Auto Air Purifier
2010
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Should one add a dehumidifier to a manufactured home with central air conditioning in a very humid climate?
Research says no. In the Rio Grande Valley of Texas many months are tropical.My central air has an A coil inside+outside condenser +proper tonnage. My man.home as with many m/h's does not have an attic, air return vents nor the proper insulation for this climate but passed state inspect. A teacher of an HVAC class brought his students to my at my request. *He cut off the fresh air intake above the A coil explaining that the a/c would be more efficient. My home has been state inspected.
I am most comfortable with humidity below 40% as I suffer from sweats due to an auto-immune disorder. Yes, I drink much water.
I have several types of air purifiers in my home & change the best type of filter each month. I’ve had qualified HVAC techs check the system. One recommendation was an intake vent installed above the filtered A-coil area. Ducts have been cleaned and repaired. A dehumidifier? No mold is present on walls, grout, etc.
Ok, just to start off with the basic opening question... yes, a de-humidifier can help remove additional moisture from the air, if your air conditioner alone is not pulling enough out to be comfortable for you. You will most likely find, that the first day you run the de-humidifier will be the day it pulls the most moisture out of the air....after several days of running it will be less and less, typically. You would see an increase during seasonal changes, and when the home is left open (doors, windows, etc, on a nice day). Most humidifiers can be regulated by humidity level, so that they shut off, when they reach a certain level of moisture in the air, much like a thermostat operates the central air system.
Now, on to a couple of other items you've mentioned.
Air return -- while it is true there are no return air ducts run to other portions of the home... the home IS DESIGNED to ciruculate the air. The front of your furnace unit will most likely have a slotted door, with the filter behind it. This is your return air duct. The air is pulled thru the rest of the home thru the doorways, and on some home designs, by small wall vents above the doors. On the majority of homes, the doors are built with about a one inch gap between the door and the floor. This is sufficient, in most cases, for proper air circulation throughout the home. If you find a particular room stays warmer during summer, or colder during winter, when the door is closed, then try leaving the door open more often, or install a vent above the door.
Installing a vent above the door to help increase air circulation is easy to do. You simply purchase a pair of vents, and cut the appropriate opening (on both wall surfaces, above the door). Installing a 2x4 is often the quickest way of sealing the top of the vent space, and is easy to do by either 'toe-screwing', or using L-brackets to hold the stud in place. Then simply screw the vent covers over the opening. This helps air circulation more than most folks would realize, especially if they have a tendency to close doors. By the way, speaking of doors... the door to the room the furnace is in must also be vented. This would be the laundry room door in most manufactured homes, where the furnace/air unit is in that room. If the unit is in a hall space, then there is usually a vented bi-fold door.
Fresh air vent -- I am somewhat concerned about this, that an HVAC teacher would cut off the fresh air supply. It is there for a reason. Today's homes are much more resistant to wind penetration....something that has come to light in recent years as being very important for the health and well being of the inhabitants. If you have a gas fueled furnace, that vent is CRITICAL for your safety, as it will supply fresh air to the flame, rather than robbing your home of much needed oxygen. I have seen some folks put a damper on the fresh air supply, so that it can be opened in winter, and shut when not using the heat. That still leaves you with the situation that you have limited fresh air entering the home...that may not be a problem if your entry doors are in constant use, hehehe, but it is definitely something you need to be aware of. There is also the possibility that tampering with the vent could impact you legally.... either with your home warranty, or your insurance coverage. Technically speaking... yes, if your air conditioner is drawing its sole air supply thru the fresh air vent, and it is several degrees hotter than your interior air, it would cause the central AC to do less work, by cutting it off.... but that fresh air vent is NOT the only source of air, as your central air system is ALSO recirculating the interior air. In my opinion it would be more beneficial to have the fresh air....and again, if you have an open flame furnace, it is extremely important. (Some homes even have a powered damper, which will open the vent when the furnace is in use, and leave it closed at other times, so hot air is not trickling down the vent when it is not being used)
Ok, on to the attic space.... I am sorry, perhaps it is my lack of knowledge, but I cannot picture ANY manufactured home that does not have an "attic space". Ive crawled around, in, under, and above, (and even in the attic spaces) of hundreds, if not a thousand or more, of manufactured homes, and they all had some sort of attic space. The way most manufactured homes are made, the roof system uses trusses, to carry the roof load to the perimeter walls, and the marriage wall...and trusses leave a space between the roofing cover (whether shingles and decking, or sheetmetal), and the ceiling drywall. There is always some form of insulation over the ceiling drywall, and there is an open air space above that. There are two methods of venting an attic space, passive and active. In an "active" ventilation system, there is an electrically powered fan, in an exhaust duct(s) that pulls the attic air out. This is usually an aluminum duct, about a foot in diameter, that vents directly up thru the roof. In a "passsive" ventilation system, the design is made to rely on thermal convection (usually)...that is, hot air rises... So, vents are placed along the roof, either at the peak, or just below it. Some homes use turbines, but most manufactured home passive vents are just flat squares...usually about 2 inches in height, and about a foot square. Most are brown or black in color. For the homes that use the peak ventilation, there is actually a gap at the peak of the roof, between the two halves of a doublewide home. There is a fiber type of product (it looks like a long layer of scotch pad material, usually black in color) that is placed over the gap, and shingles are placed over them, leaving a small gap, between the 'crown' shingles, and the top course.
With any of the ventilation systems Ive mentioned so far, there has to be a fresh air intake, to the attic space. This is normally done, on a manufactured home, by vented soffet. Some homes will have actual vent grills, but most utilize perforated soffeting. Sometimes ALL of the soffet is perforated, and sometimes they alternate solid with perforated. When I do an inspection, I'll check to see that the soffet vents are open, and not blocked by debris or insulation.
A properly vented attic space will result in the roofing material having a longer life. If there is no ventilation, then the heat and moisture will, literally, cook the roof decking and shingles. And that doesnt even begin to address the problems with mold that will result, eventually, because of the humidity problems that are present in a poorly vented attic space.
Insulation -- All manufactured homes are built to certain 'zone' requirements. This is federally, and state mandated, and ALL manufacturers must build to meet the minumum requirements for each 'zone'. A home CANNOT be sold in an area for which it is not rated. That does not mean it could not be improved upon....it just means that it meets the minimum requirements, and yes, this will result in its passing state inspection. You should have a "dataplate" somewhere in your home. Most often this is a letter size page (or 2), detailing the homes manufacturer, the plant location, the wind zone, and temperature zone for which the home was built, and also its serial number. These are often found inside a kitchen cabinet, often on the backside of a door, or on the back wall of the cabinet. Your state inspector should have looked at it when they inspected your home. For example, a 4 inch wall will meet the minimum zone requirements in most southern states... but a 6 inch wall allows for more insulation, resulting in a higher R-value insulation in the wall cavity. And, unless your home was built to be installed over a basement, there will be underbelly, and insulation, under the floor.
I hope this post has been helpful. I've traveled quite a bit, as a contractor to service manufactured home warranties, and I've often found that the dealers who sell the homes, have failed to educate the home buyers. This often leads to misunderstanding, and dissatisfied customers. The best dealerships are those that take the time to listen to their customers, and answer their questions as thoroughly as possible, and even go so far as to point out things the home owner hasn't thought to ask. Unfortunately, "profit margin" often comes ahead of "satisfied customer"....in my experience tho, the more satisfied customers one has, the more successful the enterprise is over the long run.
There ARE manufacturers that build exceptional quality homes, and there are those that try to look like these homes, while cutting corners. It is important to know the difference, and what to look for when comparing homes. 
but that's another post.
Good Luck
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