Types of Coil and Efficiency !!

HOT WATER BOOSTER COIL : Every coil has a specific, optimum velocity, so you want to make sure you are within 30% (+ or -) of that number. For example, booster coils have an optimum velocity of 800 ft/minute. That means that you can drop your velocity to 600 ft/minute, or conversely, increase the velocity to 1,000 ft/minute. The duct velocities are almost always higher, which means that you will need to transition to a larger coil. Try to get to as close to 800 ft/minute as possible, while sizing your coil to make the transition as easy as possible. Everything with coils is a balancing act.

HOT WATER & STEAM COILS : Like booster coils, hot water and steam coils should also have face velocities at approximately 800 ft/minute. Both steam & hot water coils have only sensible heating, which is why their face velocities can be the same. Face velocities ultimately control the coil’s cost, so 800 ft/minute really is a heating coil’s “sweet spot”.

If you are purchasing an air handler unit, oftentimes the heating coil is smaller than the cooling coil because the face velocities on heating coils can exceed those of cooling coils. Due to water carry-over, cooling coils cannot exceed 550 ft/minute, while heating coils only deal with sensible heat.

CHILLED WATER & DX COILS : Due to the limited face velocities of cooling coils, your choices are more limited. With cooling coils, your face velocity must be somewhere between 500 ft/minute-550 ft/minute. Remember that when dealing with cooling coils, you are dealing with both sensible and latent cooling, so the coil is wet. When you exceed 550 ft/minute, water carry-over occurs past the drain pans.

If you are purchasing an air handler unit, you probably will not have worry about the coil’s face velocity as most coils come pre-sized at the acceptable face velocities. Fan coils also come pre-sized with the correct CFM’s. However, if you are replacing an existing cooling coil, the face velocity must remain at or below 550 ft/minute!!
AIR STRATIFICATION ACROSS THE COIL

Air does not travel equally across the face of a coil. If you were to divide a coil into (9) equal sections, like a tic-tac-toe board, you would see a high percentage of air travelling through the center square, rather than the corner squares. In a perfect air flow scheme, 11% of the air would travel through each of the 9 squares, but that is not what happens. Because more air travels through the center of the coil, you want to avoid putting a fan too near the coil. Due to central air flows, most systems are draw-thru, rather than blow-thru. This is also why you want to avoid installing your coil near any 90 degree angles/turns in the ductwork. Avoid any situations that contribute more than the “natural” air stratification to help ensure your coil is at maximum efficiency.

In some situations involving cooling coils, you will have water carry-over even when the coil is sized correctly. How can this happen? Think about the tic-tac-toe board again. Air velocities are exceeding 700 ft/minute in the coil’s center, while the corners are around 300 ft/minute. This cannot and will not work.

Coils do not have any moving parts. They simply react to the air across the outside of the coil and whatever is running through the inside of the coil. Coils are 100% a function of your entire system, as well as the installation in general.



HVAC energy efficiency can produce a double or triple return on investment. If your equipment is over 10 years old, upgrading to high efficiency equipment can pay for itself in a surprisingly short period of time. But, when looking to purchase or upgrade your equipment, what do HVAC energy acronyms actually mean?


Btu (British Thermal Unit): Most commonly used unit of measure for energy use in heating and cooling equipment: one Btu is the amount of heat required to raise one pound of water by one degree Fahrenheit. The higher the Btu rating, the greater the heating capacity of the system.


SEER (Seasonal Energy Efficiency Ratio): Essentially, this measures your cooling equipment’s average efficiency over the course of a calendar year. A higher SEER rating equates to greater energy efficiency. Depending on what area of the country you’re in, your equipment should have a rating anywhere from 14-25. While a system with a higher SEER rating may have a higher initial cost, your annual energy savings will more than offset those higher upfront costs.


EER (Energy Efficient Ratio): Like SEER, EER is also used to measure your system’s efficiency. The terms differ in that EER is calculated under specific test conditions that represent peak load during the highest temperatures of the season, while SEER is measured seasonally over the course of a year. In other words, if your office or business is located in an area with extreme temperature fluctuations, such as Arizona, EER might be a more relevant efficiency rating than SEER. For EER, look for a rating anywhere from 11.5-14.5


In either case, it’s important to look at both to get an accurate idea of the unit’s performance under different operating conditions.