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Sizing Ducts in Zoned Systems

Posted by Grovest (My Page) on
Mon, Aug 8, 11 at 14:49


We obtained two quotes to install a new zoned system. Both quoted the same equipment:

AC: Carrier 24ACB7 (2 Ton 2 Stage)
Furnace: Carrier 58UVB (60,000 Btu 2 Stage)
Controls: Carrier Carrier Comfort Zone II

Although we considered an Infinity system we can't afford the nearly 40% premium this added to the pricing.

Both firms used Elite's RHVAC for sizing which calculated gains & losses as follows:

Zone_4 __680_sqft_-_15300_btuh_-_5600_btuh_-__250_cfm_-__190_cfm_-__250_cfm

**** Option #1 ******

The first firm sized the ducts as calculated by the software (Elite's RHVAC) with no oversizing. They explaned that the software takes everything into account and sizes the ducts correctly without needing to adjust the cfm for the zones. Any excess airflow is handled by a bypass connecting the supply to the return which opens when the pressure in the ducts gets to be too high.

They are proposing installing the following trunks for each zone:


Bypass 8x16 - Loop between Supply and Return

**** Option #2 ******

The other company claims that the software treats the entire system as if it wasn't zoned and that it is necessary to increase the size of the ducts in order to get the system to work properly. He explained that zoned systems should be designed such that bypassing isn't necessary and to achieve this goal each zone must be designed to handle 60% of 1st stage blower capacity less an allowance for closed damper leakage. He explained that this is calculated as follows:

1st Stage = 670 cfm
Damper Leakage = 10% x (260 + 260 + 320) = 84 cfm
Minimum Zone Capacity = (670 cfm - 84 cfmx) x 60% = 350 cfm

Which results in the following zone sizing requirement:


He explained that although a bypass technically would not be required with this design that his preference is still to install one simply to protect the equipment should a damper fail leading to a condition where the furnace is running but all zones are closed. Since the bypass is akin to a pressure relief valve on a boiler he indicated that his preference is to just dump the excess air into the room the furnace is in rather than looping it back to the return air.

***** Option #2 - Upgrade #1 *****

He then explained that although the above sizing which he said was 'by the book' would work, considering the huge peak solar gain in the sunroom he would prefer to increase the size of this zone further such that it is sized to handle the full 600 cfm of the 1st stage to avoid the system getting noisy when only the oone zone was calling. However, this would slightly increase the cost of installation for the larger duct.

***** Option #2 - Upgrade #2 *****

He has suggested that we may wish to consider rather than doing this by adding the additional two registers required off a single resized trunk that instead a second damper be used. His reasoning is that he feels that if sized for 600 cfm that in winter (when only 200 cfm is required by the room) that the velocity at the registers would be too low to ensure proper mixing (the room is 25x25 with 11' cathederal ceiling). The downside (and reason he is leaving the decision to us) is that this would add a significant additonal cost to the system.

Who is right with regards to sizing ducts within a zone?

Do either of the 'options' presented by the 2nd contractor make sense?


Follow-Up Postings:

RE: Sizing Ducts in Zoned Systems

I couldn't read all that. You definitely want to go bigger with zoning. I have an Infinity and despite its controls, I am having to add extra ducts/vents in because of noise.

I always think you want to go big on these things so you have the ability to damp down some areas.

RE: Sizing Ducts in Zoned Systems

What is 2nd stage cfm delivery?
800 cfm for 2 tons. what happens when any one zone goes high stage. with no damper bypass the velocity will be extreme.Ive done it both ways. oversize duct by 60% is great for 2 zone application but never tryied it on 4 zone.
Im all about efficiency and hate bypass for this reason however you must accept bypass rather then problems with noise and velocity.that 8x16 bypass is wayy to big and I would go 8" round ( 400cfm) The smallest zone 350 cfm +byass of 400 = 750 . carrier moves air at 350 per ton so 700 cfm wil be ok. I would check to see what 2nd stage heat cfm is gonna deliver as well.

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