Adding flex sensor to 240 HVAC - Questions

"connecting additional flex sensor to 240VAC HVAC. Do I need to clamp both L1 and L2 to the HVAC? Its not clear in the instructions although it say it can monitor up to two 120 or 240 circuits. (I connected only one sensor and it would not detect the HVAC running) And do I merge or delete the previously detected HVAC?

If it’s a 240 circuit you clamp one CT clamp onto one of the hot wires. Just hooked 2 if them up this afternoon. One for each of my Mitsubishi HVAC systems.

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Thank you. It is a bit confusing. I have both clamps on now and it works. If I remove one clamp the current draw goes in half. So if I were to try to use the second clamp for another device it seems it would just add to the single device. Once I checked 240v as the device type it didn’t have an option to setup the second device.

Well I pulled out my fluke 36 amp clamp to measure the current on L1and L2 of the HVAC. Both are 11amps 11A x 119V = 1309W. X2 = 2618W. The Sense probes measure about 1300W with one connected and 2600 with both probes connected. I don’t see how I could measure two devices accurately when it takes two probes to measure my 240V HVAC. Could I be set up wrong? The instructions need to be more clear.

I’m not near my Sense at the moment to launch the app to confirm, but dedicated monitoring supports 4 different types of monitoring. (well more than 4, but lets stick with the simple 4 for now)
1 x 240v device where the 220v device has 120v components. For this you use both clamps on the two leads to the device so it can measure both legs as they will not be the same
2 x 120v devices - 1 clamp per breaker
2 x 240v ONLY devices. 1 clamp on 1 leg for each of the two breakers.
1 x 120 and 1 x 240 device

During setup, I believe you tell the app which method you are using.
When using the 2 x 240v ONLY setup, Sense knows to double whatever it sees on the 1 clamp because it knows its a 240v only circuit, so the draw on both legs would be even.

From what you are describing, you may have missed a step in the setup so the sense is looking for 2 CT’s worth of data on a single device, but you have actually hooked up to 2 different devices

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I haven’t done the setup for my 2 240V loads I’m targeting, but only one CT is needed for a balanced (no neutral) 240V load since the current in each leg is guaranteed to be the same (KCL). Not sure how the setup process configures for 120V/120V vs 120V/240V vs 240V/240V modes.


Not an electrician but I believe with a 240 circuit with no dedicated neutral the 2 leads alternate with one as a positive and the other as a neutral which would explain your situation.

One thing that would be nice in the app is to be able to go back and check on your dedicated circuit monitoring setup to make sure you did do it right. When I installed mine yesterday I’m 90% certain I selected the 2 x, 240v selection but there’s no way to check that I can see. when I click on dedicated circuit monitoring it just tells me that it’s enabled and that if I click continue it will erase my current setup and then I can set up for two new loads. I could do that I guess and just run the setup over again and name them the same thing. Just thinking it’d be nice to be able to check it. On a side note it appears my Mitsubishi mini splits pull 70 Watts occasionally even went off, with what seems like regular intervals

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Thanks for your explanation. Problem Solved!
I did miss something in the setup. The first question when it asked if this was going to be one device or two. I chose one because I was only setting up one at that moment. I ended up deleting the dedicated circuit (lost some history that was previously merged) and redid the setup. With your explanation fresh in my head the instructions went smoother. Everything is working good now, just as you and others said, hooking to one side of a dedicated 220 only circuit it does double the wattage assuming a balanced load. Yes the app set up leaves some to be desired in this added option. I had to just pick another device for the second dedicated probe which I wanted to do later because that load is in a sub panel next to the main panel. It seems if I want to change one probe to another circuit I would have delete and set up both again. Thank you to all that have responded, all was helpful information

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I found this on the wen regarding your spikes. Apparently others have the same issue.

"My Daikin exhibits the exact same 80 - 100W standby load and
the resultant 2 kwh/day or so consumption. The bulk of it goes
to the compressor crankcase heat, which in this case [and
probably most other modern units] is done not by resistance
heating, but by the inverter itself bleeding a little extra
current through the motor coils in a way that creates heat
but doesn’t actually turn the motor. And when it does this
is intermittent, so I don’t see the current draw all the time.

If I listen very carefully at the outdoor unit I can hear a
high-pitched whining from the electronics when it’s doing
this warming stuff, and nothing when it’s not. I also have IR
shots in my house-saga clearly showing that area of the condenser
unit staying warm [which is one of the things that tends to
attract critters into them in cold weather, so screen off all
those holes!]

Ironically, the system still does this in the height of
summer when no compressor heating at all is needed to keep
the oil properly fluid. You’d think they could check the
outdoor-ambient temp sensor mounted RIGHT THERE on the back
of the unit to decide whether to bother with warming or not.
At least the compressor is all wrapped up in a thick blankie
and doesn’t lose all that heat quite as fast…"

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Looking at how much the systems seem to pull when off makes me think when I have an extended period of time when I know I won’t be running them maybe I will just kill the breakers. This is a screenshot of my multi split usage over a few hours. It seems like it pulls 66W basically for a half hour continuously then shuts off for a half hour and so on. So having two outside condensers doing this 24 hours a day seems a bit wasteful. I live in the suburbs of Philly and I Don’t really use the air conditioning or the heat for pretty much all of October.

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So just one of the little update here. here in the suburbs of Philly it’s been going into the mid-70s during the day and down into the mid to low 40s at night this past week and I’ve noticed overnight my mini split will run about 65 Watts on a regular basis every half hour or so. But once it warms up in the daytime it goes down to around 4 Watts and stops. Fwiw…

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