[hammerlane]

i know there are a lot of variables. but is there some kind of rule of thumb that states about how much a furnace should run

Your first sentence says it....a lot of variables. You could hook up an hour meter to the gas valve which is what I did. Anytime the gas valve is open...receiving voltage...the hour meter advances.

I use the hour meter to gauge run time and when to change the filter which for me is about every 125 hours.

A few photos below.

The bottom photo shows the gas valve with the hour meter wired in. I just used single to double disconnects.
As a note...the middle photo in the left column appears to show the greenfield passing thru the 4" square box. It does not...it passes behind it. The only think in the 4" box is the hour meter and its wires.

 

[hammerlane]

Also here is a thread I did last year about The Natural Gas Cost For Furnace 1 Hour Run Time


http://www.diychatroom.com/f17/natural-gas-cost-furnace-1-hour-run-time-119371/

 

[hammerlane]

i put a known good/accurate thermometer on a stick hanging out in open air. i am dealing with a 4 deg temp swing. 70 on, 74 off.

Hanging out where in open air?

What does 70 on, 74 off mean?