Without a duct system,you may have moisture condense in the cavity.You'll need a duct system to distribute the correct air flow to each room.
Help me understand the principals and forces at work here. With what I am talking about, there will be R-50 or so separating the hot attic from this cavity and this cavity will be very much in conditioned space. In that situation, do you still have to worry about moisture condensing? This is critical to my understanding. If so, is there anything magical about ductboard for lining the space if it is already airtight? For example, could the space be lined with 1/2" polystyrene and achieve the same results (again, this helps me understand the issues)?
Originally Posted by dash
You'll need a duct system to distribute the correct air flow to each room.
The reason to try to avoid a duct system, if possible, was one for simplicity and also efficiency. It is my understanding that every bit of ductwork creates system friction and resistance. It seems that, provided that you could balance the system through register sizing, just dumping the air into this cavity would be very highly efficient from a resistance perspective. It also seems like this would give a tenant more flexibility in custom balancing rooms by just adjusting the vanes on the register grills if, for example, they want extra cooling in the master bedroom and warmer in the kids room.
Originally Posted by dash
I don't think you'll find a one ton system,with dehumidification features,but a 1.5 ton may be what you need.
I hope that these new inverter based units help deal with the need in this area
Originally Posted by dash
If the actual btus required is that low,a 6" flex is likely the kargest duct you'll need for any room.If you use the cavity for an an air return,thus getting return from each room,,the supply duct won't need to be insulated,just sealed tightly,so the space will be large enough.
In this situation I was planning to have a wall mounted return mounted right under the unit since the unit is central in the space. Florida requires pass through ducts over the doors if there is not an active return system.
I've seen several homes in my area with the exact install of air tight fur down,
with no ductwork.
A friend of mine did this 10+ years ago at his home.
A little smaller than OP's build, but similar layout. Hallway was fir-d down to
7' and openings cut (..but sealed - between walls..) into eachof the bedrooms, baths & living and kitchen area. Even with a 10 year old 12 SEER and 80% furnace his utility bill
is less than $70 per month. His utility cost is approx .13 per Kwh.
Mostly I see this in commercial hvac contractor's personal homes. They 'get it'
as do you about ductwork in conditioned space.
I've yet to see the same install with ducts.
Yep, this is exactly what I am talking about. Do you have any knowledge of whether they were having any moisture problems or other negative issues? I can't imagine that the weather in Louisiana is less hot and humid then ours in central Florida. . . or is LA for Los Angeles or Lower Alabama?
Office and warehouse in both Crystal River & New Port Richey ,FL
1. Ductboard has it's moisture/air barrior on the outside,insulation on the interior.You'll have your intended moisture/air barrier on the inside(drywall),insulation on the outside,so moist/warm air in a ventilated attic,can reach the drywal,which is not a moisture barrier.
The vapor barrier on batt insulation ,isn't suffiecent,in my opinion.
Flex or metal duct has air barrier on the inside,then insulation,then moisture barrier on the outside,leakage of the moisture barrier,will cause the insulation to become wet and useless.
2. Yes, duct systems have resistance,properly designed ,no more then the fan can handle.Too little resistance,and fans won't perform properly,could be an issue if the cavity is large enough to cause one.In a large cavity you'll have low velocity,then closing registers will increase velocity,this uses up the static ability of the fan,static is what moves the air,could be an issue.
3.Registers are not designed to be the sole source of balancing.May not be able to get the correct air flow,or may become noisey when closed enough to get the correct airflow.POlus using the cavity likely means less then ideal register location,so the air pattern from the register is more critical,to have even comfort throughout the room.
4. You could use a two ton,two stage ,in low about a ton.Not sure what the inverter based units drop down to.Florida code regulates oversizing,though seldom ,if ever enforced,but leaves the contractor liable,even though it passes inspection.
Using 1" ductboard eliminates all the possible issues,return thru the bottom is fine,only suggested the cavity so uninsulated ducts could be used with no issues.
The living room/kitchen/dining room and master bedroom are contiguous to the air handler location so there would be a return directly the the bottom of the unit. The other two rooms would have pass through ducts above the doors as required by Florida code when active returns or jump duct returns are not present.
this is from buildingsciene.com
hot humid climate..this study is in Orlando Fla.
Ducts in conditioned space - The preferred method for keeping HVAC ducts and mechanical equipment inside conditioned space is moving them down from the attic. In this building profile, a conditioned attic can be used for HVAC ducts and equipment. In no case should HVAC ducts be placed within exterior wall assemblies-this is not part of what is meant by ducts in conditioned space. A vented attic assembly may be used in this climate as long as the ceiling plane is air tight and no ductwork or air handling equipment is located in the attic.
I am a building and solar contractor in Central Florida. Our desire to to build zero energy homes and we've experimented with various ways to improve efficiency.
We are getting ready to build a duplex for ourselves that will be very tightly sealed between living space and attic. We are even actually considering drywalling the ceiling before building the interior walls so that the drywall envelope is not broken where every wall is located. (The electrical contractor is the main guy affected if we do this but we would leave a sheet or two of drywall off in the middle of the rooms to make it easier to wire. The only thing the plumber has to do in the ceiling is run his stacks through to the roof.) We want to avoid putting ductwork into the attic for the obvious reasons. On an 8' ceiling it gets challenging to put ductwork in conditioned space as you can't drop your ceiling below 7' in those areas where your duct work would go. The floor plan is designed so that we would be able to hit the entire apartment with air by lowering the ceiling in a hallway and one bathroom.
So the thought is this. . . Since we will have a (virtually) completely thermally sealed separation between attic and living area at the 8' ceiling level, why don't we just hook the A/C supply to this cavity (between the 7' and 8' sheetrock. We would then penetrate the cavity on the adjoining rooms that are 8' tall with an appropriately sized register so that the air is adequately and uniformly circulated (at least that is the goal).
The reasoning is that it is simpler and cheaper than creating a duct box in the same space and we don't see what benefit the ductboard box would have since the cavity is 100% in conditioned space. Considering that a 7' ceiling is going need to be supported by 2x4s at a minimum, you really only have 8" to work with in the cavity. That's not enough height to do anything with a ductboard system.
The units are 3 bedroom/ 2 bath, 1,200 Square feet in size and they will have vinyl, double pane, Low-E windows, a galvalume natural metal color metal roof, 2' soffet overhangs, and will be VERY well sealed between living area and R-50 attic. I haven't run the numbers but I suspect that one ton of air would suffice.
Please advise on the following:
1. Is there any reason why we should not create an air distribution chase this way? Would there be moisture or condensation issues of some kind? Would be be able to adequately balance rooms by register sizes is this situation?
2. What is the best high SEER way to deal with such small BTU needs where a ton of air might suffice, considering that most A/C units are much larger than that? We want to avoid short cycling the units and we want to control humidity? Is this where an inverter based variable type of system makes sense? I've thought about using a mini-split and avoiding an air chase all together but it seems like you need a unit in each bedroom and the living area and that would be overkill for a small place such as this.
Thanks so much for your thoughts and input.
It sounds like you are trying to mimic a raised floor supply air distribution system but have it in a sheet rock enclosure above the ceiling and below a vented attic.
This is a dumb idea. In commercial buildings with a raised floor, you end up moving twice the amount of air. They cool off the standard air flow per ton down to say 55 degrees, and then they mix it with an equal volume of return air so that your supply air temperature is up around 65 degrees and well above the dewpoint of the space. 75F with 50% RH has a dewpoint of 55F, so if you were to run air temperatures close to that in a floor you would run the risk of the top of the floor sweating all the time and the floor cavity filling up with water.
So now you are considering doing this below an attic. Above the attic will be air with a dewpoint in Florida in the high 70s for long periods of time. This is going to be a rain maker. You are going to be growing mold on the back side of that drywall. You will also be pressurizing that void and air is going to leak up into that attic so it would be like running an exhaust fan for an extended period of time.
You would be better of sealing up your attic and insulating the the top or underside of the roof deck. If you do not have a hip roof, then insulate the gable walls also.