Cool Running

We surely do get spoiled by some of our modern comforts, don’t we? When I was a kid, we didn’t have air conditioning at the first 3 houses in which I lived, nor at school. Our first car with a/c was a ’65 Pontiac. Although I don’t crank the thermostat down low, I still like to have it to knock down the humidity and cool down on those muggy Florida afternoons. One of the goals I have been working toward is to get the house’s central a/c on the solar. It is a big part of the power bill, so it is a good goal. I used 215 kwh from the grid this past mild month and would like to keep it close to that through the summer, instead of the 1500 kwh (or more) that my neighbors will use.

A few years ago, as an experiment, I parked Sun King (my solar launch) next to the house and ran a line from the boat’s inverter to a window a/c. It worked great, but a 5000 BTU window unit is not what you need for a two-story house. My first attempt at running the central unit on my 24 volt system almost worked, but only because I was low on freon. All charged up, I couldn’t quite get over that startup surge. It is probably just as well because I discovered that the transformer in that cheap Chinese inverter was just a wee bit underrated and would have melted running the a/c all day.

I know my 12kw inverter would have run it because it worked for the guy from whom I bought it…but lightning fried that hope. My homemade 5548 has a huge transformer and will put out thousands of watts continuously, but it still could not deal with that starting surge. I probably could have tweaked the overload settings, but it works fine, otherwise, and I did not want to break it. Should I give up? Ohhhh, no. In a case like this, we simply need to change the rules.

This surge problem doesn’t just come up when running on an inverter, campers and boaters like their a/c, too, but sometimes end up getting a really big generator to run it. My friend Courtney has a huge bus with a big, rumbly Onan generator. Run the thermostat down and that Onan acts offended when the compressor kicks on. After that first couple of seconds, the Onan settles back into its lazy rumble, but you can tell it doesn’t like the startup.

But, dig around on the internet and you’ll hear guys talking about running a one-ton or even an 18,000 BTU a/c on a Honda 2000 generator! What? I can’t run an 18,000 BTU a/c on a 5500 watt inverter, but they can run on a 2000 watt Honda? Let me give you a hint: they cheat. They changed the rules.

If you are looking to install a new a/c, one of these new-fangled inverter mini-split rigs might be the way to go. When they say “inverter” they mean a variable speed motor controller. A few makers put inverter compressors on full size systems, too. If you’ve not seen these, or paid attention, these units start the motors up slowly and run just fast enough to maintain the temperature you set. It really is hard to tell the things are running! In addition to eliminating the starting surge, you may end up using half as much power in the long run with units up to a SEER 25 rating. For comparison, my 30 year old unit is a “High Efficiency SEER 13” model. You’ve seen the TV commercial with the old a/c unit quivering, rattling and giving off a puff of smoke? That’s mine.

Yeah, but if you are like me and are not too keen on spending 6 grand to replace something that still works and will run on FREE electricity (assuming I can get it to run at all) you may want to “cheat” like those campers and boaters are. You see, there is a thing called a “soft start” that emulates the slow spinup that the inverter units use. They sell under the trademark names of Dometic SmartStart and Hyper Engineering Sure Start. They aren’t cheap, listing around $600, but you can shop around. I found a couple for my units for $100 each from a marine salvage dealer. Buying something used without documentation may not be something you’d feel comfortable installing yourself, so you might hire that job out or get your HVAC dealer to come up with one.

Changing the Rules: SmartStart on my A/C

When the time came to give it a try, one of the golf cars was charging and the refrigerator was in auto-defrost mode, so there was already a good load on the inverter. I ran down the thermostat and the lights gave 3 little flickers as things began to run. The starter has a microprocessor that learns the load and afterward the startup was so gentle you had to watch for any flicker at all. I would say it dims the lights less with everything on the solar power than it did when everything was on grid, without the starter. Once underway, the compressor is pulling maybe 1800 watts and plugging in the golf car or firing up the Keurig while the princesses are watching Disney in the living room still leaves power to spare. The one unit upstairs will cool the whole house, but I will go ahead and convert the downstairs a/c, too, when I get the 10kw inverter built.

What about nighttime? I do not have enough battery to run a lot at night, but I have a plan. Using the programmable thermostat, I will really cool down the house during the day. When you are active, a little cooler feels good, anyway. Then, late in the afternoon, I’ll run the thermostat up a few degrees. The thermal mass of the drywall will hold that cool for quite a while, maybe even until morning, if it isn’t just awfully hot. I have no doubt the average house temp will be cooler than it has been when I was paying for those overpriced kilowatts from my power co-op.

So, success at last! The only problem is we have a cold spell for the next week or so and I don’t need a/c! That’s ok, the heat will come, eventually, and I will be ready. What about you? Are you up for getting a little more comfort out of your solar power?By: Neal Collier