Technical FAQ: HVAC: Excess Static Pressure Relief
Excessive Static Pressure Problem Definition
When the dampers are partially closed, the air pressure inside the duct increases. This leads to increasing so-called "static pressure", and if the indoor fan is not capable of handling the increased static pressure, this results in decreased airflow across the indoor coil and may lead to excessive wind noise from the registers.
Reduction of the airflow, in turn, leads to lower temperature of the indoor coil, because the amount of air moving across it is no longer sufficient to maintain the proper temperature.
This, in turn, will eventually lead to the incomplete evaporation of the refrigerant, which means that the liquid will leave the indoor coil (which it is not supposed to do) and reach the compressor. Usually, the compressors are incapable of handling liquids (with exception of "scroll" type compressors), and liquid refrigerant reaching the compressor will cause "slugging", which will eventually damage or destroy the compressor.
On the other hand, dropping temperature of the indoor coil will eventually lead to the freezing of the water vapor contained in the air on the coil surface, which will further degrade the heat transfer capability and reduce the airflow, thus triggering the domino effect.
Ways to solve the problem
Note: Credit for this section goes to T. Shannon Gilvary. The materials on this page are a digest from "Excessive static pressure relief" discussion thread on DIY-Zoning-general mailing list. The complete thread may be found here.
There is no cure from all diseases, and the solutions to the problem of relieving excess static pressure relief range from simple and workable, but not without pitfalls, to significantly more complicated and somewhat more expensive.
#1 Traditional (hardware only)
So-called "bypass damper" is installed. A bypass damper routes air from the supply ductwork back to the return so the air handling unit gets a constant flow of air, but the ductwork after the bypass sees a constant static pressure.
Pros: Simple, relatively inexpensive, allows additional humidity control. Avoids overshooting problems since cooled air is not dumped in areas where it is not needed.
Cons: Unmanageable, prone to mechanical failures. Improperly tuned bypass damper will either not serve the purpose (if it is too tight), or waste the energy (if it is too loose). Doesn't protect the unit from freezing or slugging - there's no feedback loop. Requires modification of ductwork.
#2 Simplest, least expensive (software only)
The threshold is set for the minimum total opening of the dampers. As soon as the open area of all the dampers falls below this threshold, the designated "dump zones" will start opening the dampers in such a way that the open area is no less than threshold.
Initial modeling shows that the threshold should be set around 30-40%. Anything less, and the static pressure increases unacceptably. Anything more, balancing quality degrades.
Pros: Simple (no hardware required at all), inexpensive (zero dollars zero cents), configurable.
Cons: Careful tuning required. Doesn't protect the unit - there's no feedback loop. This and following solutions will share the common problem: dump zones will be prone to overshooting (overcooling when cooling and overheating when heating).
#3 A little bit more expensive (software and hardware here and below)
Pressure sensors are installed on HVAC unit return and supply. As the static pressure increases, #2 algorithm is applied to control the dampers.
Note: This solution allows to adequately solve the problem of controlling the static pressure, but there's a problem of preventing the coil freeze and slugging the compressor which is yet to be solved.
Pros: Couple of sensors are probably less expensive than the bypass damper, this solution has all the advantages of the software. Assumption is removed from the picture - the real static pressure is measured and controlled. No ductwork modification involved. Freezing and slugging is less likely but not completely eliminated.
Cons: Still no temperature feedback from the unit.
#4 Somewhat more expensive
In addition to pressure sensors from #3, temperature sensors are installed on HVAC unit return and supply. Watching the temperature trends will allow to reliably determine the unit condition and take corrective action.
Pros: Unit condition can be determined with high probability, adding fault tolerance.
Cons: Even though the trends can be analyzed and decision made, those temperature readings do not guarantee 100% that the coil will not freeze.
Note: It is also possible to install temperature sensors not for the air stream across indoor coil, but for the refrigerant flow - one sensor immediately before the coil, the other after. It is not quite clear how to analyze the temperature patterns yet (they turned out to be quite bizarre), and how to correlate temperature readings against saturation temperature (requires further investigation), but one thing is certain: if the outgoing refrigerant temperature is getting to a water freezing point, something has to be done.
#5 Even more expensive
Here, we start intruding into the unit itself. In addition for supply/return pressure and temperature sensors from #4, additional temperature sensor is installed on the indoor coil.
Pros: The coil will not freeze.
Cons: It is still possible to slug the compressor. Indoor coil is almost always not easily accessible. Though relatively simple, this requires the HVAC unit modification, which may void your warranty, if you're not careful enough.
#6 Yet more expensive
In addition to sensors from #5, the pressure and temperature sensors are installed on the suction line. Corrective action is taken when the suction line temperature is getting close to the saturation temperature for the given pressure.
The suction line is not easily accessible in a package unit. Installing a pressure sensor on the suction line is a procedure that can be performed only by a HVAC certified technician. Kids, don't do this at home. Warranty is most probably bye-bye. Check with the manufacturer of your unit.
Pros: Almost bulletproof solution.
Cons: Careful tuning is required - the saturation temperature/pressure chart will be different for different refrigerants. It's not quite clear if this is worth the trouble, possibly, it's an overkill. Remains to be seen.
#7 The ultimate solution
Combining the methods of #6 with the bypass damper from #1, the static pressure can be maintained without over cooling any areas or wasting energy. The compressor can be controlled to maintain a constant air temperature without worry of freeze-up or slugging.
Pros: The best protection and static pressure control.
Cons: Requires ductwork modification, installation of several sensors, some of them expensive, and may be overkill.
Best Protection for the Money
Of course, "best" is a subjective term and should be taken with a grain of salt. If overshooting some areas is not a problem, then option #5 will work for most people. If you would rather avoid overshooting, then adding a bypass damper to option #5 is the best bet. Of course, if money is no object, then by all means, go for #7. Consider the cost of replacing a condensing unit, energy costs, and how much unit modification you want to get into when making your decision.
Pressure sensors are significantly more expensive than temperature sensors, so it is possible to skip option #3 and install the temperature sensors per options #4 and #5. The static pressure will go unchecked, but quite predictable, and will not matter much because the main problem is actually preventing the coil freeze and slugging the compressor. The temperature sensor is about $3, so we're looking at total of $12 plus cost of cable or wireless transceivers.