Useful aspects of the eMonitor

Tom DeMarco wrote, "you can't control what you don't measure" (widely misattributed to Peter Drucker BTW). Well hell, there's lots you can't control no mater what. A less ambitious implementation of measuring is to aid in understanding how a system operates. The eM is good for this.

One useful feature is the setting of Alerts. The eM software offers a lot of Alert choices. It can tell you when your electric cost for the month passes a certain level. I like the ones such as:

- Notify me when a particular circuit is drawing little or no power for an extended period

- Notify me if it looks like my refrigerator or freezer door was left open

- Notify me when a particular circuit or appliance seems to have been on for an extended period

- Notify me when a piece of cooling or refrigeration equipment is short-cycling, a condition that is likely to require a repair.

- Notify me when one of the selected piece(s) of equipment uses at least % more energy than it did before

On one of our SMC projects, we got an alert that the water heater was using more energy than typical, and we traced it to a hot water leak. One aspect of Alerts is that you need to be prepared for false alarms. My eM tells me almost daily

"It looks like your 'Refrigerator' door may be open; you might want to check it.If the door is closed, check the seals. This alert is triggered by a compressor that is staying on longer than usual, so if it is not a door or seal problem it could indicate another problem with the unit."

It doesn't seem to be used to modern refrigerators, which draw less power but run for longer when they are on. Also, I get an Alert that tells me

"Your SiteSage service has detected that your Minisplit heat pump circuit is not drawing any power or the expected level of power."

That's because I have a superinsulated house with good solar gains on sunny days, so the heat pump turns off most days by mid-morning, and doesn't come back on until the evening some time.

Using the eM I can see where our energy goes. I can tell if we left the basement lights on all day. I can see how much energy the preheater uses in the HRV to defrost the core. It's been most useful in diagnosing an issue with the Fujitsu heat pump.

At House 5, the outdoor compressor unit was right outside the living/dining room, about five feet from where we sat for reading or eating. We virtually never heard it run. In the new place, the compressor would periodically whine so loud we could hear it all over the house. Using the eM, this is what a day's worth of energy usage looked like:

Here I selected a day where there were minimal solar gains, so the unit ran all day. The outdoor temperature was around freezing. The pattern showed that roughly once an hour the heat pump would ramp up over seven minutes to absolute full output, in fact, the input power measured of about 2,400W was about 1/3 higher than the unit is rated for. I verified that the eM was measuring correctly by putting a ammeter on the line - it measured just under 10A, correlating nicely with the power the eM measured.

The nifty aspect of inverter-driven minisplit heat pumps is that they modulate their output depending on the building load. And running at a reduced output, using the same heat exchangers, is more efficient. And running the indoor and outdoor fans, and the compressor, at reduced speeds is much more efficient. So my unit was heating the house fine, but using more power than necessary and making a racket. How that got fixed is a whole separate story, but it was the power traces from the eM that convinced Fujitsu tech support that there was something worth looking at here. Here's what the energy use looks like in the unit running properly:

Yesterday also had little solar gain, and was about 45F outdoors. The heat pump's peak power draw here is 400W, and it averaged about 200W over the twenty-four hours. It is likely running at a COP of about five! And it is quiet, too.

After we got these control settings fixed, the heat pump used less than two-thirds of the energy per heating degree day than it did when it was cycling to full output then off. The eM helped us in the process of understanding this and ultimately solving it.

Another useful application of the eM is to be able to see how much power appliances draw when they are nominally off. I deliberately try to avoid appliances with electronics and clocks, because they draw parasitic power all the time. We own a microwave, toaster oven, refrigerator, chest freezer, dishwasher, clothes washer, clothes dryer, and heat pump water heater that all have no displays or clocks. According to the eM, none of them draw any power when off. In contrast to that, here's the March 2014 power trace for our induction range:

On thirteen of the days we didn't use the range, and because it draws about 6W continuously, on these days it uses 0.13-0.14 kWh. So the monthly baseload is about 4 kWh. This month, that will be 28% of the usage. If the eight appliances I listed above all drew 6W continuously, then we'd consume about 450 kWh/year in parasitic power (or what some people call phantom loads, or vampire loads). That would approach ten percent of the total anticipated annual energy usage here. I'm told some cable boxes draw 40W continuously!

 

Comments

[John Semmelhack]

I'd love to hear about the fix on the Fujitsu heat pump when you have the time!

[Eric]

I'd be interested in hearing more about the heat pump fix as well.

And what do you find for standby power on your unit? My 12RLS2 unit seems to draw around 40-50W this winter, although I haven't tracked it for very long. It's higher than I expected...

[Eric]

I'd also be curious to know what your home's lowest power readings are, i.e. the cumulative "always-on" loads. Right now mine's around 150W, higher than I'd like (that includes a server that's always on in the basement).

[Marc Rosenbaum]

I haven't drilled too far into minimum power draw. I've seen numbers like 42W on the Emonitor, and I know about 18W of that is the HRV, 6W is the induction range, and about 15W or so is the cable modem plus the eMonitor itself.