Some time ago I found my furnace (gas, forced air, downdraft) was getting stuck on, blowing cold air. I could reset it, but it would happen again after a while.
After calling out a service man, I was informed that the high limit switch was tripping. The furnace had been using a very light-weight filter. With a heavier duty filter, it overheated easily. With extra air-flow (bypassing the filter) it never overheated.
We opened more exit vents and even added an intake directly to the return passage of the furnace. This seems rather a drastic hack, and I still can't use standard paper filters, for example. Any ideas?
Update: Furnace is ~15 years; I had a service man to check high limit switch etc.