Water Heater Monitor

This project is a monitor for my gas water heater.  It consists of a phototransistor aimed at the burner through a small viewing window.  Since I have a new 'Flame Lock' style burner, access to the combustion chamber is very limited, and I was unsure if enough light would hit the phototransistor.  This project was started and completed in one weekend (hardware and most of the software).

Schematic of the monitor.  R1 is adjusted to produce a full logic swing with the gain and incident light of the phototransistor.  The OUT terminal is connected to the Power Monitor Node, which was already hosting the water meter monitor.  I do not have a part number for the transistor as it was from my spare parts bin.

The water heater monitor before the wiring was dressed with ty-wraps.
The phototransistor is held by a stiff 16 gauge copper wire (red) so
that it can be freely positioned in front of the window.
The black film container holds the R1 resistor and
has enough room to allow reconfiguration of
the circuit.

Close-up of the phototransistor. It is held in a two pin socket that is soldered to the
wiring and then wrapped with heat shrink.  The red copper wire is then wrapped
around the heat shrink to hold the assembly.  The glass lens of the phototransistor
does not touch the little glass window of the combustion chamber.

View along the phototransistor aimed at the burner.  The sensor is aimed at the
brightest part of the flame.  By manually dialing the thermostat on the water heater, I could fine tune the value of R1 to optimize the signal.

Cost analysis of a shower

The home control system allows me to plot any of 24 parameters that are monitored by the system.  The chart below shows only the water usage and the gas water heater run time.  The latter added as part of this project.

Chart with only the water usage and gas water heater run time plotted.  The
chart has been zoomed into the region of when the shower took place.  The user is able to position a cursor to read the data value at any time and point in the chart.  The axis for the water heater run time is on the left (cumulative minutes - red trace),
and the axis for water usage is on the right (gallons - yellow trace).

A cost analysis of this shower is as follows:

Length of shower
Amount of water used
Run time of water heater
Gas consumption rate
cubic feet/minute
Amount of gas used
cubic feet
Cost of water and sewer
dollars/1000 gallons
Cost of natural gas
dollars/100 cubic feet (CCF)
Cost of water consumed
Cost of gas consumed
Total cost of shower
Shower cost rate

The gas usage rate of the gas water heater was determined by using the dials on my gas meter and a stopwatch.  The 'half cubic foot per rev' dial makes one full revolution per minute when the gas water heater is running.

Future uses of this system

  • A monitor for a setback system to turn the temperature down during vacations or overnight periods.  The actuator for this may, for example, be an RC servo.  Proper safety measures would need to be in place to prevent the accidental setting of a very hot temperature.
  • The monitoring of the energy usage of the home if I should install a radiant heat system.
  • After some usage, I may setup a software monitor to alert me if the water heater burner runs beyond a time threshold.

Project Log

  • October 10, 2004 - Sensor installed, power node updated, and initial changes made to home control software.
  • October 11, 2004 - Graphing support added to water heater data.
  • October 12, 2004 - Cost analysis of shower.
  • January 15, 2005 - A 15 day absence provided information on the efficacy of the insulation of the water heater.  During that period, the water heater did not run a single time!  Presuming that it was about to cycle the burner upon our return, we can calculate the upper bound of the heat wasted per day through the insulation of the tank.  Since the water heater runs for about 11 minutes at a minimum (due to its hysteresis), the maximum average run time (due to energy lost) is 11/15 minute per day.  Using the data in the table above, a calculation shows that it costs less than 0.5 cents per day to maintain the water hot in the water heater.  I found this very low amount very amazing.


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