Ventilation Energy Recovery | 5 Key Questions to Ask to Determine Air-to-Air Energy Recovery Device

Nearly once a week I am confronted with this question.  My customer,  “I have an application for Air-To-Air energy recovery.  What type of energy recovery device should I use?”  My answer is nearly always the same,  “That depends”.

We have several choices.   All the technology has been around for long time now.   Pumped glycol loops were invented and used as long ago as 1949 by Konvekta, in Switzerland.  The first commercial wheel was created in 1954 by Anders Munters.  Heat pipes were invented in 1963 at Los Alamos National Laboratory (they are rocket science).  Nick Des Champs commercialized plate Air-To-Air heat exchangers during the energy crisis of 1974.  Each technology has evolved over time and all have been extensively used in our HVAC industry. 

When we look to determine the best fit for the application we start with five key questions. 

These are key factors that we consider when choosing the type of energy recovery device:

  1. Can supply air and exhaust air be run side by side?  If no, rule out everything but the glycol run around loop.  The glycol run around loop is the most effective device to use in applications where supply and exhaust air streams cannot be brought next to each other. 
  2. What is in the exhaust air stream?  There is a big difference between a laboratory and elementary school application (not just the size of the kids).  If there is something dangerous in the exhaust air stream most building owners will still shy away from enthalpy wheels.  There is a concern about cross contamination from the exhaust to the supply side on the wheel.  The best energy recovery choices for contaminated air streams have been either heat pipes (which can be sealed from one side to the other) or run around glycol loops. 
    If exhaust air is clean then we would typically use the most effective device;  the energy recovery wheel.
  3. Is summer energy recovery required? The energy recovery wheel is the most effective device (typically 70 to 80% effectiveness).  That is because this device is the only true enthalpy (water) transfer device.  If the application runs in the summer we typically use a wheel, especially in humid applications. 
  4. Are there any size constraints?  In most typical applications the size of the energy recovery device determines the size of the Energy Recovery Unit.  If space is an issue then we typically favor heat pipes and glycol run around loops.  These devices can be dimensionally changed to fit a lot of different applications. 
  5. Are there any maintenance concerns?  If there are considerable concerns about maintenance then we favor passive devices, like heat pipes and plate exchangers.  Poor quality wheels can be a maintenance nightmare.  Pumped glycol loops have to be maintained also.  

This is just a start to the process.  These are five of the key things to consider when choosing an air-to-air energy recovery device.  See links to learn more specific information about each technology.

Ask Rick: Heat Pipe Tilt Mechanism | When are they used?

Heat Pipe Eneegy RecoveryQ.  When are Tilt Mechanisms used on Heat Pipes?

A.  In the early heat pipes a tilt mechanism was used to shift the heat pipe from summer to winter use.  It did this by physically lifting one end of the heat pipe.  That allowed refrigerant fluid to run back to the warm side of the heat pipe.

Today’s heat pipes do not need to be tilted.  Tilt mechanisms are no longer used.  The heat pipe tubes were increased in size which allows refrigerant to flow freely in both directions.   That’s all it took to get rid of the tilt mechanism.

Note that it is key to have the heat pipe completely level if both summer and winter heat exchange is required.

Project Snapshot: Pfizer Blackfan | High Plume Dilution Fans

Project Name:

Pfizer, Inc., Center for Therapeutic Innovation, 18th Floor


Tsoi / Kobus & Associates

Mechanical Engineer:

AHA Consulting Engineers

Mechanical Contractor: 

J.C. Higgins


Strobic Air


High Plume Dilution Fans


Dual Plenum – 2 Fans at 12,000 CFM Each

DAC Sales Engineer:

Jim Shiminski

Project Highlights:             

Strobic Air Tri-Stack Fans - Pfizer BlackfanOvercoming Space Constraints:  This was one of the final exhaust fan installations on the Blackfan Building in downtown Boston.  There was very little room to place the fan system on the building.  The Strobic Air Tri-Stack Fan System was selected to minimize the overall system footprint. 

Height requirements:  To match existing system stacks on the roof the overall height of the system needed to be 26′ 6″.  Stack extensions and supports needed to be added to make this happen.Strobic Air Tri-Stack Fans - Pfizer Blackfan

Rigging Challenge:  The rig to the roof was complicated (usually is with a 20 story building in downtown Boston).  There was no practical way to get the system on the roof with a conventional crane.  The fans were helicopter lifted to the roof on a weekend to facilitate installation.  Several coordination meetings and pre-staging the lifts help make this a very smooth rig.

Strobic Air Tri-Stack Fans - Pfizer BlackfanStrobic Air Tri-Stack Fans - Pfizer BlackfanStrobic Air Tri-Stack Fans - Pfizer Blackfan

Ask Rick: 208v vs. 230v Motor Question

Q.  I have 208 volt service and want to run a 230 volt motor on that.  Will this work?

A.  Most motors that are rated at 230 volts will run on 208 volt power, although this is not recommended.  Most AC motors are built to tolerate a 10% variation from nameplate voltage.  The range for a 230 volt motor is 207 to 253 volts.  If the service was a pure 208 volts, at a minimum, then the motor would work.  But typically because of voltage variation the 208 volt power can vary from 187 to 229 volts.  If the service continually drops below 207 volts then the motor draws more amps, overheats and fails.  Rick


Ask Rick: Strobic Air Smart Fan System | Controls Integration

Smart System ScreenQ.  The Smart Fan brochure says ”Communicates with most building systems” (p. 1) – please elaborate.  We have an existing Johnson DDC system here.

A.       The major protocols the Smart Fan system can handle without extra modules are Modbus, BACnet (IP or MS/TP) & Metasys N2.  Lon is possible with a separate module.   Smart Fan can talk to any system as long as a gateway module exists.    Rick