Monitoring Motor On Time

This small logger monitors the on time for the motor its attached to.

The monitor is simply attached to the case of the motor with tape or a magnet -- no wiring needed. It keeps track of on time by monitoring the magnetic field of the motor.

This particular motor monitor is from Onset Computer, but I am sure other brands are available. It costs $79, but the Onset software that is used to read and plot data from their full logger family is also required.

Logging the time that an electric motor stays on can be useful for things like estimating pumping or fan power consumption, and keeping track of the on and off time pattern under various conditions.

The logger can also be used as a rough way to estimate fuel consumption for appliances in which a motor is on only during times when the appliance is using fuel -- for example, a forced air furnace. Some examples below.

An Example: Using the motor monitor to estimate water heater standby losses

My propane water heater uses a power vent fan to push combustion products up the flue. This fan is basically only on when the burner is on, so monitoring the motor on time for the power vent gives a good indication of the burner on time.

The plot below shows the on time for the power vent fan (and roughly the burner) for our propane gas water heater during the night when no hot water demands are being made.

The logger provides the on time for the power vent motor on the water heater. This power vent turns on just before the burner comes on, and remains on for a few seconds after the burner goes off.

During the middle of the night, the plot indicates that the burner comes on about every 1.25 hours for about 5.1 minutes. This is just to offset the heat losses from the tank and keep the water up to temperature.

The label on the heater states that the burner uses 40,000 BTU/hr when on.

To estimate the amount of propane used to just keep the tank water warm:

The power vent motor comes on for about 5.1 minutes every 1.25 hours.

So, the burner on time is 4.1 minutes per hour.

The total burner on time for standby losses for a year would be about (4.1 min/hr)(1 hr/60min)(24 Hr/day)(365 day/yr) = 598 hr/yr

The propane use per year is (598 hr/year)(40000 BTU/hr)(1 gal/92000 BTU) = 260 gal per year.

So, about 260 gallons per year of propane are used just to offset standby heat losses on our propane gas water heater. This rather staggering number lead to an investigation of our very wasteful hot water recirculation system. this recirculation system results in a waste of nearly 200 gallons or propane a year -- details here ...

When you actually measure things you always learn something useful, and often learn something surprising!

The motor monitor (small white box) taped to the water heater power vent motor.

Estimating Furnace Fuel Consumption

You can also use it as a rough way to estimate (for example) furnace fuel consumption by logging the furnace fan on time:

Log the on time of the furnace fan motor with the logger. Correct these per the note below.
Multiple the furnace BTU input rating per hour by the corrected fan on time to get the total BTU uses over the logged interval.

To correct the furnace fan motor on time to the burner on time:

You want the actual burner on time, which is a little different than the fan motor on time.

When the furnace starts, the burner runs for a little while to heat up the heat exchanger before the fan comes on. Note the time lag from the time the burner comes on until the fan comes on, and add this time to each of the fan on times.

When the burner shuts down, the fan runs for a while to remove the heat remaining in the heat exchanger. Note the time lag from burner shut down to fan shut down, and subtract this from the fan on times.

Gary Jan 11, 2008, Jan 17, 2008