So to prevent leaks as much as possible, an experienced engineer will specify leak testing on the air handlers provided on their jobs. This is accomplished by blocking off all openings into the unit and pressurizing it (positively or negatively) with a pressure blower and then measuring the airflow into (or out of) the unit to maintain a test pressure:
Several decisions have to be made when deciding how to test the unit:
- When to test (At factory or at jobsite)
- How many sections to test
- Positive or negative test pressure
- What pressure to test to
- How to set the failure criteria
There is a very strong argument to be made that the only pressure test that matters is the pressure test performed on the site. After all, it really matters little to the final built out project if the unit performed flawlessly on the factory floor. The actual performance in the field is all that anyone really cares about. If only a single pressure test can be fit into the budget, it stands to reason that the field test is the one most critical to the overall quality of the delivered project.
However, there is a case to be made for a factory test, too. The field pressure performance of an air handler is not only a function of the manufacturing process, but is also strongly dependent on site conditions, including the flatness of the support the air handler sits on and the rigging and mating procedures used by the contractor. Once a unit is on site, it is sometimes very difficult to determine where the failure lies if it doesn't meet the specified leakage rates. In a worst case, you might have the manufacturer, the shipper, the installing contractor and the general contractor all pointing fingers at each other. If the fault actually lies in a factory defect, the problem could be found and corrected in the most controlled environment possible with a factory test.
So the real answer? Both, if you can afford it. And if you catch a problem before it gets to the field, you might feel you couldn't afford not to do both.
How many sections to test?
Many pressure tests specifications treat the entire air handler as a single section, and require a single test for the whole unit. Some break the unit up into positive and negative pressure areas (upstream and downstream of fans, respectively) and call for them to be tested separately. Each method has its advantages and its disadvantages.
The first consideration is cost. Each test costs time and energy that will be reflected in the overall price for the job. Requiring multiple tests on a single unit will raise the cost of the air handlers to the owner. This will also require more time at the factory and on site, and could affect overall completion dates in some instances.
A second consideration is accuracy. Since multiple tests allow the unit to be tested to the actual pressure condition the sections will see, presumably this will give you a better idea of the leakage than a single test. However, there is an appreciable amount of leakage within the air handler at the internal wall that will be factored into this measurement (and double counted!) that will unrealistically penalize the performance of the unit. This is especially significant, since the internal openings within an air handler (at fan walls, usually) that need to be blocked off to perform the test are rarely built in a fashion that allows for an effective air seal to be created for these temporary tests. Even a small amount of leakage at these internal walls can mean the difference between passing or failing a tight leakage criteria.
Consider a single test for units unless special requirements drive a need for multiple positive and negative tests.
Positive or negative test pressure?
If you are making multiple tests on multiple sections of an air handler, the answer to this question is simple: Test to the conditions each section will see in operation. If you are performing a single test on the entire unit, then you may want to carefully consider how you wish to test the unit. In general, there are some leaks that will open up under one pressure condition and will close under the other. Generally it is accepted that leaks at panel seams tend to close under negative pressure and tend to open under positive. Doors that swing out tend to behave similarly, while doors that swing in behave in the opposite fashion. Seams at test closures can do either based upon the method of construction of the closure. So there is no easy rule of thumb that says one method is preferable to the other. In many cases it makes sense just to find the point of extreme pressure in the unit under normal operation, determine if this is positive of negative, and test to that condition. This condition is easy to find by simply calculating the pressure condition at each section in the unit by starting at the external static pressure and working towards the inlet, adding back pressure losses at each internal component, and subtracting the fan static pressure increase at the fan wall. Each open section of air tunnel will have a pressure associated with it, with the extremes usually falling at the inlet or discharge plenum of the supply fan. These are the maximum pressures the air handler will see in operation--and usually one will be significantly further from ambient pressure than the other.
What Pressure to test to and
How to set failure Criteria
The above two considerations go hand in hand, so I will deal with both of them together. Traditionally, specifications are written so that a certain percentage of the total air flow is allowed in leakage (usually around 1-2%) at a specific test pressure. How the leakage percentage and the test pressure are determined varies from engineer to engineer and job to job. Sometimes the test pressure is based on the total fan static, sometimes it is based on the actual cabinet pressure, and sometimes it is based on a nominal test pressure (like, say 10"). In either of the first two cases, there is usually a sizable safety factor applied.
The allowed leakage percentage varies, but it is usually in the low single digits.
While this has been the standard in the industry for many years, there are some significant weaknesses in this approach to testing. First, the actual leakage rate measured in the field is determined essentially by the total face area of all the leaks in the system--this is a function of cabinet size and construction quality, not of supply air flow. By tying success or failure of the test to the fan capacity of the system, you are favoring simpler, smaller air handlers over larger, more complicated air handlers.
Imagine a simple 10,000 CFM air handler operating at 8" of total static with just a fan, a heating coil, prefilters and a mixing box. Then imagine that same air handler, but with a cooling coil, high efficiency final filters, return fan and air blender:
(click for larger image)In the example above, you have two 10,000 CFM air handlers, one with 297 square feet of cabinet area, the other with 630 square feet of cabinet area. The large air handler has more than double the cabinet area, and more leak points such as doors, dampers, coil penetrations and shipping splits--yet both would be required to meet the same leakage rate in a test--in this case, say, 200 cfm at a 2% leakage criteria.
A further complication would arise with the pressure selection. A common pressure test criteria is 1.5x the maximum fan static pressure--or in this case, 12". It is often difficult to find a pressure blower with a static capability in this range--It might be impossible to effectively provide this test in a timely manner on a job site. Additionally, many components (especially doors, when tested in a pressure condition opposite that they would see in operation) leak uncharacteristically at higher pressures. And the unit would never see pressures anywhere near 12" in real operation, anyway, since the fan will typically create an area of negative pressure in the inlet plenum, and positive at the discharge. The maximum amplitude of either pressure is, by necessity, less than the total static pressure capability of the fan. Thus the leakage rate measured in the test will be a very significant overestimate of the actual leakage that will be experienced in operation.
A different way of specifying pressure performance can address both of these complications--and that is to tie the performance to the cabinet itself, as opposed to the air flow. There is already a criteria to do exactly this. ClimateCraft recognized the difficulty with specifying pressure test criteria to arbitrary pressures and airflow percentages. They realized that SMACNA already had a pressure test criteria, the SMACNA leak class rating. The leak class of a pressure plenum (or air handler, in this case) is calculated using the following formula (from ANSI/ASHRAE 111-1988):
|Leak Class|| |
(leak CFM) x 100
(Area sq.ft.) x (Test Pressure) ^ 0.65
|Leak CFM || |
|(Leak Class) x (Area sq.ft.) x (Test Pressure) ^ 0.65|
ClimateCraft has adopted this method of rating pressure performance and builds their units to meet or exceed a leak class of 6. For the units above, that equates to about 146 CFM at 8" for the larger unit, and 69 cfm for the smaller (or 1.5% and 0.7% of the supply air volume, respectively).
In practice, we have found that a leak class of 6 represents excellent performance for high-quality custom air handling units of any of the manufacturers we represent and quite often can be met by the high-quality foam panel semi-custom air handlers by Aaon, too. This is, however, a very high bar for traditional commercial-grade batt-insulated air handlers.
Perhaps the biggest advantage of the leak class specification is that it encompasses both the allowable leakage critiera and the test pressure in a single number. A leak class 6 air handler will perform to the same leakage class whether it is tested at 4" or 10"--the test pressure can be chosen to meet realistic pressure conditions and to facilitate testing of the unit. It is purely determined by the design of the air handler cabinet and the execution of assembly.