I don't understand the attic situation. It is a finished, but the living space is ventilated by soffit vents and a roof fan? That does not seem likely. As an alternative, you have knee walls but they communicate with the fan above a ceiling? I give up. I am not getting this picture.

One suggestion, seal up your house and you won't be as dry in the winter. You might not need the humidifier. If you have a leaky house you will have dry air in the heating season whether you have hydronic (or steam) heat or forced air. It is the same.

Radiators are not necessarily more efficient than duct heating, though they typial leaking ducts outside of the house envelope (usually attic) certainly decrease efficiency. Radiators just make for an otherwise more comfortable heating environment. If I had them, I would not be anxious to be rid of them. It is easier to have a well-zoned system with the pipes too.

If you don't need a humidifier, you can get some heat-pump mini-splits and use them for heat cool, but not the coldest weather if the electricity and gas rates agree. They can be put on interior walls. The installation is more time-consuming than the typical, simple drilling a hole through exterior walls but it can be done if you are willing to pay for it. Ceiling installation is an option too. When it is time to add the third floor, just add another set of mini-splits.

EPA laws require a valid EPA card to work with open refrigerant lines, so a full DIY is not likely.

Thanks for taking the time to reply.

juliekcmo - It is good to know about the EPA card - I wasn't really planning on a full DIY. If I went with one of the high velocity systems, I would try to cut down on as many labor costs as possible. So, I likely try to do the things I can do myself, like fish the ducts through my walls. That is probably 80% of the labor right there.

ionized - Just to give an understanding of my third floor - yes, there are knee walls up there, and there is a finished ceiling. There is a 2'x3' hatch in the ceiling that I open during the summer. When I do this, I can see the peak of the roof (and the powered fan) whuch is only 18"-24" above the hatch. There are a few spots where knee walls aren't quite finished and I could probably crawl behind them. It is very old construction up there and, if I were to do anything with that space, I would gut it and start over. So, I would have the capability of sealing everything up nice and tight. I hope that explained things a little better.

I do have my house pretty well sealed. I have all new double pane windows and have installed new entry doors. The only thing I haven't done is insulate the exterior walls (working on abandoning the old knob and tube electrical wiring first). I grew up in a house with forced air heat with a humidifier. I never realized how much I needed that humidifier until I moved here. I currently sleep with a portable humidifier during the winter and that seems to help a whole lot, but I would rather have a whole house solution so I don't have to deal with the portable one.

I am not anxious to get rid of my radiators, but I am not totally against it if it would make sense while installing a new system.

I didn't realize mini-splits could be installed on an interior wall or ceiling - that's good information to have. The need for a humidifier is still a sticking point with me, but it is good to know.

Thanks again!

When designed and installed correctly the4 Unico and Spacepak systems work well.

I have used both (Unico more often) with very good results.

You really need to get the outlets in corners of rooms and places no one will be sitting.

The air comes out plenty fast and blows hard.

brickeyee - Thanks a lot for the info, that is good to know.

I hope that your house is sealed as well as you think it is. Unfortunately, new windows and doors are often installed poorly and leak. Holes cut for plumbing and electrical are often left unsealed as well. When you insulate, be sure to do air sealing if you can. Often the powered attic fans suck a lot of air out of the living space increasing, rather than lowering. AC costs. You might consider getting a blower door test.

Installing minisplits on interior walls will mean opening the walls to get the plumbing through, but you have to do that to install ducts too,

"Installing minisplits on interior walls will mean opening the walls to get the plumbing through, but you have to do that to install ducts too,"

Make sure you see and hear mini-splits before having them installed.

Wall damage ca be kept to a minimum using high velocity systems.

The ducts are small enough to fit in a 2x4 wall if required.

A well insulated main duct in an attic (2 inch duct-board) and then the smaller flex lines to each vent.

Even if you need to run through closets from a second floor attic to a first floor, the smaller flex lines are easily boxed in and reduce the space loss in the closet to the ducts.

How are returns handled in a high velocity system? How large are they and Is one needed in each room?

brick - If I am not mistaken, I could probably prevent the main duct in my attic (3rd floor) and just run everything out of the basement, correct?

"How are returns handled in a high velocity system? How large are they and Is one needed in each room? "

They normally use a single large return.

The ones from the HV companies are designed to fit between 16 inch joists, but are longer than that.
24 inches and a run length of about 10 feet with a 90 degree turn is recommended for noise isolation.

One of the more important things is to make sure the air hanlder is isolated so it does not transmit vibration to ceins and walls.

In attics i double up some rafters (usually about 32 inches apart) and then hang a platform to hold the air handler.
A few cork & rubber isolation mounts under the air handler, and a section of vibration isolators in the liquid and vapor lines (the all copper ones) oriented vertically has been adequate.