This question was created on the page: http://www.furnacecompare.com/furnaces/rheem/classic-90-plus.html.
On 2013-03-18 JB wrote:
The flashes are indicating an error code. I believe for Rheem there are long and short flashes that establish the error code number. If you still have your user manual you should be able to find how to read the error code and what it means.
If it is simply three blinks it probably means that a limit switch is closed, which is a safety feature to prevent the furnace from firing if there is improper air flow through the heat exchanger, which can be a dangerous situation. There are a few things that could cause this situation such as a very dirty filter, blocked intake or some mechanical issue.
Since this is a safety concern it is best to call in a certified HVAC Tech to come diagnose and fix the problem.
To keep legal happy, let me just say that I am not an HVAC contractor, and you should never even look at your furnace without having a qualified, liability accepting HVAC contractor present. To keep sales happy, let me just say that you can find an HVAC contractor that's happy to take your money here: http://www.furnacecompare.com/perl/find_contractors.pl.