How much energy will continuously running a furnace fan use?

Question

We have a bi-level (tri-level? There's a small entry way and utility room, then an upstairs living area and master bedroom and a downstairs living area with bedrooms) in mid-Michigan.

The downstairs is naturally quite cooler than the upstairs, but during the summer we were having keeping the downstairs warm enough for our young son. We didn't want to raise the temp upstairs anymore but our local HVAC person suggested running the system fan continuously to help evenly distribute the air. And it worked. He did say that it shouldn't increase our costs that much and searching online seemed to come to the same conclusion. Our electric bill did end up higher than previous years but I chalked that up to higher-than-average weather temps, we were keeping things a little cooler than we had in the past due to our child, and summer electricity rates were in effect.

Now that we're getting into Winter, we just received our latest bill and it's again abnormally high. We did cut the fan usage some the billing period prior, so I'm sure it's because we're always running it still.

How much electricity usage should I really expect to see if I'm continuously running the furnace fan? Enough to see a sizeable jump in cost?

And is there a way to track how much power usage my unit is using? Do I need a killawatt or something plugged in? I'm trying to find the model number on it to see if I can find the electric specs and maybe do some calculations there. Our thermostat does tell me how often the heat or AC runs, but the heat is gas so that shouldn't affect it much anyway.

Any other suggestions or things I should try or look for? Thanks,

Answer 1

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

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_{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.}

Answer 2

I've been using the fan on my furnace much more recently, so I was curious about this as well.

I have a Luxaire G9T100014UPC13C natural gas furnace that is about 9 years old, and a Current Innovations CI300E-WIFI thermostat (which I'm controlling from my iPod).

I'm measuring the overall power use of the furnace -- right where it comes into the cabinet.

My ammeter doesn't have the fastest response, but on startup it is measuring as high as 23A.

When the furnace starts up fully (including gas firing), it is drawing about 5.2 A.

When I turn the temperature down, and it goes back to only running the fan, it drops slightly, to 4.4 A.

Now, I am not quite sure how this works out. If I wait until it turns off, and then set the fan to 'On' (instead of 'Auto'), it draws around 9 A.

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I suppose it's possible that while I have it set to 'Fan On' it runs the fan at a higher speed than it does normally, but it certainly doesn't feel like it. I let it run for 10 minutes before measuring and it was still around 5.2 A, and have repeated this a couple of times a couple hours apart just to be sure (which is why I took pictures the second time). Aside, anyone have any idea why it's so different while only the fan is on vs gas firing?

Answer 3

Nameplate Power Won't Reveal Actual Power Consumption

The horsepower, wattage, current, and/or voltage will not tell you how much power your fan is actually consuming. That is because the amount of power consumed depends on the load that the fan and your duct system imposes upon the motor. In other words, the actual power consumed depends very much on your particular installation.

Power Consumption Follows an Affinity Law

The relationship between fan RPM and power consumption follows an affinity law. Specifically power is proportionate to the cube of the RPM. So cutting your fan speed in half reduces your power consumption by 8x.

This relationship makes clear that reducing fan speed as much as possible is critical to reducing power consumption. However, affinity law alone won't help you determine power consumption in absolute quantities.

Reducing Your Furnace Fan Speed

Many household furnace fan motors are designed to operate at a variety of discrete speeds. This is usually accomplished by applying power to one of a variety of winding configurations built into the motor. Where this is the case, the furnace installation manual usually provides a table of connection wire colors and corresponding speeds for your fan.

The furnace fan speed is meant to be set to an appropriate value for the particular installation. In my experience, however, the speed installers typically set the fan to is much higher than optimal.

Fan Power Consumption Increases With Constricted Air Flow

A fan pushing air through a short length of oversized duct uses much less power than the same fan pushing the same rate of air through small, extremely long, or damped ducts. Reducing constrictions like filters, grills, and closed dampers on the entire air circuit including the return air path will reduce the fan power consumption for a given amount of air flow.

Measuring Actual Power Consumption Requires a Particular Type of Clamp Meter

Furnace fans typically get their power from a controller or relay within the furnace. It is at this location that electrical measurements can be made to determine fan power consumption. In order to measure the actual power consumed by a furnace fan, you need to do be able to do the following:

• on a common timebase,
  • measure the waveform of voltage applied to the motor, and
  • measure the waveform of current through the motor
• calculate the product of the above two waveforms
• find the time average of the product waveform over several cycles

The above is effectively what a real power meter does. The result will be in watts (W) which is what your utility power meter integrates over time to determine a watt-hour (Wh) reading which they use to calculate your bill.

The above is pretty complicated to do. Their are fairly common, specialized test instruments designed exactly for this purpose. The following are two examples:

• Mastech MS2203
• Fluke 345

There are some conceivable other ways of measuring real fan power consumption, but they are more complicated, error prone, and dangerous.

It is not possible to measure AC power consumption of a motor with a multimeter. Nor is it possible with an ammeter-only. In order to measure power consumption, a single piece of test equipment must measure both voltage and current over time (think thousands of samples per power cycle) and process that data to determine power consumption.

Answer 4

My power meter shows instantaneous real power consumption and when I run my furnace fan, it consumes around 500 watts.

If you leave it on 24x7, this will be quite noticeable on your monthly electric bill.

There is work being done on measuring and improving the power consumption of the motors used to move the air but that only helps if you buy a new system.