I know I am posting this in the boiler section, so it may be a bit bios, but I figured I would have more luck with the boiler pros then the HVAC guys knowing about hotwater heat.

Anyway... I am looking starting to look into running full or patial central air in our house which is currently hot water heated (oil boiler). There is no chance of Natural gas feed to the house, so oil, propane, wood or electric is the only available sources of heat.
I have false ceilings on the main floor which reduces the head room from 12ft to 10ft. The house is just over 4100sqft of living space (including the basement). Oh, and it was built in the 1930s and maintains most of the original paster and wood trim.

(I know, sounds totally off topic so far).

Now because I'm going to have to run duct work for the AC, I am wondering if I should run the duct work so that I can convert to forced air in the future. If I don't run the duct work to the basement, I can save a lot of headakes and cost by using an external wall mount unit and be done.

I'm kind of thinging the hotwater heat is the more cost effective means of heat, but I would like hear the opinions of the pros.

 

In terms of cost, hot water is more effiecent way to make and deliver heat
In terms of comfort, generally a well designed hot water system BLOWS away forced air (pardon the pun).
With ODR, and good controls, even a mid eff. oil burner can be cost effective.
If you don't need to worry about heating, your can put the air diffusers in the correct place, at the ceiling. You will not need to bring any sheet metal to the basement either

 

I have lived with both forced air and hot water heating systems and if given a choice I would always prefer hot water.

When using the same ductwork for both heating and cooling the duct design is always a compromise, especially in residential systems. Being able to design for only cooling is a definite plus. You might want to allow for a hot water heating coil in strategic ducts if you have trouble heating certain areas. While different spaces have different requirements generally for cooling only you would have the supply registers in the ceilings and returns near floor level. With your high ceilings an argument can be made for reversing this layout but I prefer the supply high when cooling.

What may be a problem is the sheer size of your home. As I recall you have three levels above the basement. Having even cooling on all three levels WILL be a challenge and using more than one A/C unit may be in your best interest. Generally I do not advocate zoning in residential forced air (heating or cooling) systems but it CAN be effective in large systems. Zoned systems are definitely not a DIY job. I strongly urge you to get some A/C firms with both large residential and light-to medium commercial experience to make firm proposals. After the initial proposal the selected company will need to do a complete heat gain calculation in order to make a specific design and select the system components. You should not expect the heat gain calculation in the initial proposal and you should expect to pay for the calculation. If any company tries to tell you that a full heat gain calculation is unnecessary, reject that company immediately.

Because this is (at least to you) an historical building and you want to minimize damage to walls, floors and ceilings you might want to look at a high velocity system, Unico being the best known. High velocity systems utilize small ducts with high pressure-high velocity airflow. They need to be carefully engineered and installed (even more so than more conventional systems) or they will be noisy, sometimes unbearably noisy. They have a higher cost for equipment and installation supplies but often the installation labor is less.