Front A/C BTU Output-> Adding Rear 110V A/C

[Stratosurfer]

All,
Does anyone know the BTU output of the front A/C on my Tahoe? I have no rear air and on hot days the front air barely keeps the box cool. And please don't laugh at me (too hard) but I'm considering adding one of those portable 110V A/C's back in the cargo bay. My kids are concerned I'm turning into Larry the Cable Guy as they say in some movie he has a window unit in the rear-sliding window of his truck. Not that crazy yet but the portables actually are very efficient and if I can move the rear tire outside I wouldn't lose any cargo space. I'll run it off an properly sized inverter.
I need to know the OEM unit's output to match the rear unit in output, therefore if the front unit fails altogether I'd have an aux unit that should pick up the entire load.
I believe the front dash unit is around 12,000 to 15,000 BTU's but can't find any data.
Thanks

PS (I'm seriously considering putting an R/V Low Profile roof air on the Burb when all this a/c starts to go south. Carrier now offers a 15,000 BTU unit that is only 7.5" tall; I don't think that would ruin the lines of the Burb. I could even duct it in to a newer style headliner if I wanted to update the headliner.)

[DmaxMaverick]

I don't know the rating of the OEM system, but an HVAC tech can calculate it according to evap/cond core sizes, gas volume, pressure and valve size.

Rather than trying to match the BTU, you need to match a unit according to your electrical ability. For example, a 15K BTU unit will require greater than 4KW. That's a lot of inverter! And, a single alternator won't be able to provide the juice. You'll need duals, and they will have to be high capacity. And, you'll also need higher capacity or additional batteries. Ultimately, you will spend 3x as much on the electrical system than the A/C unit. And that doesn't include installation.

If you go forward with this, the window unit is a bad idea if you intend to run it while going down the road. Window units don't work well unless installed level and stationary (not bouncing down the road at various angles). You should use a unit that's designed for it. You can get a Dometic, Coleman, or Carrier unit designed for RV's for a comparable price to the window units. I just helped a friend shop for RV units, and a 13K BTU Dometic was less than $500. And, placement on the roof is also important. It must be installed in a positive air stream (or the condenser fan will be fighting ram air).

Add to all that the additional load on the engine. Whatever BTU rating required will have to be generated by the engine. The BTU's can be translated into HP, also taking into account parasitic mechanical and electrical coupling loss. Good inverters are only about 75% efficient.

[Stratosurfer]

You should use a unit that's designed for it. You can get a Dometic, Coleman, or Carrier unit designed for RV's for a comparable price to the window units. I just helped a friend shop for RV units, and a 13K BTU Dometic was less than $500. And, placement on the roof is also important. It must be installed in a positive air stream (or the condenser fan will be fighting ram air).
DMAX,
Thanks. Yes I've considered most of what you mention, the biggest bit of data giving me hope this is possible at a reasonable is the Daimler (formerly Dodge) Sprinter vans. I notice the R/V style A/C on the rear of the passenger setups and went online and read the manual about these. The Dodge/Daimler Sprinter has a dash air, plus a rear R/V A/C running from an inverter. The manual even says that you can run the roof air with the engine off for as I recall up to 30 or 45 minutes on fully charged batts.
I had thought the newer inverters were more efficient than 75%, anyhow I looked at the replacement unit for the Sprinter Vans and they list for over $2,000, so maybe they're extra special (efficient).
Most of the portable A/C's I look at are 12,000 BTU pulling about 12 AMPS on the AC so about 150 on the 12V plus the inefficiency, so 200? or so. I suppose an extra alternator would be appropriate. I wonder if the Sprinter Vans have two?
When all is said and done, the auto industry has been looking harder at 'package units' for auto a/c. If a ducted R/V a/c was on top of my Burb, after initial installation, should the unit present problems, $600 buys a whole new unit! OEM's know this and are heading this direction. You can barely put a compressor in an std. auto A/C system for around a $1000.
Oh well, let me muck along and see what mischief I can get into...

[JohnC]

It takes a lot more capacity to start a compressor than it does to run it once started. I'd plan on at least 2 times the rated draw, maybe even 3 times. If the system can't deliver enough starting current the compressor motor with burn out early even if it does start.

[captedge_tn]

I did this with my van - I toyed with the portable A/C unit but ended up using a window unit (I'm ashamed to say...) because I couldn't bear to give up the space inside. I also rejected the R/V units as I had exactly 0" extra of clearance to get in my garage, and the thought of cutting a large hole in the roof of my non-leaking van wasn't too appealing, either. I used a small 5000 btu unit and built a little shelf to hang out the back window and strapped the whole thing with 1.5" webbing to my spare tire holder. It takes just 2 screws to remove the window and I cut a thin piece of luan plywood to replace the window and fit around the A/C. It takes about 15 minutes to put it in - works great going down the road if you want - it all survived a 6 week trip to Newfoundland and Labrador last summer/fall. The A/C unit was $90 at Lowes, but of course that was the cheap part. This unit draws about 330 watts running, about 650 to start up. I bought a Samlex/Cotek 1500 watt sine wave inverter (do NOT use a modified sine wave, you won't be happy and your electronics won't be either...) for about $600 from the Alternative Energy Store, 4 Optima yellow top batteries (wish I had 6) about $160 each at Costco (you will want these co-located with the inverter, needs to be this type or AGM if you install inside, as they don't outgas explosive and corrosive gasses) a battery combiner from Yandina (under $100) to charge the batteries while the engine is running and separate (from the starting battery) them when off. My stock 100 amp alternator was adequate for the job. I could (should) have gone with 8000 btu for the extra cooling capacity. You will need to run a battery cable from front to back with switches or disconnects each end and/or fuses (MPJA.COM) , working with a hot cable is just not fun and it can always be hit by road debris or ???. You will also need a large fuse (130-150 amp) between the battery and the inverter, a bunch of short cables and two terminal strips. A voltage meter wired in to monitor it all. Ground can be picked up from the frame. I could run my A/C for 2-3 hours with the engine off no problem, great when you're tired and it's 90 deg at the rest stop. Of course, with all this power I was able to run a 1100 watt microwave, an Engle refrigerator, my Senseo coffee maker, lights, computer, DVD/VCR (not all at the same time, mind you). It worked so well I brought along a 1600 watt inverter generator and NEVER used it. I also have a 30 amp IOTA battery charger to charge the batteries when stopped/at home/in a campground for extended times. I tried hard to avoid the Beverly Hillbillies look (well, I DO live in TN) but it IS a small house window unit and it does restrict visibility. The portable unit or a small roof-top unit would run great with this setup too. I used a Kill-A-Watt meter to measure the power usage of all of my appliances to size the inverter, at $30 it's a bargain. If you want I could take some photos and try and figure out how to post.

Scott

[arveetek]

Have you tinted the windows on your Tahoe? I did on mine, and I love the effect it has on the a/c system. I have no trouble keeping the whole compartment cool from the dash a/c now.

Another problem with using an RV roof a/c is that they're terribly loud when you ears will be right be the exaust and return air. They're not too bad in a ducted trailer or motorhome, but it'll be pretty loud in an SUV.

Casey

[rustyk]

Frankly, the 120VAC A/C will be impractical. Starting current would be in the vicinity of 10-12A (6,000 BTU would be plenty - my RV's 13,500 btu keeps a 28' motorhome cool in 105°F weather), and a 2KW inverter would handle that well, but to be safe (not frying the 120VAC A/C compressor), you'd want a pure sine wave inverter - many $$$. Then, as has been mentioned, another HO alternator, And a 2KW inverter will draw in the 180-200A range (at max output) - this means probably 2/0 fine-strand cable, which is neither cheap nor easily found - and the inverter would be best installed in the front of the truck - inside, where it could add to the cabin heat. And then, where are you gonna put the 120VAC A/C unit?

I'd suggest instead considering how much it would cost to add the rear A/C system.