Quote Originally Posted by jp102 View Post
Bison - counted the flashing red led, got 16 flashes = 1600 CFM
Does it do that both when it is in auto and when it is set with fan on (or is in the middle of or interrupted by Aprilaire calling for the blower to run?) I even wonder if the Aprilaire in the timed circulation mode is shutting off the blower at the end of its time...even if the AC compressor is still running. May be some sort of logic problem with the Aprilaire circulation mode. By checking blink counts with and without circ you might be able to catch it when it does something unexpected. For example, if the blink count is reduced by the Aprilaire when it kicks to "circ" mode in the middle of an AC call that would be a problem.

I still do not understand if a Rheem RGRM 90k will shut down the blower if it detects an iced condition. If anyone knows the answer to this question, please let me know as it will give me an indication of where to troubleshoot. Right now I still do not know if the blower is not starting; hence causing the iceup. Or if the iceup is occuring and the system is shutting down the blower.
I'm not seeing anything in the furnace/air handler manual about what it can/does detect when in AC mode. It appears that it is relying on the AC for that bit of logic. Indeed, the manual for my AC unit talks about high and low pressure protections built into the AC control board's logic and it has its own 7 segment LED display to provide codes. Your older AC might not trip the compressor on high discharge or low suction pressure.

My layman's conclusion is that the furnace/air handler will not detect the icing. So then we get back to the root question (full circle): is the thermostat or a wiring issue causing the blower to sometimes shut off or go to low flow? Or is the AC freezing with normal air flows?

What I suggest is measuring the blower LED blinks in various situations in circ mode...1. With the AC turning on while it the fan/blower is on for circ. 2. With the AC shutting off while the fan/blower is on for circ. 3. With the circ timer shutting off while the blower is supposed to still be running while the AC is running. 4. With just the blower running for circ. (To really make the difference obvious, you might want to set the fan dipswitch back to "low" during the tests...the low fan flow is so low that you won't have any doubt on the difference in blink count. If the circ timer on the tstat is overriding the normal blower speed call in some fashion then that would be a problem.)

Also while counting the flashing led I noticed that the status light was steady on. According to the manual this is a "normal fualt detected". Anyone know what this means?
My response is in the other thread. As best I can tell this indicates a normal operation state...although the wording they chose for that confuses the hell out of me too!