--209.122.xx.xxx

Often in a retrofit job a closet corner is used to accomplish this, I don't remember when it became code (late 80's?) the returns must be closed in a rental style home, the returns are ussually twice the treated air runs to prevent blower cavitation (a simple sizing reference is 6 inch tubes are 40 sq in, you will need this to finish your manual-j and -d calculations. --184.59.xxx.xx

For all of the levels in between, I will sometime use direct heaters venting to the outside or upwards to the roof, with double vents running in interior walls. --47.155.xx.xxx

As Vee says, closets are typically used. --172.56.xxx.xxx

I would build out a chase, unless there was a Large closet with extra space - or an extra coat or linen closet that isn't needed. --68.229.xxx.xxx

we have used closets and if need be it can go up anywhere you can get it then build a wall around it --74.77.xx.xx

I should have also mentioned if you are crafty you cen use a floor chase to creat the return from a position away from treated air run, when you frame the cover of the treated air tubing frame oversize - use this for return - in the basement you can use aluminized cardboard to connect stuff to the return trunk. --184.59.xxx.xx

Maybe a small chase in the corner of each floor would do it? --107.147.xx.xxx

I've seen the duct work hidden in columns. Round or square. And if you have or had pocket doors there is usually a space there too. --172.59.xxx.xx

A forced air furnace can have a lot problems where if there is smoker or smokers then the smoke will travel through the entire building. A better is to install a high efficiency gas boiler than run Uponor pex heating pipes to each suite. Convert the copper finned baseboards where increase the length. Another option if to convert to Btu then will have a rough of size of baseboards. Longer copper finned baseboards will operate at lower temperatures. If baseboards are installed in series then can install a locking non electric thermostatic radiator valve to regulate heat. The advantage with radiant hot water heater is that later can buy some more wall mounted high efficiency gas boilers than can separate out heat where tenants control and pay for heat used. Ductwork is suitable for a single family home where if they smoke then no one is effected other the other family members. With Uponor pex heating pipe it is easy DIY job to connect up radiators to building where from the boiler up where the main manifold is in the basement. As each rental unit is regulated then all factors of heat loss and thermal gain with sunshine are taken into account. --207.236.xxx.xxx

That isn't going to be a cheap date after you factor in the ductwork cost. Maybe it is a bit cheaper to install spit systems or even a small boiler system for the first two floors. --24.101.xxx.xxx

Metal ductwork also acts as a conduit for noise between all of the room duct vents. Check out high velocity which uses easier to retrofit softer ductwork. Several minisplits would also work well and would also provide cooling. --98.235.xxx.xxx

Or you can use multi head mini split ductless. --216.218.xxx.xxx

It may be that I run gas on the first floor only which I'm assuming would be an easy with the open basement and keep the 2nd and 3rd on EBB or run minisplits on those two floors. Either way, I plan on keeping the EBB in place as a backup heat source. --209.122.xx.xxx

Have a pro klay it out. Then u do it --129.222.xx.xxx

A gas boiler is easy to control where a Tekmar electronic boiler control system along with locking non electric thermostatic radiator valves where less one degree variation. If more radiation then the boiler loop temperature is lower where a window open than a large temperature drop. High efficiency gas boilers where can mount a outside sensor then the boiler out temperature is dependant upon outside temperature so the heating loop always works to keeps the heating loop temperature as low as possible. --207.236.xxx.xxx