Reduce cfm
Reduce cfm
(OP)
I have a 12 ton existing roof top unit for a 2,000 square feet retail space. my heat calc comes only to 4.0 ton. How do i reduced my cfm? please help
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RE: Reduce cfm
If you put a bypass from the supply to the return at the unit with ~3200 CFM bypassed. The problem will come with the control of the unit and cycling as the compressor may not run long enough to dehumidify the space in the summer. Depending on the location the heating side may over shoot as well depending on the type of heat (single stage, two-stage, modulating gas etc). You will in essence run into the same problems you get from oversizing equipment.
The retail space may have had a great deal of lights previously, or the unit is serving more than your 2000 sq.ft.
RE: Reduce cfm
RE: Reduce cfm
RE: Reduce cfm
While 12 tons appear to be excessive, all that should really mean is that the AC will run less efficiently than one that's properly sized because it will run at a much lower duty cycle than it should. Anything short of replacing the unit with a correctly sized one will likewise result in inefficiency, so why not leave it alone?
TTFN
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RE: Reduce cfm
RE: Reduce cfm
Nonetheless, I don't think you can make anything better by mucking with the CFM or loading. In most such cases, you wind up being more efficient, but with a higher load, which still pumps up the utility bill.
At the 10,000 ft level, nothing you do alters the heat load, so the delivered CFM has to be about the same, so the load on the AC is the same. Anything you do will not improve the wall plug efficiency of the system. Note that the store can healp itself by making the AC setpoint higher, thereby getting some AC help from the mall, but they risk losing customers because it's too warm in the store.
TTFN
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RE: Reduce cfm
RE: Reduce cfm
RE: Reduce cfm
TTFN
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RE: Reduce cfm
I wouldn't let it go to two-stage operation with reduced airflow, though, freeze-up can be expensive.
Let us know what you decide to do!
Good on ya,
Goober Dave
RE: Reduce cfm
I bet that the 12 ton rooftop is good for your retail space.
Do not forget lighting and people load. You are too low on the cooling load calcs.
Revise your load calcs.
RE: Reduce cfm
You can try turning this thing into a VVT system with a main by-pass between the supply and return (no money, no VVT boxes), then you will need to re-balance all diffusers, cause the first diffusers will suck all the air and the last one will be starving for air.
Try re-adjusting the minimum outside air on the RTU, the original 12-ton may have a high min OA compared to you 4-ton load. Then run the unit on one compressor only, make sure you have hot gas by-pass (if not buy a HGPB kit).
RE: Reduce cfm
What my manager decided to do is, and I need your input here again, is...to adjust the belts, down to 4200 cfm, where going to discharge 1,200 cfm to the return plenum and will give us 3,000 cfm left of supply to distribute to the 2,000 sq.ft. retail space.
I don't know if this is the best solution, but I'm stil in doubt.
RE: Reduce cfm
At 2,000 SF, the 62.1-2007 has a default of 30 people for outside air, at 16 CFM/PN, for minimum of 480 CFM (if this is retail sales) outside air.
While no minimum total ACH is given, I would expect 4 ACH to be typical minimum for office and light retail, so about 1,920 CFM would be needed for the 4 ACH.
For your thermal load analysis, what amount of outside air did you use? Also, are you using the 99.6% or 99.0% weather data? The tons/area rules of thumb fall apart when you go from zero to 100% outside air.
RE: Reduce cfm
This is supposedly a retail space, so there is no customer sitting at a dining table getting blasted by the AC. So, why does it need changing?
TTFN
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RE: Reduce cfm
RE: Reduce cfm
Good luck!