my HVAC calc results vs 3 estimates

[ams14]

I've recently gotten 3 estimates to replace my furnace. All companies seemed reputable, 2 were on recommendations. One recommended a 60k BTU model, one a 90k (Amana AMV), and the other (trane) claimed they did a load calculation but wouldn't tell me the recommended size until I select them.

I bought and ran the HVAC calc program for an additional opinion and to try to help me make an educated decision. I realize this is no substitution for a quality HVAC system design, but I thought additional information couldn't hurt.

Here goes:
The house is a 2 story 1901 brick, 1600 sq. feet, with a 312 sq. ft unheated/unfinished partial basement (the rest is crawlspace). The windows are leaky original single pane floated glass with no storms and there is an old fireplace. Currently there is a 40 year old Lennox 120k BTUH.

Running HVACcalc I get
62k BTUH (assuming R11 first floor and 1 degree winter temp)
72k BTUH (assuming no first floor insulation and 1 degree winter temp).

In truth, there are plaster walls with newspaper for insullation, so I'm guessing it's somewhere between. Maybe 68k BTUH?

So if I go with the 90kBTUH in an 80% efficiency (dual stage), that would give me an output of 72k BTUH. Does this sound like the right ballpark?


Ducts: As far as ducts go, everyone said they looked fine. hmmm....
I currently have 5 seperate 6" diameter runs coming from the plenum, 2 of these split to 2 registers. They are all fairly straight runs (in the crawlspace) to the 1st floor, ranging from 5' to 25' long. I will also add (and by "I" I mean the company I select) a 10" duct going to the upstairs (currently there is no heat up there).

A quick estimate shows that 6" ducts (5 x 110 CFM/duct) + 10" (1 duct x 410 CFM) = 960 CFM. Does this seem a little low for a 90kBTUH furnace? I realize this will be mostly a guess unless I can find a contractor to perform a manual D, however it didn't seem like the ones I had in intended to do that.

Again, I realize I am not performing a thorough system design, but I appreciate any input.

Thanks!

Andy

[lynn comstock]

The best info you gave is that the old furnace was 120K and 40 years old. The proposed furnace will be 25% smaller. Don't worry that 90k will be too big.

We have no way to closely evalutate a heat calculation because careful observations and assumptions must be made, and we cannot know how well you or the companies did with their observations and assumptions.

Lynn

[smokin68]

For a starting point, if the oldy is a 60%, that's 72k btuh output. Did it heat well? I wouldn't be afraid of a slight oversize in heating, but this is just my opinion.

[ams14]

thanks, for the input.

Yeah, I figured I was getting about that out of the old furnace, so that sounds right.

From the layperson's point of view, it seemed like it was well sized. The rooms were fairly evenly heated. On VERY cold nights over the past few years it would always keep up and didn't run constantly.

[ams14]

and smookin68... as a 72 Chevelle SS owner I have to take offense at your signature line

[slimwoodie]

yea ...

my '71 chevelle SS454 ... would ..

you know .....

Sssssmokein' on his ... ford

[amd]

For a starting point, if the oldy is a 60%, that's 72k btuh output. Did it heat well? I wouldn't be afraid of a slight oversize in heating, but this is just my opinion.

The output rating of a conventional furnace can be 70-80% of input; 60% takes cycling and pilot losses into consideration.

120k*0.70= 84k btuh output.

[amd]

Have you considered improving the building envelope and getting a smaller furnace? (lower gas bills)

Some things like insulating an exterior wall are very expensive with a long payback; others are much cheaper. (attic insulation, draft proofing, and storm windows come to mind)

[ams14]

Good suggestions. I've actually done about all I can in that department. The upstairs is a flat roof & knee wall setup. I insulated the entire knee wall/floor area. Can't change out the original windows. Maybe storms though, I'll look into that.