BY GEORGE LANTHIER

During last year’ s training season, I again became aware of a huge misunderstanding about the infamous‘ blower control board’ or‘ Timer’ as Resideo / Honeywell calls it. It’ s very sad that there is not enough education on these boards and on the oil board in particular, what Resideo calls the ST9103, Figure 1. I’ m going to refer to Resideo / Honeywell as Resideo for this article and from now on because that’ s their name.

These boards have been around since 1991 and they’ re not going away. If anything with the demand for more air conditioning, they are more and more common and have even been copied to death. In addition, and since Covid emerged, the demand for air purification and humidification has also increased.

I found a lack of a simple understanding that these boards all work and wire essentially the same. It really doesn’ t matter the manufacturer and although many of these boards have more features and bells and whistles, like ECM or PSC motor capability they all have a few things in common and that’ s what we’ re going to focus on.

Looking at the wiring diagram, Figure 2, for the Resideo you can easily see why apprentices and the uneducated could become easily confused. The sad part is that these are not that difficult but if you don’ t want to learn no one can teach you.

First, let’ s look at where the power comes in from, Figure 3. These boards require two power sources, 24 Vac and 120 Vac. If you look at the top of the ST9103, Figure 4, you’ ll see a terminal area on the top row called‘ NEUTRAL’ and numbered 1-2-3-4-5 and again you should think of that as just a great big neutral wire nut because that’ s all it is.

In the second row you can see terminals 1 and 2 and that’ s where your power comes in. On the right-hand side of the board, you see two terminals called X and C, that’ s where your 24 Vac is connected. Connect a thermostat

Figure 3

Figure 1

Figure 2

( for heat only) to terminals R and W and you have now connected

Figure 4

all of your‘ inputs’. Next, we’ ll look at‘ outputs’ or how the board tells everything connected to it what to do.

Okay, back to Figure 4 and let’ s look at the outputs. We’ re going to look at this as two rows, top row first moving left to right.

The ST9103 is provided with 1 / 4” quick-connect terminals and the first two are called‘ UNUSED MOTOR’. This is where any unused leads from the motor are placed. In Figure 2 you can see that Resideo shows a four-speed motor being used. The MH speed is‘ parked’ on one of these terminals. Some furnaces will come with five speed motors and that empty terminal gets used.

Next is a terminal called‘ 3’. The 24 Vac transformer needed to power the board draws its power from this terminal and the next group.

The next group of terminals are numbered‘ 1-2-3-4-5’ and as mentioned earlier these are your‘ NEUTRAL’ connections.

A terminal called‘ HUM’ is next and provides for automatic operation of a line voltage 120 Vac humidifier but be careful with high amperage draw, you may need a contactor to get the job done. You could also power the transformer for a 24 Vac humidifier from this terminal. We’ ll come back to this in Part Two. That brings us to the nine pin Molex ® connector which will come back to, on to the second row.

The first terminal is marked‘ BURNER’ but is not normally used. The next two are marked‘ 1’ and‘ 2’ and it’ s on terminal‘ 2’ that 120 vac power enters the board from your power disconnects.

We then find four terminals the first marked‘ EAC’, this is where your Electronic Air Cleaner would be connected but be aware of high amperage draws again, see Part Two.

Next is‘ CONT’ which is used to provide for continuous air circulation which when used properly can provide exceptional comfort to the homeowner, we’ ll look at this again in Part Two.

‘ COOL’ is where the highest motor speed is connected to provide for air conditioning followed by‘ HEAT’ which is where your motor speed for the proper CFM for heating is placed, in Resideo’ s drawing, Figure 2, they are using the‘ ML’ motor speed. The proper speed can be obtained from the OEMs literature or by performing a test to verify‘ temperature rise’ across the heat exchanger.

28 MAY 2025 | FUEL OIL NEWS | www. fueloilnews. com