[cam7002]

a) Goodman GMNT High Efficiency Condensing Gas (NATURAL) Furnace, 92 AFUE, model and serial # unknown;

b) Goodman Central Air Model HT-3236, s/n 0101426950; R22 refrigerant; Part # 15344-56

c) Little Giant Condensate Pump VCMA-20 UL5, serial # 554425

Last night, the temp here in Raleigh dipped into the high teens. At 10 am, the outside temp is still a brisk 26 degrees. The heater started malfunctioning in the night, but I did not notice until 6 am this morning when the baby and I awoke to a chilly home. The thermostat was blacked out. I flipped and reset all breakers at the inside panel. I determined that the thermostat is blacking out when the furnace in the crawlspace shuts down. The furnace manual indicates the following sequence of operation.

Heating Mode

1. The furnace control checks for an open main limit (this limit is normally closed). If the limit is open, the furnace will remain inoperable until the limit is closed. During an open limit, the circulating air blower will be energized. The status light will blink four (4) times.

2. The room thermostat reacts to a demand for heat.

3. The control will then check to insure that the vent pressure switch is open. If, at this point, the vent pressure switch is closed, the control will blink two (2) times and will remain inoperable until this situation is corrected.

4. The venter blower is energized.

5. The vent pressure switch will close when it detects a pressure in excess of its setting. If the pressure switch fails to close, the status light will flash three (3) times. The sequence cannot continue until the pressure switch closes.

6. After a pre-purge of about fifteen (15) seconds, the electronic ignition device will be energized.•

7. The flame rollout switches are then checked to assure they are in the closed position.

8. After a slight delay, the gas valve will open if the flame rollout switches are closed.•

9. The burners will ignite and the flame sensor will detect the presence of flame. The ignition device will deenergize. If the sensor does not detect the burner flame, the gas valve will close and the ignition cycle will be repeated for a total of three attempts. If, after the third attempt, the presence of flame is not detected, the furnace will go into a lockout condition for one (1) hour. It will then repeat the ignition cycle. This one (1) hour lockout and retry will occur indefinitely.

10. Thirty (30) seconds after the main valve is energized the circulating air blower will be activated. The furnace will remain in operation until the demand for heat is satisfied. Once the demand is satisfied the venter will shut off, and the circulating air blower will shut off after the field selectable time off is attained.

11. The furnace will remain dormant until the next demand for heat.
I believe I am reaching step 10 as the blower comes on briefly. At that point, the entire unit shuts down as if by a breaker. Shortly thereafter (five to ten minutes), the unit will try to reactivate.

I went outside to take a look in the crawlspace. The horizontal PVC vent pipe had a small icicle hanging therefrom, but I sensed this pipe is not dammed.
In the crawlspace, I found that the furnace tries to run but then shuts down.
The condensate pump, oddly it seems to me, rests on the crawlspace earth outside of the air handler. I checked the condensate pump for a dammed discharge pipe. The discharge pipe did contain about 2 cups of water, and it is filthy inside (clear ¼” aquarium-style hose that is green inside from mold, mildew). I emptied about a cup of water from the condensate pump. I did not evaluate whether the other pipe (1”?) leading into the condensate pump is clogged, but my intuition (rather than my experience) tells me this is not the problem.

I witnessed the furnace operating. It definitely gets all the way to the point that the blower starts blowing air into our home, but it shuts down within thirty seconds. At that point, the red indicator light visible through the viewing window goes completely dark. Prior to that, when the unit tries to function after a reset, I do witness several sequences of the red light flashing 4 times. The reset takes about 5 – 10 minutes before the unit gives it another go. For what it is worth, I have seen no other colored lights through the viewing window. I do see (and love!) that nice blue flame in the other viewing window before it all shuts down!

My wife and my 23-month old baby will much appreciate your expertise and advice! THANKS!

[parttime]

Hi cam and welcome, are you saying that the main burner lights before it shuts down? If so, I would look at cleaning the flame sensor using steel wool or really fine sandpaper.

[hvac1000]

A few pics of flame sensors since there are many types and styles.

[hvac1000]

Check to be sure the furnace filter is not plugged up since that can cause the unit to go out on a high limit situation shutting down the gas/furnace.

[parttime]

hvac1000, I love pictures, they makes things much clearer.