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Old 06-06-2021, 07:51 PM   #1
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Running rear (original) air conditioning off of bus and/or inverter power

Hello-Newbie here- This might be completely impossible, but I won't know until I ask, so here goes. We are converting a 2009 Blue Bird Microbus. We've installed a Growatt 3 KW Inverter with a Big Battery 24 v system. We'd like to run the 12 volt rear air conditioner (The original AC) on this system. We know we'll have to convert to 12 volts from the 120 VAC output from the Growatt. Has anyone seen this done? If so how? I'd like to be able to run it off of either power source, the bus or the inverter system. We'll appreciate any help you can give us, Thanks in advance!

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Old 06-15-2021, 10:27 AM   #2
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I don't know what specific system is used in those types of vehicles, but it is 99.999% sure they don't run on electricity. They run on engine power. There is a belt that drives a compressor straight of the crankshaft of the engine. Yes, it also needs 12V for the fans and controls and stuff, but no cold air is produced with electricity.

Electric and hybrid cars have electrically powered compressors, and there is an emerging trend to incorporate them also into higher end traditional cars and cars with start-stop systems. But it is a virtual certainty that it does not apply to your bus.

If you were a bunch of geeks that don't take no for an answer, sure, it could be done. I guess the least ridiculous way could be powering the A/C compressor directly with a, say, 2HP-or-thereabouts low-voltage BLDC motor and a timing belt.

You'd have to hack the control system to make it work, get a BLDC driver, and upgrade your alternator rather massively to be able to power it while driving.
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Old 06-15-2021, 03:15 PM   #3
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If you were a bunch of geeks that don't take no for an answer, sure, it could be done. .
This is a funny line! And it applies to a number of posts since I've been on the forum.

There is no practical way to drive the AC compressor from anything other than a powerful engine.

Most folks seem to prefer mini-splits, though a)I'm not sure how many people actually install them and b)not sure how well they cool without a lot a lot of insulating and window covering.
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Old 06-15-2021, 03:46 PM   #4
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theres been a couple projects started on this forum with people gearing up to spin an engine driven compressor with a small engine.. the idea being that the speed of the mini engine coule be altered to control the capacity of the A/C. your bus needs a lot more A/C when driving down the road than it does when parked..
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Old 06-15-2021, 04:39 PM   #5
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theres been a couple projects started on this forum with people gearing up to spin an engine driven compressor with a small engine.. the idea being that the speed of the mini engine coule be altered to control the capacity of the A/C. your bus needs a lot more A/C when driving down the road than it does when parked..
This definitely can be done. We have that original-manufacture setup on an old Flxible bus, which has a 4-cylinder gas motor to run the A/C compressor. And it works exactly as you describe, changing the engine speed to vary the output.

As far as doing what the OP wants, it must be possible. As Bert mentioned, electric vehicles do this with electricity. But I think to get the capacity needed, you'd need to pull parts from a hybrid bus.

We do install and like mini-split systems. In fact, the freight carrier dropped off another one this week. They work great, but a 12k BTU mini split obviously won't have the capacity that a 120k BTU factory bus system has.
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Old 06-15-2021, 04:43 PM   #6
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being its a Microbus that should be a Van chassis.. you can convert the system to dash-air only and then replace the rear unit with the minisplit or a single rooftop camper A/C.. between the 2 that small bus might cool nicely on the road being its small and the bus chassis Van-based dash air is usually a 15,000 BTU unit vs 9k or 12k
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Old 06-15-2021, 05:18 PM   #7
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Good catch, Christopher. I missed the microbus in the microprint on my micro-phone screen.
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Old 06-15-2021, 05:30 PM   #8
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Good catch, Christopher. I missed the microbus in the microprint on my micro-phone screen.
Still, I can totally see a four cylinder engine driving the pump, somewhere on the front bumper.

:-')

Regarding a rooftop unit, the only difference between that and a fire engine siren is a few millimeters of clearance on the shroud housing.
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Old 06-15-2021, 05:42 PM   #9
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Regarding a rooftop unit, the only difference between that and a fire engine siren is a few millimeters of clearance on the shroud housing.
Exactly! Which is why we remove them and install mini-split systems. They're loud. And, they're not very efficient.
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Old 06-15-2021, 05:43 PM   #10
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Still, I can totally see a four cylinder engine driving the pump, somewhere on the front bumper.

:-')

Regarding a rooftop unit, the only difference between that and a fire engine siren is a few millimeters of clearance on the shroud housing.

LOL never thought about noise.. im the guy who falls asleep on airplanes during taxi and never ewakes up till we land... also been known to sleep on a fishbowl with a 8V71 screamin away..


-Chris
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Old 06-19-2021, 09:28 PM   #11
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This is a funny line! And it applies to a number of posts since I've been on the forum.

There is no practical way to drive the AC compressor from anything other than a powerful engine.

Most folks seem to prefer mini-splits, though a)I'm not sure how many people actually install them and b)not sure how well they cool without a lot a lot of insulating and window covering.
We went the mini split route for our 26' bus works great for us. We do have a rooftop transit bus A/C unit that came with the bus and briefly thought about rigging it for use when the engine is off but it's more trouble than it's worth and it's LOUD. Good luck sleeping with that thing running at night.

It's a massive unit that runs on two engine driven compressors. When on the road, we'll use that engine driven A/C. When docked, we just use the mini split. To help, in the hot late summer months, we just make sure to book shady/partial-shade spots at campgrounds.

We're at a campground right now and today it was 85F and very muggy out. When the sun was directly over the bus, it was about 73F inside. When it got cloudy for a little bit, it dropped to 68F inside.

Everyone's experience will differ though because each bus is different. Our bus is white and short so a 1ton minisplit is plenty. We blocked off a decent amount of windows and have blackout/heat blocking shades throughout. We also built a mudroom around the handicap door as an additional thermal barrier between inside/outside when entering/exciting. Lastly, we have a powerful ducted fan in the rear to aid in circulating air. It works pretty well having about a 1 degree difference between the front and rear of the bus.
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Old 06-22-2021, 02:55 AM   #12
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We went the mini split route for our 26' bus works great for us. We do have a rooftop transit bus A/C unit that came with the bus and briefly thought about rigging it for use when the engine is off but it's more trouble than it's worth and it's LOUD. Good luck sleeping with that thing running at night.

It's a massive unit that runs on two engine driven compressors. When on the road, we'll use that engine driven A/C. When docked, we just use the mini split. To help, in the hot late summer months, we just make sure to book shady/partial-shade spots at campgrounds.

We're at a campground right now and today it was 85F and very muggy out. When the sun was directly over the bus, it was about 73F inside. When it got cloudy for a little bit, it dropped to 68F inside.

Everyone's experience will differ though because each bus is different. Our bus is white and short so a 1ton minisplit is plenty. We blocked off a decent amount of windows and have blackout/heat blocking shades throughout. We also built a mudroom around the handicap door as an additional thermal barrier between inside/outside when entering/exciting. Lastly, we have a powerful ducted fan in the rear to aid in circulating air. It works pretty well having about a 1 degree difference between the front and rear of the bus.

Would you define "very muggy" ?
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Old 06-22-2021, 08:05 AM   #13
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Would you define "very muggy" ?
Sure, very muggy just meant very humid. It was 85-90% humidity outside that day
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Old 06-23-2021, 10:41 AM   #14
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Sure, very muggy just meant very humid. It was 85-90% humidity outside that day

Here in the desert where the humidity ts normally 10-25%, 85 degrees F feels comfortable to me. Maybe getting a dehumidifier would help you. Some types can use less energy than an air conditioner, because they don't cool the output air (the output air is used to help cool the input air to condense the water vapor and is therefore warmed back up to the input air temperature). Anything less than 84 F feels cool to me at 10-25% humidity.


A bonus is that you don't have to cut any holes in the bus shell because there is no need for outside air exchange or to vent heat to the outside to make a dehumidifier work. The condensate can be dumped to the gray water system

Dessicant types of dehumidifiers don't usually cool the air at all, but unless you use the sun to dry out the dessicant they can use a lot of power drying out the dessicant.
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Old 06-23-2021, 11:08 AM   #15
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Here in the desert where the humidity ts normally 10-25%, 85 degrees F feels comfortable to me. Maybe getting a dehumidifier would help you. Some types can use less energy than an air conditioner, because they don't cool the output air (the output air is used to help cool the input air to condense the water vapor and is therefore warmed back up to the input air temperature). Anything less than 84 F feels cool to me at 10-25% humidity.


A bonus is that you don't have to cut any holes in the bus shell because there is no need for outside air exchange or to vent heat to the outside to make a dehumidifier work. The condensate can be dumped to the gray water system

Dessicant types of dehumidifiers don't usually cool the air at all, but unless you use the sun to dry out the dessicant they can use a lot of power drying out the dessicant.
Thanks for the reply! I actually wasn't complaining about the humidity but just sharing how well our mini split was running. The ducted fan we have in the rear of the bus stated that the humidity was roughly 63% inside the bus despite 85-90% humidity outside.

The mini-split was doing a fantastic job removing the excess moisture from the air inside the bus while still cooling the air. We are very pleased with it.
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Old 06-23-2021, 11:40 AM   #16
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Thanks for the reply! I actually wasn't complaining about the humidity but just sharing how well our mini split was running. The ducted fan we have in the rear of the bus stated that the humidity was roughly 63% inside the bus despite 85-90% humidity outside.

The mini-split was doing a fantastic job removing the excess moisture from the air inside the bus while still cooling the air. We are very pleased with it.
This is going to sound silly but theoretically you can actually save a small amount of energy by decorating with plants that get their water from the air and lower humidity at the same time. Some even provide some air filtering capacity. I tried it back in Texas (70-100% humidity). Thought that I couldn't kill them, but it turns out that not watering a plant is not the only way to kill them. Never got to prove/disprove the theory.
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Old 06-23-2021, 12:50 PM   #17
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a desert dewpoint in the 30s or low 40s versus a summer heat wave dewpoint of say 70-75 can alter the capacity of an A/C 30-40%..



in the desert your A/C can just cool air it doesnt have to condense water out of the air.. the same "state change" properties of thermodynamics that the A/C uses by condensing and evaporating the refrigerant to efficiently move heat applies to the water in the air also..



so when you condense water on the coils (and that eventually drips on the ground).. that water when it condenses from a gas in the air (humidity) has to expel quite a but of heat.. that heat gets expelled to the A/C coils making the A/C work harder to cool the air.. or if its maxxed out reducing the ability of it to cool the air..


measuring humidity by a % is a bit of a farce.. I can have 98% humidity when its 29 degrees outside and it wont feel humid at all .. I can have 50% humidity when its 98 outside and it feels quite muggy...



I use dewpoint numbers.. a dewpoint in the 50s outside typically "feels nice".. in the 40s and below and it will feel dry.. if you sweat the breeze will feel pretty cool or even cold if its below 40..



the the low 60s starts to feel "humid" by the upper 60s its feeling "muggy".. in the 70s things feel "oppressive".. except by the ocean.. something ive never understood is how 94 degrees with a dewpoint of 75 on st pete beach is hot but feels good.. yet the same in orlando or in ohio feels oppressively hot N humid..
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Old 07-26-2021, 04:50 PM   #18
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... I can have 98% humidity when its 29 degrees outside and it wont feel humid at all ...

If that's 29F I would think the humidity would be zero as environmental moisture is in solid form and therefore not present in the air. The driest deserts on earth are in the Arctic and Antarctic (though MN in February sure came close).
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Old 07-26-2021, 06:21 PM   #19
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there is still a dewpoint when its below freezing.. look up freezing fog.. its real and happens at temperatures below freezing wherew the humidity reaches 100% below freezing.
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Old 07-26-2021, 08:53 PM   #20
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