43 iB decreased then the on-pulse may decrease from that shown in Figure 4B to that shown in Figure 4C.

While the preferrèd embodiments of the present invention have been depictèd and tescribed, it will bè appreciated by those skilled in the art that many modifications, substitutions, and changes may be made thereto without departing from the true spirit and scope of the invention. For example, flow 25 sensor 66 could be located in line 62 or 64.

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Claims (2)

1. A capacity control system for a heating system having a burner, a coil for receiving heat from the burner connected in a fluid loop with a heat exchanger for transfer-ring heat to a space, the loop includes a tankless domestic hot water system receiving heat from the loop, the control system comprising:
a burner flame control means for controlling a pulse period of the burner, said burner flame control means including a flow sensor means for detecting fluid flow in the domestic hot water loop and providing a first signal to a microcomputer means indicative of said hot water flow, and a temperature sensor means for detecting the temperature of the fluid at an outlet of the coil receiving heat from the burner and providing a second signal to the microcomputer means indicative of the temperature of the fluid in the loop whereby said microcomputer provides an output signal to control the pulse period of the burner so that the fluid in the loop is maintained at a desired temperature.

2, A capacity control system as set forth in claim 1 wherein the domestic hot water system includes a tube-in-tube heat exchanger for receiving heat from the loop and said flow sensor means is located between the outlet of the tube-in-tube heat exchanger and a hot water tap in the hot water system.