Furnace downfired itself

Here is an interesting no heat/ intermittent no heat call. On Tuesday Jan 7th it was very cold here in NJ. I got a no heat call for this customer that had a furnace installed by our company about a year ago. Back in November this same customer had a pressure switch problem. This was repaired under warranty. The furnace is a Lennox SLP98 modulating 90%. I arrived to find that the furnace was firing at about 50% of its firing rate, and the CFM at around 550. The IC board was flashing two error codes. The first being the high limit had tripped, and the second error code was saying that the furnace had lowered the firing rate to match the airflow through the furnace. House temp was around 60 degrees. I asked the homeowner if she had any registers closed, turns out that about 75% of the entire house was closed off. Of course that would cause serious airflow issues. Problem solved. Why she had over half the house closed of especially with this cold weather we were having, I still do not know? You can't fix stupid. The amazing thing is that the furnace derated itself, I am still amazed that it was able to do this. Modern electronics at their best