Homebrew Demand Water Heater

<body topmargin=0 leftmargin=0 marginheight=0 marginwidth=0> <table cellpadding=0 cellspacing=0 border=0><tr> <td valign="top" height=90 BGCOLOR="#006699" TEXT="#080000" background="1fcfd5a3.jpg"> <div> <FONT SIZE="5" COLOR="#336666" FACE="Arial" ><B>BIT Company</B></FONT><B>     </B><B>|</B><B>     </B><A HREF="index.htm" TARGET="_top" TITLE="BIT Company"><U><B>home</B></U></A></div> <div> </div> </td> </tr><tr> <td valign="top" height=390 BGCOLOR="#FFFFCC" TEXT="#080000"> <div> <I>Homebrew Demand Water Heater</I></div> <bR> <div> With the exception of heating and air conditioning, the largest consumer of household electricity is usually the domestic water heater. Although newer units are somewhat superior due to better insulation, they are still energy hogs. One of the major flaws lies in the basic concept of mixing both hot and cold supplies in order to achieve a desired temperature resulting in the conventional unit being set 20-40 degrees higher than the final use requirement. This results in much higher heat losses both from the unit itself and through the distribution plumbing. In this part of the country, most distribution plumbing is by way of uninsulated copper tubing running under poured slabs. By way of example, my house had a standard 40 gallon unit and a 15 foot run of 1/2 inch copper tubing to the bathroom. With the heater set at 140 degrees, 2 gallons of water had to be run to get the first glimmer of heat and 4 additional gallons before the mixture could be set. Running through the numbers showed that a reasonably comfortable 10 minute shower cost about $1.09.</div> <bR> <div> 20 some-odd years ago when I built this house, I'd considered installing demand water heaters at the various point of use locations but the only decent commercial units ran about $800 each. There seem to be spec-wise quite a few good ones on the market today ranging from under $300 to about $600 but of course, it was more entertaining to 'grow my own'. The stuff documented below are the designs that have cut my electric bill by over 30% while actually using over twice as much hot water.</div> <bR> <div> Before you get started, you need to determine what wattage your particular situation requires.</div> <bR> <div> <B><IMG SRC="11a40da0.png" border=0 width="576" height="474" ALT="WordPad Document" ALIGN="BOTTOM" HSPACE="0" VSPACE="0"></B></div> <bR> <div> You can see from the chart that a 100 degree 2 GPM shower from a 70 degree source requires 8.8 KW. At my present electric rate, that same 10 minute shower mentioned above costs under 16 cents! ...and I never run out of hot water! However, it <U>does</U> require 37 amps from the 240 volt circuit. I have 2 - 40 amp circuits in my utility room and, since I elected to build a whole-house unit rather than change all the existing plumbing, didn't have to re-wire either. </div> <bR> <DIV ALIGN="LEFT"></DIV> <div> Design Considerations</div> <bR> <div> The heating chamber design was mostly a series of experiments to determine if schedule 40 PVC materials would be adequate as one might want to have 140 degree water available for other uses and to that end, a test cell was constructed, pressurized to 55 psi, and held at a temperature of 170 degrees for a day to see if bad things would happen. As expected, there was enough deformation in the 1" NPT threading of the heating element mounting hole and the straight thread heating element to cause a persistant seep regardless of gasketing. This was solved by replacing the 1" NPT to 2" PVC adaptor with a 1 1/4" NPT to 2" PVC adaptor and using a 1 1/4" to 1" galvanized bushing to mount the heating element. The 1" portion of the galvanized bushing needed the threads chased with a tap which may not be necessary if your local hardware store has fittings made in places other than China. </div> <bR> <div> For maximum efficiency, it was desirable to incorporate a flow sensor to disable all heating when there is zero flow. Some commercial units do not use this, indeed, the shower-only electric valve version doesn't need it at all, but for other applications like kitchen, lavatory, dishwasher, and washing machine use, it saves about half  the energy of leaving the Bunn coffee maker on all the time. </div> <bR> <div> Commercial flow sensors usually have a mechanical vane in the water flow attached to a microswitch through a flexible bushing. I decided to design a magnetically coupled unit which uses a coil-wound bobbin which is deflected upwards by flows of < 1 quart per minute into a external sense coil.</div> <div>                         </div> <div> <IMG SRC="05fb2a40.jpg" border=0 width="434" height="164" ALIGN="BOTTOM" HSPACE="0" VSPACE="0"></div> <div> <FONT SIZE="3" COLOR="#336666" FACE="Comic Sans MS" >         </FONT>                        <U> </U><A HREF="id45.htm" TARGET="_top" TITLE="Flow Coil Construction"><U><U>Flow Coil Construction</U></U></A></div> <bR> <div> The wound bobbin changes the inductance of the external coil by a few percent and varies the frequency of a simple ocillator circuit. This change is detected with a phase lock circuit and enables the thermostat/power control circuitry.</div> <bR> <div> Probably the most critical component in the system is the thermistor probe. In order to detect temperature changes rapidly, it needs to have a small thermal mass but needs to be robust enough to take the temperature changes and the pressure. The first prototypes used a NASA stainless steel probe left over from the Apollo project and was reasonably fast but hopelessly expensive unless one has a few in the old junk box! Here's a satisfactory replacement made from a Radio Shack 270-110A thermistor ($1.99), a right-angle 1/4" delrin fitting, and a dollop of JB Weld.:</div> <bR> <div> <IMG SRC="06021510.jpg" border=0 width="289" height="337" ALIGN="BOTTOM" HSPACE="0" VSPACE="0"> <IMG SRC="061d9510.jpg" border=0 width="217" height="337" ALIGN="BOTTOM" HSPACE="0" VSPACE="0"></div> <div>                        Unpotted                                 Assembled</div> <bR> <DIV ALIGN="LEFT"></DIV> <div> Electronic Packages</div> <bR> <div> The simplest version consists of only the thermostat/power controller electronics and an electric valve such as the Rain Bird CPF 075 or 100. This is probably ideal for most shower use and the only control is the temperature control potentiometer with a switch. The pot/switch control can be remoted any reasonable distance from the controller and has a maximum of 24 VAC present (valve control power). </div> <bR> <div> <IMG SRC="06f26b80.jpg" border=0 width="550" height="184" ALIGN="BOTTOM" HSPACE="0" VSPACE="0"></div> <div>          Single Element Shower Assembly with input flow control valve </div> <bR> <div> <A HREF="id34.htm" TARGET="_top" TITLE="Dual Driver Shower Control"><U><IMG SRC="0bbc8850.png" border=0 width="200" height="133" ALIGN="BOTTOM" HSPACE="0" VSPACE="0"></U></A></div> <div> Shower Control PCB Layout</div> <div> Click for larger image</div> <bR> <bR> <div> <A HREF="id38.htm" TARGET="_top" TITLE="Shower Control schematic"><U><IMG SRC="0eac8940.png" border=0 width="200" height="148" ALIGN="BOTTOM" HSPACE="0" VSPACE="0"></U></A></div> <div> Shower Control schematic</div> <div> Click for larger image</div> <bR> <div> <A HREF="id36.htm" TARGET="_top" TITLE="Shower Control Parts List"><U><U>Shower Control parts list</U></U></A></div> <bR> <div> This basic controller will handle up to 80 amps @ 240VAC through the 2 output circuits and U4, TR2, and R6 may be deleted with pins 1 & 2 of U4 jumpered if a single 40 amp circuit is adequate. The twin outputs allow one to use both the present 30 amp water heater house wiring and the 30 amp dryer wiring to supply a twin element heater  assembly as shown below:</div> <bR> <div> <IMG SRC="06e26130.jpg" border=0 width="550" height="275" ALIGN="BOTTOM" HSPACE="0" VSPACE="0"></div> <div> <A HREF="id44.htm" TARGET="_top" TITLE="Dual Chamber Parts List"><U><U>Dual Chamber parts list</U></U></A></div> <bR> <div> The Flow Sensor card is shown below and also incorporates a remote control port that overrides the output temperature to the shower control.</div> <bR> <div> <A HREF="id35.htm" TARGET="_top" TITLE=" Flow Sensor"><U><IMG SRC="0aec8850.png" border=0 width="200" height="133" ALIGN="BOTTOM" HSPACE="0" VSPACE="0"></U></A> </div> <div> Flow Sensor PCB Layout </div> <div> Click for larger image</div> <bR> <div> <A HREF="id39.htm" TARGET="_top" TITLE="Flow Sensor schematic"><U><IMG SRC="0efc8940.png" border=0 width="200" height="148" ALIGN="BOTTOM" HSPACE="0" VSPACE="0"></U></A></div> <div> Flow Sensor schematic</div> <div> Click for larger image</div> <bR> <div> <A HREF="id37.htm" TARGET="_top" TITLE="Flow Sensor Parts List"><U><U>Flow Sensor parts list</U></U></A></div> <bR> <bR> <DIV ALIGN="LEFT"></DIV> <div> Dual Driver Shower Control</div> <div> <IMG SRC="0d5dcdd0.jpg" border=0 width="220" height="221" ALIGN="BOTTOM" HSPACE="0" VSPACE="0">    <IMG SRC="0d7c8df0.jpg" border=0 width="200" height="223" ALIGN="BOTTOM" HSPACE="0" VSPACE="0">    <IMG SRC="0d9dcdf0.jpg" border=0 width="220" height="223" ALIGN="BOTTOM" HSPACE="0" VSPACE="0"></div> <bR> <bR> <bR> <DIV ALIGN="LEFT"></DIV> <div> Dual Driver with Flow Sensor and Remote Shower Control</div> <div> <IMG SRC="0dbc8d40.jpg" border=0 width="200" height="212" ALIGN="BOTTOM" HSPACE="0" VSPACE="0"> <IMG SRC="0ddc8df0.jpg" border=0 width="200" height="223" ALIGN="BOTTOM" HSPACE="0" VSPACE="0"> <IMG SRC="0e438c80.jpg" border=0 width="312" height="200" ALIGN="BOTTOM" HSPACE="0" VSPACE="0"></div> <bR> <div> As illustrated above, both units use the input cold water supply to provide heat transfer for the triac power controls. Not visible in the photographs is the JB Weld epoxy bonding between the copper tubing and the chassis to improve thermal conductivity.</div> <bR> <bR> <div> Dual Chamber unit with remote shower control and flow sensor complete. Maximum ratings:</div> <div> 11 KW, 140 degrees F, and 60 p.s.i.</div> <bR> <div> <IMG SRC="2f758db0.jpg" border=0 width="600" height="219" ALIGN="BOTTOM" HSPACE="0" VSPACE="0"></div> <bR> <div> If you wish to order engineering packages, kits, or assembled units,<U> </U><FONT SIZE="3" COLOR="#0000FF" FACE="Comic Sans MS" ><A HREF="id46.htm" TARGET="_top" TITLE="Demand Water Heater pricing"><U><U>click here</U></U></A></FONT><U>.</U></div> <div>  </div> <bR> <div>     </div> <bR> <bR> <bR> <div>                      </div> <bR> </td> </tr></table></body>