Furnace is Rheem classic 90 plus
When thermostat not calling for heat red light flashes 5 times.
When furnace is heating red light flashed 8 times.
Is this a problem?

Is it working? Does furnace work? Put out heat/ cut off as it should?

Red light? Is it in the tstat that's blinking or the led light in the furnace you are talking about? If it's the tstat you need to look at manual and find out what is causing

What make and model is this rheem. Rheem usually has two green lights and one orange lite on the control board. I think wmproop is correct that it is the lights on the stat that are blinking? Please answer his questions and we will try to help.