Furnace Rapid Flash

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This installation was on a 4-story home, and the tech had not yet gone to the basement to troubleshoot the “infinity” water source heat pump, as he was told he by the homeowner. He was in the attic at the infinity furnace above the 4th floor. He was getting his dcv in the proper range on his A and B green plug on the air handler, 3 to 4 vdc. His indoor board ohm reading was in range, 28k to 32 k. The tech ohmed out the A and B thermostat wires on the water source heat pump from the upstairs. His ohm reading was not in range of a normal infinity board. His reading was around 64.4k. The tech then made the long walk to the basement where the “geothermal heat pump” was located. Bingo.

Now get out there and organize some zone wiring!

Zone equipment would be identified as indoor unit- blue black/ outdoor unit-blue green:

Two cable ends with exposed multicolored wires, showing different wire arrangements on a white background

Customer dashboard with blue metrics tiles, left sidebar navigation, and a data table with a highlighted button.

I have cropped some screens, and modified the serial numbers to keep certain dealer and customer’s information private. Setting the customers up on the portal is an easy process and could reap many benefits. Carrier and Bryant are going to begin utilizing this portal more with new upcoming equipment.

Get out there and get connected to the portal!

Dark settings dialog with checkbox permissions and Save System button highlighted in red

Dashboard page titled “Connected Portal” with two portal tiles, blue and red, on a white background

Web dashboard with outdoor control details and a purple circled right-side action menu

At first glance, this seems simple. Just swap the two incoming power wires, right? But the furnace has been running for years at the home with no issues and swapping the wires produces the same rapid flash situation…. What gives?

Let’s tackle how to attack this fault:

FIRST, trace the wires or read the schematic to determine where L1(black) and Neutral(white) land on the board- Pointed in green. Use a voltmeter to Verify the follow:
L1-N= 120vac
L1-Ground=120vac
N-Ground=0vac
This will verify that the ground to the equipment is good and that L1 is the HOT wire and not N. ***If L1 to Ground is 0vac and N to Ground is 120vac then simply reverse the two wires as they wire nut to the furnace power in the Junction Box highlighted in Purple.

This test will verify that low-voltage R is truly the hot wire. IF this test produces roughly 145 Vac then this means that the transformer low voltage is out of Phase. *** The easiest way to correct this is to swap SEC-1 & SEC-2 on the board. (this is commonly seen when people replace transformers on furnaces)

Third, if the high voltage and low voltages have been checked for proper phasing and they are correct, and the board is still showing a rapid flash, at this point it is recommended to replace the furnace control board.

Lastly, If everything above checks good and the board is changed and the unit produces a rapid flash then you could (temporarily) try wiring the furnace to an extension cord and plugging it into a receptacle within the home. After ensuring the L1 and N are correct once more, if this corrects the rapid flash then the issue is with the breaker feeding the furnace.

There will/can always be a “one in a million” situation but for the vast majority this guide will assist in finding the reason for the furnace rapid flash.