furnace and heating duct questions?

Question

This is a two part question.
My home has a Bryant Quietline furnace, you can tell its very old. It runs quite a long time to heat my 750sq foot home and I don't keep the heat very high, it just works hard to do a very little.
For savings and saftey reasons I would like to replace it soon.
First questions, what size btu unit would I be looking at for about 750sq feet? The home is very well insulated in the attic, but has windows that are getting close to needed replaced. I would think that 45,000 3 ton or 70,000 4 ton would be good since the home is fairly small. What would you recommend?
I just glanced at http://www.acdirect.com/goodman_ruud_gas_furnaces_.php for prices and those units run about $800 on that site, is about what I should expect?

2. I've questioned the duct work since I moved in. Heres why. The house is built on a concrete slab. The duct work appears to consist of areas left open within the concrete slab to allow air flow. Hard to describe,

duct work cont.
but from what i can tell, i would describe it as if they blocked out areas for the duct work. poured concrete for the foundation slab and then removed boards and had a maze concrete based tunnels for duct work. I can't see how this would be efficent wouldn't the heat be lost in the concrete? I was thinking to have some duct work placed in the attic and have the vents come from the ceiling. Since the home is not that big it could be accomplished with 8 ceiling vents thats about 2 per room. Is this thousands of dollars worth of work just in doing this?
Please give me your advise on anything you can. I've wondering about this stuff for a very long time
Thanks
Ryan

Answer 1

A 2 ton would do it for 750 sq.ft That would be 8 regular sized vents.The air has to be vented because if it is not you would have backflow and this would ruin your new unit.The vents are 100 cfm per vent and every ton is 400 cfm.So a 4 ton would require 16 vents.A Goodman unit is a pretty good one for the price these are what I install.

Answer 2

First, you want a furnace that runs a lot and an A/c that runs a lot, that is the most comfortable kind keeping the house at a more constant temp. Too large a furnace is Noisy and too large an AC will not keep the humidity down as it runs so little.

May i suggest that you use the same ducting that is below in the concrete and purchase a furnace of 90% efficiency with a VARIABLE SPEED MOTOR and a VARIABLE CAPACITY GAS VALVE .(2 stage furnace) That type of a furnace will not only compensate for your ducting which might be slightly inadequate, but the variable gas valve will allow that house to be very comfortable and run at low low gas consumption and low low blower speed. So quiet you cannot even hear it So comfortable its unimmaginable something could do tht good a job.
This is what i own and it is a really superior furnace.

I dont like the low end Goodman furnaces, You are only going to replace these new furnaces once in 20 years so spend the money on a Top line Trane(lowes sells these) or Carrier. Product. Sp spend the money on a Variable SpeedBlower type ++++A variable Volume gas type furnace often called a 2 stage furnace. Dont fool with ducts!!!