I asked about the performance of the current system because if it is adequate, the 4 ton would certainly be adequate. Since it is not, there is no way to tell if it is not doing the job because it is too small, or because there are other problems like duct leaks or inadequate ducts to the furthest ends of the upstairs runs. Insist on a full evaluation of the system including a physical inspection of the duct system, heat transfer calculations, and the adequacy of the size of the ducts for each room. You should see reference to "manual M' and 'manual D" Ducts outside of the house envelope are particularly problematic because duct leakage turns your HVAC system into a power vent. Even with very good ducts there is leakage and then there are room to room pressure differentials to deal with that can do the same thing. That is why Jackfre suggested mini-splits. That will be pretty costly for 12 rooms. Heat pumps will also be more costly to operate than gas furnaces in the current situation. I have learned on this board that in some areas the current building code requires a system for each floor in a home. That should tell you something about the difficult of heating and cooling two floors with one system. One of the most common complaints you see posted here is a big differential between upstairs and downstairs. To get heating and cooling right, you really have to move dampers when switching from one mode to another. There is just no way around the fact that you need more air flow upstairs in the summer than in the winter. For those that don't like the idea of a duct in the chimney, I understand that you would not want a duct in a working chimney. Is there any big problem with re-tasking a disused chimney for a duct chase?

I do think from your description that the failure of the drywall ceiling is a result of lack of venting between the top of the insulation material and the underside of the roof. For starters, I would poke around and see if you have pre-formed (often pink, but possibly blue) foam pieces with ridges tucked up under the roof. These are installed between the rafters in cathedral ceilings (which is technically what you have from an insulation point of view, no matter how garrett-y your rooms feel). If not, then that's your problem (or problems) in a nut shell. You can research this on the Owens-Corning website which has lots of instructions about venting cathedral-style roofs. And you can look over these pieces at any Home Depot/Lowes. It will make sense when you see them. You may need to remove, install roof venting and replace the wall/ceiling surfaces. The plaster may be damaged more from aggressive removal of adjacent surfaces than from roof deflection, but one can't tell without studying it. And you may also need additional venting on the kneewalls now that the attic has been converted into living space which generates a good deal of moisture. You can cut down on this by installing proper vent fans for the kitchenette and bathroom areas, but you will still need under-roof ventilation, if not soffit vents, as well. It sounds as though the attic work was done by DIY amateurs. And that brings me to the most serious point that occurred to me: is it possible your attic has been converted into living space on the QT, and without benefit of building inspections, permits, (and unfortunately, increased property taxes) etc.? It may seem like a persnickety issue, but the chief risk in my opinion is that your attic living space - if done sub-rosa -might not have sufficient, legal, means of egress in an emergency. If you never have an emergency, it might not matter, but in the case of fire, inadequate escape routes can lead to the death of occupants and firefighters attempting a rescue. This is a very serious risk, and not one I'd care to undertake; it has the potential for devastating financial, legal, and even criminal consequences. While you noodle on that risk, you can research the catherdral ceiling vent issues on line. If you think structural compromise might be the reason for the plaster cracking, then an old-house friendly structural engineer (and many are not) would be the one you need. Consultations with roofers, insulation contractors and plasterers will not resolve the central issues and may lead to unnecessary, off-point work at this stage. Aside from permitting issues, however it doesn't sound that dire, though ripping and venting and replacing the wall and ceiling surfaces won't be any fun in the heat of the summer. HTH, Molly~

Just to tell you where I am coming from on this - I wrote the book! (It's fun to say that.) "Residential Ventilation Handbook" It's been said pretty well already, but bath fans must vent all the way to the outside. Not just near an opening. Use insulated ducting to reduce the possibility of condensation in the ducting. Use 3 screws to attach the ducting to the hood. That will keep the ducting from turning or pulling off. Be careful that the screws don't interfere with backdraft dampers. (Cloth duck tape was made from the same duck material sailor's pants were made from. Not good at all for duct work.) If you can do it, transition from the 4" nozzle on the fan to 6" ducting. The flow will be much better. Point the nozzle of the fan in the direction of the exhaust hood. Air is lazy. It hates to make 180 degree turns. Here is a link that might be useful: Heyoka Solutions

So I'm replacing my un-insulated vent tubing with insulated because i live in wisconsin and it condenses and drips through ceiling, comes out in low areas at seems of tubing because it goes across attic a distance of 15 - 18 feet on a curve with multiple low spots. I present have a sled under the seem to collect water from last winter tabokkal ! Choice is go up to roof 5-6 feet or out the present vent 15-18 feet with insulated pipe Or shorten the line by going straight to side of home but this requires going up high with latter to drill a new vent hole, as tight quarters in attic prevent drilling from attic. Going to the soffet is 12 feet. I was considering soffet only because I was fearful of too much slow covering my vent. Do I have a legitimate concern as sometimes roof is covered with 10 to 12 inches? My pitch is lower 12-18 I think thats right. Is there a certain vent style or type recommendation? Is there a height that is recommended for safety and efficiency with going up to roof? Thanks for any advice I'll wait patiently