Changing wattage draw on bathroom exhaust fan

One of my bathroom exhaust fans was detected, awesome. However it has an rather odd watt usage that I don’t think is accurate. It starts out using a fairly steady 40 watts which sounds about right. However the power consumption then gets erratic and starts dropping consumption then settles down to a flat line of 13 watts. This does not seem correct at all, the fan has no speed control and does not change sounds as it runs, so it is not slowing up. Once it settles in at 13 watts according to Sense, it stays perfect on 13 watts.

I will try to attach some pictures to show what I am talking about.

Any Thoughts?

1 Like

Venting questions:
Where does the fan exhaust to?
Is there a mechanical flap valve at the exit? Does the fan have to work harder to open the flapper?
Is the flapper valve stuck down?
Is there a dust build-up in the line?
Does the vent line have a dip that allows moisture build-up? Is the fan working harder at the beginning to blow out the moisture condensation in the vent line?

Vent fan questions:
Have you recently oiled the bearings on the motor?
Have the motor bushings gone bad?

What type/model of fan do you own?
Have you checked the manufacturer’s specs against your SENSE readings?

Interesting problem. Probably more ideas will be provided by others.


I ancedotally have seen similar detected device power profiles which I know can’t possibly be accurate, and confirmed by plugging a HS110 smart plug on the device. Not sure what to do in cases like this. Deleting the device has been suggested numerous times on this board as not being the correct thing to do. Waiting it out to see if Sense ever updates what it thinks the device power profile is, or using a smart plug (which has its own issues for devices that Sense has detected), are the only other options. I wish there was feedback like “close, but no cigar” that users could inform the Sense borg collective that the device on detection is correct, but the power profile is erroneous, and that it should retrain the device.



Thanks for letting me know, I can only imagine how hard it is to accurately detect wattage used based off the noise the device makes on the power signal. However I was kinda just trusting it was working fairly accurately, but this fan is a great example that it is not very accurate in this case. 13 watts vs 40 watts is a big difference. I think they have a thread for this sort of thing, but you have to be a veteran member in order to post in it. I would gladly let them look at the data to see if it is something they could possible correct.

Thanks Again for letting me know I am not the only one experiencing this anomaly.


All great questions, I do not know the model number, but it is a standard ceiling exhaust fan. I would have guessed anywhere from 30 to 60 watts would be normal.

I don’t think the exhaust line itself is effecting anything major, there is no change in sound of the motor and after quite a few minutes when it drops down to 13 watts, it is a flat line 13 watts, vs a noisy 40 watts that it starts out as.

The vent is unobstructed, vents in attic, no exhaust flap, goes straight up then I have it hung near peak of roof and then draped back down about 1 foot (kinda like a candy cane). FYI : i know it is not code to vent in attic anymore, but this place was built 25 years ago and it was pretty common place to vent back then. My attic is well ventilated, so I think it is okay.

I have never oiled the bearings, I don’t think it has any place to oil. I did clean it out once really well. I don’t think the bushings are bad, that noise would drive me batty, I love silence. The motor never changes sound, so I think it may just be a sense issue.

By the way I keep saying 13 watts, even though the picture I captured shows 12 watts. It is actually 12 watts now that I looked back over it. Not sure where I got the 13 watts from.

Our house is 30 years old. Last year I removed the face plate covers on all our bathroom vents and sprayed WD-40 on the motor bearings. There is no designated oiling point on my motors (Broan). That reduced the noise and improved the fan startup time. I’m sure moisture and age was effecting the efficiency of the fan motors. When you take the cover plate off, you should see the manufacturer’s model stamped on the mounting-box label. Most motors require a higher current when they are initially turned on. However, I don’t think that your motor would pull a higher load for that long of a period.

It could be a SENSE detection issue, but it never hurts to do the fan maintenance. Maintenance is cheaper than replacing the whole unit. Good Luck!

1 Like

@Dcdyer, Agreed on the maintenance, I did just clean this one out about 2 years ago. Sense is taking a look into this for me by looking at the data. It does appear to be an anomaly.

I just had another anomaly show up last night, my hybrid water heater I assumed was only using 233 watts, which is what Sense has been showing me for the past month or so. I was quite impressed and proud I switched out. However all the sudden last night it decided to more than double all the way up to 597watts the last three times it has run. Only thing I can think of is, it was only showing one leg and not the blower fans on the evaporator core. Now it realized it was the same device and showing both compressor legs plus the blower fans. It still is a great reduction in power consumption over standard cold water heater, but not as good as I thought, lol.

I will have to tame my excitement on how little power it was using… lol