Need HVAC advice, any pros?

[16onRockandRoll]

My natural gas furnace has been on the fritz, and I'm having a heck of a time figuring out whats wrong with it.  It wouldn't light, so I cleaned the heat sensor, and broke the hotwire igniter.  I replaced the hotwire/heat sensor, and it will light the pilot no problem, and sometimes the burners will light, but it runs for no more than about 10 minutes (usually far less) then they go out.  I have removed the filters to make sure it's not an airflow problem, to no avail.  The best it has done was right after I blew dust out of the vacuum hose from the blower to the pressure switch, that was when it ran for about 10 minutes.  I'm hoping someone here can help me, or at least give me some advice should I decide to replace it.  I will most likely be installing myself, I'm pretty handy (6-7 years in other areas of construction, very little exp with HVAC), and my brother will come help me.

BTW, it is a Heil model NNE075B12A1

[Skunkeye]

I'm no pro, but does your furnace have a flame sensor?  Most gas furnaces have a sensor that sits directly in the flame, and if that sensor doesn't detect flame within a certain time limit after the gas valve is opened, the controller shuts the valve, to prevent filling the house with gas in the case of failed ignition.

Check to make sure that sensor is placed where it will get hit by the flame when it's lit, and that it's not cracked or dirty.  You can test it with a multimeter - just check for voltage between the wire on the sensor and the socket where it plugs into the furnace.  With the flame on (or a suitable flame source directed on the sensor), you should get a couple volts out of it.  If it's under two volts, it's probably a bad or dirty sensor.  You can try to clean it with steel wool if it's sooty. 

[Virginian]

If the problem continues after  checking the sensor (thermocouple) you should check the vacuum switch as it verifies that the blower is running and will shut the gas valve off if it fails to read the blower pressure.

Is the vac switch two wire or three?

[16onRockandRoll]

Flame sensor is new, and doesnt kill the pilot. That was my first guess. Vac switch works.

[Virginian]

Check the thermostat in the heat chamber
It will sense to much heat and shut off gas valve