Duct Size and Furnace Size

Hello,

As a prospective buyer of a replacement gas furnace, I have been reading many of the posts in this forum for several weeks now. I have learned a great deal about a number of things, including what to look for in an installer/vendor.

I have obtained about half a dozen estimates to replace a 23 year old 120,000 BTU Carrier furnace. I live in a 2100 sq ft condominium in a renovated 1920s brick building in the Boston area. Our current furnace works all right, but it has very short cycles and the condo gets a bit cold in between cycles. i am thinking that it may not last much longer and that a 2 stage variable speed model may provide more comfort. Also, we have to go with an 80% model because there is no way to vent the high efficiency furnaces in our building.

Only two of the potential installers have taken measurements of the condo and done heat load calcs. Heat loss was calculated at about 59,000 BTUs by both installers. However,they are each recommending different size units -- and this leads up to my question.
The first installer recommends an 80,000 BTU 2 stage variable speed. The second installer recommends a 100,000 BTU 2 stage variable speed and states that "because of the inherent ductwork a larger blower will be needed."
Now the ducts are quite large, and i cannot afford to change them. So my question is: Is it true that the size of the ductwork determines the size of the furnace -- and that i may have to go with an oversized furnace because of the ductwork??

All the other installers kind of eye-balled the place and then gave me an estimate for a 120,000 BTU furnace, which i assume is oversized. However, this new issue with the size of the ductwork now makes me wonder if they somehow took that into account in staying with a 120,000 unit.

So anyway, i am quite confused, and would appreciate any input on this question!

Only Manual J can determine the size of your unit.

Click on the red tab above and do your own.

Once you have the size down then and only then will you know if the existing ducts can handle it.

Manual J is not always the solution for me , by the square footage you are giving and the age of the building and how you say it seems to get cold in between cycles the one contractor you have had that recomended the 100,000 btu 2 stage seems to be the on the right track, since you can only use 80% in your building i would probably up it to a 120,000 but i would have to know the duct size to determine how many ton drive i would use, and i would keep it a two stage just for better efficiency. The only thing is with the age of your building and furnace you would have to line the chimney unless it is flued through the roof with double walled pipe , inspectors are enforcing the chimney liners very regularly due to all the new codes in Mass. Hope this helps

I agree on the Manual J statement and am glad to see that you got two contractors to actually do the calc. Walk away from the rest, and certainly walk away from any suggestion of a new 120k unit. Square footage based estimates are a waste of time. We're talking about a condo, which means you're going to be sharing common walls with other heated space; even though the building is old, it sounds like it has been rehabbed already, which means it's insulated. Unless somebody made a huge mistake on their Manual J, it'll be right- and if it's wrong and you somehow end up undersized, it's their responsibility to fix their mistake.

When I hear "the furnace cycles a lot, and it gets cold between cycles" I usually assume that the existing furnace output is already oversized. A smaller unit will yield steadier temperature regulation, with long, steady heating cycles. That's the kind of operation where people tend to say "I can't even tell it's on, except that the house stays warm". What you have now sounds like the classic roast-freeze-roast-freeze cycles of an oversized unit. Slow and steady really is much more comfortable. Using a two-stage furnace will mask some of the effects of oversizing, but it will act like a smaller single-stage furnace because you'll never have enough demand for it to need the high stage at all.

From the perspective of a 23 year old 120k input furnace, a new 100k 80% unit will have similar output. Your statements on comfort suggest that you really need smaller-output unit than what you have now, and Manual J strongly agrees. In your shoes, I'd use an 80k input or at most a 90k (Carrier 58CVA-090 is an 80% two-stage variable with 90k input, 71k output, and a four ton drive instead of three if you're really worried about that).

Going to 100k because of ductwork is a chicken and egg question. A 100k furnace will probably have a larger blower, but it will also probably *need* more airflow because it will have more heat output in the first place.

[Edited by wyounger on 01-25-2006 at 11:16 AM]

Quote:

---

Originally posted by ir_boston
I have learned a great deal about a number of things, including what to look for in an installer/vendor.

I have obtained about half a dozen estimates to replace a 23 year old 120,000 BTU Carrier furnace. I live in a 2100 sq ft condominium in a renovated 1920s brick building in the Boston area.

i am thinking that it may not last much longer and that a 2 stage variable speed model may provide more comfort. Also, we have to go with an 80% model because there is no way to vent the high efficiency furnaces in our building.

Heat loss was calculated at about 59,000 BTUs by both installers.

The first installer recommends an 80,000 BTU 2 stage variable speed.

---

You will be quite pleased with 80k BTU 2-stage
http://www.trane.com/Residential/Pro...aces/XV80.aspx

or add a heat pump in a dual fuel set-up
if you wish to have A/C.

Hi,

Thanks so much for the replies. It seems that more people agree with the recommendation for an 80K unit than a 100K unit. Regarding insulation, I dont think the walls are insulated but the windows are newer windows (double-pane).
It is very helpful for me to know that a current 100K input unit will have equivalent output to a 23 year old 120K unit. If the current one is oversized then maybe i should go down to the 80,000 BTU. I will probably try to get another one or two contractors over now that i know all the issues to consider.

As for the chimney liner, yup, i am working on getting that done as well!

thanks