AU2007231526B2

AU2007231526B2 - Dual heater, on demand water heating unit

Info

Publication number: AU2007231526B2
Application number: AU2007231526A
Authority: AU
Inventors: Patrick Michael Conrick; Jeff Elliott
Original assignee: Rheem Australia Pty Ltd
Current assignee: Rheem Australia Pty Ltd
Priority date: 2006-03-24
Filing date: 2007-02-27
Publication date: 2011-07-07
Anticipated expiration: 2027-02-27
Also published as: AU2007231526A1; AU2011200433A1; AU2011200433B2; NZ570975A; WO2007109829A1; NZ590704A

Abstract

An instantaneous water heater (100) includes a gas heater (102) in series with an electrical heater (122). A flow sensor (118) indicates to control means (108, 112) the amount of heating energy required, and the control means control the heaters (102, 122) to heat the water accordingly.

Description

On-Demand Water Heater Field of the invention [001] This invention relates to improvements in on-demand water heaters. Background of the Invention [002] On-demand water heaters do not have a large storage tank which is maintained at the operating temperature, but supply heat to the water stream as it is being drawn off by a user. This mode of operation requires a large heat input. In some on-demand heaters, the heat is supplied by a gas burner. In one arrangement, the combustion gasses heat the fins of a heat exchanger while the water flows through ducts in the heat exchanger which are thermally connected to the fins. [003] Historically, the gas burner is usually of an on-off type which is either fully on or off. The gas burner also has to be designed to heat the maximum water flow to the operating temperature. The maximum flow can be above 10 L/min, and can be up to or above 30 L/min. Typically, the flow rate is of the order of 15 L/min. When instantaneous gas heaters are marketed with a nominal flow rate of, say; 20 litre/min, 24 litre/min etc, the understanding is that this is for a temperature rise (AT) of 25 0 c. This is typically the flow rate available at showering temperatures. The official Australian Gas Association (AGA) rating is done for a AT of 40*c. So a 24 1/min heater at 25 *c is equivalent to 15 litre/min at of 40 0 c AT. [004] The heater must be able to heat water from its ambient temperature to the operating temperature in the time it takes the water to flow through the heat exchanger. Typically the instantaneous heater takes 20 to 30 seconds to get to operating temperature so may waste up to 10 litres before set point is reached. [005] Because the heater supplies sufficient heat to heat the maximum flow, where the flow is significantly lower, for example, less than 5 L/min, the water can become overheated. This problem becomes more significant as the flow drops below 2 L/min. [006] Even where the gas burner has a controlled gas flow, it is also difficult to control the gas burner to supply low levels of heat at a properly regulated temperature. [007] On demand gas burners also need to trade efficiency for performance in rapidly heating the water to the operating temperature. [008] It is desirable to provide an on-demand water heater which is capable of supplying low flow rates at a regulated temperature. [009] It is also desirable to improve the efficiency of on-demand gas water heaters.

2 [010] Any reference herein to known prior art does not, unless the contrary indication appears, constitute an admission that such prior art is commonly known by those skilled in the art to which the invention relates, at the priority date of this application. Summary of the invention [011] In this specification, the term "controllable heater" refers to a heater in which the heat energy output of the heater can be controlled. Depending on the context, reference to a threshold temperature can indicate a single temperature or a range of temperatures. (012] The present invention provides an on-demand water heater having a first heater in series with a second heater. [013] The first heater can be a gas heater. [014] The second heater can be an electric heater. [015] The first heater can be located on the upstream side of the second heater. [016] The first heater can be on the downstream side of the second heater. [017] The on-demand heater can include a first controller to control the first heater. [018] The on-demand heater can include a second controller to control the second heater. [019] The first heater can be a controllable heater. [020] The second heater can be a controllable heater. [021] The invention also provides a method of operating an on-demand water heater having a first heater and a second heater and a flow rate detector, wherein the second heater is a controllable heater adapted to operate at lower flow rates than the first heater, the method including the steps of: detecting the initiation of water flow; turning the first and second heaters on each at maximum power; detecting when the outlet flow of water reaches a predetermined temperature threshold, monitoring the water flow rate; detecting whether the water flow rate is below a first predetermined threshold flow rate, referred to as a low flow threshold; and where the water flow rate is below the low threshold, turning the first heater off. [022] The method can include regulating the output of the second heater in relation to the flow rate of the water.

3 (023] The invention further provides a method of operating an on-demand water heater having a first heater and a second heater and a flow rate detector, wherein the second heater is a controllable heater, the method including the steps of: detecting the initiation of water flow; turning the first and second heaters on each at maximum power; detecting when the outlet flow of water reaches a predetermined temperature threshold, monitoring the water flow rate; detecting whether the water flow rate is above a predetermined low threshold and below a predetermined upper flow rate threshold; and where the water flow rate is between the upper and lower flow rate thresholds, optimizing the energy provided by the first and second heaters. [024] The invention further provides a method of operating an on-demand water heater including a main heater and an auxiliary heater, wherein the auxiliary heater includes an electric element, the method including regulating the electrical power to the element to maintain the water in the vicinity of the element within an operating temperature range when there is no water flow. [025] The method may, including the step of regulating the element to heat the water to within the operating temperature range during low flow usage. [026] The invention further provides an on-demand water heater having first and second controllers adapted to control the heating energy delivered by the gas heater and the electrical heater on the basis of the tariff of the electricity and/or the gas. Brief description of the drawings [027] An embodiment or embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: [028] Figure 1 is a schematic illustration of an on-demand water heater embodying the invention. [029] Figure 2 is a schematic illustration of an alternative embodiment of an on-demand water heater embodying the invention. Detailed description of the embodiment or embodiments [030] Figure 1 schematically illustrates an on-demand water heater 100 embodying the invention.

4 [031] Water enters the heater 100 at the water inlet indicated at arrow 126, and exits via hot water outlet indicated at 128. [032] A gas heater 102 includes a gas burner 104 supplied with gas from a gas inlet 130 via a valve 106. A heat exchanger including fins 105 and water duct 107 transfers the heat from the combustion products to the water. An ignition device 103 is provided to ignite the burner 104 when gas starts to flow. The ignition device can be directly responsive to the commencement of gas flow or can be controlled by controller 108. Alternatively, a pilot flame can be provided. A flame sensor (not shown) can also be provided to ensure that the gas is burning while the gas is being supplied to the burner 104. [033] The gas valve 106 can be a simple on/off valve, but in a preferred embodiment, gas valve 106 is controllable to control the amount of gas supplied to the burner. A first controller 108 controls the operation of gas valve 106. [034] A first temperature sensor 124 senses the temperature of the water entering the gas heater 102, and a second temperature sensor 116 measures the temperature of the water leaving the gas heater 102. Temperature sensors 124 and 126 are connected to controller 108. [035] A flow sensor 118 detects the flow of water. In the preferred embodiment, the flow sensor not only detects whether the water is flowing, but also measures the flow rate. The output of flow sensor 118 is connected to controller 108. [036] The first controller 108 can be set to maintain the temperature of the output water within a predetermined operating range by regulating the amount of gas supplied to the burner via valve 106. The control can be a feedback control where the controller increases gas flow when the temperature when the output water temperature falls below a lower threshold band or value, and decreases the gas flow if an upper temperature threshold is exceeded. [037] In an alternative arrangement, the first controller can use an algorithm to calculate the gas flow required using parameters such as the heat exchanger and burner characteristics, the inlet water temperature from sensor 124 and the required outlet temperature range. [038] In an enhancement of the arrangement described above, a further heater is provided. In the embodiment of Figure 1, the additional heater is an electric element 110 in housing 122. The water inlet enters the housing 122 at inlet 126 and flows around the element 110 before entering the gas heater 102. [039] A third temperature sensor 120 can be provided to measure the temperature of the water entering the electrical heater.

5 [040] A second controller 112 is provided to control the element 110. The second controller 112 is responsive to the flow detector 118 to turn the element 110 on when water flow is detected. The controller reacts to the initial detection of water flow by supplying electric power from the electrical utility 114 to the element 110. While controllers 108, 112 are shown as separate devices, they can be implemented by a single device. [041] The second heater 110 can be operated in an on/off mode, but, in accordance with an embodiment of the invention, it is operated in a controllable manner. In the controlled power mode, the controller 112 regulates the electrical power supplied to the element 110. (042] The second controller 112 can be adapted to regulate the power in response to the flow rate detected by the flow sensor 118. Additional control parameters can be provided from the temperature sensor 124 sensing the temperature of the water leaving the housing 122, and/or the temperature sensor 120 sensing the temperature of the water entering the housing 122. (043] The water heater illustrated in Figure 1 can be operated in a number of modes. [044] In a low flow mode, where the regulation of the gas heater is not sufficiently accurate, the element 110 can be used on its own. [045] In an intermediate flow range, the gas heater 103 can be used on its own, or both the electrical heater 110 and the gas heater 102 can be used in combination to maximize efficiency. [046] At high flow rates, both the gas and electric heaters can be used together to meet a high flow rate. [047] Preferably, the controllers 108, 112 include programmable processors to facilitate the different modes of operation. [048] In one embodiment, we have added an electrical element to an existing gas on-demand heater in a configuration similar to that illustrated in Figure 1. The gas heater uses a Paloma governor and we have modified the software of the governor to permit the greater flow rate available with the use of the electrical element. With a 4.8 kw electrical heater, we have found that an additional 2.7 litres/min flow within the operating temperature range can be achieved. [049] In Figure 2, elements which correspond to elements in Figure 1 have similar numerical labels except that the prefix is "2" instead of "1". [050] Figure 2 illustrates a second embodiment of the invention, in which the electric element 210 is located downstream of the gas heater 202. [051] In Figure 2, the temperature sensor 216 serves as the output sensor for the gas heater 202, and as the input sensor for the electric element 210.

6 [052] Gas controller 208 has temperature inputs from temperature sensors 216, 220, and 224. Thus the gas supply can be regulated to take account of the outlet temperature information from sensor 216. [053] In Figure 2, the electric element 210 is controlled by controller 212. The power can be controlled by the use of pulse width modulation. The electric heater shell 222 can be insulated to reduce heat loss when there is no water flow. The element 210 can be controlled to maintain the water in the heater 222 at the operating point, for example by using a sensor located to measure the temperature of the water in the electric heater. Preferably, the pipe length between the gas heater and the electric heater 222 is minimized. The electric heater can be located adjacent to the gas heater. Thus the electric heater can be used to maintain the water within the operating range when there is no flow, and the electric heater can be used to heat the water when the flow rate is low. [054] With the electrical element fitted to the outlet of the gas heater, the 2.7 litres/min additional flow rate described in relation to the embodiment of Figure 1 is provided. In addition, there is a reduction in the initial waste water which is delivered before the water reaches the operating temperature range, particularly at low flow rates. [055] In a further improvement, the controllers 108, 112 can be adapted to optimize the operating cost of operating the heater by allocating the load between the gas and electric heaters in accordance with the current tariffs for the gas and electricity. [056] Thus where off-peak electricity tariffs apply, greater use may be made of the electrical heater than would be the case when the peak electrical tariff applies. [057] The controllers 108, 112 can have time-of-day tariffs stored in memory or the tariffs can be notified to the controllers via a communication link to the energy supplier, shown illustratively in Figure 2 as wireless modem 250. [058] The decision as to which energy source to use or the proportions of each source to use can be calculated based on cost per unit of usable heat delivered. For example, the comparison can be made on the basis of the following: [059] Cost of electricity CE = TE * EE where TE = Current electricity tariff EE = Electrical energy delivered [060] Cost of gas CG = TG * EG where TG = Current gas tariff EG = Gas energy delivered 7 [061] UE = EE * OE [062] CE TE UE /E UG = EG * lG CG = TG * UG /flG where UE = Usable electrical energy IIE = efficiency of electrical heater UG = Usable gas energy rIG = efficiency of gas heater [063] Electricity only is used where CE < CG and Peq <= PEMax where Peq = required power PEMax = maximum power of electrical heater [064] Where the gas supply is controllable, gas only is used where CE > CG and Preq <= PGMax [065] Electricity and gas are used in other cases, the maximum available energy from the cheaper source being used, supplemented by the other source up to the required amount of energy. [066] If the gas supply is not controllable, electricity is used for energy requirements less than PEMax. [067] Other control schemes can be used without departing from the inventive concept. [068] Where ever it is used, the word "comprising" is to be understood in its "open" sense, that is, in the sense of "including", and thus not limited to its "closed" sense, that is the sense of "consisting only of'. A corresponding meaning is to be attributed to the corresponding words "comprise", "comprised" and "comprises" where they appear. [069] It will be understood that the invention disclosed and defined herein extends to all alternative combinations of two or more of the individual features mentioned or evident from the text. All of these different combinations constitute various alternative aspects of the invention. [070] While particular embodiments of this invention have been described, it will be evident to those skilled in the art that the present invention may be embodied in other specific forms without departing from the essential characteristics thereof. The present embodiments and examples are therefore to be considered in all respects as illustrative and not restrictive, and all modifications which would be obvious to those skilled in the art are therefore intended to be embraced therein.

Claims

1. An on-demand water heater including a first heater in series with a second heater and a flow rate detector for controlling said heaters.

2. An on-demand water heater as claimed in claim 1, wherein the first heater is a gas heater.

3. An on-demand water heater as claimed in claim 1 or claim 2, wherein the second heater is an electric heater.

4. An on-demand water heater as claimed in any one of the preceding claims, wherein the first heater is located on the upstream side of the second heater.

5. An on-demand water heater as claimed in any one of claims 1 to 3, wherein the first heater is located on the downstream side of the second heater.

6. An on-demand water heater as claimed in any one of the preceding claims, including a first controller to control the first heater.

7. An on-demand water heater as claimed in any one of the preceding claims, including a second controller to control the second heater.

8. An on-demand water heater as claimed in any one of the preceding claims, wherein the first heater is a controllable heater.

9. An on-demand water heater as claimed in any one of the preceding claims, wherein the second heater is a controllable heater.

10. A method of operating an on-demand water heater as claimed in any one of the preceding claims, wherein the second heater is a controllable heater adapted to operate at lower flow rates than the first heater, the method including the steps of: detecting the initiation of water flow; turning the first and second heaters on each at maximum power; detecting when the outlet flow of water reaches a predetermined temperature threshold, monitoring the water flow rate; detecting whether the water flow rate is below a first predetermined threshold, referred to as a low flow threshold; and where the water flow rate is below the low threshold, turning the first heater off.

11. A method as claimed in claim 10, including regulating the output of the second heater in relation to the flow rate of the water. 9

12. A method of operating an on-demand water heater as claimed in any one of claims 1 to 9 having a first heater and a second heater and a flow rate detector, wherein the second heater is a controllable heater, the method including the steps of: detecting the initiation of water flow; turning the first and second heaters on each at maximum power; detecting when the outlet flow of water reaches a predetermined temperature threshold, monitoring the water flow rate; detecting whether the water flow rate is above a predetermined low threshold and below a predetermined upper flow rate threshold; and where the water flow rate is between the upper and lower flow rate thresholds, optimizing the energy provided by the first and second heaters.

13. A method of operating an on-demand water heater as claimed in any one claims 1 to 9 including a main heater and an auxiliary heater, wherein the auxiliary heater includes an electric element, the method including regulating the electrical power to the element to maintain the water in the vicinity of the element within an operating temperature range when there is no water flow.

14. A method as claimed in claim 13, including the step of regulating the element to heat the water to within the operating temperature range during low flow usage.

15. An on-demand water heater as claimed in any one of claims 7 to 9 when dependent on claim 6 wherein said first and second controllers are adapted to control the heating energy delivered by the gas heater and the electrical heater on the basis of the tariff of the electricity and/or the gas.

16. An on-demand water heater substantially as herein described with reference to the embodiments of the invention illustrated in the drawings.

17. A method of operating an on-demand water heater substantially as herein described with reference to the embodiments of the invention illustrated in the drawings.