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BillyHvac
BillyHvac, Journeyman HVAC Tech
Category: HVAC
Satisfied Customers: 22097
Experience:  Endorsed for unlimited heating, cooling, oil burners, boilers, refrigeration, hydronics
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My Tempstar furnace keeps shutting off. How to fix it?

Customer Question

My Tempstar furnace keeps shutting off. I have replaced the main electronic valve and the pressure switch. It calls for heat, ignites the furnace, runs for 5-10 seconds and shuts down. The WC is constant at .6. Suggestions?

Submitted: 7 years ago.
Category: HVAC
Expert:  BillyHvac replied 7 years ago.

Have you sanded flame sensor off?

Customer: replied 7 years ago.
No, should I?
Expert:  BillyHvac replied 7 years ago.

The main things that shut down the flame after ignition is:

Flame sensor is coated and does nor recognize flame thus shuts unit down.

Pressure switch faults due to extra pressure in the system, but if properly vented, no obstructions, or bad switch it should be fine.

Limit trips, but this is after an extended run where it overheats.

The flame sensor trips in 3-5 seconds

Pressure switch trips upon firing

Limit trips after extended run

Customer: replied 7 years ago.

So, from my observations the furnace turns on, preheats, ignites, and begins running. After 5 seconds or so it shuts down and restarts. Like I said, pressure switch replaced, electronic valve replaced. I noticed the vent motor drawing nearly 3 amps when it is supposed to peak at 2.5 amp. Any other thoughts?

Expert:  BillyHvac replied 7 years ago.

The vent motor drawing 3 amps is on the 120v side of operation and the flame/ignition control is on the low voltage side. So even if the motor were to amp up, as long as the transformer is still delivering 24v to safety circuit and ignition control it should be fine.

My first bet is flame sense. Sand the sensor off and if I am wrong I will remain available for follow up.

Customer: replied 7 years ago.

OK, I will sand the sensor.

If the flame sensor is next to the ignitor and is flat then I sanded it. Or is it the round sensor going into the side of the sheet metal chamber?
Here is what happens...
1) turn furnace on
2) vent blower turns on
3) ignitor heats up
4) furnace begins to run..
5) after 2 minutes it cycles off for 10 seconds and restarts
6) after 45 seconds it cycles off for 10 seconds and restarts
7) after 25 seconds it cycles off for 10 seconds and restarts
8) after 45 seconds it cycles off and I get a 6+2 flashing code
9) 6+2 states...
Soft lockout -
Flame sense lost during run
Cycling pressure switch or blocked condensate.

Expert:  BillyHvac replied 7 years ago.

Is this a hot surface that glows and lights pilot and then pilot lights burner.

Customer: replied 7 years ago.

There are 3 parts bolted to the same tab.
1) the pilot gas tube
2) the ignitor
3) the thin flat surface that maybe the heated glow plug.
The other sensor I see is about a foot away and is about 3/4 round bolted to the main sheet metal duct work where the heats get pulled from by the main blower.

Expert:  BillyHvac replied 7 years ago.

This is the Honeywell smart valve.

Change the circuit board

Customer: replied 7 years ago.

The circuit board is contained in the valve assembly... part # 1 0 1 3 3 5 0. I changed the valve with board this morning.
Furnace #s N8MPN075B12A1
A032003677

Expert:  BillyHvac replied 7 years ago.

The only thing left in the flame rectification system is the pilot/sensor assembly.

Customer: replied 7 years ago.
Yeah but that would not cause the furnace to recycle after the length of time it is runing.
Expert:  BillyHvac replied 7 years ago.

This is like saying I have a noise in the tires somewhere and I changed 3 and it is still there. There is only the pilot assembly left. And also you do have the board in the valve but also the board controlling power to the valve and the inducer assembly in the blower section. You say you have changed the gas valve, the circuit board, and a pressure switch but the code is not pressure switch.

There is only one component left.