Honeywell Power Box Assembly for F50A F50E with AFS

The 208420 Compatible with the W8600

The 208420A power box provides the terminal strip for the optional W8600E remote indicator. The 120v 208420A replaces any dual cell F50A or F50E power box with or without the terminal strip as shown in photo below.

If your power box does not have a terminal strip or has an unused terminal strip, then you can use the 208420A and simply leave the terminal strip unconnected. Otherwise a W8600E would connect with a 4 conductor thermostat wire whereas a W8600F would use a 3 conductor wire.

The 208420A can also be used with the newer W8600F by replacing the internal daughter card, wiring harness and terminal strip with that included with the W8600F. More...

Photo shows power box with and without the terminal strip.

208420 Air Flow Sensor Compatible

The 208420A has a built-in air flow sensor. The air flow sensor detects air flow by sensing a vacuum (differential pressure drop). This power box will wait until air flow is sensed and then turn on. This power box can be used in an F50A-E that is wired to a full-time 120 volts or used when 120v power is only supplied when the furnace blower is running.

Some earlier F50A, F50E power boxes did not have the air flow sensor. The 208420A replaces power boxes with or without the air flow sensor. If your prior power box does not have the air flow sensor, then the 208420A built-in sensor will simply be redundant. The animated photo below shows a power box with and without the air flow sensor.

208420 Air Flow Sensor Nozzle Retrofit

The 208420A replaces power boxes with and without the air flow sensor. In 2004 Honeywell discontinued the power box without the AFS (208421C) to consolidate inventory.

The air flow sensor feature was introduced 1991. The F50A and F50E prior to 1991 did not have the AFS and do not have the hole for the nozzle. The AFS senses the vacuum of the return duct and signals the power supply to turn on.

If replacing a power box without the AFS, then check that your frame has the hole for the nozzle. Without the hole the 208420A will not sense the vacuum and turn on. See the Tech-Note.

208420 Air Flow Sensor Disable

The procedure to permanently enable the power box so that it powers up without air flow is shown in the animated photo below. Perform these 2 steps:

1. Cut the resistor jumper J8
2. Disconnect the plug at J3

Setting Ionizer Current Jumper J6

The 208420A can be configured for four sizes: 12½x20, 16x25, 20x20, 20x25. A jumper on the power supply board needs to be set based on the overall size of your collector cells. This jumper J6 sets the ionizer current.

Power Supply Inspection

First visually inspect the board. Most problems are visible such as a melted transformer winding or a burnt component such as the surge suppressors RV1 RV2.

Check the thin white ceramic resistor R3, the corner wire can break away. This can cause the collector plates to get power but not the ionizer wires.

Continuous cell sparking (for months) can burn out components or wiring. Resonant sparking causes elevated voltage swings that can break down the insulation on the high voltage wires. Check for scorching.

Often the fuse blows when the switching FET Q1 shorts, we believe from line spikes. Check the fuse continuity. A blown fuse typically reads 4.4M ohm because of circuitry in parallel.

The fuse is soldered in and not replaceable. If there is an internal short the fuse blows so to prevent any components from flaring up. Replacing a blown fuse will not fix the problem.

Honeywell does not release the circuit board schematics. Service tech's want to replace the complete board and be confident. Honeywell does not sell the discrete components. We do not do component level repair of the boards.

Replacing the complete board with a new one with a 2 year Honeywell warranty would be the most reliable repair.

Power Supply Bench Test

Here is our procedure to test the 208420A power box. Warning high voltage shock can cause death.

Bypass the air flow sensor to force the power supply ON by grounding the jumper resistor J8, see photo and pdf below.

Connect 240 vac to the 2-prong connector at the back of the power box. Check the following outputs with respect to chassis ground.

Terminal P3 will output 8.2k vdc
Terminal P2 will be 4.2k vdc
Terminal P5 for the neon lamp outputs 100 vdc

Since high voltage meters are not common, Honeywell provides the neon lamp as reliable voltage indicator. If the high voltage output falls below spec the neon lamp voltage will drop out.

Shorting P3 or P4 to ground has the same effect as the test button. The short will temporarily cause the neon indicator to go out. By tapping P3 to ground you should be able to draw an arc 1/32". Tapping P3 to ground can produce a larger arc near 3/16". These tests are with out the cells connected, the arc will be twice the length with cells connected. See Spark Test pdf.

Warranty on Replacement Parts

The Honeywell warranty for F50 Replacement Parts is 2 years. Our customers simply need to ship us the defective part. We test and confirm defective. We then ship back a brand new part at no charge.