Adventures with a Lennox G20 Series furnace

I am not an HVAC professional. I have decades of extensive troubleshooting experience, mostly in electronics/audio/Macintosh computers, though also as a dedicated non-professional in automotive, irrigation, appliances, and other fields… yes, including HVAC. I am cautious and learn as much as i readily can about a repair topic before jumping in. My track record of success is very high. It often takes me vastly longer than it would a professional to reach the same result, yet with a labor rate being a non-issue, i have less need to “throw parts at” a problem to keep overall costs in check, and have the luxury of more precisely determining the failure and in most cases only replacing what is necessary.

Lennox intended their Blower Control Center (BCC) circuit boards to be modular replacements, which makes a lot of sense for busy HVAC professionals. However the equation changes greatly when a homeowner-equivalent with deep (some would say mad) electronics and component-level troubleshooting skills is doing the work. It’s not a complicated circuit, but i found i needed to do a bit of reverse-engineering to be able to diagnose and repair it properly. This page covers my findings.

BCC3-1 Pictorial Diagram & Schematic

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Detailed Sequence of Operation

The original Lennox Installation Instructions includes a nice troubleshooting flowchart. I found that i needed more detail.

Normal Heat Sequence

1. Thermostat closes R to W on call for heat. This provides 24VAC to W, unless limit switch or F1 are open (or there is a related failure in the 24V supply or powerline 120V feed).
2. Relay K40 closes. Damper door opens via motor driven off 120VAC via CAB terminal.
3. Assuming that the vent is not blocked (S62 is closed), there is no flame rollout (S47 is closed), once the damper door is fully open and damper prove switch S64 closes, WI terminal sends 24V through this three series switch chain to the ignition module THS or TH-W terminal.
4. Pilot ignites, followed by main burner, under control of ignition module.
5. Pilot valve signal PV from valve feeds Valve Sense terminal (and PV on ignition module), starting timer U1 on fixed 45 second countdown to blower start.
6. U1 turns on Q1 which turns on K2 which switches line voltage from CF (Continuous Fan) to H (Heat) terminal, running blower on heat speed.
7. When call for heat ends, thermostat opens R to W connection. K40 drops out, dropping THS/TH-W ignition module signal as 24V disappears from terminal WI via one pair of K40 contacts. At the same time, 120V via CAB goes away via the other pair of K40 contacts opening, allowing the damper door to close.
8. Valve sense goes low, triggering U1 to commence the 90, 150, 210, or 270 second countdown. At the end of this countdown, U1 turns Q1 off, K2 turns off, (heat) blower stops.

Cooling or Ventilation Fan

1. Thermostat closes R to G, providing 24VAC to the G terminal.
2. Relays K1 and K3 (in parallel) activate. K1 switches ACC terminal from H over to line power. K3 switches line power from K2 (heat blower relay) to terminal A (cooling blower).

Alternate BCC Circuit Board Test Procedure

There’s nothing wrong with the Lennox procedure. I wanted an out-of-furnace bench test, thus i came up with this alternative procedure:

1. Disconnect the P.C.B. from the furnace wiring.
2. Jumper terminals: Limit to 24V.
3. Supply 24VAC between 24V and C terminals of P.C.B. and 120V or your choice of test voltage to L1 and your other common-point reference (you could use the Neutral terminals if nothing else is connected to them). Or, you could do resistance measurements from L1 to the various output terminals.
4. Connect R to W. Check for:
   • K40 closes
   • 120V on CAB
   • 24V on WI
5. Apply 24V to Valve Sense. After 45 sec., K2 should close. There should be 120V (or your other reference) on H.
6. Remove 24V from Valve Sense. After time-out jumper setting, K2 should open.

Troubleshooting & Repair

If the blower runs on vent fan/cooling setting but not during a heat cycle, check for a burned open solder joint on K2. I found this failure on a BCC3-1 45K48 board. Clean the connections well and resolder with plenty of solder to repair. Might be a good idea to check blower motor current draw.

Thoughts on this page? As long as it’s related to this page (and not spam), you’re welcome to email me.