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Old 06-27-2009, 03:17 PM   #1
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60 year old duct system with 4 inch ducts


My house used to have a low pressure GE oil furance. Nice alumn. ductwork, but all the runs to the boots are 4 inches. In 2001 the former owner put in a new Lennox oil furance. The fun cycled constantly, even though she had a smaller fan put in (5 inch). Now that I'm remodeling I want to switch to gas and was hoping to use the existing duct work. I understand that there MAY BE variable speed gas furnaces that will do OK with the 4 inch ducting? Some contractors have told me I don't have any options and that I must size up the ducting to 6 inch. I can't get to the majority of the ducting and I have lath and plaster, so I don't want to start tearing into the walls at this point. I don't want to purchase a new furnance and find out I am still going to have the same problem with the fan cycling etc. Any thoughts on this?

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Old 06-27-2009, 03:29 PM   #2
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60 year old duct system with 4 inch ducts


How many 4" supplies do you have.

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Old 06-27-2009, 04:43 PM   #3
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60 year old duct system with 4 inch ducts


Upstairs: 8 o- 4 inch
downstairs: 3 - 4 inch
downstairs: 2 -6 x12 robbers
4 return total - 6 x 12 (2 up and 2 down)

I can add some supply(s) if needed as well as returns - just don't want to switch out the 4 inch I have now. Some exposed duct work in the downstairs I could access. The house is just under 2000 square feet.
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Old 06-27-2009, 05:47 PM   #4
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60 year old duct system with 4 inch ducts


I doubt if a ECM unit will work well with those pipes. My technical rep from Lennox has told us that some newer cheap built homes with undersized ductwork are having problems with the ECM motor not being able to find a constant speed from a lack of airflow. Causes the motor to hunt. May also void the unit warranty. We have a few of those 4" systems in Wpg and they are terrible, Sorry.

Last edited by yuri; 06-27-2009 at 05:50 PM.
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Old 06-27-2009, 06:05 PM   #5
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60 year old duct system with 4 inch ducts


You don't really have enough supplies for a VS blower to work riight.

Your supply static would be too high even if your furnace was only a 60,000BTU.
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Old 06-27-2009, 10:08 PM   #6
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60 year old duct system with 4 inch ducts


How big a furnace do you need? If you have enough 4" ducts, it could work, but currently you have 132 sq in of supply from the 4" ducts and that's less than half of what you would need for a 70,000BTU furnace. Not familiar with the term "robber" but if it's a supply duct, you are getting another 144 sq in of supply from those. Problem with 4" ductwork with a gas furnace is the velocities will be quite high, producing drafts and noise.


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