Sense for AC diagnostics?

This is really mostly a call for help from folks who understand HVAC. We have had a couple days of unseasonably hot weather so my 2 AC units have been madly running. Initially they weren’t helping much so I had the AC guy out and he put 2-3 pounds of refrigerant into both of them (20 year old single stage units). Now they are cooling again, but the larger compressor occasionally pops the 20 yo circuit breaker in my main panel. It has happened 3-4 times so far, so a I have been using Sense to try and watch for when the breaker pops somI can reset it and so I might get some insight into why.

Here’s the AC compressor breaker - It’s the ganged 50 in slots 2 and 4.

Here is the breaker tripping around 3:40 AM. Note that the other AC compressor keeps going (slightly smaller and slightly lower power consumption 4000W vs. 5000W)

Here is me resetting the breaker this morning. It went for a bit, then tripped again. I reset it once more and it’s been running nicely.

Any thoughts or opinions on why my breaker is tripping ? The HVAC guy is coming out this aft. He thinks replacing the breaker should fix things.


Ask your HVAC guy about a “Hard Start Kit” for your AC compressor. Link below will help explain it and why you would need one.


Thanks… I will ask him. I’m not sure the issue is associated with startup though. It seems to happen minutes to hours into compressor operation.

Really curious to follow this thread. Let us know what happens. I ran my AC briefly this past weekend just to test it and the breaker tripped. It ran fine after I reset, but I haven’t had much of a need to run it since.

another consideration is that when breakers are old and going out, they trip easier and sometimes for no reason… my dad’s air compressor used to trip the breaker all the time on initial startup of the pump… changed the breaker, no issues since… sometimes they just wear out…

I"m not an electrician, but my dad is an HVAC guy his entire career… he says breakers wear out, they don’t last forever…


Here’s another trip and reset cycle after running for about 3 hours… Sometimes, the breaker trips immediately when I first reset it, but stays on the second time (at least for a few hours)


I do find Sense useful, at least for watching for breaker trips.

My thoughts (same as other comments):

  1. Need a “run start capacitor” - Hard Start Kit
  2. Need to replace the “run start capacitor” - They also wear out.
  3. Replace the 50-amp breaker.
  4. Too much Freon added.
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Thanks ! This is 20 yo unit so it comes with a fairly sizable run start capacitor. The HVAC guy replaced the run start on the other unit because he could see it starting to bow outward, but thought the downstairs unit was OK.

He replaced both breaker and run-start capacitor today, so if things are fixed, we may never know the cause. But it was a cheaper route to go than two separate service calls.


So your HVAC guy did a visual test and didn’t perform an electrical test using a meter on the Run Start capacitor?[Testing a Run Start capacitor]

Maybe this YouTube video can assist him on his next call-out.

Good point… But the bulge is also pretty much a giveaway for a failing capacitor.

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After working valiantly for two unseasonably hot days, with a couple breaker resets required (after the new breaker and freon was installed), the compressor in our downstairs unit has given up the ghost. The AC guy believes that the compressors end-of-days was coming and that pushing the unit to run at-spec for two very hot days accelerated it’s demise.

I recognized the compressor failure when the on-cycle for the compressor was only showing a 1000W (compressor fan only) uptick in the power vs. 4000W when fully functional. The repair guy identified the failure via the burned out prongs on the plug into the compressor and lack of compressor noise. Sense never identified that AC compressor because it hadn’t run very much since my April reset, so I don’t have a good view of whether the waveform changed.

Update: New upstairs and downstairs compressors installed on July 9th ! Working like a charm.

Now I’m going to see if I can use my Ecobee data to isolate all my compressor starts in the past two months to see if the waveform changed over time. Since Sense hasn’t detected it, I’m going to need to rely on the 5 min resolution of the Ecobee export to narrow down my search. But first, I’m going to double check that Sense and the Ecobee data correlate because I’m suspicious of a few data issues:

  • Sense data dropouts - Not all my HS110 data has been picked up over time by Sense, and made it into Sense export - I have seen some dropouts but I really don’t know how many relative to the total volume of data. My plan is to compare my HS110 ‘Furnace Down’ hourly data against aggregated hourly Ecobee furnace fan runtime.

  • Ecobee data dropouts - My Ecobee also has data dropouts due to it rebooting when my furnace overtemps (not enough airflow) and short cycles during heating. But the good news is that Ecobee inserts NA (not available) in the data when this happens, so I at least know there has been a dropout and can choose to ignore that chunk.

  • Different types of fan cycles - Sense actually ‘sees’ slightly different cycles via the HS110, for heating, cooling and fan-only. Each uses a slightly different power profile.

But it looks as if things correlate reasonably well most of the time. Heating, as expected correlates less well and is missing a number of points (from Ecobee dropouts), but it also seems that the basic fan cycle (5 min every hour) has some discrepancies as well.

You can see the vertical lines at 300 sec (5 min fan run time) and at 3600 sec (fan run for a full hour). But the line of points is relative linear and indicates a furnace blower that runs at about 500W (or 0.5 kWh/h)

If I add a little horizontal jitter into the points, you can see how many datapoints are really clustered into the green and blue vertical lines, and that there is also a very strong horizontal line, underlying the vertical green line. There are lots and lots of points on top of one another at (300 sec, 0.036kWh), reflecting nominal 5 min furnace blower operation per hour in ‘Fan’ mode.

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I dealt with replacing hard start capacitors on our single unit for about five years before I broke down and had the unit replaced last October before cold weather came. Once they start eating capacitors, they are on their last leg. I wish we had replaced the unit sooner as there was a huge decrease in electricity use as soon as we did. This was pre-sense so I don’t have historical data except for electric bills

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I’ve been in the hvac field for 15 yrs. A breaker trip is usually the compressor pulling high amps. A few things could cause this. 1 failing compressor 2 high head pressure 3 bad run start cap. Since he changed the run cap already let’s go to the next possible cause the High head pressure. To fix this problem all your hvac guy needs to do is clean the condenser coils with a power washer. To check a failing compressor he can mega ohm the compressor and that usually will tell you what shape it’s in. Hope that helps


Thanks for your answer. The HVAC guy did indeed check the head pressure and the answer turns out to be #1, a failing compressor. It gave up the ghost after marshalling through two of the hottest days on record here, with a couple more breaker resets. Thank goodness we’re back to cool nights and reasonable days. Now it’s time to search for a reasonable replacement.

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In regard to the immanent replacement of your

I’m excited for your upgrade story and the improved-efficiency charts you’re going to post! :wink:

I was recently involved in a new HVAC install for a sizeable city apartment and was surprised that the MEP spec’d a single big compressor … an 800lb Daikin beast that had to be manhandled into the building and up stairs vs craning to the roof due to various practicalities. I was pushing for using smaller ganged-up Mitsubishi compressors to both make the physical install easier and to create some kind of redundancy in the system. Not that it may apply in your case, but it seems not all engineers lean toward “distributed systems”.

That was also my first experience with sizing HACR (HVAC-specific) breakers.

On another note, I think your story is good example of a case for Sense having a “spike alert” and by extension the ability to tag a potential “breaker trip” and alert you to the fact.


I recently replaced our unit (October) and got different and very wild prices. Our electric cooperative wanted $10,000, 2 contractors at $8,000 and the low ball at $4,800. I would usually show from a low ball like that but went with him. He had all the same qualifications, licenses and insurance as the others. The unit he was selling me was an American Standard/Trane that was 1 deer rating less. Doing the math on energy saved on one Seer wasn’t worth the extra $3,000 in cost.
The low ball was also recommended to us by two friends that were very happy with the unit’s he installed for them in the past ten years for one and three years for the other.
If I had stopped at just three bids, I would have lost a lot so shop around.

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I use the AHRI database to compare what’s available:

For my own small-scale install, for cooling & heating, I settled on using 2 completely separate Mitsubishi mini-split heat pumps vs 1 larger dual-zone unit because the compressors are easier to carry up stairs and there are added efficiencies when considering the refrigerant pipe lengths (minimized). 33 SEER & 13.5 HSPF. If one system fails I’ll also have something working. My space is only 600sqft.

Just wish it could selectively dump heat into my hot water (there’s an “Air to Water Heat Pump” that Daikin makes but it’s way too big for my space)

Th Mitsubishi splits are great but I’m wondering Why 2 systems for 600 ft? Even the smallest system could easily handle that area. Do you ha e two rooms with poor circulation?