stock ac and heat

[Austin]

I am currently converting an MCI 12 I was planning on leaving factory heat and a/c so I don't have to run a generator while driving any suggestions?

[Dallas]

About what?

[buswarrior]

he's driving suggestions.

Seriously, Austin, go for it. You do need to keep that system in top notch condition, or the leaking gas will be an ongoing drain of your bus $$$$$ resources.

happy coaching!
buswarrior

[Buffalo SpaceShip]

Austin, there's simply no comparable substitute for factory air and heat. If your coach's systems work well, by all means keep them in place. And like buswarrior mentions, keep 'em maintained and you'll be very happy indeed when it's hot (or cold) out... and you're not! 

Keep us posted!
Brian B.

[TomC]

I hear this misnomer many times on this board.  What's the difference between running the generator while you're driving or having a 20hp drain on the main engine with that big Carrier refrigeration compressor of the buses air conditioning system?  Actually, in my book, the generator running will take less fuel, since I know I can keep my bus cool at temperatures up to 100 degrees with two roof top air conditioners running.  When hotter, I can use all three.  The horsepower draw from the generator is maybe about 9hp once they are running.  Sounds alot less power than the 15-20hp needed for the 10 tons of cooling with the factory air.  Plus, in the 12 years I've had the roof tops in, I haven't done anything to them, up and beyond cleaning the filters and inspecting the coils for dirt.  Can anyone with factory air on their coach say that they've gone 12 years without some sort of expensive repair?  I thought so.  Good Luck, TomC

[Dreamscape]

I agree with TomC somewhat. Although if it works great and everything is up to snuff, I would leave it until a major breakdown in the system. Tearing it out and replacing it with something else is costly.

If it aint broke don't fix it!

Happy Trails,

Paul

Dreamscape

[Buffalo SpaceShip]

Sounds alot less power than the 15-20hp needed for the 10 tons of cooling with the factory air. 

True, Tom, but that 10tons of A/C will only have to be cycled on for a few minutes per hour and won't be taxing the engine at all during the unloaded state. A genset sucks fuel whether the A/Cs are "compressing" or not. Plus, it will cool the bus down from a heated state in a fraction of the time of rooftops. I know you like the performance of your system, but a properly-maintained stock system will run rings around aftermarket solutions.

Like Paul, I say, 'keep it 'til it's broke.'

-bb

[niles500]

Posted on Sunday, April 30, 2006 - 3:58 am:       

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I think some people are getting confused about compressor HP and btu/hrs - let me see if I can confuse it more -

The btu's you are discussing are "cooling capacity" btu's - not the number of btu/hr or HP needed to drive the compressor

Using 1 HP = 2540 btu/hr

Then taking a "cooling capacity" of 76,200 btu/hr (6+ tons)

You first have to figure the "coefficient of performance" - this is the efficiency attained in actual 'cooling' by a particular refrigeration system - late model compressors are capable of achieving a factor of up to 3

A cooling capacity of 76,200 btu/hr with a "coefficient of performance" factor of 3 would require 25,400 btu/hr or 10 HP

Furthermore a compressor which unloads to 33% after the initial 'cool down' would only require 3.3 HP

*** DISCLAIMER (yes, this is for newer efficiency units operating at 100% efficiency - I am not commenting on the take it out/ leave it in discussion - only trying to clarify the actual loads placed on the mill - and yes I know the motors add more load via the alternator - but the COMPRESSOR ITSELF is more accurately a 5-15 HP load plus any inefficiencies in a 9 ton system)
truthhunter@shaw.ca

Rating: N/A
Votes: 0 (Vote!)
Posted on Sunday, April 30, 2006 - 4:36 pm:       

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Much better put NS. thanks for the elegance and simplicity
Tom Caffrey (Pvcces)

Rating: N/A
Votes: 0 (Vote!)
Posted on Monday, May 01, 2006 - 3:25 pm:       

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Niles, that was a very nice explanation. I would only add that the way these were used in our coach, the compressor never gets to unload; it runs at full cooling as long as it is on.

The eight tons or 96,000 BTU would require at least 32,000 BTU or 12 3/4 hp to drive it and a few more hp for the air handling.

I would figure that it would take a gallon of fuel per hour when operating.

For what it's worth.

Tom Caffrey PD4106-2576
Suncatcher
Ketchikan, Alaska
niles steckbauer (Niles500)

   

[Jerry32]

I found with stock AC system I had to shut it off as it got too cold in the bus in 100 F temps. I had a small MH with two rooftop units and never could get very cool on the road with generator operating them. Jerry

[TomC]

A very important point on air conditioning-if you have an over sized unit-like what would be the stock bus system-yes it would run for a relatively short amount of time to get it cool, but it wouldn't have time to dehumidify the air.  You want the best combination of sizing that allows the air conditioning to cool relatively quickly, but also to have long compressor runs that will dehumidify the air.  Granted, many buses control the temperature with the heat and have the compressor run continuously for dehumidification.  But then, I go back to my original point-why run a 15-20hp compressor when 2-3 one horsepower compressors will do the trick?  I can tell you that even here in Calif, with only about 40% humidity, the water just rolls off the roof when the airs are on.  Whichever way you go, whatever works best for you is the way.  Good Luck, TomC

[Tony LEE]

The standard mechanical 8V71 puts out less than 300 hp on a good day and you would think that switching the 30hp oem unit on would cause a very noticeable decrease in performance going along the road at 60mph. Strangely, I can't really pick the difference, and given the lousy fuel consumption anyway, a bit extra to be comfortable is not worth worrying about.

It would be interesting to know exactly what the fuel penalty is in extra wind resistance for having three roof units fitted all the time, not just when they are running while on the road. The savings may be illusory given the claims that having a car A/C on is more fuel efficient than opening the windows.

[Stan]

Austin: You have now heard all the debate about retaining the original A/C system so now is the time to consider how it will affect your interior conversion. A standard MCI A/C system has a large duct on each side at floor level and it discharges air at window level. This has to be taken into consideration in every step of building the interior.  If you cover up the original system with walls, cabinets or furniture, it will not work properly.

[DrivingMissLazy]

The one item that I do not see discussed is the fact that normally the OTR S/C system is only used probably 10% of the time that you are on a trip. Only while on the road. What about all the time at the campground? What do you use then? Still have to have the rooftops or something similar, and the OTR system sure occupies a lot of space that well could have been used for something else. If it is left in, then someone suggests removing it if it fails, but it is hard to use this space after the conversion is completed. A well insulated bus with a good genset, properly sized, is the way to go in my opinion.
Richard

[DrivingMissLazy]

It would be interesting to know exactly what the fuel penalty is in extra wind resistance for having three roof units fitted all the time, not just when they are running while on the road. The savings may be illusory given the claims that having a car A/C on is more fuel efficient than opening the windows.

A test was run a few years ago by an engineering type bus nut on a very long run from Texas back east and return. No roof airs going one way and they were then installed for the return trip. He detected absolutely no difference in fuel mileage and also explained the reasons why. He was a very well respected poster and I do not think anyone challenged his results. If I could just remember his name, the information is probably still in the files at BNO. Seems like the first name was Steve, but I really do not remember for sure.
Richard

[DavidInWilmNC]

There's one point that hasn't been made about the stock vs. roof airs + genset: redundancy.  With the stock coach air, there is the option of using it and/or the roof air + genset.  I knocked the muffler off my generator on one of my trips (on that damned root).  Although I didn't lose it, it wasn't really an issue, as we were using the coach air for cooling and an inverter for the 'fridge.  With a large inverter, there is the third option of redundancy: roof air + inverter.

As to the original air distribution system on an MCI... the original system does use the walls as ducts.  I removed all that when I removed the inside aluminum panels and insulated.  I also removed the air channels.  I've converted the original center return to a supply duct.  I have a step where the front part of the ramp was, and that's part of my air return.  I've also got a return on each side of the living room area.  I've installed registers in the bedroom, bathroom, kitchen, and living room, although I haven't tied it all together yet.  At the moment, I have a baseboard register over each supply opening from the blower.  It's very noisy and I don't think it's designed to operate with such a small load on the motor.  When I block part of the supply opening, it runs a lot 'smoother' and quieter.  When completed, this duct system will be used for my diesel furnace.  Another option is to have a basement A/C system using these same ducts. 

As to the additional load on the engine, the Carrier compressors unload in two stages, according to my MCI manual.  When maximum cooling's not needed, the compressor unloads to a lower capacity and adds heat to the supply air when needed.  I have the heater circuit disabled, as something's wrong with my heater control valve at present.  In other words, the full load of the compressor on the engine isn't always present.  Obviously, the blowers are running and drawing current whether the compressor is running at full or reduced capacity.  Like Tonylee, I couldn't detect the difference in power with the AC system on or off.  I averaged 7.2 mpg on my most recent trip with as much time spent against the governor (73mph) as interstate traffic conditions would allow and the coach A/C on.  There was a bit of stop and go traffic through towns once off the interstate, then a lot of 65+ mph driving.  Maybe this fall, when I'm not using the system much at all, I'll remember to check the mpg without any part of the HVAC system running.

I do know that on a 97° day, driving with sunlight right on me, I only used the driver's A/C about 10 minutes here and there.  After I was out of the direct sunlight, I cycled the compressor to keep from freezing.  I don't know how much I'd spend to repair a system that I hadn't seen run.  There could be lots more wrong with it than initially thought.  Since mine works, there was no way I was going to scrap it. 

David