Common problems with Intertherm furnaces include a furnace blower that constantly runs and circulates cool air and a furnace that only functions in short spurts. Intertherm furnaces also commonly have burners that won't light.
To troubleshoot a constantly running furnace blower, use a screwdriver to open the furnace's access panel. Find the temperature limit switch, and lift its cover to find a dial containing metal markers. Starting from the left, reset all the metal arms to 90, 120 and 200 in order to communicate to the furnace when it should come on, shut off and turn off the burner, respectively. Pull the white switch to change the furnace to automatic so it follows those settings. If the furnace blower still runs constantly within 15 to 20 minutes of the fix, call a professional technician.
To fix a furnace running in short spurts, check the anticipator, which is likely set on 0.2 or lower. Change it to 0.3 to 0.4 depending on the manufacturer's recommendations.
If the furnace's burner does not light, first check that the thermostat is set to heat and turned to an appropriate temperature. Examine the pilot light to determine if it is lit. If it is not, turn the furnace on, open the gas valve, and light the pilot by holding the knob while inserting a long match. Hold the knob for 30 more seconds, and slowly release. If the pilot light stays lit, turn the knob to on; if it goes out, try lighting it again after five minutes.