[A/S] Re: A/S Furnace problem

["nickcrowhurst" <NickandSandra@xxxxxxxxxxx>]

Henry, as many of us know to our cost, there are many possible 
reasons for this failure, but before getting to the complicated stuff, 
it may be worth trying a few obvious ones. For starters:-

1. Do you have a gas supply at the right pressure? The cold weather 
may have caused a drop in pressure. Check that you can light all 
burners on the hob and the furnace at the same time. If so, maybe 
you have the required 11" of water head pressure.Try taking the full 
cylinder indoors overnight to get it warm. Bear in mind that the 
arrow on the regulator does not automatically point to the tank that 
is supplying gas. It points to the one you first chose to be the supply 
tank. The arrow will only point to the other tank if you manually 
rotate it. Both tanks should be turned on. I think the word for this 
is "counter-intuitive"!

2. Do you have at least 12.6 volts at the furnace? If the trailer 
battery is down in voltage, particularly in the cold, the fan will not 
turn fast enough to operate the sail switch inside the furnace. 
Charge the battery if required, and keep plugged into the mains 
electricity.

3. Is the thermostat calling for heat? At this stage, I remove the 
thermostat from the wall, remove the two wires to the furnace, and 
twist them together with a wirenut. This shorts out the thermostat, 
and enables the furnace to fire, regardless of temperature. 

If you have enough gas pressure, at least 12.6 volts, a shorted out 
thermoststat, and still no ignition, you probably have a problem with 
the furnace. You then either have to get your digital voltmeter 
active in the furnace, or you need to find an experienced person to 
fix it. To help us help you, we will need to know exactly what you 
hear and see when the furnace should be operating, and what make 
and model is your furnace. I will describe what happens with my 
Suburban NT30K. 

As soon as the thermostat wires are joined (or the thermostat is 
turned up, if connected), the time delay relay is activated. After a 
few seconds, you should hear and feel the fan blower pushing cold 
air out of the vents. When the fan gets up to speed, the wind blows 
on a small sail (inside the furnace), which blows open the sail 
switch, allowing current to pass to the circuit board and the gas 
valve solenoids. About 15 seconds after the fan starts, there is a 
click as the gas valve solenoids open, and at the same time, the 
circuit board delivers a current to the ignition probe. There is an 
inspection window at the front of the furnace. If you look through 
this window, you should see a spark like a 1" streak of lightning 
pass between the two wires of the ignition probe for a couple of 
seconds, followed by a wooshing sound and a mass of flames.  
There is a limit switch in the circuitry to provide a safety cut-out if 
the furnace overheats.

Any one of these items can fail, preventing ignition. If it turns 
out to be a circuit board failure, I recommend the Dinosaur brand, 
for reasons discussed previously on this site. 

Let us know what happens, and someone on this group will surely 
be able to help with the diagnosis. Good luck. Nick.