Whoa… I think the AC not starting is only part of the problem and I’m not talking about technical issues.
3.5 tons at 13 seer is going to have a running kW of 4kW, give or take and on hot days that’s going to take the battery to 50% in less then 5 hrs assuming no other loads are on. We’ve recently looked into this for a customer that spontaneously added 3 ton 13 seer unit to his part time off grid system without telling us until it was already installed. Of course it was never part of the original design but that’s an entirely different story. Thank heavens we have a good relationship with this customer further bolstered by contract clearly detailing what the system was designed to do. According to everything we can find and the HVAC installer you need a minimum of 5 to 7.5 times the running current to start an AC unit. A “hard start” capacitor will not help because all it does is give the AC unit more amps to start but do not actually reduce the starting surge. Apparently as AC’s age they can actually get harder to start so these units help with that. I’ve not come across the “soft start” you mention unless your talking about 3 phase stuff. The Xantrex XW4024 at this project will not start his 3 ton unit. But of course, it’s not that simple. This is a tiny off grid system with one string of L16’s. When the AC attempts to start we observe the battery voltage dropping on the fully charged battery bank to 21v while the kW reading on the XW never goes past the mid 2’s. While this is happening the XW’s voltage drops L-L voltage drops to 156v. The inverter does not fault out and only shows a Low battery voltage error message. So my theory is the XW can’t hold the 240 L-L during the surge which inherently reduces the surge thus self limiting the AC unit and preventing the inverter from going into a fault mode. But that’s just my guess. Xantrex technical did pick up the phone but they were pretty puzzled why the inverter didn’t go into fault mode. Our solution is to install a 17kW generator and run the AC off the generator only. The generator and a propane tank are a 100% solution and they were cheaper then adding inverters and batteries which we were pretty sure had a pretty good chance of not being a 100% solution. Since it’s a rarely used off grid cabin the propane usage wont’ be terrible. IMHO, Max, I suggest that you don’t even try to run the AC off of the inverter or even think about adding a 2nd one because it’s always going to be problematic. Best, Travis Creswell Ozark Energy Services _____ From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Max Balchowsky Sent: Monday, August 18, 2008 3:01 PM To: RE-wrenches Subject: Re: [RE-wrenches] offgrid system question Dear wrenches, going to look at a system installed by others and would like some input from some of you with more off grid experience than I have ( 4 systems over the years). the system was put on a new home - off grid 20 sharp ND123UJF panels 4 modules in series - 5 parrallel strings, 2460 total array dc voltage. mppt 60-150 charge controller, 12 sun extender sealed AGM batteries, 305 AH at 12 vdc, wired for 915 AH at 48 volt DC. The system is tripping off when the AC unit trys to start - AC is comfortmaker 3.5 ton 13 seer. They say start up inrush is 104 amps (sounds high to me) and they put a DST 10 soft start on the unit (I think that's just a big capacitor). I haven't been on site yet but it sounds to me that they just didn't design enough inverter capacity into the system...... What are your thoughts? Max Balchowsky SEE Systems No virus found in this incoming message. Checked by AVG. Version: 7.5.526 / Virus Database: 270.6.4/1617 - Release Date: 8/17/2008 12:58 PM No virus found in this outgoing message. Checked by AVG. Version: 7.5.526 / Virus Database: 270.6.4/1617 - Release Date: 8/17/2008 12:58 PM
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