[See UPDATE on Manual J results, page 2 of thread.]
I am in a 2000 sq. foot, three story townhouse in Boston with a 19-year old Lennox hot air furnace, with 137,000 BTUs. The a/c is 5 ton. Thinking it is near the end of its life, I have started shopping for replacements. From reading this board, I am leaning toward a Rheem modulating furnace, but would consider a Trane or American Standard. Being a cheapstake, I only heat the place to 60 degrees day and night in the winter, and only use the a/c five days a year. (I use window a/c's instead a lot.)
The best I can tell, the sheet metal ductwork was improperly routed and sized when this place was built, and realistically is NOT changeable because it is all contained behind walls and ceilings.
From the top of the furnace, the two primary metal ducts going out are 18x12, and 18x9, with one flexible soft round duct, maybe 8". A few outlying registers give little or no output. The two main trunks are zoned ass-backwards with two dampers: front half of the house and back half, rather than by floor, so I basically keep them open wide.
The air returns, from what I can tell are grossly undersized. As it enters the furnace, the sheetmetal return duct is 16"x18". Working backwards, that leads up the three stories, with a few bends. With internal insulation, the actual internal dimension 12" x 17.5".
Given these issues, I have a few questions:
(1) Is the Rheem modulating furnace (RGFD) a good or bad idea? Those slower fan speeds may suck in too little air, no? Or, is it because it can self-adjust, it will be smart enough to run higher fan speed to suck in more air?
(2) Given my ducting problems, realistically NOT changeable without tearing up the walls of the whole house, do you have any other general recommendations?
(3) One installer said I should stick to the 5 ton a/c (which I think more right-sized should be 3.5 to 4 tons), because a smaller unit won't have the power for those large outgoing ducts. That doesn't make sense to me. Does it to you?
(4) From a few phone calls, and the one installer who visited so far, I get the sense that NO ONE will do a real heat load/loss calculation. They see what the ducting is, see how large the old unit is, speak to the distributor, and come up with a size. What do I realistically do, and how should my duct problems be factored into figuring out what the right size units are?
Thanks in advance.
[Edited by mrconsumer on 08-25-2006 at 07:22 AM]