Far better it is to dare mighty things, to win glorious triumphs, even though checkered by failure, than to take rank with those poor spirits who neither enjoy much nor suffer much, because they live in the gray twilight that knows neither victory nor defeat.

Theodore Roosevelt, The Strenuous Life

Winter is approaching and cold weather is here already. I often build a wood fire in the evening or morning, heat the open area of the house, then turn on the furnace fan. The return air grille is high in the vaulted ceiling not far from the wood stove, allowing hot air generated from the wood stove to spread through all rooms of the house. About two weeks ago, I switched the fan on and nothing happened.

My diagnostic approach was elimination of potential failures, doing the easy steps first. I learned on line which thermostat wires controlled the fan, removed the thermostat from the back plane, and crossed the two terminals (red and green). The fan didn’t work. This proved (almost conclusively) that the thermostat was not the problem. I then shut off power and  checked the blower and motor. There was no evidence of a burned motor or shaft failure; the squirrel cage turned freely. I bypassed the fan controls and energized the blower leads. The fan worked, proving the starting capacitor and the motor were fine. I then turned the power back on and heard a relay operate on the fan control board. I concluded that the transformer was alright and that the fan control board had failed.

I searched the Internet, simply inserting the old part number into the Google search box. My first hit was a used board on e-Bay for $199. The second was a new one for $187. Information on that site revealed that the part had been replaced with a “universal” fan timer. (Universal is an industry term for a part that won’t fit like your old one.) My third search was a great site that was clear. They offered the new universal part (the part number is in the title) for $104 plus $10 shipping. My wife ordered it standard shipping; we have a wood stove so there was no rush.

If you undertake this job yourself, do read the directions before you start. They include the advice of removing the mounting screws of the old board and installing the new board before disconnecting any wires from the old one. This is sound advice. I used a big cable tie to hold the old one out of the way. You should move the leads one at a time and have a high confidence that each is properly connected before moving to the next. Here are some other pointers:

  • Find the part number for your old board in the dip switch setting  table of the instructions in case your appliance is not listed in the directions.
  • The dip switches are covered with a brownish translucent protective cover. It is not obvious, especially if your eyes are older than an antique car. Remove it before attempting to operate the dip switches.
  • I used a utility knife to set the switches, using a lateral and slightly upper force on the switch.
  • The new board’s terminals are in different positions. You will think that all the leads are sufficiently lengthy to fit the new board, as I did. I was wrong. I mounted the new board in a different position, had to move the transformer, mounted the board upside down and moved the ground terminal on the furnace chassis to get it to fit.
  • The neutral terminals are all identical. Connect the white wires to them.
  • IND is the label for the combustion air fan. Honeywell calls it the induction fan. Connect the black lead from the induction fan here.
  • The common on the secondary side of the transformer is defined by its connection to ground with a dual lug. It goes to the common transformer terminal. The other secondary lead is X1. These are both 24 Volt leads and connect at the low Voltage side of the board.
  • The safety interlocks were on the six wire plug on my job. The wiring did not match the “typical” diagram. It had 4 terminals in use and mine had 5. However, the four they showed were connected similarly through normally closed interlocks and the fifth was shown connected identically to my furnace on another wiring diagram. Voila.
  • Connect the black power wire from your supply circuit to the L1 terminal.

The universal board has simple on-board diagnostics, with a green LED that shows normal operation and displays trouble codes. I didn’t see the trouble codes; mine worked perfectly when tested. This includes success with pre-purge (combustion air fan goes on before spark production to purge any explosive mixture prior to the generation of the spark), ignition, blower starting after ignition, flame going out when thermostat is satisfied, and the blower operation enduring for a short time to cool the heat exchanger after the flame goes out. Of course, my fan switch starts and stops the blower as it should.

My guess is that this do-it-yourself effort saved about $100 in parts and probably $200 in labor, yielding roughly a savings of  75% . This is enough for great dinner with fine wine for two, lots of gas, or some darn necessity. I also avoided the sales pitch with the mechanic arguing that my 16 year old furnace should be replaced because it isn’t that much more money. The truth is that I need new ductwork, so a new furnace and the needed duct improvements will cost about $10,000.  That’s the subject of a future post.



1 Comment to “Honeywell ST9120U1011 Fan Timer Control for Gas Furnace”

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