[adamsqh]

I have a model lf24 that runs for about 2-3 min then shuts down and starts over will not stay lit though the fan blows constantly.I have changed the heat sensor the flame sensor the board the gas valve. The first time it comes on it runs for about 2.5min then as it warms up it gets down to running only about 30 sec any ideas?

 

[danpik]

Well, since you have changed out almost everything accept possibly the high limit or the filter I would start there. Sounds like the furnace is shutting down on overtemp. How is the airflow in and out of the furnace?

 

[Marty S.]

About all you have left is inducer and pressure switch.

 

[64pvolvo1800]

I have a model lf24 that runs for about 2-3 min then shuts down and starts over will not stay lit though the fan blows constantly.I have changed the heat sensor the flame sensor the board the gas valve. The first time it comes on it runs for about 2.5min then as it warms up it gets down to running only about 30 sec any ideas?

Sounds like a tired gas valve to me. Throw a voltmeter on the GV leads and see what comes first; loss of fire or loss of voltage. If voltage stays then your GV is at fault. Voltage goes first, then maybe overheat sensor?

 

[adamsqh]

Ok I changed the high limit and the pressure switch and got good air flow through he vent and still wont stay on and it wont give me a blinking code it blinks everything normal. Im getting ready to swap the whole thing out.

 

[64pvolvo1800]

Gas valve test.?

 

[adamsqh]

didnt check voltage on gv just changed it.

 

[how]

If your gas valve is shutting off without a code on the board either the
thermostat circuit is being interupted (pigtail R & W on the furnace) or the board is bad.

 

[adamsqh]

Ok I changed the board already but have not changed the thermostat I wired it constant so it should have ran nonstop and it still shut down.

 

[how]

Jumpering R & W where the stat wires connect to the furnace eliminates the chance its a partial break in the thermostat line. Make sure to disconnect the stat wires before you jump R to W.
A faulty thermostat circuit won't get picked up by the board diagnostics. Beyond that, the board should be giving you a fault led reading.

 

[adamsqh]

ok I just replaced the stat and still no bueno I am at a loss I think I am going to have to change the unit out with another one that I have if no other sugestions.

 

[adamsqh]

could it be that im not getting enough gas pressure?

 

[how]

Pigtail the R & W stat wire connections inside the furnace, Not at the thermostat. Turn off furnace power/ detach stat wires from R & W inside the furnace/ connect a wire between the R & W/ put furnace doors back in place/ turn power back on.
If it still shuts itself off...Thermostat circuit is OK but Board is suspect!
If it runs continuously...thermostat wiring or thermostat is the problem.

 

[adamsqh]

Thanks for all the advise i found out the inducer vent that goes to the pressure switch was plugged. Thanks

 

[how]

Hey adamsqh
I hope all is now well.
Anyone have an idea why the board diagnostic led didn't give a code for a pressure switch shut down?

 

[Houston204]

I don't see an LED on this unit heater...

 

[how]

Yes, It makes post 5 a bit of a mystery.

 

[JScotty]

I don't see an LED on this unit heater...

Oddly enough I just had to order a fan blade for this unit so I happen to have the parts list already up on this unit. Without knowing the full model number I don't know which board his unit uses but the "dash 5" units use this board:

You can see the LED right in the middle. Not sure why it wouldn't flash a code if the pressure switch was actually the problem though.