Does a continuously running furnace pilot light need a spark ignitor

furnace

I have a very old gas furnace with a continuously running pilot light. The system has a spark ignitor that has recently gone bad but I'm wondering why the furnace ever had the ignitor installed if the pilot light is constantly on? I assumed spark ignitors were only necessary for furnaces that didn't have a continuously running pilot light.

The picture does not have the pilot light running.

Best Answer

Does a continuously running furnace pilot light need a spark igniter?

It does not "NEED" one as the pilot light can also be lit with match or a lighter but it is more convenient to use the igniter. The igniter is only needed if the pilot goes out and needs to be re-lit.

The igniter is for lighting the pilot light, not the furnace.

If you ever have to shut your gas off for any reason, such as service or vacation or if the pilot just goes out, you can use the igniter to light the pilot light.

Once the pilot light is lit it stays on (should stay on) and when the furnace needs to fire up the constant pilot provides the flame needed to ignite the burners.

Most, if not all, modern appliances that have combustion now have electronic spark igniters so there is no need to have a constantly lit pilot.

Pilot Lights Wikipedia

Pilot Light Safety

Where to find the specifications

Listed on the furnace

Somewhere inside the service panel of the furnace, there should be a schematic for the furnace. This may list the electrical specifications of the blower motor. If not, the blower motor itself will have a label on it. It should list the voltage, amperage, horsepower, etc.

Using an Ammeter

An ammeter is used to measure the amount of current flowing through an electrical circuit. You can use a clamp on ammeter to measure the amount of current being used by the blower motor while it's running.

Simply set the meter to measure current (A in the pictured meter), and clamp it on the hot wire feeding the motor.
_{This method may require working on or near live wires, please use caution and/or consult an expert to take the readings}

Calculate Usage

With the above information and Ohm's law, it's easy to determine how much power the motor will use over a given time period.

Ohm's law says that Power (P) = Voltage (V) x Current (I), so we can determine how much power the motor uses at a single instant in time.

ex.

P = 115V * 5.2A
P = 598 Watts

Next we have to determine how many kilowatt hours the motor will use, which we do by dividing by 1000.

P = 598 W / 1000
P = 0.598 kWh

So for every hour the motor runs, it will use 0.943 kilowatts of power.

Total power/day = 0.598kWh * 24h
Total power/day = 14.352kWh
Total power/month = 14.352Kwh * 30
Total power/month = 430.56kWh

Finally, if you multiply this number by the amount you pay per kilowatt hour. You'll see just about how much it costs to run the motor constantly.

Cost = 430.56kWh * $0.07
Cost = $30.14 per month

_Notes:
_{Results using this process will be an estimate only.}
_{Values used are not actual values, they are example values only.}
_{Calculations may be incorrect, nobody checked my math.}

Why isn’t the spark gap lighting the pilot on the furnace

I finally found some time yesterday to take a closer look at the furnace.

Gas Pressure

I started by measuring the incoming gas pressure using a gas pressure gauge. I found it to be 6 WC (inch water column), which is well within the acceptable range specified on the furnace.

Gas pressure gauge

Dirty Gap

Next I took a closer look at the spark gap itself, and watched closely as the spark was generated. I even recorded the spark with my video camera, which allowed me to zoom in fairly close. This is when I noticed a bit of carbon buildup on the electrodes, and wondered if it could be that simple...

Cleaning the Gap

I grabbed a bit of 220 grit sandpaper, and cleaned the electrodes up a bit. steel wool is probably the tool of choice for this job, but I didn't happen to have any on hand at the moment (not yet unpacked).

Success!

After testing the pilot ignition a few times, the problem seems to be fixed. We had a cold night last night, and the furnace fired up fine all night. As I write this, the furnace has just cycled successfully yet again.

Update

I found this YouTube video from Grayfurnaceman that describes the problem exactly, and says that the only solution is to replace the assembly. I'll be replacing the pilot assembly once the weather warms up, so we'll see if that solves the problem.