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Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 5648
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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I just purchased a new air handler and ac pump. The unit I

Customer Question

I just purchased a new air handler and ac pump. The unit I believe is a 3 ton unit. I am wanting to know what size of return lines do I need to run and also some questions with my supply duct work.
Submitted: 4 months ago.
Category: HVAC
Expert:  Phil replied 4 months ago.

hello, are you talking about refrigerant lines (copper tube)

supply and return duct size depends also on approximate length and how many elbows.

We can go from there

Customer: replied 4 months ago.
Supply and return duct work. Wanting I with metal duct work for both return and supply. The return length will be 20 feet total with 2 90 degree bends. The return grate register is 20.25 x 20.25. I will be installing two of them. The house is 1700 sq ft.
Expert:  Phil replied 4 months ago.

Thanks, ***** ***** tune it a bit, I need to the region of the country you are in

For instance in south florida we run 350 cfm per ton, because of the heat load from the humid air, and in arizona, 500 cfm per ton

Expert:  Phil replied 4 months ago.

If you like you can put a damper in the supply air duct to the bedrooms and adjust it for only 20% air flow when the bedrooms are not in use, then divert the rest of the air to the living areas... that allows you to set the AC to 85F when you are away, and when you get home dump 90% of the cold air into the living areas... thats called 'zoning' a system. it can save 30% or so on fuel costs

If you go that route the ducts must be enlarged a bit.

Customer: replied 4 months ago.
I live in eastern Oregon.
Customer: replied 4 months ago.
Shouldn't I be able to add up all the sq ft of the rooms. Install one main duct line that is rated for the cfm's needed then branch of that to each room with correct duct size for the rooms.
Expert:  Phil replied 4 months ago.

thanks, ***** ***** borderline valuable in that would be a bit of a frill in your case.

If you want to send me a photo of a pencil sketch you draw for your floor plan and duct runs I will mark it up with duct sizes.

Use the paper clip icon at the right end of the tool bar or the attach files link to the left of your send button for that.

Your idea of how to run and size the ducts is correct, however depending on the lay out it is possible to starve the distant rooms for airflow.... larger rooms and with rooms with windows facing south or west need more air flow than others

Expert:  Phil replied 4 months ago.

show NS EW on your sketch please and location and size of windows, and type of glass double or triple insulated etc.

Expert:  Phil replied 4 months ago.

Is this a heat pump? which brand and model is the air handler, if you have a variable speed blower, I will size the air ducts differently that if you the typical 3 speed blower

Mark the location of the air handler on your sketch please.

Expert:  Phil replied 4 months ago.

Depending on where the tee's are located air rushing past the tee can greatly reduce air flow into that side branch, and air flow that goes straight down a main header and dead ends at branch can force a lot more than you would normally get into that duct.... and these effects can change from heating to cooling etc, resulting in erratic temperature control.

The change is because we need less air flow for heating than we do cooling so this venturi effect on side branches and velocity pressure at the ends of runs changes summer to winter, and daily a few times as a system goes from heat to cool and back.

Its not costing you a dime more to have me do it for you correctly than to do it by rules of thumb...

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