Call it out or not?

I was inspecting a Lennox furnace and brushed against the coil on the side of the gas valve. It was hot enough that it made me jerk a little. It measured about 164 F with the IR thermometer but I didn’t initially think anything of it. The next day, yesterday, I happen to come across the very same furnace, photo is below. After running about the same amount of time (15-20 minutes straight), the coil on this gas valve was only at 76 F.

So now I’m wondering if the first one was overheating or if it was normal. I’m not familiar with the purpose of the coil so I’m at a loss. Any HVAC guys out there who could shed some light on this?

I got about ten feet away from the exterior condenser and noticed an audible hum coming from it. When I got up to it, it was loud. Not as loud as the transformer boxes throughout the neighborhood, but the same kind of humming or muffled buzzing. I can’t remember anytime in nine years where I noticed a unit humming away when it wasn’t operating. Any thoughts? Bad transformer inside?

I can’t see it on this computer, but 164F on a 24 VAC 50VA circuit is “Hot”.

When the coil breaks down it sometimes shorts out to other wire (but still works) in the coil making it shorter, reducing resistance and increases current which heats the wire.

Is #2 a furnace or heat pump?
Relays energized on the off cycle will humm at 60 cycles. Does not mean they will fail, just that they get out of line when they get older.

1.The coil on the gas valve is the holding or Operating coil when the stat calls for heat that coil is what opens the valve and holds it open until the stat satisfies or a safety opens that will also cause the coil to close the valve and kill the burner. The electrical load on the coil creates heat from the 24 volt circuit much like a transformer I would think 167 degrees is pretty warm if the coil gets to hot it will fail the windings in the coil will open (short) out gas valve will not open

  1. The exterior condensing unit has one contactor with a 24 volt holding or operating coil that will hum prior to fail, If humming I write them up. Hard to hear sometimes with the compressor and fan operating. If you want to know just how loud the humming is leave the thermostat in the cooling mode turned down and kill the exterior 240 volt circuit to the unit then you can really hear the contactor hum because it will still be calling for cooling 24 volts is still active with the disconnect pulled

Some coils that stay energized all the time can get hot…

Not likely in a residential gas valve though.

This is a high voltage coil…


David, just an a/c unit. It was ten years old.

The unit wasn’t in operation at all but the hum was noticeable. Enough out of normal that it got my attention over biting winds.

Thanks again guys. HVAC is definitely my area of least experience so it’s a real help to have these answers.

The only reason I reconsidered it was because I had a second identical unit to compare to. I don’t see valves with coils like this on the side very often. Guess I’ll have to start giving them the lick test when I do. :slight_smile: (ouch)

True but in a residential furnace its strictly 24 volts and it is normally closed device opens on demand from the stat

On a straight A/C unit with the thermostat in the off or satisfied position there should have been no humming because the operating contactor is a normally open device and closes on demand from the thermostat, are you sure the 240 volt supply was off at the unit and the thermostat calling for cooling

I was asking type of unit because a Heat Pump reversing valve stays energized in most units.

Generally it is energized in summer, but a few energize in the winter.

There is nothing in an old A/C condensing unit that would be energized when off (except the compressor heater , installed).

The thermostat was set at 70 and the furnace was in operation at the time. There was power at the A/C disconnect.

There should have been nothing humming in that situation nothing should have had power energized on the T side of the contactor just the line side. Most of those 10 year old units had a contactor with just a single set of points on one side of the contactor and one side was hot all the time