Size duct based on air-handler (5-ton) or condenser (4-ton)?

[Shoester]

I bought a used system to put in my shop, it's a 5-ton air handler and a 4-ton condenser unit. My understanding is that this is not terribly uncommon. I will be doing the ductwork myself, but I am a total amateur. Do I size the ductwork based on the air-handler or condenser? My gut is telling me air-handler, as everything appears to be based on CFM, which is driven by the air handler, but you know what they say about assuming! Thanks!

 

[MrFreeze]

Size your ductwork by the cfm put out by the air handler. Condenser presumably sits outdoors and sheds heat directly to ambient air.

Good luck,
MrFreeze

 

[TRWham]

That air handler probably has multiple speeds, so you need to decide how much air to move, then base your sizing on that. You need to match the airflow to the evaporator and the cooling capacity you want. More air yields more sensible capacity, but less dehumidification (latent capacity).

 

[bonneyman]

That air handler probably has multiple speeds, so you need to decide how much air to move, then base your sizing on that. You need to match the airflow to the evaporator and the cooling capacity you want. More air yields more sensible capacity, but less dehumidification (latent capacity).

Agreed. Most newer air handlers have multiple speed taps on the circuit board to adjust CFM in the field as manufacturers never know exactly what conditions the eventual location will have. So they give the install techs a way to "tune" the AH to a building.

 

[fitter30]

1600cfm = 4 tons cooling
2000cfm = 5 tons
There probably isn't much difference between sizes of duct.

 

[Shoester]

Thanks guys, appreciate the feedback. I have a follow-up question - how do I size/plan for where to reduce the duct to maintain pressure? For example, my air handler will be at the end of a 60' long duct run across the length of the shop. If I start with 18" duct coming off the plenum running down the length, how do I know when to reduce to a smaller size?

 

[PoorUB]

Thanks guys, appreciate the feedback. I have a follow-up question - how do I size/plan for where to reduce the duct to maintain pressure? For example, my air handler will be at the end of a 60' long duct run across the length of the shop. If I start with 18" duct coming off the plenum running down the length, how do I know when to reduce to a smaller size?

How large is your shop?

For a one a two or three car garage I don't see a reason for much duct. My furnace sits about 1/3rd the way and a 40 foot wall. The duct consists of a 24" run, the splits into two ducts, but they are short. One is straight, the other has a 45 degree elbow. both have vanes at the openings to deflect the air down. The whole duct run is 3-4 feet. I also have a ceiling fan and the shop is comfortable.

If you insist on running duct across the shop, size your duct for the CFM at the air handler, run it about half way, install half of the registers, then resize the duct for 1/2 the CFM and mount the other half of the registers.

Your AHU should have fan speed information available. You mentioned a 5 ton AHU, but i would assume you will run the blower motor on 3rd speed. 1600 CFM for 4 ton requires a 12"x20" duct, or something similar, 16" square would work too. The other half of the run requires a 12" square duct.

 

[Shoester]

How large is your shop?

For a one a two or three car garage I don't see a reason for much duct. My furnace sits about 1/3rd the way and a 40 foot wall. The duct consists of a 24" run, the splits into two ducts, but they are short. One is straight, the other has a 45 degree elbow. both have vanes at the openings to deflect the air down. The whole duct run is 3-4 feet. I also have a ceiling fan and the shop is comfortable.

If you insist on running duct across the shop, size your duct for the CFM at the air handler, run it about half way, install half of the registers, then resize the duct for 1/2 the CFM and mount the other half of the registers.

Your AHU should have fan speed information available. You mentioned a 5 ton AHU, but i would assume you will run the blower motor on 3rd speed. 1600 CFM for 4 ton requires a 12"x20" duct, or something similar, 16" square would work too. The other half of the run requires a 12" square duct.

Thank you, this very helpful. Shop is 44x64, with an enclosed 20x20 bump out on one corner.

 

[TRWham]

Thanks guys, appreciate the feedback. I have a follow-up question - how do I size/plan for where to reduce the duct to maintain pressure? For example, my air handler will be at the end of a 60' long duct run across the length of the shop. If I start with 18" duct coming off the plenum running down the length, how do I know when to reduce to a smaller size?

Typically you recalculate the remaining air every time you remove air from the flow and size the remaining duct based on the reduced number.

 

[Shoester]

Typically you recalculate the remaining air every time you remove air from the flow and size the remaining duct based on the reduced number.

And the 'reduced number' is as simple as subtracting a certain CFM based on how many registers each section has?

For example, if I have a total of 5 registers for 2,000 CFM, I would subtract 400 CFM past the first register, another 400 CFM past the second register, etc? Or does the size of the register dictate the CFM to subtract?

 

[TRWham]

And the 'reduced number' is as simple as subtracting a certain CFM based on how many registers each section has?

For example, if I have a total of 5 registers for 2,000 CFM, I would subtract 400 CFM past the first register, another 400 CFM past the second register, etc?

Exactly.

 

[Shoester]

Exactly.

Thank you for the help.

 

[PoorUB]

And the 'reduced number' is as simple as subtracting a certain CFM based on how many registers each section has?

For example, if I have a total of 5 registers for 2,000 CFM, I would subtract 400 CFM past the first register, another 400 CFM past the second register, etc? Or does the size of the register dictate the CFM to subtract?

Yes, that is correct, but in the real world nobody resizes the duct after every register, unless it is a large percentage of flow. I suppose if you are only doing 4 or 5 registers you could, or after every two registers would suffice too.

 

[gagecalman]

How large is your shop?

For a one a two or three car garage I don't see a reason for much duct. My furnace sits about 1/3rd the way and a 40 foot wall. The duct consists of a 24" run, the splits into two ducts, but they are short. One is straight, the other has a 45 degree elbow. both have vanes at the openings to deflect the air down. The whole duct run is 3-4 feet. I also have a ceiling fan and the shop is comfortable.

If you insist on running duct across the shop, size your duct for the CFM at the air handler, run it about half way, install half of the registers, then resize the duct for 1/2 the CFM and mount the other half of the registers.

Your AHU should have fan speed information available. You mentioned a 5 ton AHU, but i would assume you will run the blower motor on 3rd speed. 1600 CFM for 4 ton requires a 12"x20" duct, or something similar, 16" square would work too. The other half of the run requires a 12" square duct.

Any chance you can post a few pics of your furnace setup?

 

[PoorUB]

Any chance you can post a few pics of your furnace setup?

It isn't pretty, but functional!

 

[gagecalman]

Thanks. I like it.
My buddy bought a regular 2-ton heat pump for his 24 x 36 x 10 cape cod pole building. He wants to have it installed with elaborate ductwork to try to condition the upstairs and downstairs. The building is uninsulated. I'm not a HVAC guy but I don't think it will work.
I'm trying to convince him to just install the HP for the downstairs, do a mini split for the upstairs, and at least insulate above the downstairs.