Monday, May 14, 2018

Supercooling a House in the Desert: part 2

Last post I examined how much my house's internal temperature rose over an eight hour period of temperatures that exceeded 100F with no AC at all during the period. That gives me a feel for how much I'll need to cool the house down during the peak period, but to really understand how to control things a closer examination of some other things needs to be done. First, lets look at recirculating the air inside the house between rooms.

Recirculation can be done easily by simply leaving the AC fan on and set the mode of the compressor to 'OFF'. This will move the air around the house and even out the temperature of the various rooms. Essentially you're taking cool air and displacing some warmer air in areas that might be hot spots. I tried this and found it to really works well for evening out the house.

However, I'm really frugal (aka cheap) and having both of my AC motors running all the time was too much for my personal sensibilities. I stagger them so only one is ever running, and I don't run them all the time. Here's a chart to show you what is happening.

Those little steps in there are each fan running as measured by my whole house device. It actually monitors the house's incoming power real time <link>. There's other stuff going on in the house, so the graph isn't a perfect set of steps. If you look closely, you'll notice that one of the fans pulls more power than the other; that's because one AC is a 5 ton and the other a 3, so they use different sizes of motors in the air handler.

There are thermostats that can handle recirculation for you, but they are rare. I had a couple for a while before I built my own and they worked fine. A bit hard to set up though; the manual was many pages long. My plan over time is to get rid of the thermostats entirely in favor of a computer that monitors the temperature in each room and adjusts the fans and compressor for the most comfortable environment. I'm probably a year away from that goal. Folk that don't want to bother with that can look around, there are lots of good thermostats out there.

Note that I DO NOT recirculate during the peak period. I'm just too cheap for that. Instead I use ceiling fans to move the air around in a single room and keep it feeling fresh. No, this doesn't even the entire house out, but it does well in the most used rooms. This is another of the list of personal preference items you need to consider.

Now, let's talk about the high surge current that an AC compressor uses when starting up. It's an absolute fact that most electric motors pull very high current when they first start up. When I looked at my AC compressors, they pull around 4kW when running, but the peak surge getting them started was way up around 14kW. Yes, that's nothing to laugh at; it's a lot of power.

But, it's only for a second or two. Here's a chart that shows the spikes created by starting an electric motor. In this case, it's my refrigerator, freezer and the freezer out in the garage.

I especially like this graph because I caught the garage freezer for the entire starting spike. A limitation of measuring power is that you have to capture discreet instances of power, you can't get everything. I capture it in about 1 minute intervals and often miss most of the rising spike. It's just impossible to grab everything all the time; there isn't enough storage to keep it. The reason I used the appliances is that I don't filter the readings as much. If you don't filter the reading, they're all over the place and you can't get any accuracy at all because of the inherent noise. The whole house power would be a real mess to try and read with the start up spikes from everything in the house showing up.

But back to the spike, notice how much higher it is than the run current just next to it? Here's a cutout of that particular piece all by itself.

While the normal run current is only around 100W, the starting surge is almost 900W, but also notice that it is only for less than a minute. Since the peak demand number is for an entire hour, that's not enough to impact it. The surge peaks of  electric motors is so small in actual power used that it just doesn't matter on your power bill.

There is another reason to avoid the incessant cycling of larger motors though, wear. On something like a freezer it doesn't matter much because they are fractional horsepower motors, but on an AC unit, starting and stopping a compressor is tough on the mechanics. Bearing, armatures, pulleys, etc wear out often enough, we don't want to increase the wear by 'short-cycles' on the motors. When you start it, let it run a bit if possible.

But ignore the uninformed when they speak about the huge power usage of the AC starting; it just isn't a problem. Unfortunately, many of the uninformed are AC 'experts' that we all have to listen to from time to time.

Now, since I mentioned the appliances, let me expand a bit on them. I have separate appliances for the refrigerator and freezer as well as a chest freezer in the garage. These have to run all the time to keep my food safe. I don't want to spend the money on spoiled food instead of power. I've already charted the usage of all of them above, let's look at a stacked chart of their power usage during a typical peak period. Stacking means that I add each chart line to the others so you can see the total usage of all the appliances at a glance.

I also filled the items so they would be easier to see. The fridge and garage freezer use around 100 W each and the house freezer uses about 130 W. Since they don't always run, there are actual periods where no power is used by the appliances, but not very often. The fridge runs about half the time in long periods, while the garage freezer runs in predictable periods, but still about half the time. The house freezer runs the most, partly because it has two defrost cycles a day that it has to recover from opening the door to get ice which lets the cold out. You can actually feel the cold air hit your feet when you open the door on this stand up freezer. Very convenient though.

The appliances create an almost constant draw on power that is roughly 350 watts. Combine this with ceiling fans (a necessity in the desert to keep the air moving) and the usual passive devices and my particular baseline power usage is close to 500 watts ... constantly. Everything else I run just adds to it. I don't think I can get it below that without some risk to my food supply.

It's important to note that the fridge and freezer in the house vent right into the kitchen. That takes the heat that may be inside the appliance and dumps it into the house making it necessary to remove that heat with the AC. This is good in the winter since it will keep the house warm, but not so good in the summer where you have to get rid of that heat. I may someday have to look at that as a factor, but it's a really hard thing to measure.

So, factoring all this with the appliances, the very best demand number I can possibly get is .5kW. That would actually be great, there's always something. microwave popcorn, lunch, TV, whatever makes life more fun and comfortable will creep into my power bill.

Remember, your mileage WILL vary. Your lifestyle, house, habits and the way you hold your tongue will conspire to create your own personal profile. Use this as data for decisions, not as an absolute.

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  1. Thanks for your blog posts. Usually quite interesting (yes... even the heavy machinery :) )
    About this post... althout at lower "residual" power consumption, I found myself in the same spot as you.
    Solution: a couple of cheap solar panels (2x100w) and a Grid Tie Inversor. You may need a bit more that that, but basically it would offset the power consumption in the peak hours...
    I know you're not a big fan of solar, but that worked for me.

    Best regards!

    1. thk, actually, I'm a huge fan of solar. I'm NOT a huge fan of solar salesmen. People have no idea how many solar spam comments I've had to pull off this site. A neighbor and I have been looking into installing a few solar panels to offset the cost of energy, but we're going to do it in an off-grid fashion.

      The list of requirements to install solar with a grid tie system is incredible, because the solar political lobby in my area is forcing it. They were a huge factor of the last rate increase by the power company as well.

      So, folk like you that did it themselves as a project to save money are not only allowed, their welcomed. Solar companies are just deleted.

    2. Whilst the lobby groups may force a long list for you to use a grid tie, it will be cheaper and more efficient in the long term. A rule of thumb in Europe (can't see why it would be different in the USA) is double the unit cost for stored energy vs bought. Decent batteries cost a lot, must be nursed (never more than 10%-15% depth of discharge) levels checked and topped up, cells evened out. Sure you can buy car batteries re-labelled leisure, and they may last the 2 year warranty at regular 100% DOD. As your supply wont be synchronised to the grid, you'll need to install new distribution and contactors and control how that works.
      I agree with thk, and a small GTI and a few panels help with base consumption - it's what I do now. I was off grid 2 years.
      I guess a desert system would be less efficient (compared with other parts of USA) as PV gets less efficient the hotter it gets.

    3. Anonymous, You're probably totally correct in that the long term could create a savings, but around here many of the solar companies are still charging $5 a watt. $300 for a 60W light bulb? Excuse me, no thanks.

      On a forum recently I issued a challenge: if a solar company can actually save me money right now vs. my conservation work, I'd buy it. Strangely, none of them called me.

      I really like the idea of a few solar panels to wipe out my baseline usage of 500-600 watts. That means I'd probably have to have a 1500W system to allow power credits to take care of the nighttime. That could be worth looking into. Especially if I use micro-inverters that could be added to over time.

  2. Hello Dave,

    I really like your blog, I'm from the other side of the globe (the Netherlands), so we don't have this "problems" with dry heat.

    But in your post you talk about these spikes when starting the AC fans's.
    Have you ever thought about using softstarters to switch those AC fan's. This will reduce the start spike. Or when you want to do it very fancy, use frequency convertors, this will eliminate this start spike completely.

    They could even reduce the total powerconsumption from the AC fan's, by letting them run on lower (adjustable) speeds.


    1. I don't envy you your winters. I'm pretty sure you don't envy my summers.

      I actually have thought about using a soft start device for my home, but for this series of posts I want my house to be as much like other folk as reasonable. That includes the nasty spikes.

      I really like the idea of using a three phase motor and a motor controller to control the speed. It WOULD use less power and I could control the amount of air produced by the systems. The problem is cost, but that is coming down all the time. A little one horsepower 3 phase motor and phase controller would be sweet

      Thanks for the suggestion.

  3. A friend recently found a good deal on a 2hp, 3 phase motor on eBay. You might get lucky too if you're not in a rush. He plans to add a VFD for speed control but they're good for soft starting a motor too.

    Now that I'm thinking about it, jcw did a lot of power measurements and optimizations on this blog:

    The real reason I commented was to show off this cool company that monitors the power into your house and uses those turn-on spikes to determine what appliances turn on and when. Judging from your colorful chart, you're monitoring each appliance seperately, but just thought I'd mention it:

    1. Thanks for the link to Sense. That device is intriguing, and if it actually works, could be useful. Too bad they don't offer it in a version that doesn't use cloud services.

      I think everyone know how I feel about depending on a cloud service. It's ok for things like the Amazon Dot, or something that doesn't accumulate data, but I won't rely on a cloud service... I've been bitten too many times already.