BY GEORGE LANTHIER
as a point of reference all of our drawings would place the thermostat terminals at the bottom of the drawing like Resideo does, Figure 7. Second, we would call out the Molex ®‘ pin-out’ similar to a clock, Figure 8. So, let’ s see how this plays out.
In Figure 6 we have two yellow wires coming from the pins at the 1 o’ clock and 7 o’ clock positions, which is
Figure 7 your limit switch, Figure 9. We’ ll come back to this.
A jumper wire in red has been placed between the 11 o’ clock position and the 6 o’ clock position and from the 6 o’ clock position we have the power lead to an interrupted ignition control. The neutral wire is in the 9 o’ clock position leading to the neutral on the primary control and that wires up your oil-fired furnace.
One of the service calls
Figure 8 that can be annoying to the uneducated is a faulty Limit control. It’ s quite confusing to some to find the system blower running on arrival and the burner off. You’ ll find that the‘ reset button’ on the primary does nothing and that’ s because you are‘ off on Limit’. The furnace is cold, but the board thinks it’ s hot because it’ s following its program that if the Limit is open, you run the blower and keep the burner off. Biggest misunderstanding with these boards, FACT!
To check the high-limit try it first with the power off. Using the ohmmeter function of a V-O-M insert a pin probe into the Molex ® connector at the one o’ clock and seven o’ clock positions as shown in Figure 8. If the switch is open, you should read‘ OL’. If the switch is closed, you should obtain a small resistance reading that reflects the circuitry of the board.
Next, verify it with the power on. Place your meter leads between one of the‘ NEUTRAL’ terminals numbered‘ 1-2-3-4-5’ as mentioned earlier and the one o’ clock position on the Molex ® connector, you should get 120 Vac. At the seven o’ clock
Figure 9 position of the Molex ® connector and
‘ NEUTRAL’ you should get a reading of‘ O’. Both readings are based on an incoming voltage 0f 120 Vac.
Using this method, you can diagnose all of the circuits entering and leaving the Molex ® connector on the ST9100 Series boards.
Finally, it is worth noting that some furnace manufacturers( OEMs) use the 3 o’ clock and 5 o’ clock positions on the Molex ® connector to control the primary control on low voltage by pulling in the‘ T-T’ terminals of the primary control. I don’ t know why you’ d have to ask them.
In researching this article, I found that there are only a few OEMs making oil-fired furnaces anymore. Many have given up on liquid fuels and are now focusing on producing only products for gas and electric fuel sources. I was sadly surprised, but it goes with the territory, I guess.
In further research we found that the gas furnace market is not that strong either, but it used to be the number 1 way to heat a house in the USA. Most new construction is headed towards the mini-split revolution and air-to-air heat pumps, the future it’ s said‘ hangs on the wall’.
With further research we found boards made not only by Resideo but also ECR, Hydrolevel, Figure 10, and NrgMax but they all pretty much wire up the same. And for many of you, including the new guys the R8991 mentioned in the wiring diagram, Figure 2, was an attempt by Honeywell and Phelon( now Carlin) to build a combination primary control and burner ignitor, Figure 11, but it didn’ t turn out well.
Much of this article is taken from our training manual called‘ The Warm Air Handbook’ and is included in our
Figure 10
Figure 11
‘ Warm AIR Controls for GAS & OIL’ seminar. Avoid being among the confused and undereducated and get taught correctly on these boards. There’ s still a lot of these boards out there and will be for a few years yet, we hope.
We’ re back in business and will be doing the seminar once or twice this season. Come join us at our new location in Webster, MA. See ya! l FON
George Lanthier is the owner of Firedragon Academy. For more information, visit www. FiredragonEnt. com.
30 MAY 2025 | FUEL OIL NEWS | www. fueloilnews. com