Then the furnace is completely dead. I have an electronic (intermittent) spark ignition furnace, the sort that you can hear tick, tick, tick. After avoiding the engine wiring harness diagram for my car for many years, suddenly it made sense to me. After staring at the furnace diagram for so many years, it suddenly make sense to me. I'm sort of professional but I don't know what a pilot relit looks like and what it means. Suddenly I can relate the blocks in the wiring diagram to the boxes in the furnace. The rest is simple.
I checked that the pilot relit has 24V to it when heating is demanded. Basically it's a low to high voltage transformer to create the spark. I immediately ordered one replacement because it is cheap.
Then I thought of lighting it manually until the replacement arrive from the wrong coast. You should NOT attempt this unless someone is in danger of dying from cold and the sort. There is little difference between ignition by a spark and by a BBQ lighter. BUT, the ignition happens inside a room with a door closed. With a lighter you have to open the door and get to within 1 feet of the burner. And also, the furnace cover is open. If anything happens, there's a huge difference. So make sure there's no gas accumulation. Follow manufacturer's procedure to shut down completely, including the gas line, let possible gas leak dissipate, set thermostat to minimum and follow procedure to restart. Set thermostat to above room temperature. Finally put the BBQ lighter tip to where the pilot is and ignite.
After that, I discovered a youtube video showing the same thing. I cannot agree more that you cannot leave the furnace alone after starting. You must shut it down also after desired temperature is reached. Otherwise, the relit will try to spark, the pilot gas will come on, and the whole control will be in some sort of cycle. The gas from the pilot should be adequately dissipate from the vent and that the wast of gas will be minimal compared with when the furnace is started.
After some work, I can see that the sparks aren't completely dead. That led me to wonder if the relit is the only possibility of fault. From the web I vaguely read about that the relit isn't probably earthed, and cleaning the igniter with pin, brush, or sand paper. Now I know that they are talking about, or they don't want to talk about too much just like replacing the spark igniter with a BBQ lighter. Here's what I do.
Of course you check if 24V AC reaches the relit terminals and they are not rusty. Check if the earth terminals have 0 ohm with earth, usually the metal chassis and bare metal gas pipes are earthed. Now the high voltage wire aren't soldered into the relit. You can just pull it out. You may want to cut off a little bit wire and push it back in to ensure better contact.
Inside the ignition hole there are three components in the assembly. The pilot is basically a tough gas tube with a funny shaped end to direct the flames. The flame sensor connects to the gas valve via a bare earth wire like connection. The flame sensor itself is like a thicker stiff earth wire.
The igniter is at the end of the high voltage wire, which is itself a piece of stiff wire. It is usually thin and bent to direct the spark. When it was sparking, I observed that the spark goes to either the pilot or the flame sensor. Indeed both of them are earthed. And my igniter is midway between the flame sensor and the pilot.
So the principle is like lightning find it's way to earth. So I think the cause of not sparking is that the high voltage conductor or the earth conductor are not conducting due to oxidation. If you restore them to shiny metal surface, they should spark if everything else are working properly.
Without unmounting anything, you can see all three tips at an angle from above. You can scratch them to expose a little bare metal using a long pointed object. You can use a long narrow file or sand paper rolled into a tube. If it sparks better, you could take out the assembly and restore the metal surfaces so it will last another 20 years.
To take the assembly out, the minimum is to take out the pilot gas connector. Taking out the whole gas line together with the valve controller saves you a lot of grief.
If scratching a little doesn't work. I thought of mounting an earthed metal tip beside the igniter as a quick fix. It could be a narrow piece of steel sheet metal that can bend toward the igniter tip.
Now for the upgrade. Now manufacturers are all using hot surface igniter, and still some direct spark igniter (that starts the main burners). My intermittent spark igniter (that start a pilot first) are long gone. There's really not much good reason other than that you don't like the tick tick tick any more. A spark igniter last 20 years while a hot surface igniter last a few years at most, though the newer materials last a bit longer.
May be you can't find parts any more. May be your igniter is so bad that you need to replace it. And of course the valve. If you can't find a valve with main and pilot control anymore, you have to replace one with only main valve. Then you need hot surface igniter for the main gas line directly.
It's conceptually simpler if you upgrade to a hot surface. You just need to find a HSI that can be mounted onto the metal sheet chassis and place it close to the pilot tube. The newer rounded type are more likely to fit. A longer one is surer.
Most of them use line voltage at 120V. You probably have that control signal in your system. But to make it simple, you can use the relit input voltage at 24V AC to control a 120V relay, that in turn drives the HSI. The whole thing cost may be $20 that is about the price of a relit.
You need a HSI that is fast acting like a spark igniter. The relay should match the output current of the HSI. One caution is that relays have different types. Solid state relays cannot handle even small motors though their current rating are not exceeded.
Anyway, my relit is arriving next week but I probably do not need that. My furnace is working now after taking out the igniter assembly and sand paper all over.
After sanding off some oxidation, the relit sparks better but not consistence. I put in my new relit and it works perfectly. This is the 2nd time now. The components was new but in a box that may be 20 years old, from the original parts supplier and original parts number - a collectable. That's how I locate the part on eBay with my 20 year old user manual. Did someone got it from an estate sale of a furnace technician? Did someone retired and clear his stuff? Amazing!
Thank you very much for writing such an interesting article on this topic. This has really made me think and I hope to read more. furnace repairs
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