Blower too slow?

[micro]

I have a 2 stage Frigidaire gas furnace that keeps shutting down after 10 to 15 minutes. Lights indicate that some limit switch has tripped. I used a meter to find that the "supply air" switch opens up for a minute then resets itself. From that I assume that either the limit switch is faulty or the furnace is overheating.

Items checked:
  filter: clean
  air conditioning coils: clean
  blower: clean
  vents in the house: open
  flames don't come out the front while the blower is on so the heat exchanger likely isn't cracked

I disabled the second stage by disconnecting the W2 wire. The furnace works fine.

So I wonder if there's enough airflow to cool the furnace. Perhaps the blower doesn't turn fast enough. I'd like to be able to measure the RPM of the blower and the airspeed to compare them to specs but I don't know how to do that. Any suggestions on how to proceed from here?

[Admin]

I would first check manifold pressure and temperature rise.

Enable W2 and try running the furnace with the blower door off.  If it runs without tripping, you could have a ductwork issue.  There are usually options to increase heating speed as well. 

If this is a high efficiency furnace have you checked the secondary heat exchanger coil?

[micro]

I doubt if the secondary heat exchanger coil was checked. I'll have call someone in to check the things you suggested.

Here's a manual that matches my furnace:
http://pdf.lowes.com/installationguides/663132234202_install.pdf

The blower speed puzzles me. It's variable speed so I would expect it to change according to conditions but it always sounds the same and doesn't seem to be very fast. I don't see anything in the diagram that allows it to sense temperature so it can adjust its speed. What's the point of a variable speed blower if it never changes? 

[Admin]

See pages 27 and 34.

Use pins 1 to 4 to set the blower speed for heating and pins 5 to 8 to set the speed for cooling. To determine the appropriate switch settings for your installation, see Table 7 (page 34) for heating or Table 8 (page 35) for cooling.

Turning Pins 1 to 4 all to 1 (ON) would give you high speed.  Adjust according to the temperature rise.  Page 28 will explain how to measure temperature rise.