Water Heater thermostat setting (Now: how water heaters work and half detecting 220V devices)

My water heater has two heating elements, each with it’s own thermostat setting. I would like feedback from Sense on how the individual elements are running. If one is running significantly more then the other that could indicate a thermostat set incorrectly or malfunctioning.

EDIT: post moved from Product Suggestions to Device Detection

Is this a tank water heater?

If so, they are configured that the upper element always runs more than the lower element. This is because the lower element cuts on only after the upper element cuts off. And the upper element always comes on first because hot water exits from the top.

The vast majority of water heaters are wired so that only one element at a time can be turned on. That is normally the bottom element. The upper element rarely turns on except in cases of extreme water draw down such as at the end of a long shower. In this case, cold water coming in the bottom of the tank has not been warmed enough by the bottom element and makes it way to the top element which then turns on and also turns off the bottom element. The bottom element is typically of a higher wattage than the upper element.

Some heaters are designed for “simultaneous” operation where the elements operate independently and both can be turned on together. This means that the power mains feeding the heater must be sized to handle the increased power of both elements turning on at the same time. Its not a particularly common configuration…

So if you have a normally wired heater, you should see the bottom element doing most of the heating which is not considered a failure.

You have this backwards. It is the lower element that is turned on second. Here’s some information on the sequence of element use in electric water heaters:


Interesting, thank you both for the education…my OP clearly shows my ignorance of the topic. I always thought they run at the same time typically.

Now that I understand it better, I think dmagerl is referring to figures 3 & 4 in your link…when the tank is hot and cold water is introduced, the bottom element kicks on first. Seem you are referring to figure 1 where the tank starts cold and the top element kicks on first.

Very helpful though, thanks both!

You’re right the top element turns on first when starting with a cold tank. Once the top of the tank reaches temperature, the top element turns off and switches to the bottom element. From there on, as long as the top of the water column stays hot, it is the bottom element that switches on and off.

The top element usually never cools down enough to turn on again unless there is a large draw down of water and a lot of cold water floods the tank.

The bottom element is the workhorse of the tank.

A number of people, me included, have seen where Sense seems to report half the power draw of heater, almost as if its reporting only one leg of the 220v, It could be that the upper element turns on and it is lower power than the bottom element. But Sense reports almost exactly half the power so it seems it really is only one AC leg being reported and not the upper element.

Yes. That explains it.

I have two 50 gallon electric water heaters that are controlled by zwave relays. I turn them on for just 45 minutes a day. So in my setup, the UE always kicks on first, runs for about 15-18 minutes, and then the LE of active for 22-25 minutes. So frequently, both elements shut off before the relay is turned off.

Here’s an example from earlier this week. Water heater came on at 7:00 AM and the elements were off before 7:45 AM.

Controlling my water heaters has saved me a lot on my electric bill over the last three years.

Yea, something like that is happening for me…that’s what had me confused. My Sense detected two different devices, one “Water Heater” and one “Heat2”. My water heater says 4500W on the side and the two devices are both close to 2250W, so I thought they’d run together and add up to 4500W.

Nope, they take turns running and in fact Heat2 runs WAY more then Water Heater, so it must be the bottom element. The problem is it’s only showing 1leg of the 220 in each device…the other leg shows up as a 2250W bump in Other.

Anyone had any luck resolving this? Does it help to delete devices and have them re-detect? I’m hesitant to try that cause I’d REALLY like to track my water heater usage and I’m afraid it’s not going to come back if I delete it! :slight_smile: …even if it only tracks half the actual usage, I can easily double those numbers and figure it out.

Do you ever miss the on-demand hot water?..washing hands, etc? It’s a great idea and saves a ton of electricity, but I dunno if I could talk the wife into it! :smile:

…maybe once I have hard data on how much that on-demand hot water actually costs…

We’re out all day - and have never felt the lack of hot water. But we do have 100 gallons (2 heaters) and live in a warm clime.

Very expensive to install (thousands of dollars, depending on capacity…e.g. hot water flow requirements, how much re-plumbing is required, whether you can do it yourself, etc), quite efficient to operate (no standing tank losses. Also wouldn’t touch on-demand electric (those are basically a cruel joke based on my experience) and would be shy about propane, depending on where you live. Up here in New England propane is quite pricy.

I deleted my water heater because I thought it was not read right, it has been two or three weeks and has not re-detected it, at least it is not showing up. I wish I has watched it closer. At first it look like it was reading right and like when taking a bath it was recording about 4500 -5000 watts use, I hope this helps.

This happens with most of my 220V appliances from time to time - only one leg is detected. I’ve seen it with the water heater, dryer, as I write this the oven is running at 2800 watts but reporting 1400.

It would be a fairly simple fix in the Sense software to allow users to declare a device as 220V, which would tell the unit to automatically set both legs to “on” if one leg is detected as “on”. Alternatively this could be a simple “if-then” rule in the programming: IF a device is ever detected as drawing an equal current from both legs, THEN it can be assumed that it will always do so. I can think of few exceptions to this rule; some devices use both 120V and 240V power, but the 120V loads are typically minor compared to the 240V loads.


I’m having the same problems with the timeline graph showing half the actual wattage. Mine shows about 2300 like yours but if you look at the bottom right of the graph at the total current usage, the number will be correct there, reflecting the full 4500. As far as I can tell all the calculations in the device page of the app about usage and costs are also reflecting the correct amount. The problem just appears to be with the timeline itself

To make sure I’m understanding correctly, your water heater device bubble shows 2300 but all calculations and power meter data on the usage page indicate a 4500w draw?

Can you post some screenshots?

Here are screenshots of it turning off showing 2312 watts and totalwattage of 5635. Then immediately after where it shows total at 999 watts. 5635-999=4636. This happens with many appliances and not just the 240 ones, also with 120 devices.

I have sent reports through the app many times with no response about this. Not every time it happens because it’s almost always. I have a device that is not detected which is floor heat that I know operates at 540 watts of 120ac. I can see it on the timeline show 240-270 as this comes on every minute for just a few seconds. The timeline bubble will be off but the total will be correct with this device. I originally though it was measuring just one leg for some reason but it appears that is not the case.

Ah, ok. I see. But the Usage screen or the Water Heater device under the Devices tab isn’t showing the full 4500W, right? That doesn’t sound like a UI or timeline problem, if so, but rather just that Sense hasn’t correctly IDd both legs of the water heater.

It has to be software otherwise the timeline and the device screens would reflect the same information. It appears the device screen is correct and it’s just the timeline and the tags there that are off. It has to be picking up both legs for the full 4500 to be calculated anywhere. I had originally though it was a problem with one leg but that does t explains 120 devices with the identicalreporting of half the wattage.

I might be misunderstanding then. Can you post a screenshot of the usage stats for the device? The images you posted show the 2600W being detected as water heater and I’m guessing the other 3000W are relegated to Always On, other devices, and Other.