I am adding approx 1000 sq ft to my 1800 sq ft house. It is 90 yrs old, 2 floors, with an oil furnace and hot water radiators all on one zone. No central air. We want to add central air and that is the problem. Our contractor says we have two choices. (A) Keep the existing system (new radiators in the addition) and install high velocity a/c. He says this is necessary because the existing pipes for the radiators leave little room for normal size air ducts. (B) Completely replace the heating system we have with a hydro-air system, which would clear the way for traditional ducts. Option A would give us 3 zones (existing and 2 for the addition), while Option B would give is 2 zones (1 up and 1 down). I am nervous about switching heating systems as the current one works great. What is the typical installation cost between Options A and B? Are there efficiency differences? Are there other options?
Plan C would be to install an attic fan, if you have an attic or loft space.
Plan D would be to install a Mini-split system on one of the upper floors, and keep the indoor doors open as best you can. Cooling the second floor will help a lot.
I can't comment on cost options -- I was the installer, not the pricer.
Is there a basement or crawl space under the first floor and/or an attic space above the second floor? If you have both, you could be in luck because you might be able to add central air with two separate zones and drop the air down to the second floor rooms from the attic and up to the first floor rooms from the basement or crawl space.
Many HVAC people refer to hot air heat as "scorched air" heat. They often say that, for heat, hot water baseboard/radiator heat is the best, gives the most comfortable and even heat, and shouldn't be replaced with "scorched air" heat unless absolutely necessary.
You also wrote that one option was, "(B) Completely replace the heating system we have with a hydro-air system, which would clear the way for traditional ducts." I have a hunch that the idea of taking out the existing pipes to "clear the way for traditional ducts" doesn't seem logical or make sense. Most existing older hot water radiator heat systems have piping in tiny channels within the walls and removing them won't create any new duct space that couldn't be created elsewhere.
You might want to try also posting on alt.HVAC for a variety of HVAC contractor opinions. Be sure to include whether or not there is a crawl space or basement and/or attic.
I am by no means an expert or contractor, so don't rely on this as any kind of expert opinion. But I had to have new HVAC ac systems installed in a few different houses and commercial buildings, including two with existing hot water radiator heat. In one of the houses, I elected to have the existing baseboard heat system removed for other reasons (aesthetics, etc). The result was okay for my purposes, but the heat is definitely not as even and not as comfortable as before. Removing the pipes didn't do anything in terms of creating access for new air ducts, etc.
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