Guest house attic mini split unit

[Jonathan28]

Hello everyone,

Hope everbody is having a great evening!

I am new to the forum, just found it tonight.

I am having trouble finding a split system that will fit into a small attic
space of a guest house (in Palm Desert, CA) and have the ability to be ducted to 3 locations. the unit that I have shown is a Mitsubishi PEAA18AA which is a 18000
BTU (more than enough) I won't know the exact load until tomorrow.
Does anyone know of a better solution than this one (a mini split), this thing is shaped
like a bed its 8" high 28" wide & 47" long and it is unclear where the service side is, it looks like it is the whole bottom side. Maybe I should use a regular unit
which I thought were more like 18" square x maybe 4' or 5' long and have
service sides more conducive to a small attic application like mine.

I would also like to put my return air down at the other end of the
guest house which would require a long return air duct in the scissor trusses
above the main space.

Maybe I am just not finding the right mini split manufacturer?
Someone must make a unit that will work for this application, and I must admit that I am not fond of the ceiling cassette, or the wall unit, I much prefer
having the hardware hidden with only the registers visible.

I have included a few pdf files which show the conditions.

Thank you for any help in advance!

Jonathan

[AC/Chas/SC]

You may want to look at the Samsung or Sanyo ducted mini splits and maybe bump up the size 1/2 of a ton. These systems have true variable speed compressors and would be very hard to oversize. ChecK out the btu output and and the service access data.

[mchild]

Hello everyone,

Hope everbody is having a great evening!

I am new to the forum, just found it tonight.

I am having trouble finding a split system that will fit into a small attic
space of a guest house (in Palm Desert, CA) and have the ability to be ducted to 3 locations. the unit that I have shown is a Mitsubishi PEAA18AA which is a 18000
BTU (more than enough) I won't know the exact load until tomorrow.
Does anyone know of a better solution than this one (a mini split), this thing is shaped
like a bed its 8" high 28" wide & 47" long and it is unclear where the service side is, it looks like it is the whole bottom side. Maybe I should use a regular unit
which I thought were more like 18" square x maybe 4' or 5' long and have
service sides more conducive to a small attic application like mine.

I would also like to put my return air down at the other end of the
guest house which would require a long return air duct in the scissor trusses
above the main space.

Maybe I am just not finding the right mini split manufacturer?
Someone must make a unit that will work for this application, and I must admit that I am not fond of the ceiling cassette, or the wall unit, I much prefer
having the hardware hidden with only the registers visible.

I have included a few pdf files which show the conditions.

Thank you for any help in advance!

Jonathan

Service side will be on the right side as you look into the supply opening. Return is setup to come into the bottom but can be moved to the end (directly opposite the supply) by removing a knock out panel on the rear and covering over the opening on the bottom.

The way you show it suspended looks good and will allow for access around the entire unit. I don't know the static pressure rating on the unit you are considering, but often they are somewhat low so make sure you have ducting that will accommodate the low static requirement.

While over sizing is not as much of an issue with these units, get the load calc done and size only as much it as it takes to meet the greater of your cooling or heating load.

[tedkidd]

You may want to look at the Samsung or Sanyo ducted mini splits and maybe bump up the size 1/2 of a ton. These systems have true variable speed compressors and would be very hard to oversize. ChecK out the btu output and and the service access data.

I'd like to understand that better and it sounds like you really have these doped out.

Because they are variable speed they can modulate to the exact load? Do they run at the same efficiency throughout their range? How do they know how much to output - thermostat communicate delta between actual and set point and ramp up or down depending upon how close?

[ebierley]

Another option is a conventional condensing unit paired with a high velocity air handler like Unico which would use 4" flex and small ceilling vents.

[Jonathan28]

Another option is a conventional condensing unit paired with a high velocity air handler like Unico which would use 4" flex and small ceilling vents.

Is 4" flex the duct size? and if so, you must mean to supply the two bathroom
compartments. Why do I need high velocity? Is that so I can have a long
(30 foot) return air duct? I'm looking up Unico now.

[beenthere]

Unico doesn't use 4" flex.
Flex duct won't with stand the pressure of a high velocity system.

High velocity systems use a special supply duct.

[mchild]

I'd like to understand that better and it sounds like you really have these doped out.

Because they are variable speed they can modulate to the exact load? Do they run at the same efficiency throughout their range? How do they know how much to output - thermostat communicate delta between actual and set point and ramp up or down depending upon how close?

Ted,

I am most familiar with the VRV/VRF systems but the mini's have similarities. In general you are correct with the differences being how the manufacturer measures and communicates the load. On the subject Mitsu unit I believe it is done through the Delta T as you suggest.

Operating efficiency should be greater at part load conditions - it is for VRV/VRF systems where the electric consumption is about 30% at 50% load. This increase is due to the effective increase in coil surface area vs compressor output.

All of these systems are communicating. Less so with the mini's, but a substantial amount of communication with the top end VRV/VRF systems. Thus, controls are always proprietary.

Hope this helps.

[mchild]

Is 4" flex the duct size? and if so, you must mean to supply the two bathroom
compartments. Why do I need high velocity? Is that so I can have a long
(30 foot) return air duct? I'm looking up Unico now.

Why a 30' return run? Your renderings show it at about 6'. But, they also show a single supply in the main room. If you are going with something in the 1.5 ton range then that room is going to get about 500 CFM. That is a whole lot of air for a single supply.

It looks like you will have built in soffits. If it is possible to use those you could run ducting in them and get your supply air distributed through out the room.

[crmont]

I'd resist the urge to stick a mini-split air handler up in that attic. They aren't really designed for it.

A wall unit is a much better choice.

[mchild]

I'd resist the urge to stick a mini-split air handler up in that attic. They aren't really designed for it.

A wall unit is a much better choice.

Why do you feel they are not designed for attic installation. The unit being considered is foam insulated. Are there other issues?

[crmont]

Why do you feel they are not designed for attic installation. The unit being considered is foam insulated. Are there other issues?

They are designed to be installed in the conditioned space like in a dropped ceiling. The access is from the bottom. The fan doesn't have the static pressure necessary to deliver air through long runs of ducting.

A wall unit or ceiling cassette is a much more efficient choice; No duct loss.

You don't want to create a headache for a service person to have to work on your unit.

This is just my opinion.

[Jonathan28]

Why a 30' return run? Your renderings show it at about 6'. But, they also show a single supply in the main room. If you are going with something in the 1.5 ton range then that room is going to get about 500 CFM. That is a whole lot of air for a single supply.

It looks like you will have built in soffits. If it is possible to use those you could run ducting in them and get your supply air distributed through out the room.

I would rather have the return air at the other end of the large space which would be more like 25 feet. Did you miss the two supply registers in the
bathroom compartments? I need to get some air to those spaces, although I have seen hotel rooms that only had a window unit to cover about the same area and I don't think they had supply to the bathroom and dressing spaces. Do you think it would be okay to leave those rooms
without any air? If I did that, all I would need is a wall mounted unit, no attic access, no return air duct, problem solved. I just feel bad about not providing air in those bathroom spaces. Most hotel rooms I have seen don't have a
door on the dressing area (the space with the sink) here in the desert it will get awfully hot in that toilet and shower room if those doors are closed.