furnace replacement ideas for 2 unit bilevel home

thanks in advance to all here, this place is invaluable for learning new things, heres todays question... i own a 2 unit bilevel home (one unit above the other) both units are pretty much identical in layout and size, currently both units are heated with one furnace located in the basement, to be a certified rental unit here (grand rapids, mich.) both units must have their own heat(or more specifically each unit must have their own control of the heat, currently thats pretty much impossible with the one furnace. i was thinking of replacing the one furnace with two separate units. but to install both in the basement would make quite a bit of work regarding the hot and cold plenums and vents and returns. so i thought.... one unit in basement for lower apt. and installing a downdraft furnace in the attic for the upper unit, i hear that downdraft systems arent as efficient due to basic physics (heat rises) would the savings at install time be worth it compared to cost of running a downdraft system? oh ya, both apts are approx. 900 sq. feet , pretty small.
thanks again for all the knowledge and insight and even for the opinions (we all have one haha)

 

handigrguy

Obviously your equipment choice is important. I would only consider 92+ AFUE gas furnaces unless there are installation/application issues. But even more important is the selection of a dealer who has expert ductwork design experience. For a system to work properly, ductwork must be sized correctly to handle the air flow of conditioned air. You may be advised to redo and resize your ductwork that will handle the lower floor unit. I am not as concerned about the new ductwork in the attic for the second floor. Do you plan on air conditioning? Insist upon a manual J heat/cool load calculation for both floors.

Good LucK!

 

i dont plan on air conditioning, and the 92+ afue thats efficiency rating right? as for the existing ductwork.....if the upstairs units heater goes in the attic, then even a complete redo of the lower level ductwork wouldnt be so bad, its the idea of ripping up all kinds of walls in order to get ducts from basement to upper unit that scares me,
thanks a lot for the info.
cause knowing is half the battle

 

I would normally agree with Tiger Dunes about the 92+% efficient equipment but not in an attic in Grand Rapids. I've seen way too many freeze. For the upstairs, I'd go with an 80% efficient unit.

 

thanks grady for your tip, i have never heard of furnaces "freezing" . is this to do with the gas supply or with the temp. around the furnace? i guess i dont understand what would freeze, would it be the control unit? or what, more input would be greatly appreciated. thanks again.

 

92%+ furnaces condense water due to their design. This condensate will be in the cold space of the attic thus it may freeze and stop the operation of the furnace.

 

Thermofridge is right. The exhaust on these furnaces is so cool a lot of water condenses out of the exhaust. If the furnace is off for a length of time due to someone turning the thermostat back or a power failure, the water can freeze & damage the heater. Some manufacturers specificly state not to install their 90+% efficient equipment in an area where temperatures can drop below freezing. I have seen furnaces split open due to freezing. Frozen condensate drain lines are common.