What You Might Not Know About Coils & Air Handlers

swcoil_superadminAir Handlers

Steam Coil

With the daily hustle and bustle of business and all the new technologies, there are many aspects of equipment, applications and systems that may get overlooked or may not be completely understood.  They are important, so let’s look at a few of them.

What type of coil needs to be replaced in less operating time on average than other types?

The answer is “the steam heating coil” which can cause corrosion and freeze events.  There are many steam systems that build up corrosive agents over time and start to attack copper.  Often, this is created by the steam heating process itself.  The treatment that’s required causes the buildup of ammonia, carbon dioxide and oxygen to increase to a ‘parts per thousand (volume)’ and pin hole leaks to appear in the tubes.  Example: A coil system is leak free for 9 years and multiple leaks appear in years 10, 11 and 12.  The problem continues to get worse each year.  The problem is exasperated when the owner replaces the coil assuming it’s time for a replacement, and the leaks continue to appear in the new coil within 2 or 3 years.  If you don’t clean out the containment, you don’t solve the problem.

Freeze-ups occur because the condensate is not removed as quickly as it’s formed and standing condensate with below freezing air temperatures equals possible coil damage from a freeze event.  It’s very important, especially on preheat coils with low pressure steam, that the vacuum breakers and traps be properly located to insure condensate removal.

Why don’t fan coil units perform as advertised?

Fan coil units are one of the most economic ways to cool or heat a space.  There is limited ductwork and piping, so it seems to be a simple way to produce comfort within a space. Not true.  Remember that fan coil units have direct connected motors, and the air flow volume is greatly reduced when even a short run of ductwork is attached.  Less air flow is less BTUH capacity – that’s a given.  The other aspect is that less “air flow” creates less “air throw” and that can cause hot and cold spots within a space.  It’s very important for a designer to add a service factor for air flow.  Example: Designing every unit on medium speed which allows you to go to high speed after installation if needed, and especially as coils and filters load each year. 

What are the major differences between today’s central station air handlers versus those from the 80’s?

There are so many differences it could take 3 pages to list them all. Here are the big ones.

  • Units are quieter based on better metal to metal design, better insulation, and quieter fans and motors.
  • Increased coil and filter selections, including all types of filters from 30% up to high efficiency design for specific system filtration. The advanced computer coil design provides efficiency and ability for a cleanable design.
  • Better mixing and handling of outside, return, and exhaust air
  • Improved access with actual hinged and latched access doors versus the old removable panels that caused stripped screws after a few removals
  • More efficient fan performance based on the myriad of fan types and the advanced design of VFD’s that can save money during intermediate system conditions
Central Station Air Handler

An AC unit’s high refrigerant pressures may come from loaded condenser coils.  Why? 

Most refrigerant systems don’t have any margin for error. Capacity and proper operating temperatures and pressures are all based on a new unit. Condenser coil loading is very prominent and can cause high pressures and ultimately compressor failures if the problem persists.

Remember that condenser coils are the only coil mounted in an HVAC system that doesn’t have a filter on its upstream side.  They are mounted outside and are usually very dense, since secondary surface (fins) are required to meet capacity.  Many condenser coils are now 20 to 22 fins per inch.  Also remember that condenser coil air flow is based on direct connected propeller fans that lose efficiency with extra resistance.  You can have a new condenser coil with .25” air resistance that can load to .75” resistance.  This increase may plummet the system CFM flow which will translate to the condenser not performing and create high pressures.  It’s very important to keep coils clean, and the number of times for cleaning may vary based on each specific installation and environment.

A few lines of wisdom:

“I used to be indecisive, now I’m not so sure”.  W.C. Fields

“In the book of life, the answers are not in the back”.  Charlie Brown

“Never take life seriously.  No one gets out alive anyway”.  Anonymous

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