Tech Forum

It depends a lot on what you want to spend. If you do a search on "hour meter" you will find lots of suitable products. In general, hour meters are not resettable.

www.ingramproducts.com/Hour_Meters_Counters-Rectangular_Hour_Meter_12_to_48VDC.html is an hour meter that's non-resettable, cheap and easy to install. The datasheet says it will operate on AC power and there are only two connections. You could just connect between C and Y in your furnace if you want to monitor cooling. C and G if you want to monitor the fan and C and W if you want to monitor heat. There will be 24 VAC present between these terminals in the modes above. Between C and R there is 24 V all the time. You would also have to decide if you wanted to ignore the time the fan was on by itself. That would require creating an OR condition between the C and Y and C and W contacts using two relays by putting the Normally open contacts in parallel.

www.kep.com/catalog/ii/pages/products/HB26-hour-run-meter.html is an Electro-mechanical timer that also meets the specs with reset.

Some furnaces such as some Carrier models don't use the traditional terminals and the motors may be 3 phase internally and would be harder to interface. A heat pump would be slightly different. Some furnaces have humidifier or 120 VAC electronic air cleaner voltages available.

Some timers come with non-replaceable batteries with a 5+ year life. A "contact closure" is a common way to cause timing. A "contact closure" is easy to get. Just connect a 24 VAC relay across the terminals above. Some of the smaller rectangular meters would have a "standard" cut-out.

www.laurels.com/magna_clock.htm would meet your spec, but it has way too many features. With electronics, the case, power supply and "real estate" (the size of the PCB) make up most of the cost so, obviously, a thow-away, battery powered meter with no reset is the simplest.

The key here is knowing what to search for, "hour meter" and some interfacing possibilities and it looks like 120 VAC and 24 VAC are options for power and timing and a contact closure is an option for timing with one or two additional relays.

DC meters might work on low voltage AC too. Check the datasheet.

In answer to your question, the easiest way is to go with a digital display hour meter. The Eaton E5-224-C0450 fits your needs exactly. It can be powered by the 24 VAC normally found on furnaces, will display the time and is resettable. The time range is 0.1sec to 100000hr which should take care of your needs for a couple of years.

The advantages of going this route are you do not need to build a circuit, you can mount the unit where you like,  and the wiring from the furnace to the meter can be done with normal thermostat wiring.

I would wire the meter up to the coil of the fan relay so that when power is applied to the coil, the timer will use that as it's input.

Newark Electronics (www.newark.com) has them available for $50.44, their catalog number is 73R4520. You can see the catalog page at this site, www.newark.com/catalog_129/index.html?page=1904.

I hope that this information helps you out.

I've built up a MicroChip PicChip PIC18F23K22 for this purpose. The PicChip directly interfaces to a 2 line 16 character LCD display.

It shows running time in 99999 hours, 59 minutes, 59 seconds format.

The PicChip simply counts cycles on the 24 VAC thermostat. Circuitry is simple, 5 volt regulator, a transistor, a few resistors and not much of anything else.

In case of power failure, the PicChip writes the current hours to EErom every hour.

The whole project was built on a Radio Shack 276-150 perf board.

The simplest way to determine cumulative furnace running time is to use an elapsed time meter. Two sources are:

Grainger p/n 6X143:
www.grainger.com/Grainger/wwg/search.shtml?searchQuery=6X143&op=search&Ntt=6X143&N=0&GlobalSearch=true&sst=subset

Digi-Key p/n CRA216-ND or CRA-217-ND:
www.digikey.com/product-detail/en/10075/CRA216-ND/1657654  or
www.digikey.com/scripts/DkSearch/dksus.dll?vendor=0&keywords=Cramer+10083

Any of these will run you about $100. Installation is simple: The elapsed time meter operates from the regular 120VAC mains in your home, and runs whenever it is powered. Just connect it to the same switched circuit that powers the furnace blower motor. These meters are equipped with a mechanical reset button.

Remember to power down your furnace system before making connections to the blower circuit.

A Hobbs meter will do most of what you want to do, i.e., keep track of the total time. They are used on Piston aircraft to sum the total engine-on time for maintenance purposes. They cost from $16 up on-line. They are only as remote as the pair of wires running to them and often are not resettable. I have a new Cramer one which goes t 99999.9 hours, is 110 volts 60 Hz, not resettable, and never used. It is , thus now at zero. I am the time of life when I should be getting rid of my "stuff" so if wnat it and contact me at [email protected] I'll send it to you.

I think I found exactly what you need, see Multifunction Counter-Timer kit K8035. I found it at apogeekits.com, under timing kits and modules index on the left side of the web page.

Check out the Curtis Instruments hour meters that are available from Digikey. You need to select the proper operating voltage. They are available with a rest option.

How complex do you want it to be? A microP with a digital display and many hours work, or a home brew digital circuit (many many hours), or an old PC with a serial port and a couple hours for programing (just use the switch to ground the RTS pin and have a QBasic periodically check if RTS is high or low), or hack a $5 digital kitchen timer to do the job?

I would go with $5 digital kitchen timer (or digital stopwatch). Rip the case open and explore how the Start/Stop button works. It may be pretty easy to interface that button with the switch that is operated when the motor is turned on.