How bad did I goof?

funfool

I was asking for some help last week about sizing the fresh air return, was decided thay a 20"x24" with a 14" duct would work.
Last week I got the all the ducts and floor register installed, did not have time to build the chase for return air. This is the only piece missing right now, will finish this weekend.
Is down draft, I left the top plenum off, and have been using it through the last week

Weird weather we have in fall, is 40's at night, low 80's in afternoon. So is not used much.
I am seeing a problem and trying to troubleshoot it. The t-stat set to 70, it comes on and runs for about 5-10 min and then shuts down, then 20 min later it comes on for 5 min and shuts down.
Just seems like it is not running long enough.
Or could it be the old used t-stat that has been riding around in the van for 2 years, I used just to see if the furnace worked?
Both will be fixed today, I suspect the fresh air, wondering if it will cause this problem?

funfool

seems it was the t-stat, I replaced the wire and t-stat with new and moved it a few feet away from the furnace. And turned it up all the way and it never turned off until I turned it down.
I was just concerned I was causing damage by leaving the top plenum open.
At the local mom and pop plumbing and heating store, they assured me it would not cause damage.

beenthere

See how it cycles tonight.

Yep, no harm running it without the return plenum on yet.