If you move this system there is a high chance it will throw your duct system out of balance.

Having the HVAC centrally located in the home sends warm or cool air outward while drawing return air to the center of the home. If you move a system to a corner it will upset this balance.

It's not saying you can't do this, but realize you will likely pay for it in comfort especially at OAT extremes (hot or cold weather snaps). The area in which you move it to will have short duct runs, while the other side of the house will have long duct runs. This will effect the comfort level of the home.

I realize that open floor plans are all the rage, but they come at a 'comfort' price. These nice pretty looks are things that cost additional money in the way of comfort solutions.

If the furnace is more than 10 years old, and you plan to live in this house more than 5 years, then you are better off buying a new furnace. I suggest you find a contractor who will do a load calculation. There is a good chance the current furnace is oversized and you may be able to reduce the size especially if you are going to replace it with a condensing furnace.

I don't under why the venting through the roof would work with a condensing furnace but not with a conventional one. In both cases the slope has to be in an upward direction. Maybe if you had a drawing it would make sense.

Don't forget that no matter where you move the furnace in time it will have to be serviced. It needs to have sufficient accessibility.

What is your location and are you trying heat a large open space? Another option to consider is a mini split. That would take up only some wall space. But that may not be a solution for the rest of the house.

I would move the hot water heater and avoid a tankless system. The small amount of space savings is not worth it in my opinion.

You should not have an CO issues if the installation is done by professionals who know what they are doing. Get a combination CO and smoke detector if you have any concerns. Every house should have one.

Yes, a condensing furnace needs to be lower than the flue outlet so condensate can drain back to the furnace. Furthermore, the flue needs to have some minimum slope back to the furnace. No local low spots or sags are allowed. Keep in mind that you also need to have a drain or condensate pump near the furnace. One problem I ran into in trying to retrofit a condensing boiler was the vertical flue through the roof. Horizontal venting was going to be difficult. Existing chimneys can be used in some cases. This one was a partially-exposed chimney (to the outdoors in central New York). My understanding is that the flue could get too cold under those conditions and that a flue fully in surrounded by the building would be OK. It came as a big surprise to me and really threw the project off. Maybe it is not a problem in warmer areas.

Mike, perhaps skinny plastic flue can somehow be made to reach the existing flue with the relocated furnace in a way that a new, high-temperature flue cannot.

Thanks for your replies folks. This is a tricky and expensive problem. I was told that the reusing the flue as a chase may not work as according to the code the PVC vent has to be strapped every 3 feet or so to guard against breaking as releasing hazardous substances into the flue. Now I am thinking back to regular 80% furnaces. Probably just install a new one in another location to the corner and extend the ducts and supplies. This means that I have to keep the tanked water heater as well. It can either be all condensed or none at all.

There are methods to allow installation of concentric, flexible plastic flue inside of existing chimneys. Support is provided for with the system that is accepted in the US and Canada. Maybe not in your area, however. How, exactly is the existing flue gas exhausted?

Here is one example.

flex, plastic, concentric chimney liker