BY GEORGE LANTHIER

Editor’ s Note: Part One appeared in the May 2025 issue of Fuel Oil News.

We’ re back to our discussion of blower control boards, specifically a couple more comments on a few of the terminals and what they do and precautions to be aware of.

Let’ s look at the terminal called‘ HUM’, Figure 1. It provides for automatic operation of a line voltage 120 Vac humidifier. But as we said in Part One 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, and I’ ve rarely had any problems doing it this way, it’ s the 120 Vac units that can cause a problem. We then, continuing on, find four terminals, the first marked‘ EAC’, Figure1, this is where your Electronic Air Cleaner would be connected but again be aware of high amperage draws.

Let’ s look at what you do about those high amperage draws. These boards are not designed to handle many of the high amperage loads that many of us try to attach to these boards because many of us do not have a good working knowledge of relays and loads and that’ s what good continuing education is all about.

When powering a humidifier or Electronic Air Cleaner( EAC), Figure 2, using one of these boards, the board’ s internal switching should be used to make a relay or‘ contactor’, Figure 3. An example of the suggested wiring for such an application is shown in Figure 4. The EAC or humidifier should not be powered directly from the board. A RIB Relay, Figure 5, can also make this job very easy.

When selecting a contactor for this purpose choose one that will properly power the humidifier or Electronic Air Cleaner. Using an example; if the humidifier has a Rated draw of

Figure 1

Figure 2

Figure 3

1.5 amps at 120 Volts AC, then the contactor should have a similar or larger Rating. Most of these boards cannot have a load on the humidifier( HUM) terminal or EAC terminal of more than 0.8 amps. Failure to observe these cautions can cause fatal damage to the board, they’ ll blow up, literally.

When you find yourself having to apply a 24 Vac humidifier to these boards wire the power transformer between the HUM terminal and neutral as shown in Figure 4. This will enable the board to control the humidifier during heat cycles.

As we mentioned in Part One there are several ways used by OEMs to wire an oilburner, or a power gasburner into the circuit of a furnace equipped with one of the ST9103 boards. Figure 6 shows the wiring terminals for a current state-of-the-art primary control and how it could update and upgrade the

Figure 4

Figure 5

original Honeywell drawing shown in Figure 7 showing the use of a primary control with interrupted ignition and purge cycles through the use of an oil valve, Figure 8.

Next is that‘ CONT’ terminal, Figure 1 and Figure 7, which is used to provide for Continuous Air Circulation which when used properly can provide exceptional comfort to the homeowner. Before we look at that let’ s review the other motor terminals from Part One.

‘ COOL’ is where the highest motor speed is connected to provide for air conditioning followed by‘ HEAT’ which is where your

30 JULY 2025 | FUEL OIL NEWS | www. fueloilnews. com