Data collection question

I am trying to map the furnace run time at my house and would like to sense when the furnace motor is running and record it with my Arduino. Any suggestions for a cheap way to know when the motor is running, I thought about a current sensor and could use one but is there a simpler solution I am missing?

Does the motor have a LED or control light that is on when it is running ?

If it has you can sense it with a simple LDR hooked up as a voltage divider to an analog pin.

Yes, I am not sure it works but if it doesn't I will fix it and use the LDR as you suggest. Thank you very much.

@Richard Crowley

Using the thermostat would be pretty easy. The systems I'm used to use 24 volts on the controls. An optoisolator with a series resistor on the LED side would provide the sensor, Thermostat open = Optoisolator conducting = heating system off.

Is that 24v DC or 24v AC...?

I was monitoring a heating system with a 24v AC system, and had attached the AC signal to my monitoring gear via an opto-isolator for various reasons... and had problems with the monitor not "seeing" that the heating was on (when it was), about half the time. You guessed it... the 50% of the sine wave when the LED was more diode than "Light emitting."

You can get opto-isolators with two LEDs in them, one "facing" each "way" so that the out put stays on throghout the AC cycle.

e.g.... Manufacturer Toshiba Manufacturers Part No. TLP620-2(F)

In my mind the only thing worth considering is when the motor is running and therefore the pump is delivering oil. There may be times when one of the zones of the thermostat is calling for heat but the furnace is not running. Several of the zones circulate hot water through a radiant floor and the furnace may not be required to make up the heat until later since the furnace runs when the reserve has been reduced. Since there are multiple zones measuring the thermostat wires wouldn't be the simplest solution.

On the motor control and there is an indicator light which is not working but perhaps I can put something in parallel with the Thermostat wires on the terminal strip.( or fix the non working light) In this case the motor controller thermostat wires are controlled by a master controller which integrates all the zones. So the thermostat wire is generic in this case. Could I just simply put two LED's one forward and one reverse and a photocell to essentially make my own optoisolater and then run phone wire back to the arduino with a simple voltage divider? I would think that would work and be simple if I ever had a call to use it on someone else's furnace.

I am interested in a easy to implement system which could be used on other people's furnaces since when talking to buddies I can't imagine they won't want to either duplicate or borrow the system to measure their own energy usage and savings when implementing energy conservation or technologies. Thanks for all the replies.