I have an old furnace (1979 Rudd 140,000 BTU capacity) powered by LP gas. The home delivery system is forced hot air, so there is duct work down in the basement which some contractors have suggested as being adequate for both the heating needs of the house, and for a new AC system where one had never been in place, while others have suggested replacement of complete existing ducting, or moderations to it. That is one area of ignorance on my part, which I'd like to learn about.
Another is some knowledge about the actual components of the HVAC system that are proposed by the various contractors I have dealt with. Even when they use the same brands (Lennox is the most common, also 2 Goodman, 1 Trane and 1 Carrier) the capacity in terms of BTUs for the furnance and tonnage for the AC system have been different, and yet they are all measuring the same size house. Therefore, I need some feedback in this area. The Wall Area on the original architectural drawing for this house = 1406 sq ft with a Glass area of 277 sq ft. Square foot measurements that these contractors have taken vary between 1550 to 1650 -- do not know whose measures are absolutely accurate. Furnaces suggested have been 70,000 BTUs to 135,000. AC tonnage has been 2 1/2 tons to 4 tons with the inner coils matching.
Also suggested has been Aprilaire Humidifier 400 & 600 and Air Cleaner by Merv a 10 or 16, or an Aprilaire.
Where do you live, what type of house construction? It's hard to second-guess a Manual J when we don't even know if you're in Texas or Ottawa. The design temps differ a bit between the two.
Originally Posted by pacesq
Gas furnace sizes may differ a bit between, say, an 81% efficient furnace and a 95% furnace. You might need the next size larger 81 to match the output of the 95.
Still, such a range of values indicates that some of the folks doing the bidding are using rule-of-thumb, other are using varying qualities of Manual J, and the rest are just smoking crack.