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furnace duct sizing


Ken Meyer
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This may be a dumb idea, but I thought I'd throw it out there and see what you guys think. About 15 years ago, we converted from an oil furnace to a high efficiency gas furnace. The basement ceiling is 87" high,and one of the heating ducts is 17"wide by 11" high, leaving roughly 76" of headroom. Since we changed the furnace, we have remodeled two bathrooms and removed the ductwork to them, using a toekick heater in one of them, and radiant floor heat in the other. Both bathrooms previously were fed off this duct. It occurs to me that I could reduce the size of the heating duct to gain some headroom, since the demand is lower now. How would I go about calculating the size required for the duct?

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Hi,

Well, I don't know how to calculate it, but I can tell you that most HVAC guys around here prefer to tear out and replace the large ducts used for oil heating systems with smaller ducting that's more appropriate for gas systems, so I have to believe that what you want to do seems to make sense. Terry might know.

OT - OF!!!

M.

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It has been too long since my HVAC training to even begin to properly calculate duct size, but I might provide some pointers in general theory.

Assuming the furnace is correctly sized for the entire house, each room duct will be sized based on the CFM supplied by the blower. You have to have enough outlets in the house to avoid throttling the output from the furnace. This is also important if you have central A/C, maybe even more important since the system will ice up with too low of air flow. If you restrict the duct size in one room without a corresponding reduction of airflow at the other rooms, the static pressure in the system has to be taken into account in order to make the new duct work properly. This means you might need to throttle down the other rooms with in-line or grill dampers to obtain a higher pressure for the new duct.

There is a proper method of calculating duct size but most tech around here use a rule of thumb approach which may not give good results in a conversion situation like yours.

Remember faster moving air yields noisier systems.

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Get a Trane Ductulator http://www.professionalequipment.com/du ... KNC-Google They're kinda neat to play around with anyway.

First you need to know the CFM of the furnace - on high speed if you have central air and a two speed blower. The main duct should be big enough to handle the total CFM of the furnace and is typically rectangle in shape. Then as you keep running branch lines to feed the rooms the main duct can be reduced accordingly ie. 800 CFM=10x12 main duct, 1st branch line is 6" round @ 200 CFM, then the main duct goes down to the corresponding size for 600 CFM. These numbers are pulled out of the air but you get the point. If you're looking at gaining head room you can play with the rectangle shape to adjust for headroom.

In your case with removing duct work you have also decreased the load. If you did the remodel after the furnace change the furnace is now over-sized. While this may not affect the furnace in heating choking back on air flow will come back to bite you in the cooling mode. Shutting down airflow =ice on the evap.

It's not complicated to figure out but is really best left to an HVAC contractor or sheet metal company.

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Originally posted by inspector57

Assuming the furnace is correctly sized for the entire house, each room duct will be sized based on the CFM supplied by the blower.

That's not quite right. A heat load calc determines how much air the room will need, to maintain design criteria, followed by what size of duct will deliver the needed CFM. Adding up all the rooms will determine the size of the furnace (or you can wag at X cfm per square foot).

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Originally posted by Terence McCann

Originally posted by inspector57

Assuming the furnace is correctly sized for the entire house, each room duct will be sized based on the CFM supplied by the blower.

That's not quite right. A heat load calc determines how much air the room will need, to maintain design criteria, followed by what size of duct will deliver the needed CFM. Adding up all the rooms will determine the size of the furnace (or you can wag at X cfm per square foot).

Thanks for the info. There is no central air, we get along fine without it, so it's just a heating system. I'll gather more information about the furnace and see what I can figure out. I just want to know if this is practical. Maybe if and when the furnace ever goes out, we'll replace it and move it to a new location other than the middle of the basement, and redo the ductwork at that time, we would gain a lot more room that way.
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