Nomad Joatman is a DIYer and motorhead who is HVAC and air conditioning certified. He lives on a rolling lab of sorts. Folks looking to buy back up power should understand the importance of load management before they buy a generator, less is usually more if you do it right.
Here’s an email sent to me by the Nomad:
Think of it this way: A 3 ton A/C is 36,000 BTU and requires a compressor motor that puts out 4.5hp to 5.5hp. That’s going to eat up a 6/1’s hp to start it, but it will start it. The biggest hog at the trough is the big central air systems. They cool an entire house, regardless of whether someone is in the room. We need to change our thinking a little here. Let’s step back to the days before central air. Window units were the rage. You had one where you lived, whichever room that was. The bedroom got one, you spent 1/3 of your life there and you slept comfortably. If you had one in the parlor/living room, you were up-town! We sold people in the late 1960s on the idea of going to central air for the convenience. Yeah, we did the numbers game….totaled up the LRA & RLA on all their window units compared that to the central unit we were selling and “showed” them how inefficient their window units were, never mind that they were never all on at the same time trying to cool areas were they weren’t living. That was my first experience with “numbers can mean whatever you want them too.” I digress….
My “house” is a 40′ Motorcoach conversion. I cool it with a 2 ton or 24,000 BTU 2 stage heat pump that is powered by a 7.25KW Chinese diesel. My generator has NEVER loaded down! The key here is that the unit is two stage, having two small compressors. The thermostat demands cooling, the first compressor and fans kick on, if after two minutes the temperature hasn’t dropped, the second compressor kicks in to lend support. Each compressor is 115VAC and I run them on each leg of the 220VAC, balancing the load on the system.
Now, the cost initially for the system is usually higher. Let’s face it, there’s more hardware involved. Two compressors and some added controls, but the advantages are clear. Instead of taking one big bite, you take two smaller ones. A couple of the big A/C manufactures started experimenting with variable speed compressors a few years ago. They we’re a turbine type of compressor rather than a piston driven. The advantage was a spin-up start rather a big jolt to get the thing going. They were called “Scroll Compressors” and their start up was a lot more gentle. The downside was the same as a jet turbine…Noise. Carrier actually came out with a “blanket” to wrap them to deaden the noise. They may still make them, I know they sure promoted the heck out of them.
There are some of the big A/C companies that are working on variable speed compressors. There may be some out now. Same principle, start slower and increase speed and output as demanded.
Now as for your “customers” that say they need 15KW to push a 36,000 BTU or 3 ton system….Bullshit! I’m reminded of a CD I bought from a so called “Guru of bus conversion.”
When sizing a generator for your bus conversion he told people to add up the wattage of everything in their rig, add 20 percent and buy accordingly. Now I ask you, are you going to run everything in your house at once?!?! I actually saw a rig that a guy (using the CD) had done a conversion on an MCI bus. He had a 20KW diesel genset that weighed 1100 lbs mounted in one of the storage bays! It took up half of a 6′ x 7′ bay and was so loud even with sound deadening and a custom muffler, that you couldn’t have a conversation near his rig……I digress again.
My belt drive compressor system will utilize three automotive compressors. I will use time delays (variable voltage & time) to sequence them on at 1 minute intervals. All of it is over the counter (OTC) stuff, if you know what you need.
I need to correct something or rather clarify:
(1) Locked Rotor Amps or LRA is not a measure of starting requirements on a compressor. It is how much amperage a compressor will pull if the rotor locked in failure before the compressors internal circuit breaker will trip. It’s valuable in sizing the line wiring and breaker requirements, but is not how much amperage a unit will pull at startup!
(2) A “soft start” kit is not the same as a hard start kit. A soft start kit actually reduces the amount of “kick” a compressor receives to start. They are usually used in situations where there is a loud compressor start-up to quiet a noisy compressor. They are hard on the start windings and will kill a compressor quicker.
(3) The start capacitor can be increased safely by 20 percent in both VAC and MFD. But only the start capacitor, not the run capacitor! The spike on the start will be so quick and short that some recording devices won’t even register it.
Let’s not air condition places we aren’t living. Short of wearing space suits, the next best thing is window units. They condition the areas where we are, not where we may go. They are a simple method of sequencing, taking little bites, instead of one big one.
Use time delays on A/C’s. When a compressor runs the pressure on the “high side” is running about 270-325PSI and the “low side” is 70-100PSI. When the system shuts down it takes time for the pressures to equalize. The longer a compressor is off the better. That’s why the owners manual on any refrigeration product recommends a 3 minute delay before re-starting. If you don’t wait, the system is starting against a load. Use a delay on break and set it for 5 minutes or more to reduce start-up load. The delay on break will keep kids and the relatives from short cycling the system.
That’s all for now George,
Oh I almost forgot…..
My system for using the waste exhaust for air conditioning……well, think about two drip coffee makers; one leading to the other, one’s outlet goes to the reservoir of the other and vice versa. As one empties the other fills, the full one switches on and the empty off until it’s full again……think about it