I have an older Thermo Pride IGH2-75 NG furnace that malfunctioned. It exhibited TWO separate problems before being a no heat situation.

First problem: I call for heat on t-stat. Exhaust blower motor runs and igniter clicks rapidly for about five seconds and gas is admitted into combustion tubes. No ignition. It will do it again, but there is a noticeably loud whoosh with flames coming out the burner tube and into the room. Furnace does this every time. It runs often as its 10 degrees out.

Second problem: Now furnace is totally no heat. I call for heat. Exhaust blower runs and igniter clicks rapidly for about five seconds. No gas is admitted into combustion tubes. Five seconds after igniter stops, gas is admitted into combustion tubes but igniter fails to operate. I used a long neck lighter and am able to light each combustion tube individually. However, the flame is weak, intermittent and blows itself out in each combustion tube. The squirrel cage motor runs but blows cold air. The combustion vent stack is warm to the touch. I shut off the furnace at this point and switches to electric heat.

I do see a couple of obvious things wrong. The old circle dialed Honeywell t-stat is set to 65° and the heat out put, during problem one, was 73°. The built in thermometer is completely broken. Also, the flames looked the way I described because they are most likely dirty. I rent here and this is an older (1960 era) unit that had never been cleaned. The filter is new and is installed correctly. I do not believe this unit is not computer controlled. Gas pressure from meter presumed good as water heater pilot is strong with a blue cone inside the large cone and it heats correctly.

I like to think I know the theory of operation of a NG FA furnace. The only things I can think of is the t-stat, t-couple,heat sensor or the White Rogers valve is shot, but that doesn't explain the delayed ignition/bang! Please help! John

John, welcome to the forums. It sounds like you may need the furnace cleaned & possibly a gas valve. Since you rent, this is something the landlord should be made aware of. In most cases it is the landlords resposibility to make sure you have a functioning & safe furnace. You should not work on it.