Motor Stall - Incorrectly identifying tankless water heater as motor stall

How was your issue detected?: Getting 6-8 “motor stalls” per day, all wrong.

Did you noticed any other signs of this issue around your home?: n/a

I have been getting false 6-8 stalls identified per day that are spikes caused by my 240V tankless water heater turning on and off in short succession (e.g. washing hands for 10 seconds). These spikes are around 10,000W which is normal for my water heater. Sense detects the water heater perfectly if it’s on for longer than a few seconds. Obviously a water heater isn’t a motor so it’s not correct for it to identify here.

Is there any way to reduce the false detection? I would like to use this feature, but with so much “noise” it’s not worth watching the alerts.

Sense people would need to comment on your specific case, but I would like to point out something about the above comment.

Quite a few gas tankless heaters, and electric tankless as well, have motors in them. For example the high efficiency units actually pull heat from the heat exchanger using fans to re-introduce the heat into the water. They also have pumps (motors) for recirc purposes.

Most of the time the motors within these units kick on after brief usage of water. If you have a high efficiency water heater it likely does have motor(s)

Here’s an example of what a gas tankless looks like. As you can imagine there’s no use of electricity other than the motors.


Hi @rsim - as @DevOpsTodd mentions, electric tankless heaters often have motors in them.
Either way, I would like to have our Data Science team look at this to see if we can provide a little more information on what you’re seeing here to see if this could be a false positive on our end OR if there’s more information we can pass along to you.

Can I give our Data Science team permission to view your data?

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@JustinAtSense: yes, please.

Thanks @DevOpsTodd as well for the motor info, makes sense.

Hi @rsim - I spoke with Data Science this morning about the motor stalls diagnosis you’re seeing in the app.

We looked at your stalls for 08/23 and they are all your water heater, which is not what we should be looking at.

For some reason beyond our understanding, your water heater onsets look like they are inductive (water heaters are primarily electric resistors).

Since this is a Labs feature, we’re unable to issue a fix for this but we’re going to take note for future iterations. For the time-being, I would disregard the motor stalls that you can attribute to your water heater.

Do you mind sharing any brand/technical details of the water heater you have?

@JustinAtSense it’s a Stiebel Eltron “Tempra 24 Plus” older model #224199
24.0kW Single Phase AC 240V 100A (2x50A). FWIW it does not have a recirculating pump.

Thanks for looking into it, any updates later on will be appreciated!

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We all assume that heating elements are purely resistive, but if the heating elements are truly coil-shaped, they can have an inductive component to them.

Our team here noticed some peculiarities when inspecting this a bit further.

  • Do you have a 3-phase system? If you’re unsure, sending a photo of your Sense installation/inside of your panel would be a huge help!

No, I do not have 3-phase supply.

You do mean two-phase, right?

Here’s the specification sticker.

Single phase means only two ports where the power comes in. Single phase, 240V means each leg enters, but not the neutral. Single phase, 208V (three phase) means two phases enter, but no neutral (208V phase to phase)


Wanted to touch base one more time with our data science teams findings here:
The water heater uses 18 kW when it turns on, which matches your house being 208V (not 240V.) So, from the back-end, for some reason it looks like you have a 2-of-3 supply and not a typical split-phase 240V supply.

An example of the behavior we’re seeing: Your water heater ran at 4:16AM this morning and was using just above 18 kW, not 24 kW. Everything points to you having 207V, 2-of-3 supply and not a 240V supply.

Nothing is technically wrong here outside of the motor stalls notifications you’re receiving, just wanted to convey the additional information that Data Science passed along.

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Would the line voltage show up that way in the “Power Quality” section of labs? That should put any confusion to rest.

The challenge with 3 phase is that each phase to neutral is the standard 120V AC. It’s only when you measure from phase to phase (without looking at neutral reference) that the discrepancy due to 120 degree phase alignment vs typical 180 degree is visible. Both the Signals section in the My Sense display and the Power Quality look at single leg/phase to neutral measurements. I’m guessing Sense is using internal tools to look at the leg to leg (really phase to phase) AC voltage.