@Adam, the static rise and
@Adam, the static rise and drop (resistance) of multiple supply or return branches are not additive! Allison covers this in the next article in this series (see link at end of article) where he writes: “choose the (duct run) that has the greatest total effective length. You do NOT use the sum of all the ducts and fittings.”

With 9 returns with filter grilles, you are correct that filters won’t use up much of the available static. Aside from considering the resistance of the ‘greatest total effective length’ path, you should calculate average filter face velocity (CFM / filter face area in ft2). Ideally, you’ll want face velocity to be less than 200 feet per minute, which works out to 6 ft2 if system airflow is 1,200 CFM. Check out Allison’s article “The Path to Low Pressure Drop Across a High-MERV Filter” on that topic.

IF YOU ARE DESIGNING A SYSTEM
IF YOU ARE DESIGNING A SYSTEM, REGARDLESS OF THE AMOUNT OF RUNS OR CFM, THEN YOU CAN DO IT WITH A STANDARD DUCTULATOR. MATCH 600 CFM WITH .1 FRICTION LOSS FOR SUPPLY RUN OFF UNIT AND 600 CFM AT.05 FOR RETURNS. THAT WILL GIVE YOU ROUND OR SQUARE DUCT SIZE THAT IS OPTIMAL. THEN IF YOU NEED LETS SAY TWO 100 CFM SUPPLY REGISTERS AND YOU BRANCH THOSE OFF ABOUT 2 FEET DOWN THE LINE, THEN FIRST YOU WOULD CHECK 100 CFM AT .1 (BC IT IS SUPPLY) AND FIND YOU WOULD RUN A 6 IN ROUND PROB. THEN AFTER THE TWO TAKE-OFFS AT 100 CFM A PIECE, YOU NOW HAVE 400 CFM TO DEAL WITH… NOW YOU CAN RUN DUCT SIZED FOR 400 CFM AT .1 FR INSTEAD OF 600. AND SO ON AND SO FORTH.

Dumping relief air in the ceiling void avoids the issue with more common bypass ducts (i.e., routing relief air directly back into the return side close to the AHU, which is a bad idea on several levels). However, your plan has other risks: in particular, creating a pressure imbalance between ceiling void and rooms, which will induce more infiltration that if the system were balanced. You can mitigate (but not eliminate) this issue by ensuring the ceiling void communicates with room air, but that in turn could lead to excess cooling (or heating) in the spaces where that air exchange occurs. There’s no free lunch with fixed capacity equipment.

BTW, if your AHU is variable speed, constant volume (i.e., ECM motor with constant volume logic), then it will attempt to maintain the selected airflow volume, but at a cost of higher energy consumption. In that case, you’ll set the relieve damper using a power meter rather than based on air volume, keeping in mind that you still need to make sure external static at the AHU doesn’t exceed the maximum for the unit.

Lastly, you asked about positioning the relief damper… It doesn’t make any difference where it’s located as long as it’s upstream from all VAV boxes.

Mark wrote: “I would take my
Mark wrote: “I would take my measurements with a probe in the blower compartment for the negative side and a probe above the coil, but below the heat pack for my positive pressure, and add them together…”

Just to clarify, most AHU’s have the coil on the negative side of the blower (Trane Hyperion is one exception that comes to mind).

Regarding probe placement, it’s easier/better to drill the plenums instead of the AHU cabinet. In that case, subtract the heat strip kit static spec from the SP numbers given in the blower table. Either way yields the same result. Also, if the blower table includes a factory filter (typically @ 0.10 IWC) and it’s not being used, you must add that back to the blower table numbers.

In my experience, most air handlers have a higher maximum static than 0.5 IWC. Also, virtually all air handlers have more than one speed tap or setting, sometimes with different maximum ESP per speed. In any case, never use the AHU’s maximum ESP as your target. Keep in mind that the filter resistance will increase over time as it collects dust. More importantly, blowers are less efficient at the top of the fan curve, especially ECM’s. It’s best to stay in the middle of the range. If you have to use max static to get the design airflow, consider using a different air handler!

Once you determine the appropriate design airflow, study the blower table and determine which speed yields the desired airflow at an acceptable ESP, interpolating as necessary. I typically specify ‘not to exceed 0.4 IWC TESP’ for conventional air handlers, but I go lower for ducted mini-splits and other low static AHU’s. The objective is to stay within the blower’s ‘sweet spot’ in terms of efficiency. Unfortunately, residential equipment manufacturers don’t publish fan curves. However, if the blower table includes Watts for each static-speed combination, you can easily calculate blower efficiency (CFM/Watt) for a few points within your target range.

@David — So, if I have an AH
@David — So, if I have an AH which indicates that it has a max allowable static pressure of .5, then the only static pressure measurements I need to be concerned with exceeding .5 are the non-factory filter racks, and whatever heat pack I have installed, and the ductwork itself? Therefore, I would take my measurements with a probe in the blower compartment for the negative side and a probe above the coil, but below the heat pack for my positive pressure, and add them together to get the less than .5 (hopefully) SP that is max for that unit??

@Mark, external static, by
@Mark, external static, by definition, accounts for permanently affixed internal components such as the heat exchanger in the case of a furnace, or the refrigerant coil in the case of an air handler. So to answer your question, the coil in Allison’s example has already accounted for in the blower table’s external static specs.

OTOH, a heat strip kit, if present, is considered ‘external’ because the size and thus pressure drop is unknown by the manufacturer. The installation manual should have a table showing how much static to subtract for each kit.

Cabinet mounted filters, especially when shipped with the unit, are typically accounted for in external static, although designers should always verify one way or the other. In many cases blower tables include a footnote indicating how much static was subtracted for the filter. That way if a more (or less) restrictive filter is installed, only the difference would be subtracted from available static.