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Tom Hand 11-20-2011 02:18 PM

G50UH-36B-090 Furnace No Start; Yuri?
Good day, it is cold in KC. and my parents furnace will not start. New flame sensor, new over temp switch, inducer blower tries to come on and shuts off. It has two flashing lights, rapidly. The exhaust is open, I shorted and opened the pressure switch. Randomly, but not often now, it will come on and get all the way with flame on and heating the tubes and then when the blower is to kick in, the inducer blower shuts down and the regular blower stays on to cool it all off then shuts down. The voltage that tries to kick on the inducer blower is very low and it just enough to kick the inducer a revolution or so.

Does it sound like the only thing left is the Sure Start board?

Thank you very much!

Doc Holliday 11-20-2011 02:41 PM

What do you mean that the voltage is low to turn on the inducer? How much voltage is to the furnace and how much voltage is coming out of the inducer terminal on the board? If you have a meter, you pull of the wires to the inducer from the board and check the voltage at the ID terminals on board.

Tom Hand 11-20-2011 03:09 PM

Hi Doc, thanks for the comments. I unplugged the two wires to the inducer motor and when time was right for it to power up, the voltage supplied barely jumped to maybe 10 volt AC. With it plugged back in, the inducer motor barely spun then immediately went off. It is free to turn.

Put a new breaker in the house box to be sure low voltage to the furnace was not an issue.

Thank you Doc for piping in to help me soon.

yuri 11-20-2011 03:15 PM

What is the voltage coming into the furnace? Also monitor this voltage to see if it drops when the furnace is acting up. You may have a power problem with the house or elec panel. Hopefully you do not have aluminum wiring in the house. Poor connections in the elec panel/buss bars or at the furnace may be the problem. I have hundreds of those units installed and have not run into that problem but anything is possible. is a good Lennox parts guy.

Make sure the furnace is grounded properly and has a good neutral connection. The board will act up if not grounded properly. Check for continuity to a water line to the plate on the furnace the board attaches to and use a cheater piece of wire if necessary.

Tom Hand 11-20-2011 03:23 PM

I believe the voltage is correct and it was also before the breaker was changed. The furnace is only 5 years old, the house maybe 15. Both are immaculate.

The furnace has been working correctly most of yesterday after having changed the flame sensor earlier this week. Just today it went into the odd mode. Both lights are flashing quickly.

I followed the ground wire all the way into the furnace wiring and then into all the internal grounds.

And, a couple of times today we got it to fire up correctly then it would shut off right before the main blower kicked on. The inducer had shut off at this time.

The motors would not operate correctly if the voltage was significantly low; I am almost 95% sure it is right and can have Dad take voltage readings later.

Thanks guys!

Doc Holliday 11-20-2011 03:25 PM

The inducer motor runs on 115 volts. If you can take one hot and the neutral coming straight into the FURNACE, (check to make sure you only have one hot used as if there is 240 originally supplied and you wire two hots to the inducer than you'll fry the inducer) and the ground or neutral, cutting the two wires from the inducer back a few inches from their respective spade endings which plug into the board (so you can wire nut the spades back together) and wire the inducer straight hot from the incoming 120 volts to the furnace to inducer.

Do this with the breaker off. What you're doing is supplying the inducer it's correct power while bypassing the board. All the board does is tell the high voltage where to go, same voltage as through the board as the 120 coming into the furnace ( or at least one leg of it if furnace runs on 240) but now without the board.

If the inducer comes on normally while wired to a direct 120 volts than your board is bad.

Double check the required voltage on the inducer first, making sure it is 115 or 120 volts.

Flip the breaker back on and again, if the inducer comes on than the board is bad.

yuri 11-20-2011 03:26 PM

poor neutral or grounding can cause problems. Lennox recommends checking between C on the board to neutral and there should be less than 2 volts reading. Usually I go with the copper water line trick as it is better.

Tom Hand 11-20-2011 03:30 PM

1. When there is a call for heat, W1 of the thermostat energizes W of the furnace control with 24VAC. Does this.

2 − S10 primary limit switch, S47 rollout switch and S21 secondary limit are closed. Call for heat can continue.

3 − SureLight control (A92) energizes combustion air inducer. B6. Combustion air inducer runs until S18 combustion air prove switch closes (switch must close within 2−1/2 minutes or control goes into 5 minute Watchguard Pressure Switch delay). Once S18 closes, a 15−second pre−purge follows. Inducer tries to energize but stops short.

4 − SureLight control (A92) energizes ignitor. A 20−second warm−up period begins

Tom Hand 11-20-2011 03:31 PM

I'll have Dad check the ground on the actual board. Is it clearly marked on the top or side section of the board? I seem to recall seeing it.

yuri 11-20-2011 03:35 PM

After the pre-purge the inducer must keep running B4 the igniter starts to warm up. If it drops out the board should give you a pressure switch fault code. He can check from the C terminal on the board by RWGY to the neutral coming into the furnace. Should not get any voltage reading but at 2 there is a grounding problem. Do you have proper #18 wire to the tstat. I had a unit wired with telephone wire that damaged a board from low voltage.

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