My Basement (walkout) has transite heat ducts. Manual J heat loss at design load (Minneapolis Minn.)is about 32,000 btu's, at 30 degrees the loss is only about 15,000 btu's. The existing furnace is a 70% 108,000 btu input, about 75,600 out. Seems WAY oversized to me.
I'm considering replacing existing with a Rheem MOD downflow, and getting different opinions as to size. The original contractor recomends staying with about the same size output. He feels that because of the transite you really need MORE output to overcome the "Cold Ducts" as it takes a while to get everything heated up. The other guy says "downsize". I have about 160 Ln Feet of duct and it is insulated with about R8 foam.
Because the existing is oversized now, the system does seem to short cycle as far as I'm concerned, and because of this it does blow a lot of cold air every time it starts up. I would think wth a smaller MOD I will get longer run times that would keep the ground temps up and in turn be more comfortable.
One thing I DO like about this furnace being oversized is that it gives me a backup source of heat when I'm out of town in during Jan/Feb. and would probably keep from having a freeze-up if the main furnace crapped out (Manual J 100,000 btu out).
Does the transite furnace REALLY need to be oversized to keep up?
How small dare I go? Smallest MOD is 21,888/54,720 output, next up is 27600/69,000 output, which is not that far off the existing 75,600.