Do you mind revealing the brand name/mfg?
Static pressure does not indicate true air flow. If you do not have a flow hood than you should simply be able to calculate CFM by the Equal Friction method
Equal Friction Method:
Area (Sq Ft.) = CFM divided by velocity FPM
where as: area = 15,000 CFM divided by 1500 FPM = 10 Sq
10 Sq. Ft x 144 (Sq. Inches per Sq. Ft)/24" = 60"
Therefore, select 60"x24" supply duct
5400 CFM ÷ 1700 FPM velocity = 3.18 Sq Ft. Duct Area
Aspect Ratio = 48/12= 4
Aspect Ratio 4 = 144%
Round duct = 66%
Difference = 78%
Velocity FPM = airflow CFM/ area in Ft Squared
Velocity x Area = CFM
To Figure CFM by known Sq Ft area and Static Pressure.
To Measure Air Volume there are two easy methods:
1. Temperature Rise:
In the temperature Rise Method, the heating section is ran with the indor fan speed set to cooling speed.
The temperature of air entering and leaving the air handler or furnace is measured to find the temperature rise. A formula is computed and air volume is determined.
2. External Static Pressure Method:
This called total external static Pressure.
A differential manometer is used to measure the air pressure at the inlet and outlet of the blower assembly. This pressure is the same pressure a balloon has pushing against its internal walls. Air Blowers have the same type of pressure at the inlets and outlets. The measured air is compared to design charts provided by the equipment manufacture to determine how much air volume is flowing in cubic feet per minute. (CFM).
As the static pressure across the fan goes up, the ability to move air goes down. This static pressure at the inlet and outlet is called total external static pressure.
The higher the Static the less air will be able to move through the air handler. The less static pressure that is present, the greater the ability to move air through the air handler.
For every cubic foot of air, the air has weight.
Every pound of air carries so many BTU's of heat in it.
When a system is designed the amount of air is set and the heat in the air is measured. This is important as the system can only absorb as much heat as is available. There are two ways the amount of heat fluctuates, by the amount of pounds of air flowing and by the heat in the air. If the heat in the air the system capacity falls. If the amount of pounds of air falls, the system capacity falls.
please tell us the brand you are having issues with so we can stay away from them.. thx :)
Airflow has been confirmed and re-confirmed...reply #'s 3 and 13.
Trouble shooting is not part of the repair...understand the symptoms and you will find a solution.
Another question: is this equipment brand/model/design something that's been around a while or something out of China :grin2: that you decided to try?
Does this look like your elements?
I agree with the guys on checking the temperature rise. That should tell you everything you need to know. If I had that problem I would temporarily bypass all the limits long enough to do the test. It shouldn't take more than 15 minutes. For reference a 10kw strip at 800 cfms should give about a 40 deg. Temp rise. If you figure the CFM's by the temp. rise method then if your rise is too high you've got airflow problems & if its too low you've got air bypassing the strips. If its in the ballpark then I would think the limits are probably the problem & the factory reps recommendation is correct.
Thanks Mr. Bill, Saturatedpsi and Gary for your replies... The units were sold to me by one of the largest (if not the largest) mfgr/distributors in the country, however, the units were manufactured by another company under another label. I don't want to yet mention their name because I don't want to bring upon them any bad press. I have bought hundreds if not thousands of fine air handlers and condensing units from them and probably will continue to do so. When you look at this unit from the outside and for the most part from the inside, it looks almost identical to the units manufactured here in the country by other big name brands. The one exception is the heat strips. The heat strip that you depicted is not what is in this unit we are having trouble with. The heat strip you depicted is in the brands that I replaced the troubled unit with. Notice how the strips you posted have thin curely cue wires stragically fashioned in size & shape and placed to be directly in front of the two blower ports. Every inch of these curely cue ht strips are washed in the air stream thus giving up all their heat to the supply air; unlike what is happening with the troubled units. I'm not sure the factory could up the safety temeratures after being tested and labeled, if they could I think they would have. Besides if the customer is not getting the benifit of all the heat being generated being brought out in the room and rather just heating up the furnice and ceiling cavity its really not fair to them for the wasted energy cost.