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Author Topic: powering roof top ac from bus engine  (Read 1678 times)
Larry B
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« on: March 02, 2014, 07:05:48 PM »

  Good evening everyone-- very cold here today-31C on way to shop this morning.  I know many other have run their roof top ac units from bus engine before me I just want to make sure I have all the parts in right order before starting job. Steamfitting and welding takes a lot less thinking than this electic ****. A few months ago by chance  I found a new 24volt -4000 watt Magnum inverter at a  good price. The only thing this inverter will run is my front roof top ac unit for down the road travel. It is likely easier for me to make a sketch than to try and explain my thoughts. I have an extra main battery disconnect in my stash. I would like to use my start batteries as bank batteries.  Will this be a problem as long as I open the main battery disconnects as soon as I shut bus off.? Will this shorten life of start batteries to use them as bank batteries? I am planning on shutting inverter off near end of destination to give alt chance to fully recharge start batteries. Batteries are 8D sealed type not flooded. I was thinking of adding a 15ampx120 volt extension cord type plug to inverter ac in to recharge start batteries if ever needed. I will likely mount the remote at front in view for driver for road travel. Bus alt. is rated at a little over 200 amp @24 volt (50 dn I think-- BIG & with 4 drive belts). Thanks for taking time to look at sketch and what needs to change to make better.
            Larry B
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« Reply #1 on: March 02, 2014, 09:07:30 PM »

I am certainly no expert on this, but I can give some thoughts.  When you run a AC through an inverter, you want to be running it on power from the alternator; the batteries act as a sort of middle man.  Since a 50DN alternator puts out around 250+ amps at 24v, there is probably enough power to run 2 AC's without effecting the batteries.  Now if the alternator fails, you will run your batteries down very quickly.  This is something I might be concerned about.

From your drawing, which I had some trouble reading probably due to my poor resolution screen, it looks like you plan to run your house electrical system off the start batteries too.  Since, from the former post I gather you have a separate house bank with another inverter, are you planning to have two inverter systems going into the same panel?
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« Reply #2 on: March 03, 2014, 03:06:33 AM »

There is no problem with doing exactly what you have drawn out and described.  You say that your only purpose and use for the inverter is to run the front AC while the bus engine is running.  What you've drawn will allow that with no problems.  You can obviously go further than that.  A Magnum 4024 will probably run two air conditioners while being fed from the bus batteries and the alternator, or one AC and a coffee maker or whatever.  It's got more capacity than just one AC.  Your two 8D batteries will be fine, and your alternator will support the load.

I do exactly this, with a very similar system using a MS4024.  The only difference is that I have added a house bank of 4 batteries for times when the engine is off and I want to use the inverter for small loads - TV, computers, etc.  The house bank is simply connected to the inverter side of it's disconnect switch.  Note also that if you want to you can leave the disconnect switches on and use the inverter to charge your start batteries.  There is absolutely no need to shut the inverter off to allow the alternator to fully charge the start batteries, they will be just fine.  If all they did was start the bus they will be fully charged within around a half hour of starting the bus - inverter on or off - and will remain healthy.  Your inverter use will load the alternator and the batteries less than using the stock OTR AC setup would have.

Edit:  just noticed you have a residential type fridge.  You should run that from the inverter as well as the AC, no reason in the world not to.

Brian
« Last Edit: March 03, 2014, 07:08:51 AM by bevans6 » Logged

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« Reply #3 on: March 03, 2014, 06:11:11 AM »

I found 2 Sea Power 7.kw units I bought yesterday at a buy they are pure sine I am going to give it a try www.meps.com looks like they may work
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« Reply #4 on: March 03, 2014, 12:43:00 PM »

  I found 2 Sea Power 7.kw units I bought yesterday at a buy they are pure sine I am going to give it a try www.meps.com looks like they may work 


     Looks like a great system - I wonder what the drawback is (I'm guessing $$$$).  Even the smallest "Road Power" system will make 5K at RPM slightly above idle, with a 3:1 pulley drive to the alternator.
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Larry B
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« Reply #5 on: March 03, 2014, 08:51:08 PM »

   That is good news about not needing house batteries for my system.The only thing left is to get busy and install the pieces.  I need to think about this a little while. Right now I am leaning toward everything to one panel box with four possible ac sources.---24volt inverter, 12volt inverter, 30 amps shore and 6500 watt generator.  The panel to power source will be by manual (twist lock) plugs. It will not be long before you know which power source can handle which or how many loads. The thought you had -Lin-  a 24 volt power loss and dead start batteries was the reason I was considering locating the remote control within driveer view. This remote reads voltage and ampress (charging or inverting), This should tell if system is functioning correct. If I had of had the info a couple of years ago that I have gained from this site I would likely only have a 24 volt inverter and a system similar to brian's. That is why this site is so great. There is someone who will know the answer to your question. Thanks again for all the help
          Larry B
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« Reply #6 on: March 04, 2014, 02:49:06 AM »

delete - forgot you had the MS2812 and the huge 12v battery.
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Nick Badame Refrig/ACC
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« Reply #7 on: March 04, 2014, 02:51:24 AM »

Hi Larry,

Not to be the negative one but, after all that work and $$ you will end up with a little over 1 ton

of cooling capacity! Roof air's work well when you are sitting still but when you are faced with 60mph

forced air coming through every little crack.. (god forbid you open the driver vent) They don't do so well.

A better solution would be a dash air engine driven system. Even the smallest dash air system is about

4 tons of capacity. Much less operating parts, and is a failure happens the trouble stops right there.

Red dot systems are not too expensive these days.

FWIW
Nick-

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« Reply #8 on: March 04, 2014, 02:56:57 AM »

Nick, what would you quote to install such a Red Dot system in an MCI, complete turnkey?  I got the impression that it would be around $8K to have done.

Brian
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1980 MCI MC-5C, 8V-71T from a M-110 self propelled howitzer
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Vintage race cars -
1978 Lola T440 Formula Ford
1972 NTM MK-4 B/SR
Nick Badame Refrig/ACC
1989, MCI 102C3, 8V92T, HT740, 06' conversion FMCA# F-27317-S "Wife- 1969 Italian/German Style"
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« Reply #9 on: March 04, 2014, 05:29:57 AM »

Hi Brian,

This system could be modified with longer refrig. lines to accomadate the drivers area.

I did a simular system 4 years ago on an ice cream truck and worked very well.

To figure out cost to install, one must know how much time install will require.. On a bus,

I would figure 2 days to complete. Shop rate of $105.00hr

http://www.classicautoair.com/classic-under-dash-air-conditioner.html

Nick-
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« Reply #10 on: March 04, 2014, 05:54:23 AM »

I don't understand why it is necessary to suffer.

Unless a coach is driven only in moderate temperatures, can be driven with the windows open, and the sun is not shining through the windshield of a southbound coach an owner has to have a plan for air conditioning. With all due respect, one roof air is not a plan.

Nick is correct in that a good driver's air is important. Unless a roof air has ducting to provide the driver and passenger cool air flow the heat from the sun when southbound is oppressive. But dash air or driver's air is a lousy solution to providing comfort for the rest of the coach. I recognize everyone defines their comfort level differently, but I can say with conviction unless you keep the entire coach cool it is damn near impossible to cool a heat soaked coach with three air conditioners, much less one.

Unless you have a large bank of house batteries and a lot of inverter capacity you are not going to run three AC units while driving, and then you will be working your alternator at its max. Why not plan on running a generator? Simple solution, will not use a lot of fuel, and it makes no sense to suffer. Life's too short.
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Jon

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« Reply #11 on: March 04, 2014, 01:12:16 PM »

I solved the problem by installing a home made a/c unit automotive type using two evaporators and a single compresser, one evaporator in the dash and one in the center of the coach, kept the whole thing cool in hi temp climate, bought all the parts from a NC a/c man near charlotte nc, sanyo compresser , installed it myself and never had to touch it again. also had a 7 kw gen set and two roof airs I could also run on road but had no need to do so, this is the same set up used on conversion vans.
Frank Allen
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« Reply #12 on: March 04, 2014, 06:41:22 PM »

Could you use the transmission PTO to run a larger alternator or generator to run a dedicated front dash mount larger A/C?  I do not think one roof A/C in the front will be enough going west down the road in the late afternoon in the hot summer fighting that massive solar gain.  Just me.  HB of CJ (old coot)
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« Reply #13 on: March 05, 2014, 02:21:21 PM »

Hi Nick, have you heard anything of Artic Breeze 24 volt bus AC? Good or bad? Thanks, tom, lvmci...
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Nick Badame Refrig/ACC
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« Reply #14 on: March 06, 2014, 03:05:48 AM »

Hi Tom,

kinda small at 8,000btu's.  What do you plan on cooling with it?

Like any other compressor driven cooling system, they consume electric.

Nick-

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