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.
Thank you for your quick response. The other problem is the owner doesnt want to replace the existing roof top unit. I was thinking is there a way of redirecting the excess cfm? I'm not sure..please help I need more options. Thanks again
4 tons is what you might proffer for a residential application, i.e., 2000sqft/(500sqft/ton), but a commercial application usually has a higher load due to lighting requirements and occupancy. Moreover, in many commercial applications, the heat gain from outside is usually much higher than residential, due to doors cycling, and more window area, and often, much poorer insulation.
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?
Actually its 4 point something. To round it off, it will come up to 5 ton. It's actually inside the shopping mall, so there's no exposures. it's only a typical retail space with minimal lighting and equipment.
"inside the shopping mall, so there's no exposures" Well, I'll argue that many stores have their doors open to the mall during the entire business day, so yes, there is an exposure. Now, some malls, possibly those that go bankrupt from their AC bills, help out by keeping the general access areas heavily AC'd, and your heat loss might actually be flipped, and you're getting cooling help. I've been in malls where the walkways were substantially cooler than some of the stores.
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.
Sorry, so replacing the unit to a lower cfm is not an option because the owner doesnt want to spend. My question remains, is there a way to reduce the cfm? can you adjust anything? can you add something to the unit to reduce it's cfm? can you redirect the air? I'm NOT really sure, I'm new to hvac. SEE url for unit spec. GCS16-1603. Please help
Try to turn off cooling and leave on heating and ventilating and try to see if the mall cooling with the doors open will make a difference but if that doesn't work then a replacement may be the best option
The unit has two stages of cooling (i.e., two compressors). You may be able to slow the fan down to give a decent supply air temperature with only one stage running, and re-do the controls to eliminate the starting of the 2nd stage? You'll have to look at the coil geometry to be sure.
I wouldn't let it go to two-stage operation with reduced airflow, though, freeze-up can be expensive.
Autokad :
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.
A client that does not want to spend money you say? Can't make an omelet without breaking the eggs.
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).
Thank you all for your inputs. But it's true, reality is most of the clients doesnt understand, all they want is cheap, cheap, cheap...don't care just make it work. Anyway enough with my vent.
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.
The cooling load and the total air flow requirements do not always match up.
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.
I still don't see why you need to change anything. A higher CFM means that you get your cooling air in quicker, and then the thermostat stops the AC. Over the course of an hour, roughly the same amount of air is supplied, regardless of the actual tonnage.
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?
When you have an oversized unit, you are constantly cycling the compressor on and off. Bad for dehumidifciation.. horrible if you are in a humid climate (Florida). If this is an enclosed multi-story mall, yeah, 12 tons is overkill. I am all too familiar with owners having shallow pockets. Be sure you and/or your boss send something to the owner in writing, letter, email, something, so if/when something goes wrong, you have something to fall back on. Ask me how i learned that one!
