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Old 01-01-2023, 02:15 PM   #1
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Mini split air circulation fan ideas?

I like many others here have a mini split installed in the back of my bus over the master bed. In the summer, and even in the winter when the heat is on, the temperature is great in the back, but it’s a 10 degree differential up to the front of the bus. Obviously the front is inherently leaky so I’m never going to get perfection, but I was thinking of coming up with a circulation fan along the lines of the cold air return used in home HVAC systems.

My question is how to lay out the vents. My thought is to put a duct as close to the floor as possible in the back bedroom, then put another duct as close to the front living area as possible near the ceiling. I would use some sort of reversible fan so in the summer I would draw air from the back and bring it to the front, and in the winter I would draw air from the front and bring it to the back. The winter duct placement doesn’t seem ideal but I have a diesel heater to do the heavy heating work anyway so this is more of a supplementary idea.

Anyone have specific experiences and tips here or other ideas?

I’m not sure how to place or conceal this ductwork, even a 3” duct from vent to vent will take a lot of space.

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Old 01-01-2023, 04:59 PM   #2
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It would be a mistake not to mention you should first do everything possible to insulate your windows against heat gain in the summer and heat loss in the winter. Assuming you are aware of that, air handling is definitely still a challenge, and may not help with your situation.

I have a speed-adjustable marine inline exhaust fan in the bus, not yet installed, for either internal air circulation or additional exhaust of hot air. The fan moves about 450 CFM at full power, roaring like a leaf blower, and at low power it is still pretty loud so I'm not sure they're good for more than temporary use. It's a Seaflo brand 4" diameter. I see no practical way to duct it but have considered a flex line running along the floor on the passenger side to push cold air forward (for cooling) or pull cold air backward (for heating).

Because a mini-split only puts out so much cooling or heat, improving air circulation will likely make the whole bus too cold in the winter or too warm in the summer.

I recommend starting with a floor fan that allows you to experiment first with air circulation and overall system performance. That way you can do large volume air handling to determine if your systems can deliver the overall heating and cooling needed before committing to what could be a suboptimal and potentially costly air handling solution.

I have a shuttle bus and I'm sure longer buses are more difficult to set up. For what it's worth here's the state of my shuttle bus systems.

Starting with heat, I have a diesel heater mounted under the bed in the rear of the bus, ducted to about halfway up so it blows forward. The return is a space between the bed and the rear wall of the bus. The force of the blower fan coupled with the directional blowing does a good job of both mixing and circulating the air.

The natural flow is forward along the floor, then rising; and as it cools and as the intake of the diesel heater sucks air from the back wall of the bus, the upper air moves generally backward. I think this is the most common layout for diesel heaters and does not require return air ducting.

I block off the driver's area, and the heater works well, given my climate and level of insulation-mild climate, minimum insulation. As mentioned, if the driver's area is part of the living space and not blocked off, window coverings are a must have to manage internal temps without resorting to larger cooling or heating systems.

For A/C my minisplit is in the back like yours. I also experience that temp differential, but up until recently I didn't have the curtains and insulating window coverings and haven't been able to test them in summer heat. I'll try the floor fan before doing anything with the marine exhaust fan.
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Old 01-01-2023, 07:52 PM   #3
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they make ducted minisplits.. I have one in my house.. (and 2 that I disassembled from their original wall mount cabinets and built my own ducted cabinets for..

I have seen one bus where a ducted minisplit was mounted in an underfloor stporage bay and ducted through the bus... you could try making a return duct for yours, however the only way to get true air flow is by creating a pressure differential... ie return from the front of the bus and supply at the back,. if you choose to run a duct to say the middle of the bus.. you will need a booster fan... those crossflow blowers in minisplits can build a decent positive pressure but they dont do well at suction... (they dont need to in a standard wall mounted config)
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Old 01-02-2023, 08:09 AM   #4
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Originally Posted by cadillackid View Post
they make ducted minisplits.. I have one in my house.. (and 2 that I disassembled from their original wall mount cabinets and built my own ducted cabinets for..

I have seen one bus where a ducted minisplit was mounted in an underfloor stporage bay and ducted through the bus... you could try making a return duct for yours, however the only way to get true air flow is by creating a pressure differential... ie return from the front of the bus and supply at the back,. if you choose to run a duct to say the middle of the bus.. you will need a booster fan... those crossflow blowers in minisplits can build a decent positive pressure but they dont do well at suction... (they dont need to in a standard wall mounted config)
Yeah, I couldn’t find a ducted mini split that ran on 120v at the time. I was definitely planning on putting a blower (ideally one with reversible flow) in the middle of my circulation system so I could suck air or blow it out either direction.

I guess one question is whether it’s best to try to draw in cold air from the mini split in the back and push it out toward the front of the bus, or if it’s a better idea to try to draw in warm air at the front and push it toward the back of the bus.
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Old 01-02-2023, 04:11 PM   #5
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Originally Posted by dbsoundman View Post
Yeah, I couldn’t find a ducted mini split that ran on 120v at the time. I was definitely planning on putting a blower (ideally one with reversible flow) in the middle of my circulation system so I could suck air or blow it out either direction.

I guess one question is whether it’s best to try to draw in cold air from the mini split in the back and push it out toward the front of the bus, or if it’s a better idea to try to draw in warm air at the front and push it toward the back of the bus.

good point... the 2 of mine that are 120 volt are the ones I built myself from reworking regular wall mounted..



ultimately you want to create a 2 way flow.. you need to get air up front and pull front air to the back.. I cant remember does your bus have a factory ducted A/C system overhead? if so you could use the blowers from that to move air around..



the issue comes about that whats good cool isnt great for heat and vice versa...



you could try simply putting a fan or 2 up high blowinfg the same direction.. one in the doorway to the back room and another a bit further up and see if it will create flow lanes.. otherwise id be inclined to run a duct front to back that pushes air towards to the intake of the split.. the split unit ramps up and down (and shuts off) based on a sensor that is in the return air stream.. pushing unconditioned air at its intake will have the effect of treanding the unit to run more or run at a higher output longer.. with hoipefully some of its supply air being pushes out of the room toward the front.. the risk of course is that it ends up super-conditioning the one area and not moving the air far enough...



these type balance issues are the same things many HVAC engineers try to solve with multi-level homes and a single central air system...



each room in my house has supply AND return vents.. houses in my neighborhood with the builder-grade stuff have supply in each room and only one central return in a hall.. those homes dont condition nearly as well...



the obvious "best-case" would be if you had a second split in the front.. but I realize this is much easier said than done
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Old 01-02-2023, 08:29 PM   #6
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Hi Chris, did or can you post some pics of your duct modification of a regular mini split wall unit.

Johan
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Old 01-03-2023, 12:00 PM   #7
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Hi Chris, did or can you post some pics of your duct modification of a regular mini split wall unit.

Johan

im on the road now but I can do it when I return home.. I pretty much completely dismantled the unit and made my own cabinet and use my own blower.. it is not compact as it was to become ducted in a basement so I didnt care about space constraints.. just that it worked...
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Old 08-04-2023, 07:35 PM   #8
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Has anyone installed one in front and back? I’m troubleshooting the same issues with one in the back barely cooling even the far back area.
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Old 08-04-2023, 08:14 PM   #9
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Originally Posted by Nick5272 View Post
Has anyone installed one in front and back? I’m troubleshooting the same issues with one in the back barely cooling even the far back area.

Hi Nick,


I have one 12k Btu in the back and one up front. The back one cools the back no problem and the font cools the front 1/2 with no problems. As other stated i think airflow is the main issue. If I remember correctly I read somewhere: you want to push the unconditioned air into the conditioned part to get better efficiency, I do not know if that actually works though. the two minisplits solved my issues and was planned for from the start, as 1/2 of the bus will be shut off from the other half for for conditioned storage. I did not want to heat/cool the all of it.
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Old 08-05-2023, 04:38 PM   #10
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I'm not sure a 3 inch duct would move enough air to help much with distributing the air from the back of the bus to the front. A box fan on the floor in the doorway / hallway to your bedroom blowing cooler air from back to the front in the summer may be more effective. The warmer air from the front would return to the mini split along the roof.

In winter turn the fan around and blow the cooler air from the front to the back to be heated and returned to the front along the roof.

Ted
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Old 08-17-2023, 02:32 PM   #11
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I wonder if two fans without ducts would help - one low in the back blowing cold air to the front and one near the ceiling blowing the warm air to the back. It might be worth trying before attempting to add a duct.
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Old 08-17-2023, 05:38 PM   #12
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I wonder if two fans without ducts would help - one low in the back blowing cold air to the front and one near the ceiling blowing the warm air to the back. It might be worth trying before attempting to add a duct.
The only reason I considered ducting was because head and body space will be tight where the fans need to go, in the hallway between the rear bedroom and the kitchen.
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