Rules of Duct Design, Why?

  I went many years as a Service Tech and never had a clue as to these rules.  As a matter of fact I went years as a Service Manager, and quite a while as an Instructor, without seeing these all in one place.  I had this problem with learning the hard way - who knew you could pick up a book (or read a blog!) and learn this so easy?  If you haven't been able to tell, I am all for giving this type of information to those that need it; so please share with your service managers, techs, sales, and even the greenest of tin knockers.
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Sleep through the Static

  In my last post, Under Pressure, I talked about available blower static pressure and PSC motor maximums.  But, what if you have correctly designed the ductwork and set up your airflow perfectly - only to get a call years later about an uncomfortable couple of rooms?  A contractor in RI had just this problem a week ago, but he was armed with the knowledge of static pressure and the ability to measure it!
  Static pressure is a simple measurement that can be taken with a static pressure probe and a dual-input manometer.  I have seen guys out there still using magnahelics, u-tube and incline manometers.  These mechanical means will give you the same answer - but I wouldn't use an abacus to do what I can with a calculator!  Anyhow, the manometer you are using to measure gas pressure can be used for static pressure too, as long as the resolution is .01" w.c. (25 pascals) or better.  With static pressure, we are able to see how much (or little) pressure is in the ductwork.  We can also identify restrictions like clogged evaporator coils or hidden air filters.  I once used a static pressure probe to locate a filter that had been insulated over and remained untouched for years!  When measuring, I had a very low negative static pressure before the filter, and extremely high negative static after.  This identified an abnormal restriction.  Most pleated filters have a .12"-.15" pressure drop when clean.

Dirty Coil Angle #1 (.76"w.c. pressure drop)
  Last week I got a call from the contractor in RI with a couple of static pressure measurements that didn't sound quite right.  He explained that during the recent cold spell, most of the rooms being fed by this unit were uncomfortable.  He checked the static pressure in the supply duct and only had a .01" w.c. - almost nothing.  Measuring before the add-on A/C coil, doing so by removing the high limit switch of the furnace, the tech was able to record a static pressure of .77"w.c.!  Most coils have a pressure drop of .20"-.30"w.c. when clean.  Since the pressure drop of the coil is .76" (.77 - .01), it was easy to determine that the clogged evaporator was the culprit.  Since removing this 10 year old coil that was obviously never cleaned was so labor intensive, the tech was able to sell a new coil that will be compatible with DOE SEER minimums when the time comes to replace the condensing unit.
Dirty Coil Angle #2

  If the technician was never armed with the necessary tools and knowledge, it would have been easy to start replacing motors and searching for disconnected ductwork.  After all, mechanical systems are much easier to fix when you know what the answers are.  Static Pressure, like temperatures or amperage, is an invisible thing and we must have the right tools to even quantify them.  Start measuring static pressure when commissioning the unit, and during maintenance.  It really is amazing what you will find!

Under Pressure

     A recent conversation with a high quality HVAC contractor reminded me that every piece of equipment (and people for that matter) has their limits.  A 1/2 ton truck can only pull so much weight, a man can watch only so many movie award shows, and a Permanent Split-Capacitor (PSC) motor can overcome only so much static pressure.  The conversation started centered around figuring the design friction loss for a newly installed unit.  For those seasoned system designers, you should be aware that this very important number is traditionally figured prior to duct installation.  Anyhow, in order to calculate the friction loss, one must know the available blower static pressure and the total equivalent length of the longest duct run.  Here in lies the problem: How can a blower overcome more static pressure than it is "designed" for?
     Most technicians have looked at furnace manufacturer data plates to write down model numbers and even find the required temperature rise.  System designers usually peer over manufacturer ratings and instructions, long before the homeowner makes that last payment - never mind by the time a technician completes a service call.  The issue is that many system designers and technicians overlook an important detail on every forced-air furnace's label: Maximum Total Static Pressure.  This is the highest amount of restriction the motor can overcome before seeing performance issues, like frozen a/c coils for instance.  When designing the system, and figuring the available blower static, it is most important to be able to deliver your design airflow (CFM) at half of an inch of water column (.50" w.c.) on medium speed.  All PSC blower motors have manufacturer blower charts that generally end at .50" w.c. and have clearly labeled data plates stating this maximum restriction.
Sounds like a lot of pressure to go around, right?  Well, the maximum is not all that is available. What if we added a new high efficient 4" or 5" pleated filter?  This is a restriction in our airflow, and the higher the airflow the more of a restriction.  In this particular example, we lost .20" w.c. for the air filter, when it is clean and new!  Also, since there is ductwork on that furnace, why not add an evaporator for air-conditioning?  That's right, another restriction and we lost .25" w.c. for this device.  Oh, and don't forget the return grille, balancing damper, and supply register on that longest duct run.  This added an additional loss of .09" w.c.  Some quick math, even for the greenest of technicians, will show that we now have negative available blower static pressure (.50" - .20" - .25" - .09" =  -.04").  This means there is no static to overcome any ductwork!  Unfortunately, by this point all of the ducts are installed and the unit is operating - providing the much needed heat over the past week of single digit weather.  So, it is determined that the ductwork is undersized and modifications are needed.  This is usually quite the conversation to have with the homeowner, and do you think they will be paying to fix that ductwork?  Good luck!  The problem could have been found long before this point by installing equipment at or below it's limits.  Technicians could find this issue much quicker by measuring total static pressure, or even measuring airflow for that matter.  The problem is not that the ductwork is too small, it is usually the fact that it was designed using a different or incorrect friction rate.  Electronically Commuted Motors (ECM's) can help deliver the airflow and comfort, but higher static (max should be .80" or less) translates to more noise and increased Kwh.  Think about these things before setting your equipment up for failure!  Remember:  a PSC motor can only overcome .50" w.c. and most men have a (1) movie award show limit per year...        

The Drop Light

Well, the mild Winter couldn't last forever in New England - and so starts all the panic phone calls from those that did not have their equipment maintained.  I used to dread waking up on days like today, something about snow that brings out all the crazy homeowners.  Don't get me wrong, companies I have worked for made plenty of money while I ran around like a chicken with my head cut-off.  I used to do well selling maintenance contracts on the first snow day, and the first scorcher after Memorial Day too!  Imagine telling a homeowner that if they had completed preventive maintenance back in October they wouldn't have had to call you today - and would have been much more comfortable when trying to roll out of that warm bed this morning.  As a technician, I started out not finding the value in maintenance or service contracts.  But, I firmly believe that is because no one showed me how to correctly service a furnace, heat-pump, A/C, etc.  The old man I first carried tools for was more concerned that I knew how to wrap up his drop light than teaching me how to set a heat anticipator!  I just didn't see what the big secret was and set out to find out on my own. 
When you get right down to it there is much more to maintenance than changing the air filter and kicking the ductwork while mumbling to yourself that "it's still here!".  A lot of companies out there have what we call "A" and "B" techs.  The "B" techs tend to be the ones doing the maintenance and are viewed not as profitable as the "A's".  But, completing proper maintenance can take quite the load off of your Service Departments on these first days of snow.  How many times have you had to go back to homes on days like today under warranty because the system was just serviced?  If you are starting to count any number above zero than it sounds to me like you showed your "B" techs how to wrap up a drop light!  Don't leave it as a mystery with these guys, start making your service department profitable: invest in them.  We're not talking about increasing hourly wages here, we're talking updated testing equipment and knowledge.  Completing proper maintenance prior to the heating and cooling seasons will free-up your "A" techs to respond to the emergencies, and give your staff the ability to respond to new customers at a time like this.  We all have to start somewhere, and believe me when I say I can wrap a drop light with the best of them!