[fintstone]

Originally Posted by fintstone

You simply do not have enough return capacity to run on high heat. You need another return to increase capacity. Look at Appendix E.

http://www.graycoolingman.com/uploads/1/0/6/6/10667336/58mvp-_2_stage_variable_speed_troubleshoot_guide.pdf

It sure looks to me like the previous owner increased the size of the house and added supply ducts (and probably upsized the furnace) and did not increase supply air to match. If that one return is all you have, it is likely just not enough. You would need to cut a hole in the return plenum and add another return...preferably from a different floor/part of the house (if possible).

I could not really see your return air plenum in the photos (assume you have one). If not, you probably need to add one. If you can access it easily, depending on design (and the area it is located), you might be able to open it fairly easily and allow more air in to test if the additional air would solve your problem.

Also, I would suggest that you update the grill on your existing return to one with larger spaces between the fins. Those with such narrow slots also restrict air flow.

Of course, I assume that by not you have confirmed the ductwork you were examining is not collapsed or blocked.

Good luck.