US3421691A - Boiler temperature modulating control - Google Patents

Description

Jan. M, i969 N. A. FORBES BOILER TEMPERATURE MODULATING CONTROL Filed Aug. 2. 1966 Euh Norman A. Tes

ATTORNEY United States Patent Olce Patented Jan. 14, 1969 3,421,691 BOILER TEMPERATURE MODULATING CONTROL Norman A. Forbes, Louisville, Ky., assignor to American Standard Inc., a corporation of Delaware Filed Aug. 2, 1966, Ser. No. 569,623 U.S. Cl. 236-9 13 Claims Int. Cl. F23n 5/00; G05d 23/00 ABSTRACT F THE DISCLOSURE This invention provides an arrangement for operating the fuel-feeding means of a boiler for a hydronic heating system which is intended to supply heat to a plurality of zones (such as apartments or offices of a single building or group of associated buildings), each zone, or at least one zone, being provided with a separate thermostat. The apparatus includes a heat accumulating means enclosed in an insulated housing to provide thermal inertia. The heating system, which is under the control of the thermostat or thermostats, is combined with one or more heaters to heat the heat accumulating means. Two effective signals are generated in the system, one related to the temperature of the heat accumulating means and the other related to the boiler water temperature. These two signals are coordinated to control the operation of the fuel-feeding means.

This invention pertains to temperature control apparatus and more particularly to apparatus for controlling the operation of a heating system.

An object of the invention is to solve p-roblems of expansion noise and wall streaking caused by introducing excessively hot boiler water into the baseboard radiators in zone-controlled hydronio heating systems.

Another object of the invention is to minimize nucleate boiling noise which is also caused by high boiler water temperatures.

In the single-zone systems of fifteen years ago these problems did not exist; the large mass of water and cast iron Vcoupled to the boiler prevented the boiler water temperature from changing rapidly. In effect, the boiler water smoothed out the variations in demand, resulting in a condition that was almost ideal from the standpoint of comfort and noise. By comparison, modern hydronic heating systems have been designed with ever-decreasing quantities of water and cast iron, so that even in a singlezone system the boiler water temperature rises rapidly in response to a demand for heat. This problem is magnilied by the tendency to install oversized boilers in residential hydronic heating systems.

Furthermore, problems of noise and streaking are made worse by the trend toward zone-controlled systems, which now represent over 50% of the sales of residential boilers. Conditions are worse in a zone-controlled system when only a single zone is calling for heat; since the boiler is capable of heating the entire house, its heat output is far greater than a single zone can absorb. Under these conditions the boiler water temperature rises rapidly to the setting of the temperature limit control, normally between 200 F. and 230 F. This hot |boiler water is the basic cause of all the problems discussed above. For example, if the second zone calls for heat while the boiler water temperature is near its upper limit, the second zone suddenly receives a charge of hot water, leading to expansion noises and wall streaking.

In a modern hydronic heating system still another noise problem can exist: with high water temperatures, nucleate boiling inside the boiler causes noises that are transferred to the baseboard radiators in the room. In systems where such high water temperatures are reached nearly every time a zone calls for heat, noise from this source is a major source of complaints. Ordinarily, such noise should be restricted to the relatively few days when high water temperatures are actually needed to meet the heat load of the house.

To sum yup the basic water temperature control problem, an ideal control should permit only those boiler water temperatures that are actually required by the heating system. High water temperatures should be produced only in cold weather, or during pickup periods when the house temperature is being raised. Water temperatures should be made to rise and fall gradually to minimize expansion noises, and high water temperatures should be produced only rarely so as to minimize complaints about nucleate boiling noise and wall streaking.

In order to solve these problems the invention contemplates providing a thermal analog of an old-fashioned boiler having a slow response to calls for heat. In particular, there is contemplated apparatus for controlling the activation of the fuel-feeding means to the Water boiler of a hydro-nir,` heating system which heats at least one zone provided with a thermostat. The apparatus comprises a heat accumulating means having thermal inertia (the thermal analog). Heating means under the control of the thermostat are provided to heat the accumulating means. First signal generating means generates a signal related to the temperature of the heat accumulating means. Second signal generating means generates a signal related to the temperature of the water in the water boiler. Controller means is responsive to the signals generated by the first and second signal generating means for activating the fuel-feeding means.

Other objects, the features and a-dvantages of the invention will be apparent from the lfollowing detailed description when read with the accompanying drawings which show, by way of example and not limitation, the now preferred embodiment of the invention.

In the drawings:

FIGURE 1 shows a hybrid block and schematic diagram of the pertinent portions of a heating control system; and

FIGURE 2 shows waveform diagrams at certain points in the control system of FIG..1.

Referring now to FIGURE 1, the system comprises a hot Water boiler 10 which feeds, upon demand, hot water via conduits 12 and 14 to zones 1 and 2 respectively. Each of the zones has a thermostat, a zone valve motor, a hot Water circulating system (not shown) and a zone valve end switch. For example, conduit 12 is connected via zone 1 valve (operated by motor 16) to zone 1 circulating system under control of zone 1 thermostat 18. The associated zone valve end switch is provided to close a contact when the associated zone valve is open. All such contacts for all the zone valves are connected in parallel in block 20, so that relay 1R can be energized only when one or more zones is calling for heat, and gas valve 26 is positively and instantly closed when all zone thermostats are satisfied, thereby preventing temperature overshoot in the boiler.

Boiler 10 is red by gas fed from gas source 22 via conduit 24, gas valve 26 (fuel-feeding means) and conduit 28 to the gas burner (not shown) of boiler 10 under control of the remainder of the system.

The system is powered by a source of alternating current AC which is fed to the primary winding TRIP of transformer TR1 and, via limit controls 30 to the primary winding TR2P of transformer TR2. Limit controls 30 include the systems on-off Switch and a high temperature limit control to prevent unsafe water temperaturcs.

Gas valve 26 which is opened by the flow of alternating current through a solenoid winding is connected in series with contact set 1R1 of relay 1R across the terminals of the secondary winding TRZS. When relay 1R is energized, contact set 1R1 closes and gas valve 26 is actuated.

The winding of relay 1R has one end connected via zone valve end switches to bus 32 connected to one terminal of the secondary winding TRIS of transformer TR1. The other end of the winding of relay 1R is connected essentially via silicon controlled rectifier SCR to bus 34 connected to the other terminal of the secondary winding TR1S. More particularly, the anode terminal of silicon controlled rectifier SCR is connected to the winding of relay 1R and the cathode of the silicon controlled rectifier SCR is connected via diode D1 to bus 34. Diode D1 is provided to permit the SCR to be fired by the cathode terminal rather than the gate, thereby permitting a low resistance R3 (typically 1000 ohms) to be connected between the gate and the cathode of the SCR to minimize the tendency of the SCR to be fired by its own leakage without reducing the sensitivity of the SCR as much as such a resistor would reduce it in a conventional gate-drive circuit.

The firing of silicon controlled rectifier SCR and, therefore, the operation of relay 1R (the controller means) is accomplished by two signal generating means. The first signal generating means comprises: diode D2 and capacitor C1 serially connected across busses 32 and 34, through the agency of resistor R1; and thermistor T1 thermally coupled to the water in boiler 10 and electrically connected between junction S1 between diode D2 and capacitor C1 and the cathode C of silicon controlled rectifier SCR. The second signal generating means comprises: diode D3 and capacitor C2 serially connected across busses 32 and 34 through the agency of resistor R1; and thermistor T2, thermally coupled to heat accumulating means 36 electrically connected between the junction S2 of diode D3 and capacitor C2 and the cathode C of silicon controlled rectifier SCR.

It should be noted that diodes D2 and D3 are oppositely polarized. Accordingly, oppositely-phased ripple waveforms are generated at junctions S1 and S2 in response to the AC waveform generated across busses 32 and 34. (See the similarly labeled waveforms of FIG- URE 2.) These waveforms are combined at the cathode C of silicon controlled rectifier SCR to form a square wave indicated by waveform C of FIGURE 2. When one of the thermistors changes resistance the waveform C is moved positive or negative; also the waveform reaches a negative-going peak before the start of the conduction cycle. Hence, by using cathode drive for the silicon controlled rectifier, it is insured that the silicon controlled rectifier conducts for the entire conduction cycle or not at all, thereby insuring snap action of relay 1R and preventing chatter in relay 1R. By virtue of the on-off conduction of the silicon controlled rectifier SCR halfwave current flows through the winding of relay 1R. Therefore, a free-wheeling diode D4 iS shunted across the winding of relay 1R to smooth the pulsations of current. The function of resistor R1 is to prevent excessively large ripple voltages when the resistances of thermistors T1 and T2 are very small. Otherwise, the linearity of the system would be poor in that range.

Resistor R2 connected between resistor R1 and the anode of silicon controlled rectifier SCR is for the purpose of giving the circuit a predictable span between the thermistor resistance ratio at which relay 1R pulls in and the ratio at which it drops out; in other words the purpose of R2 is to set the circuit differential. The control without resistor R2 has essentially zero differential and so is not suitable for some applications. To give it the required differential, the circuit takes advantage of the fact that the average anode-to-cathode voltage of the silicon controlled rectifier SCR changes when the silicon controlled rectifier conducts. When it is not conducting, the anode-to-cathode voltage is a sine wave with an average value of zero. When it is conducting, the anode-tocathode voltage is approximately the negative-going half of the sine wave with an average value of about 10 volts in a 24-volt AC system. By tapping off a portion of this voltage change and applying it to the diode-capacitor networks (this is the purpose of the voltage divider made up of resistors R1 and R2), the circuit acquires `a differential that is independent of the set point. Although the differential could be made adjustable, it is preferably fixed at about 20% of the pull-in resistance. Capacitor C3 serves to protect the silicon controlled rectifier from firing on impulse noise, such as is generated by brush motors and automobiles.

The heat accumulating means 36 comprises a slab 40 of metal against which are placed heating resistors HR1 and HR2. One end of resistor HR1 is connected via line 42 to thermostat 18: the other end of resistor HR1 is connected via temperature limit control 44 and line 46 to bus 34. One end of resistor HR2 is connected via line 48 to thermostat 19; the other end of resistor HR2 is connected via control 44 and line 46 to bus 34. The thermistor T2, the metal slab 40, the resistors HR1 and HR2 are enclosed in a heat insulating housing 50.

When zone 1 requires heat thermostat 18 closes and current fiows from bus 32 via valve motor 16 to bus 34. Valve motor 16 is energized and boiler Iwater flows from conduit 12 into the circulating systemI of zone 1. At the same time, current fiows through resistor HR1 which gradually heats slab 49. In the embodiment shown in FIGURE 1, the resistance of thermistor T2 which is based on the use of negative temperature coefficient thermistors gradually changes. It is noted that positive temperatures coefiicient thermistors can be used simply by interchanging the functions of T1 and T2 so as to fire SCR on an increase of T2 with respect to T1. The resistance of thermistor T2 gradually decreases. Eventually the resistance of thermistor T2 is less than the resistance of thermistor T1 by the slight ratio necessary to fire the SCR and relay 1R operates to close contact 1R1, energizing gas valve 26 and delivering fuel to boiler 10. The burning fuel raises the temperature of the boiler water, causing the resistance of thermistor T1 to decrease, eventually reaching the ratio of thermistor T1 and T2 resistance at which the SCR ceases to tire. At this point, relay 1R is de-energized, contacts 1R1 open and fuel ceases to flow from gas valve 26 to boiler 10. If the thermostat stays closed for some of the time, the heat accumulating means 36 continues to rise slowly (the rate of rise depending on the rate of heat input from heater HR1, the thermal capacity in B.t.u./ F. of 20 and the thermal resistance of the heat insulation slab 40 means), the resistance of thermistor T2 continues to fall slowly and eventually the SCR fires again. A similar phenomenon occurs for zone 2. In fact, the heat input into heat accumulating means 36 depends on the number of zones calling for heat and on the percentage of the total time they call for heat. In effect, the boiler Water temperature cannot rise above the temperature of the heat accumulating means by more than a small amount, which depends on the differential of the firing circuit as discussed above. Since the temperature of the heat accu-mulating means is forced to rise slowly because of thermal inertia in slab 40', the mean (averaged over a period of, say, 10 minutes) boiler Water temperature is also forced to rise slowly. The slow rise of boiler water temperature is the desired result. In effect,

the boiler is forced to cycle on and ofi so that the mean boiler water temperature rises slowly.

It should be noted that'such a system requires no outside temperature sensors. Accordingly, the system according to the invention is much simpler than present systems which measure the outside temperature and preset the boiler water temperature to a value based on the outside te-mperature.

In addition, the system allows full heat capacity of the boiler to be used for unscheduled heat loads (pickup), for example, in raising house temperature in the morning, thereby overcoming a basic flaw in conventiona l indoor outdoor control systems which cannot accommodate unscheduled heat loads.

In some cases it is desirable to de-energize the hot water circulator WC when no water is being delivered to any zone. To accomplishthis result, relay 2R is connected so that it is energized-when any one zone valve end switch closes (that is, in effect, when any one zone thermostat calls for heat by closing its contact), thereby closing contact 2K1 and energizing water circulator WC.

While only one embodiment of the invention has been shown and described in detail, there will now be obvious to those skilled in the art many modifications and variations which satisfy many or all of the objects but which do not depart fromthe spirit of the invention as defined by the appended claims.

What is claimed is:

1. Apparatus for controlling the activation of a fuelfeeding means to the Iwater boiler of a hydronicheating system for heating a plurality of zones at least one of which zones has a thermostat comprising, in addition to the thermostat, heat accumulating means having thermal inertia, a heating means under the control of said thermostat for heating said heat accumulating meansiirst signal generating means for generating a signal related to the temperature of said heat accumulating means, second signal generating means'for generating a signal related to the temperature of the water in the water boiler, and controller means responsive to the signals generated by said first and second signal generating means for activating the fuel-feeding meaiis.l

2. The apparatus of claim 1 wherein the hydronic heating system heats first and 'second zones, each having a thermostat and comprising first and second heating means respectively controlled b'y 'one of the thermostats.

3. The apparatus of clairnr2 including means for changing the temperature in the"water of the water boiler substantially only as rapidly'as the temperature of the heat accumulating means changes and including means whereby said heating means generates heat substaiiitially in accordance with the degree of operation of said'thermostats.

4. The apparatus of claim 1 further comprising a source of alternating current, and wherein said first signal generating means comprises a first diode and a first capacitor serially connected across said source of alternating current and a first temperature sensitive resistor having a first end and a second end connected to the junction'of, said first diode and said first capacitor, said second signal generating means comprising a second diode and a second capacitor serially connected across said source of alternating current, a second temperature sensitive resistor having a first end and a second end connected to the junction of said second diode and said second capacitor, said diodes being oppositely polarized, and means for connecting the first ends of said temperature sensitive resistors to said controller means.

'5. The apparatus of claim 1 further comprising a source of alternating current and wherein said controller means comprises a relay including la winding and a contact set, the state of said contact set controlling the activation of the fuel-feeding means, a silicon controlled rectifier having an anode terminal, a cathode terminal 'and a gate terminal, means for serially connecting the winding of said relay and said silicon controlled rectifier across said source of alternating current wherein said anode terminal is connected to one end of said winding and means for applying the signals from said signal generating means to one of the other terminals of said silicon controlled rectifier.

6. The apparatus 4ofi claim 5 wherein the signals generated by said first and second signal generating means are applied to said cathode terminal and further including resistor means for connecting said gate terminal to said source of alternating current.

7. The apparatus of claim 1 further comprising a source of alternating current having first and second output -terminals and wherein said first signal generating means comprising a first diode and a first capacitor serially connected across the terminals of said source of alterhating current and a first thermistor having a first terminal and a second terminal connected to the junction of said first diode and said first capacitor, said second signal generating means comprises a second diode and la second capacitor serially connected across the terminals of said source of alternating current and a second thermistor having a first terminal and a second terminal connected to the junction of said second diode and said second capacitor, said diodes being oppositely polarized, said controller means comprising a relay including a winding and a contact set, the state of said contact set controlling the activation of the fuel-feeding means, a silicon controlled rectifier having anode, cathode and gate terminals, means for connecting one end of said relay winding to one terminal of said signal source, means for connecting the .other end of said relay winding to said anode terminal, means for connecting the first terminals of said thermistors to one of the other terminals of said silicon controlled rectifier, and means for connecting said cathode terminal to the other terminal of said source .of alternating current.

8. The apparatus of claim 7 wherein the first terminals of said thermistors are connected to said cathode terminal and further comprising resistor means for connecting said gate terminal to the other terminal of said source of alternating current.

9. The apparatus ,of claim 7 wherein the hydronic heating system heats first and second zones each having a thermostat and comprising first and second heating means respectively controlled by one of the thermostats.

10. The apparatus of claim 9 wherein said heat accumulatinglineans is a slab of heat storage material, said heating means are heat generating resistors thermally coupled to said slab, said first thermistor -being thermally coupled to said slab, and heat insulating means enveloping said slab, said first thermistor-fand said heat generating resistors.

11. The apparatus of claim 1 wherein said heat accumulating means comprises a slab of metal and heat insulating means enveloping said slab.

12. Apparatus for controlling the temperature of any or all of a plurality of regions which are to be heated by a common heat source which has a first temperature sensing means for generating control indicia, said apparatus comprising common heat accumulating means having thermal inertia, a plurality of heating means each corresponding to and controlled by one of said regions and employed for heating said heat accumulating means, a second temperature sensing means sensitive to at least the temperature of said heat 'accumulating means, and means controlled jointly by said first and second temperature sensing means for controlling the operation of said common heat source.

13. The apparatus of claim 12 including controller means responsive to the first and second temperature sensing means which includes a differential means for setting the ofi and on cycles of the heat source during normal operation.

(References on following page) 7 8 References Cited 2,564,120 8/ 1951 McLean 236--9 UNITED STATES PATENTS 3,158,319 11/1964 NelSOn. `1

2,038,067 4/ 1936 Taylor 2326-91 EDWARD J. MICHAEL, prima@ Examiner, 2,172,108 9/1939 Praud 236-91 5 v 2,263,422 11/1941 Harris 236-91 U-S- Cl- X-R- 2,286,296 6/1942 McGrath. 236-68, 78; 237-8

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