Furnace - odd electrical current behavior

DavidRNovember 4, 2012

I have a fairly conventional Comfortmaker gas furnace, 80% AFUE, about 15-20 years old. I'm rebuilding my backup power system and decided to add the furnace to it. As part of this project I popped a clamp-ammeter onto the line from the furnace breaker and started it up to see what kind of current it needed.

What I saw was puzzling.

When the furnace started, it started both the blower and draft fan immediately (this is normal for this furnace). At this point my ammeter read 6.2 amps.

When I heard the gas click on and light, the current DROPPED to 1.6 amps! I didn't hear any change in the sound of the blower or draft fan.

The current stayed at that level for a good 15-20 seconds, then went back up, but this time to something more like 5.7 amps. Then over the next few minutes it gradually declined to a bit more than 5 amps.

I'm far from an expert, but this makes no sense to me. I expected the current to increase when the gas valve opened and the ignitor came on, but it *dropped* instead.

So what gives with the 1.6 amps?

And why would the current rise again later? Why would it fall a bit over time?

Sure would be nice if it would run at 1.6 amps all the time!

Thanks for any insight anyone can offer.

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I am going to take a speculate as to what is going on. First I don't think it is normal for the blower to to turn on before the furnace has ignited. It it does then it is blowing cold air into the house everytime it starts up.

I think the blower is shutting off when the furnace ignited and then turns on again. Are you sure the blower is is not stopping for a few seconds when the igniter turns on?

    Bookmark   November 4, 2012 at 9:15AM
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Some of the newer furnaces run tests before starting the burner. I know few of the details, but one of the tests is to be sure the exhaust vent is clear. Another could be (I don't know for sure) that the blower is working-- no need for the burner to light if the heat will not get into the house. The control board has some functions such as these that it performs. I suspect that your furnace is functioning normally. Experts may offer differing or additional perspectives.

    Bookmark   November 4, 2012 at 6:43PM
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Thanks for the responses! It turns out that Mike was close to the answer. My aging ears were playing tricks on me. A hand on a register confirmed that the blower does NOT start immediately when the thermostat calls for heat.

A little cheating of the access panel interlock shows that when the furnace starts, the ignitor immediately begins to heat. This, with the draft fan, accounts for the initial 6+ amps. After some seconds (10-20?) the ignitor reaches ignition temperature, the gas valve opens, and poof, the burner ignites. Then the ignitor shuts off. Now only the draft fan and gas valve are drawing current, hence the drop to 1.6 amps.

Several more seconds pass and then the heat exchanger temperature rises enough for the blower to start. Then the operating current again rises, to around 5.7 amps.

I still don't know why the current gradually falls from there to around 5 amps as the furnace operates, but at least the first part of the cycle makes sense.

As an aside, I wondered whether the AFUE rating of modern furnaces takes into account the electrical energy used to burn gas more efficiently, and how this would shake out in the efficiency game.

If the gas valve and draft blower together draw 1.6 amps, that's about 192 watts, equivalent to about 655 BTU/hr if it were passed through a resistive heating element.

That's not a huge percentage of the furnace's heat output, if my math is right - for a 40k BTU, maybe ~1.5%, if my math is right. So if a traditional draft hood furnace was, say, 60% efficient (I'm making that number up; how [in]efficient *were* those old furnaces?) obviously we're ahead with the modern furnace.

The next question is, does a 96% AFUE unit use more electricity than an 80% AFUE? How much more efficient is it really, after considering the additional electrical energy input (if any)? There I have no data - anyone?

    Bookmark   November 16, 2012 at 3:29AM
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The AFUE rating is a measure of how efficiently the fuel is used. For a gas furnace it does not take into account how much electricity is used. I suppose it would for a electric furnace.

The answer to your second question is sometimes yes and sometimes no. It depends on what type of blower motor is used in the furnace. The ECM motors (DC driven) are more efficient than PSC motors (AC driven). You would have to look a the data sheet of a particular furnace to determine this information. It is a small amount of energy compared to the fuel costs regardless of which motor is used.

There are times when the ECM motor in a variable speed furnace can use more electricity than a PSC motor in a single stage furnace. Some variable speed furnaces are programmed to push out a specified amount of air flow (CFMs) If the static pressure is high, the motor will increase the speed (RPMs) in order to achieve this air flow. The motor is using more energy to overcome the restrictions than a motor in a furnace that does not have this feature.

    Bookmark   November 16, 2012 at 10:10AM
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In general, a fan motor will use more juice to start than to run. Not including the digital control models.

    Bookmark   November 16, 2012 at 3:25PM
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Many motors pull less current as they heat up (though often at a slightly worse power factor).

Keep in mind fr AC power, current is NOT power.

power = current * voltage * POWER FACTOR

Power factor varies with temperature and the load on the motor (among other things).

Variable speed AC motors are even worse, since they are often no longer operating on a clean sine wave (and it takes a higher end meter to measure RMS current from a non-sinusoidal waveform).

    Bookmark   November 17, 2012 at 10:31AM
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