Title:
Water heater for removing particulates and returning water without particulates to heater, and method of operating same
Kind Code:
A1


Abstract:
A water heater for heating water containing particulates comprises a recirculating path responsive to water and particulates in the heater. The recirculating path removes particulates from the water flowing therein and returns water with particulates removed from it to the heater. The path includes a particulate filter for removing particulates from the water flowing in the path and a pump for pumping water in the path back into the heater. The path is responsive to a liquid cleaning agent applied to the heater with the water returned to the heater.



Inventors:
Frasure, Charles J. (Blackfoot, ID, US)
Frasure, Greg A. (Blackfoot, ID, US)
Frasure, Paul J. (Blackfoot, ID, US)
Frasure, Blake D. (Pocatello, ID, US)
Application Number:
10/345370
Publication Date:
07/22/2004
Filing Date:
01/16/2003
Assignee:
FRASURE CHARLES J.
FRASURE GREG A.
FRASURE PAUL J.
FRASURE BLAKE D.
Primary Class:
International Classes:
F24H9/00; (IPC1-7): F22B23/06
View Patent Images:
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Primary Examiner:
YUEN, JESSICA JIPING
Attorney, Agent or Firm:
LOWE HAUPTMAN GILMAN & BERNER, LLP (Alexandria, VA, US)
Claims:

We claim:



1. A water heater particularly adapted to heat water containing particulates, comprising a recirculating path arranged to be responsive to water and particulates in the heater, the recirculating path being arranged to remove particulates from the water flowing therein and to return water with particulates removed from it to the heater.

2. The water heater of claim 1 wherein the recirculating path includes a particulate filter for removing particulates from the water flowing in the path.

3. The water heater of claim 2 wherein the recirculating path includes a pump for pumping water in the path back into the heater.

4. The water heater of claim 3 wherein the pump is downstream of the filter.

5. The water heater of claim 4 wherein the recirculating path is arranged to be responsive to a liquid cleaning agent and the pump is arranged to pump the agent to the heater.

6. The water heater of claim 5 wherein the recirculating path is arranged so the liquid cleaning agent is introduced into the recirculating path between an outlet of the filter and an inlet of the pump.

7. The water heater of claim 3 wherein the recirculating path is arranged to be responsive to a liquid cleaning agent and the pump is arranged to pump the agent to the heater.

8. The water heater of claim 1 wherein the recirculating path is arranged to (a) be responsive to a liquid cleaning agent and (b) supply the cleaning agent to the heater.

9. The water heater of claim 8 wherein the recirculating path includes a pump for pumping water and the liquid cleaning agent in the path back into the heater.

10. The water heater of claim 1 wherein the water heater includes a closed tank having a vertical exterior wall arrangement and a bottom portion with a wall extending horizontally and vertically to a normally closed bottom drain, a valve arrangement coupled to the bottom drain for selectively enabling water flowing through the bottom drain to flow through it to the recirculating path, a hot water outlet connected to the tank interior, a cold water inlet having a segment in proximity to the horizontally and vertically extending wall, the cold water inlet being arranged so that in response to water being removed from the tank, cold water flows from the inlet into the bottom portion to gently wash sediment in the bottom portion toward the horizontally and vertically extending wall, the horizontally and vertically extending wall being arranged so the washed sediment drifts by gravity to the drain and the drifted sediment accumulates at and in proximity to the drain, and a controller for opening the valve from time to time for removing the washed sediment from the bottom portion via the drain and preventing accumulation of substantial sediment in the tank.

11. The water heater of claim 10 wherein the recirculating path is arranged for causing water flowing through it to flow into the cold water inlet.

12. The water heater of claim 1 wherein the cold water inlet segment comprises a generally horizontally extending manifold.

13. The water heater of claim 12 wherein the manifold includes a plurality of openings dimensioned and arranged so the cold water flows gently through them without causing turbulence in the water in the remainder of the tank.

14. The water heater of claim 13 wherein the manifold comprises a tube, the openings being only in a portion of the tube such that water flowing through the openings does not flow upwardly.

15. The water heater of claim 14 wherein the openings do not have a nozzle effect and have an area such that the flow rate of water passing through them does not cause water or sediment turbulence.

16. The water heater of claim 11 wherein the water heater includes an electric heating coil, the bottom portion wall intersecting the outer vertical wall arrangement and sloping downwardly from its intersection with the outer vertical wall toward a central axis of the tank where the drain is located.

17. The water heater of claim 11 wherein the water heater includes a gas burner and a central flue in fluid flow relation with combustion products of the gas burner, the gas burner being below the bottom portion wall, the bottom portion wall having an intersection with the flue and sloping downwardly from its intersection with the flue toward the outer vertical wall arrangement, the bottom portion wall having a sloping bottom edge having a nadir at the drain and a runway between the sloping bottom edge and the outer vertical wall arrangement; the cold water inlet segment, the bottom portion wall, the runway and the drain being arranged so that the washed sediment drifts to the bottom portion wall, thence drifts by gravity to the runway, thence drifts by gravity to the drain.

18. The water heater of claim 1 wherein the water heater includes a gas burner.

19. A method of operating a water heater comprising supplying water including particulates to the water heater, heating the water in the water heater, removing some water and particulates from the water heater from time to time, from time to time removing particulates from the water removed from the water heater, and from time to time returning to the water heater the water removed from the water heater.

20. The method of claim 19 wherein the particulates are removed from time to time from the water removed from the water heater by a filter.

21. The method of claim 20 wherein the water is returned to the water heater by pumping.

22. The method of claim 21 further including supplying a liquid cleaning agent to the water returned to the water heater by pumping.

23. The method of claim 22 wherein the liquid cleaning agent and the water returned to the water heater by pumping are simultaneously supplied to the water heater.

24. The method of claim 20 further including supplying a liquid cleaning agent to the water returned to the water heater.

25. The method of claim 24 wherein the liquid cleaning agent and the water returned to the water heater are simultaneously supplied to the water heater.

26. The method of claim 19 wherein the surfaces in the water heater are cleaned by supplying a liquid cleaning agent to the water returned to the water heater.

Description:

RELATION TO PREVIOUSLY FILED APPLICATION

[0001] The present application includes subject matter disclosed in the co-pending application of Frasure et al., Ser. No. 09/929,495, filed Aug. 15, 2001.

FIELD OF INVENTION

[0002] The present invention relates generally to water heaters having provisions for minimizing the effects of particulate material in the heater and to a method of operating same and, more particularly, to such a water heater and operating method wherein particulate material is removed from the water and the water from which the particulate material has been removed flows back to the tank.

BACKGROUND ART

[0003] Particulate material enters water heaters with cold water or is produced in the heater in response to elevated water temperatures that produce carbonates. In household water heaters, the particulate material forms sediment that settles to the bottom of a tank where the particulates accumulate. Frequently, the sediment accumulates in household water heater tanks to such an extent that an electric water heater coil becomes completely covered, reducing heat exchange efficiency materially, and possibly causing the coil to become overheated to such an extent that the coil breaks. In gas water heaters, the sediment accumulates between a burner and the water to reduce the heat transfer efficiency. In electric and gas water heater tanks, the sediment coats the interior walls of the tank, adversely affecting heater efficiency. Particulates also have an adverse effect on so-called instantaneous water heaters, which are frequently used for industrial purposes; an example of such an instantaneous hot water heater is available from Rinnai Kabushiki Kaisha, Nagoya, Japan. In an instantaneous water heater, cold water flows into a heater exchanger heated by gas flames.

[0004] In some water heaters, the particulates are accumulated as sediment that is evacuated from the water heater through the water outlet, by being discharged through a water faucet or trapped in the faucet strainers. The sediment trapped in the strainers should be eventually cleaned by unscrewing the strainer.

[0005] Other water heaters, e.g., Sigler, U.S. Pat. No. 4,505,231, and Taylor, U.S. Pat. No. 3,762,395, have manually operated sediment drain valves to drain the particulates in the form of accumulated sediment from the bottom of the water heater. The previously mentioned co-pending application discloses an arrangement wherein particulates are accumulated as sediment at the bottom of a tank and are automatically removed through a valve from time to time. A problem with these water heaters wherein particulates are removed from the tank is that a substantial amount of water is usually drained from the tank with the particulates.

[0006] It is, accordingly, an object of the present invention to provide a new and improved method of and apparatus for minimizing adverse effects of particulates in a water heater.

[0007] Another object of the invention is to provide a new and improved method of and apparatus for operating a water heater wherein particulates are removed from the water heater without removing a substantial amount of water from the heater.

SUMMARY OF THE INVENTION

[0008] In accordance with one aspect of the invention, a gas or electric water heater particularly adapted to heat water containing particulates comprises a recirculating path arranged to be responsive to water and particulates in the heater. The recirculating path is arranged to remove particulates from the water flowing therein and to return water with particulates removed from it to the heater.

[0009] Another aspect of the invention relates to a method of operating a water heater. The method comprises supplying water including particulates to the water heater. The water in the water heater is heated. Some water and particulates from the water heater are removed from time to time and some particulates from the water are removed from the water heater from time to time. The water removed from the water heater is returned to the water heater from time to time.

[0010] In a preferred embodiment, the recirculating path includes a particulate filter for removing particulates from the water flowing in the path. The recirculating path is preferably arranged to be responsive to a liquid cleaning agent for dislodging particulates from surfaces in the water heater. The recirculating path preferably includes a pump for pumping water and/or the cleaning agent in the path back into the heater. The pump is preferably downstream of the filter.

[0011] The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed descriptions of several specific embodiments thereof, especially when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING

[0012] FIG. 1 is a front elevation view of a household electric water heater in accordance with a preferred embodiment of the invention;

[0013] FIG. 2 is a top sectional view of FIG. 1 taken along lines 2-2;

[0014] FIG. 3 is a bottom sectional view of FIG. 1, taken along lines 3-3;

[0015] FIG. 4 is a side view of a manifold included in the heater of FIGS. 1-3;

[0016] FIG. 5 is a front elevation and section view of a household gas-fired water heater in accordance with a preferred embodiment of the invention; and

[0017] FIG. 6 is a top section view taken along lines 6-6 of FIG. 5.

DETAILED DESCRIPTION OF THE DRAWING

[0018] The household electric water heater of FIGS. 1-4 includes closed tank 10 having an upper section containing outer vertically extending cylindrical wall 15 and a lower section having a conical wall 20 that has a downward constant slope, i.e., inclination, angle α of at least 42 degrees from the horizontal. The bottom of conical wall 20, at central drain 21, is connected to pipe 24, thence to enlarged elbow 25, which is connected to manual ball valve 30, in turn connected to automatic solenoid operated drain valve 35 which is connected to pipe 36. Thus, the electric water heater of FIGS. 1-4 includes: bottom portion conical wall 20 intersecting outer vertical wall 15, and electric heater coils 45. The conical wall slopes downwardly from its intersection 26 with outer vertical wall 15 toward a central vertical axis of tank 10 where drain 21 is located. Drain valve 35 is actuated by timer controller 40 which can be adjusted for the length of valve opening and the time of day. Typically, valve 35 is opened daily, e.g. in the middle of the night, for a period sufficient to cause about a half gallon of water to flow from tank 10 to pipe 36.

[0019] Water and particulates in pipe 36 flow into an inlet of particulate filter 37 having an outlet connected to an inlet arm of T joint 38. T joint 38 has a leg connected to source 39 of a leg connected to source 39 of liquid cleaning agent, e.g., vinegar or acetic acid, that removes particulates from surfaces in tank 10 by a chemical reaction. Source 39 is connected to joint 38 via solenoid valve 41 that is opened by timer controller 40 simultaneously with opening of valve 35. T joint 38 has an outlet arm connected to an inlet of pump 42 that timer controller 40 turns on at the same time it opens valves 35 and 41. Pump 42, when turned on by controller 40, sucks (1) water that flows through filter 37 and which has had particulates removed from it by the filter and (2) cleaning liquid from source 39. The liquid cleaning agent chemically reacts with and dislodges impurities on surfaces in the water heater to clean these surfaces and provide a flow of particulates to filter 37. The dislodged particulates are trapped by filter 37 so the water at the outlet of pump 42 has no substantial particulates therein. The liquid at the outlet of pump 42 flows via pipe 44 and T joint 43 back into tank 10 to form a recirculation path as described in more detail infra.

[0020] The water temperature is set by electric heaters 45 and adjustable temperature controller 50. Heaters 45 are in heat exchange relation with the water inside tank 10. For clarity, the drawing does not include heater insulation which covers all sections of the heater and hot water pipe 60. Penetrating heater roof 65 are pressure and temperature relief valves 70, vertically extending cold water inlet pipe 75 (frequently referred to as a dip tube) and corrosion reducing anode 80. T joint 43 is inserted in pipe 75, about half between roof 65 and the bottom end of pipe 75.

[0021] The opposite arms of T joint 43 are connected to the center portion of pipe 75, while the leg of T joint 43 is connected to the outlet of pump 42 via pipe 44. Thereby water that is relatively free of particulates and is clean flows from pump 42 to the lower portion of tube 75 via T joint 43. While the outlet of pump 42 is preferably applied to a center portion of pipe 75 to reduce the load on the pump and somewhat isolate the pump output from the head of the water applied to the inlet of pipe 75 and slits in the tube 92, it is to be understood that the outlet of pump 42 can be connected to any appropriate point in tank 10.

[0022] Hand hole cover 85 provides access to the tank interior for manual cleaning and inspection.

[0023] As illustrated in FIGS. 1 and 2, the bottom end of dip tube 75, just above horizontal intersection 26 between cylindrical wall 15 and conical wall 20, is connected to T joint 90, having horizontally disposed opposite ends connected to opposite ends of metal tube 92. Tube 92 is shaped as a ring, held in place by hangers 89 (not shown in FIG. 1 to simplify the drawing) so the tube outer surface is about an inch from the interior surface of wall 15, to hold tube 92 so that the tube is mounted essentially horizontally about an inch above intersection 26.

[0024] Tube 92 is a generally horizontally extending manifold including a plurality of openings dimensioned and arranged so the water entering tube 92 from tube 75 flows gently from tube 92 through the openings therein without causing turbulence in the water in the remainder of tank 10. The openings do not have a nozzle effect and have an area such that the flow rate of water passing through them does not cause turbulence to the water or sediment in tank 10. The openings are only in a bottom portion of tube 92, such that water flowing through the opening does not flow upwardly in tank 10.

[0025] To these ends, tube 92 includes three sets 96, 97 and 98 of slits. In an actual water heater, tube 92 included a total of 48 slits.

[0026] In essence, tube 92 includes first and second segments 93 and 94, respectively connected to the opposite ends of T joint 90 so that the water in the bottom of dip tube 75 flows from the dip tube into segments 93 and 94 in opposite directions from the T joint, which thus forms a common point for the water to flow in opposite directions.

[0027] The angled slits in set 96 are in segments 93 and the angled slits in set 97 are in segment 94. The slits in sets 96 and 99 in both segments 93 and 94 are angled in the same direction as the laminar flow of water in these segments. The slits in set 98 that are in segments 93 and 94 are substantially perpendicular to the direction of laminar flow of water in tube 92. In the actually constructed water heater, there was a total of 48 slits; 22 slits in set 96, 22 slits in set 97, and four interspersed right angle slits in set 98. Each slit had a width of about {fraction (1/16)} inch and a length of about 1 ½ inches. The slits of sets 96 and 97 are tilted about 20 degrees relative to the direction of laminar water flow in tube 92.

[0028] In response to water being removed from tank 10 through hot water pipe 60 while valves 35 and 41 are closed and pump 42 is off or through pipe 36 while valves 35 and 41 are open and pump 42 is on, water flows from the bottom of dip tube 75 into tube 92, thence through the slits in sets 96-98 into the bottom portion of tank 10. Water flows through the slits in sets 96-98 in response to water flowing out of hot water pipe 60 or opening of drain valve 35. In the former case, water enters the top of cold water pipe 75 as shown at arrow 110. In the latter case, water enters pipe 75 from pump 42 through T joint 43. In both cases, water in the bottom of pipe 75 flows through the slits in sets 96-98. The water flowing through the slits in sets 96-98 gently washes sediment in the bottom portion of the tank toward conical horizontally and vertically extending wall 20. The inclination angle, α, of wall 20 below the horizontal plane is such that the washed sediment 95 accumulates at and in proximity to drain 21.

[0029] Controller 40 automatically and simultaneously (1) opens valves 35 and 41 and (2) activates pump 42 from time to time, e.g., periodically, to remove the washed sediment 95 (FIG. 2) from the bottom portion of tank 10 via drain 21 to prevent accumulation of substantial sediment in the tank. Because the washed sediment lodges in the filter 37, which is outside of the heat exchange region of tank 10, the adverse effects of sediment accumulation in tank 10 are avoided.

[0030] Timer and valve controller 40 activate solenoid valve 35 for varying durations and frequencies depending on the hardness of the water and amount of particulate residue in the water. The timer is typically set to actuate the solenoid valves 35 and 41, as well as pump 42, for about 3 seconds during the middle of each night. Depending on water pressure and component sizes, in this 3-second period about one cubic foot of water flows in a recirculating path from valve 35 to T joint 43 via filter 27 and pump 42. This one cubic foot of water drains only the cool water located in the lower portion of tank where conical wall 20 is located. The hot water above lower heating coil 45 is not discharged since the lower coil 45 is about 4 inches above the top of inverted cone 20.

[0031] The household gas fired heater illustrated in FIGS. 5 and 6 includes tank 140 having cylindrical wall 145 and lower section including an inverted conical wall 150 having a minimum downward slope angle, β, of at least 42 degrees from the horizontal for optimum operation. Drain 152, at the bottom of inverted cone 150, is adjacent to elbow 155, connected to manual ball valve 160, in turn connected to automatic solenoid operated drain valve 165. Drain valve 165 is actuated by timer/controller 170 which is adjusted to control the valve opening duration and the time of day the valve is opened. Water flowing through valve 165 flows via pipe 166 to an inlet of particulate filter 167. Filter 167 has an outlet connected to an inlet arm of T joint 168, having a leg connected via solenoid valve 172 to source 169 of a liquid cleaning agent, e.g., acetic acid or vinegar. T joint 168 has an outlet arm for supplying water that flows through filter 167 and has had particulates removed from it and liquid from source 169 to an inlet of pump 171. Pump 171 has an outlet for supplying the water from filter 167 and the liquid from source 169 back to tank 140, to form a recirculation path for water removed from tank 140 via valve 165. Timer controller 170 turns on pump 171 and opens valves 165 and 172 simultaneously, preferably during the middle of each night, a period when hot water is not usually flowing through pipe 185.

[0032] The water heater temperature is set by (1) gas control valve 175, (2) annular gas jet manifold 176, which is located under cone 150 to preclude water contact with the flame, and (3) adjustable temperature controller 180. For clarity, the drawing does not show heater insulation which covers all sections of the heater and hot water outlet pipe 185. Penetrating the heater top section 186 are pressure and temperature relief valves 190, cold water inlet pipe 195, and corrosion reducing anode 200. Flue pipe 201 penetrates the center of the top section 186 and extends down to the top of inverted cone 150. Within flue pipe 201, is tubing coil 202 that, by convection, moves cool water in tank 140 from inlet 203 of flue pipe 201, to an outlet (not shown) near the top cover 186. Handhold cover 205 provides access to the tank interior for manual cleaning and inspection. The water in tank 140 is in heat exchange relation with hot gas from manifold 176 by virtue of metal cone 150 and metal flue pipe, being in contact with the water and heated primarily by convection by the hot gas.

[0033] Because gas jet manifold 176 is below cone 150 and flue pipe 201 is located in the center of tank 140, drain 152 cannot be centrally located. Consequently, drain 152 is located in proximity to exterior wall 145, at the lowest portion 220 of flange 240 that extends from the lowest edge of cone 150 and is bonded, e.g., by seam welding or soldering, to wall 145. Cone 150 forms a vertically and horizontally extending bottom wall portion of tank 140. The bottom edge of cone 150 has a zenith point 222 diametrically opposite from drain 152, which is at the nadir of the cone bottom edge. In each vertical cross section of tank 140, flange 240 extends horizontally between the bottom edge of cone 150 and wall 145. Flange 240 extends continuously and smoothly around the circumference of the bottom edge of cone 150, between zenith point 222 and drain 152 to, in effect, provide a runway for sediment incident on the flange and cone 150. The inclination angle β of the horizontally and vertically extending wall of cone 150 relative to the horizontal plane is such that washed sediment in tank 140 drifts by gravity along the wall of cone 150 to the runway flange 240 forms. Inclination angle β continuously varies from a minimum angle along a straight line of the wall segment between flue 201 and zenith point 222 to a maximum angle along a straight line of the wall segment between flue 201 and nadir 220. The inclination angle of the runway between zenith point 222 and drain 152 is such that the washed sediment incident on the runway also drifts by gravity to the drain. The optimum minimum inclination angle β is 42 degrees below a horizontal plane extending through a horizontal intersection of cone 150 and flue 201.

[0034] At the lowest end of dip tube 195 is horizontally extending T joint 210 for directing cold water horizontally in two directions into tube or manifold 212. Manifold 212 is connected to the bottom of cold water inlet tube 195 and fixedly mounted by hangers (not shown) just above zenith point 222. Manifold 212 is shown as being horizontally disposed, but it is to be understood that the manifold could be inclined so it is a fixed distance above flange 240. Manifold 212 includes many slits 214 completely along its length. The slits 214 are only in the lower half of the metal tubing forming manifold 212. Manifold 212 is similar to manifold 92 in that slits 214 are dimensioned and arranged so the cold water flows gently through slits 214 without causing turbulence to the sediment and/or water in tank 140. Slits 214 in manifold 212 can achieve this result by having the same dimensions as the slits of manifold 92. Slits 214 differ from the slits of manifold 92 because all of slits 214 are perpendicular to the direction of laminar water flow in the annular tube forming manifold 212. One actually built manifold 212 has 48 slits 214, spaced 1 inch from each other along the circumference of the manifold.

[0035] In response to water exiting hot water pipe 185, shown by arrow 230, cold water enters cold water pipe 195 as shown at arrow 235. In response to water and particulates flowing through drain valve 165, the particulates are removed from the water by filter 167. The water flowing through filter 167 and the cleaning liquid flowing through open valve 172 are sucked by pump 171 to flow via T joint 173 into a midpoint of pipe 195 about half way between roof 186 and manifold 212. In both cases, water flows to the bottom of pipe 195, thence to manifold 212 and through slits 214 to gently wash sediment in tank 140 to the wall of cone 150, thence to the runway that flange 240 forms and to drain 152.

[0036] The gas water heater has convex roof 186 and vertical sides of about 40 inches. The bottom edge of cone 150 at zenith point 222 is about 8 inches below the bottom of flue 210; at nadir 220, the cone bottom edge is about 12 inches below the bottom of flue 210. A 1.5 inch diameter outlet and a 90 degree elbow 155 are connected adjacent to drain 152, at nadir 220 of cone 150. A bell reducer reduces the piping from 1.5 inch diameter to 1.25 inch diameter. Stainless steel ball valve 160 isolates stainless solenoid valve 165 for maintenance or replacement. Tank 140 is about 2 feet in diameter and has a volume of about 33 gallons. Stainless steel inlet dip tube 195 terminates at the 90 degree T joint 210 about one inch above the bottom edge of cone 150. Three legs support the tank and can therefore accommodate uneven floors. The preferred tank material is stainless steel surrounded by foam insulation and a think outer metal shell.

[0037] The electrical components include pump 171, as well as solenoid valves 165 and 172, and timer and valve controller 170. Timer and valve controller 170 is adjusted to activate pump 171 as well as solenoid valves 165 and 172 for varying durations and frequencies depending on the hardness of the water and amount of particulate residue in the water.

[0038] Although the materials referred to for construction are stainless steel, a less expensive heater could be made from a glass-lined carbon steel body using copper pipe and bronze valves.

[0039] While the present invention has been described by reference to specific embodiments, it will be apparent that other alternative embodiments and methods of implementation or modification may be employed without departing from the true spirit and scope of the invention. For example, the recirculating path for removing particulates can be employed to remove particulates that lodge in heat exchangers of so-called instantaneous industrial hot water heaters, of the type previously discussed in the Background Art portion of this document. In such a case, a first T joint is connected in the cold water pipe leading to the heat exchanger. A second T joint is connected in a by-pass pipe connected, via a by-pass valve, between the heat exchanger hot water outlet pipe and the cold water inlet of the heater. A particulate filter has an inlet connected to a leg of the second T joint and an outlet connected via a third T joint to an inlet of a pump having an outlet connected to a leg of the first T joint. The third T joint is also responsive to cleaning liquid that flows through a suitable solenoid valve that is opened simultaneously with activation of the pump, at a time when hot water is not removed from the heat exchanger. It is also to be understood that in some circumstances the cleaning liquid source and plumbing associated therewith are not necessary. The particulate filters are typically of a type wherein a filter element is easily removed. Further, backwash techniques can be employed for removing particulates from the particulate filter from time to time, at intervals much less frequent than activation of the recirculation path. In a practical household water heater, the recirculation path is typically integral with the tank, and the filter and cleaning agent are located so they can be easily accessed.