Topic: Putting a Window AC unit in the back seat, how can this fail?

Since selling the Duff car, I needed another track beater for non-Lemons non-race track stuff, so I dragged an old Civic out of the weeds and put some tires on it. I share the tow rig with another team member who also has a track beater, so I'd like to be able to drive my track beater to and from events, but it's too hot for me to drive it a few hundred miles in the summer, especially after a long weekend at the track. I tried it last month at an HPDE at CMP and it was miserable. At some point it was gutted to be a race car, so there's nothing left of the original ventilation system. It's just an empty shell inside.

I have a relatively low-hours small window unit AC sitting around from my air-conditioned tent project. I made a duct box out of rigid foam board and metal duct tape and taped it to the unit. Using a couple of thermometers to measure the temperature differential, it looks like it is happy with two 4" ducts on each side of the condenser. Any less ducting and it heat soaks the condenser.

My current plan is to build a metal shelf where the back seat used to be, right behind the roll bar, put the window unit on the shelf, secure it with a metal bracket, and route two sets of 4" semi-rigid ducting out the back of the car through the floor on either side of the spare tire well.

The AC unit uses 455 watts, and the factory alternator makes 840 watts. I can get an aftermarket alternator that makes 1560 watts relatively cheaply, so I figure that should cover the differential. A 2000/1000 watts power inverter is $85 and is very compact.

I won't drive this on track with the AC on, just on the highway to and from the track.

Is there something that I'm not thinking of here?

Everybody grab your brooms, it's shenanigans!

Re: Putting a Window AC unit in the back seat, how can this fail?

Why stop at air conditioning the Civic? Surely there's room in the glovebox for a fax machine and space under the seat for a George Foreman grill? You could sell your home to pay for it - think of the money you'd save by living in the Civic!

On a slightly more sensible note - won't the AC unit generate quite a bit of heat on the back seats? You might want to add a small sprinker system to suppress any potential fires.

Good luck smile

Re: Putting a Window AC unit in the back seat, how can this fail?

That's not right, obviously the George Foreman grill should be in the glove box and the fax machine should be under the seat. Think man, THINK!

There aren't actually back seats, or even front seats until yesterday; it's just an empty beer can inside the car. I did try some rigid aluminum ducting and the exhaust ducting gets up to 150 degrees, so that will need some foam wrapped around it.

This is actually based on something that a guy I know did at his shop when his central AC went out. He built a ducted box on the back of a big-ass window unit and cools his whole work space with it. When I asked him about it, he said that window units are $/BTU the cheapest way to cool anything. I did the math and he appears to be correct. I can get 2 tons of cooling capacity for $439 at Sam's club. You just have to duct them if you're not going to mount them in a window. I'm thinking of doing this for a small warehouse that I bought.

Everybody grab your brooms, it's shenanigans!

Re: Putting a Window AC unit in the back seat, how can this fail?

You're talking heat transfer here, and the ability of the back end of the A/C unit to disperse with the heat it's effectively removing from the car's interior.  I'm thinking perhaps rather than just having two ducts that sort-of flow away from the condenser, have one duct forcing fresh air to the condenser, and the other going away, sort of a cross-flow arrangement, to bring fresh air in and lead warmed air away.  You might step up to a 6" duct to make it work really well.  You don't want to overheat the sucker. 

Don't forget, removing heat is only half of an A/C unit's job.  The other half is dehumidification.  The A/C unit is going to generate a fair bit of condensate, operating in the southeast corner of our country, and that's going to need to be plumbed out of the car somewhere.  Make sure the unit is angled similarly to the way it would e in a window, so you don't get condensate going in weird places.

2008- 2010:  1966 Volvo 122, "Charlie"
2010-present:  1975 Ford LTD Landau...doesn't have a name?

Re: Putting a Window AC unit in the back seat, how can this fail?

why not just stick it through Driver side rear window?  You only need a small amount sticking out.  Should be able to find a small window unit.

Silent But Deadly Racing- Chief cook and bottle washer, Former Flyin Turd Race Team Captain 
Ricky Bobby's Laughing Clown Malt Liquor Thunderbird Turbo Coupe, Datsun 510, 87 Mustang (The Race Team Formerly Known as Prince), 72 Pinto Squire waggy, Parnelli Jones 67 Galaxie
Besmirching race tracks in the Eastern US since 2001

Re: Putting a Window AC unit in the back seat, how can this fail?

Sounds fun, but why not put together a cool-suit system that you can use during the race as well?  I drove our race car an hour to the track with the heat index at 104 degrees and was completely comfortable.

Electric Mayhem Racing

Re: Putting a Window AC unit in the back seat, how can this fail?

One issue is that it is designed for stationary use.  It is not designed to be installed in a car.  While it will likely work for a while, the constant vibration and bumps from road use will lead to early failure.

Re: Putting a Window AC unit in the back seat, how can this fail?

I don't have a rear window, it's a 2-door, and I'm not sure having something sticking out of the car will fly at tech. Hell, I'm not sure having this thing bolted down inside the car in a steel bracket will fly at tech.

I was thinking about a big-ass vent with a scoop on it, and having something bigger than 4" duct would make this a whole lot easier, but I'd like to keep the car relatively normal-looking for club events. and I haven't found anywhere that I can do more than a 4" hole without losing some space that I want to use or putting an obvious big gaping hole in it. I did put my intake duct openings and my outlet duct openings on opposite sides of the car, so those aren't near each other.

For heat transfer, I tried different duct sizes and numbers of ducts with it on a bench outside while I monitored the temperature differentials through the evaporator and condenser, comparing them to temp differentials with one side of the duct boxes removed. You can really see when you have too little duct for heat transfer; the condenser temps shoot up and the evaporator output does as well. I think that two 4" ducts should be enough, but I left thermometers in the evaporator and condenser output streams so I can keep an eye on it. If I start to see temps that are out of line with unducted temps, then I may have to go with a couple of hood scoops on the roof feeding a couple of 8" ducts.

I read somewhere that the compressor might not handle bumps and vibration very well. I'll have to see how that. plays out. It's a $95 window unit that I got at Sam's Club, so if car conditions cause it to only work for three years instead of for ten years, then I'll be okay with that. At some point I'll probably go junkyarding for the correct parts for a proper automotive AC system,  but I've got four tracks to go to in the next two months, and this can be made from stuff that I have sitting around the shop, so I'll give it a shot. If it goes horribly wrong, then I should be able to rip it all out of there in a few minutes. Of course driving to Memphis in August with no AC will royally suck, but I can still roll down the windows.

Everybody grab your brooms, it's shenanigans!

Re: Putting a Window AC unit in the back seat, how can this fail?

Any way you could make it removable, so you can take the unit out once you get to the track?

2008- 2010:  1966 Volvo 122, "Charlie"
2010-present:  1975 Ford LTD Landau...doesn't have a name?

Re: Putting a Window AC unit in the back seat, how can this fail?

I thought about that. I can remove four nuts and loosen four hose clamps and have it out of the car in about five minutes, but I'd prefer not to if I don't have to. It only weighs about forty pounds, so it won't be hard.

With it on the shelf that I made, the condensate drain is over a shallow depression that used to be the rear seat, so I drilled a hole through the floor to drain the water out of it.

The BMW that we were prepping for the Targa Southland next week just started missing and dying after it warms up, so this may be our cross-country ride next week. I can't wait to see what the SCCA tech guys think of this contraption.

Everybody grab your brooms, it's shenanigans!

Re: Putting a Window AC unit in the back seat, how can this fail?

Lots of ways to fail with this.....................

I set it up, here's some pics:

http://www.carolinahondas.com/members/roger-albums-stuff-picture6646-lumpy1.jpg
http://www.carolinahondas.com/members/roger-albums-stuff-picture6647-lumpy2.jpg
http://www.carolinahondas.com/members/roger-albums-stuff-picture6648-lumpy3.jpg


The 4-year-old $69 WalMart "Everstart" battery was the first weak link. At idle, it drops too low and the inverter shuts down. The inverter instructions do say to only use a deep cycle battery with it, so I put an Optima Yellow Top in the car and that fixed the problem. I don't know if it was the age of the old battery or the cheapness/crappiness.

Voltage drop is also a problem, mostly due to my inverter location. DC line losses are about 10x AC line losses, which is why we use AC for most stuff. The guide with the inverter says to use thicker stuff than we use as traction cable in the EV for power, but I'm using 4AWG scavenged from an old set of jumper cables. I've got about 2% line loss, so around 11.75v at idle, which is enough to keep it working and the DC lines run cool. If I had the room, then I'd mount the inverter closer to the battery and run AC back to the AC, but this works; it no longer dies at every stop light.

It works in its own way. These were never designed to quickly cool a hot space, they can't transfer as much heat as a car AC. It will slowly cool the car. I get in the car and it's 100+, and within a few minutes it's in the eighties, and slowly drops after that. That said, with the car in the eighties and a 50-degree breeze blowing on the back of my neck, it's pretty comfortable, much better than without it. For what I want, which is cooling on long trips, it may be a good solution.

I think that the two 4" ducts are not enough. After about 20 minutes, condenser temps start rising, and after about 20 more minutes it's heat soaked and useless. I'm going to look at how to get more air in and out of it next.

Everybody grab your brooms, it's shenanigans!

Re: Putting a Window AC unit in the back seat, how can this fail?

ARRGH!

It looks like mechimike is correct about the 6" duct.

I've been doing my airflow math based on area without taking anything else in to consideration. I really should know better; I've done this for vehicle intake systems before.

The best math for this application looks like HVAC math, which uses "friction rate" tables.

So while two 4" ducts are only slightly smaller in area than a single 6" duct, they flow less than half as well. And nobody makes radiused HVAC flanges or anything else that would make this stuff work better.

I'm off to Lowes for a 6" hole saw and some bigger ducting.

Everybody grab your brooms, it's shenanigans!

Re: Putting a Window AC unit in the back seat, how can this fail?

I really don't enjoy being right; as an engineer, it's just part of my job.

It's ok, though.  Having been married for almost 6 years now, I'll take a "you're right" anytime I can.

2008- 2010:  1966 Volvo 122, "Charlie"
2010-present:  1975 Ford LTD Landau...doesn't have a name?

Re: Putting a Window AC unit in the back seat, how can this fail?

With all due respect, you guys are making things way too complicated.
This is really not that difficult. 
Here are some examples.  Good luck:

https://uniim1.shutterfly.com/ng/services/mediarender/THISLIFE/003008623257/media/103158203205/medium/1501899965073/enhance

https://uniim1.shutterfly.com/ng/services/mediarender/THISLIFE/003008623257/media/103158207932/small/1501900118149/enhance

https://uniim1.shutterfly.com/ng/services/mediarender/THISLIFE/003008623257/media/103158204275/medium/1501900145020/enhance

https://uniim1.shutterfly.com/ng/services/mediarender/THISLIFE/003008623257/media/103158205838/medium/1501900170848/enhance

https://uniim1.shutterfly.com/ng/services/mediarender/THISLIFE/003008623257/media/103158212351/medium/1501900197205/enhance

MarioKart Driving School: 1987 Honda Prelude Opus #28  (still can't win anything...)

Re: Putting a Window AC unit in the back seat, how can this fail?

The two sedans with the A/C unit in the passenger rear window...man, talk about hella blindspot.

2008- 2010:  1966 Volvo 122, "Charlie"
2010-present:  1975 Ford LTD Landau...doesn't have a name?

Re: Putting a Window AC unit in the back seat, how can this fail?

I love the generator mounts. That's giving me some ideas for the EV in the September race.

6" ducts are much better. While I was in there re-working the duct box, I also extended the intake duct box around the top and other side of the AC, giving it more intake area. That seems to have helped as well. I had taped over those sides, only leaving the big opening that flows over the compressor open. I was in WV all weekend, but I'll try it on the street when I get home.

Everybody grab your brooms, it's shenanigans!

Re: Putting a Window AC unit in the back seat, how can this fail?

I feel like this calls for a giant NACA duct on the driver's side behind the door for the inlet.

Quad4 CRX - Wartburg 311 - Civic Wagovan - Parnelli Jones Galaxie - LS400 - Lancia MR2 - Boat - Sentra - 56 Ford Victoria
Known Associate of 3pedal Mafia, Speedycop, and the Russians.  Maybe even NSF.

Re: Putting a Window AC unit in the back seat, how can this fail?

That probably would work best, if I could find a duct with a 6" outlet.

After driving around for a bit, results were mixed. Sometimes it acted like everything was fine, and sometimes the condenser would heat soak.

The rear fan was making more of a racket than usual, so I thought that maybe it was unhappy with the non-sinusoidal power. I plugged it in at the shop and got the same racket; it looks like the fan is not well, probably from being submerged when the shop flooded. I pulled it and put another window unit in the car and it's nice and quiet now and works much better.

It's up on jack stands for some exhaust repairs, but I'll try it again tomorrow after I get the exhaust fixed. Theoretically I'm going to drive this thing to Charlotte on Thursday night for three days of Targa Southland, but now we may have to take my brother's Focus ST.

Everybody grab your brooms, it's shenanigans!

Re: Putting a Window AC unit in the back seat, how can this fail?

After driving the new setup for a bit, this is useless for around town driving. The electrical load at idle is too high, causing the inverter to drop the load if traffic is bad, and the A/C really doesn't like getting power cycled.

Tonight I'll drive it a few hours to Charlotte on the first leg of my 5-day trip to Memphis and back and see how it does.

Everybody grab your brooms, it's shenanigans!

Re: Putting a Window AC unit in the back seat, how can this fail?

Put in a few EV cells to keep the voltage to the inverter up! big_smile

Re: Putting a Window AC unit in the back seat, how can this fail?

Just got to Charlotte, and the AC really worked well. Get it going on the highway, set the thermostat to 68 degrees, and it quickly cools the car to 68 degrees and holds it there (I hung a thermometer off the rear view mirror to verify it). We'll see how the weekend goes; we've still got to drive this thing to Atlanta and then to Memphis and back, but I can say that after 2-3 hours driving it to Charlotte, it's so much nicer to be in a nice cool car than it was driving the Duff car across the country last year.

Everybody grab your brooms, it's shenanigans!

Re: Putting a Window AC unit in the back seat, how can this fail?

It was cold when I went to Charlotte; on our way to Atlanta it didn't go as well. After taking some temperature readings, it turns out that the air under a car is really f***ing hot and probably a bad place to take your AC intake air from. I probably should have known that.

In Atlanta I re-routed the intake duct to the roof. I was in the paddock area at AMP with a 6" hole saw working on it between runs when someone walked over and asked what I was doing. I looked at the hole saw, then looked at the roof with a 6" hole in it, then looked back at him and said "Is that a trick question?" I don't think he got the Calvin and Hobbes reference.

With the air intake on top of the car, we set out for Memphis. Around 80 MPH on the highway, we got a great lesson in Bernoulli's Principle as the low pressure area over the roof tried to reverse the flow through the condenser, fighting with the fan until the condenser heat soaked and the compressor shut down. Again, I really should have anticipated that. A quick stop at an Advance Auto Parts in Dalton, GA for one of those fake plastic hood scoops that you stick on your hood with double-sided tape, stick it on the roof over the intake duct hole, and problem solved, AC blows cold and life is good. It worked all the way to Memphis, where we ate a lot of ribs at Rendezvous Saturday night and kicked ass at Memphis International Raceway on Sunday.

Here's a picture with the scoop on the roof. We took second in the Unlimited class behind Randy Pobst and Jason Owens in an Exocet (the car behind us), which, while obviously nobody can be happy with second place, was not a bad outcome and better than anyone (including us) expected. Robert drove a brilliant lap at AMP on Saturday morning in the Time Trial and we got a big class win (and 3rd place overall!) in the Rally stage on Saturday afternoon to bump us past the Ariel Atom team, and we were withing spitting distance of Pobst/Owens on the final day. They equaled our score in the participant voting contest (Handing out Duff Beer will make you as popular as a famous race car driver!) and Randy beat us by two seconds in the track-cross, but we stayed ahead of the Atom team to hold on to 2nd:

http://www.carolinahondas.com/members/roger-albums-stuff-picture6650-alldone2.jpg

Everybody grab your brooms, it's shenanigans!

Re: Putting a Window AC unit in the back seat, how can this fail?

One thing I noted during the 1600-mile trip that may be important:

Removing the carpet and padding from the interior of a car gets rid of everything that was insulating you from the heat of the powertrain and the heat radiating off of the pavement. The floor and firewall are really hot, which probably makes it harder to cool the car. On a hot sunny day, it only gets down to the 80's, if overcast or raining it will get in to the 70's, and at night it will get in to the 60's.

I'm going to put the heat/noise padding and carpet back in the car and see how that works.

Everybody grab your brooms, it's shenanigans!