effect on downsizing

19 years ago

I want to replace my furnace & a/c. The current a/c & blower is 4 ton and the ductwork will handle about 1400 CFMs. Both furnace & a/c are oversized.

My heat calc shows that I could get by with a 3 ton unit. My upstairs is more comfortable when the weather is really hot and my unit runs much longer.

If I go with a 3 ton a/c & matched furnace (blower), my airflow will be about 1200 CFMs. With a variable speed blower, it might be set a little higher.

Will a decrease in CFMs through my duct system cause a drop in static pressure (good?). Will this result in less frictional loss of air flow to the upstairs?

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Comments (3)

eths
19 years ago
I would not downsize, but I lean toward oversizing. Don't take my word for it. Read posts from udarrell, or solicit his opinion. He has lots to say about this topic. Also, he has mentioned (previously) advanced Tstat settings that allow you to control humidity better by having the unit run longer without freezing...
don_b_1
19 years ago
I can set my 2Â½-3 ton variable speed blower to produce anything from a low of 1020 CFM to a high of 1980 CFM with a multitude of choices in between.
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veesubotee
Original Author
19 years ago
Posted by don_b_1 (My Page) on Wed, Sep 19, 07 at 23:44

I can set my 2Â½-3 ton variable speed blower to produce anything from a low of 1020 CFM to a high of 1980 CFM with a multitude of choices in between.

I don't doubt that you can do that, but would you? If your furnace or a/c blower speeds are not set to operate within an ideal range based on your heat input or cooling capacity, I suspect that you might have some problems.

My original question concerned reducing size (4 ton to 3 ton) with accompanying reduction in CFMs and how that would equate to (more)less comfort.

V