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home>Mini-ERV

SUSTAINABLE LIVING
Installing a Single Room Energy Recovery Vent
in the kitchen/dining area

October 15, 2014
It is fall in Maine, and a week or so after the first frost I take down all the window screens and seal up the house by installing double pane interior storm windows.  I purchased these well-made units many years ago, but since then have been encouraging people to build their own based on a design created by my friend Topher Belknap - here are my instructions.  Once these are installed, the house becomes very tight and we notice that kitchen smells tend to linger for many hours after cooking since we do not have a kitchen vent hood. 

The reason for sealing up the house so tightly in the winter is that temperatures here remain below freezing for most of December through March and remain very cold from October through May.  Here is a chart taken from my personal weather station for the year of 2013 to put this in context:


When we had an energy audit done several years ago, they used a blower door test from which we learned that the air exchange rate for the house was around .35 ACH (Air Changes per Hour) which is at the threshold that ASHRAE consider to be healthful.  Since then I have sealed up the leaks around the windows that we identified and I have become concerned about interior air quality in the winter.  We may not be getting enough fresh air.
TwinFresh Comfo ERVI considered installing a complete fresh air ventilation system in the house, but it would be expensive (over $5000) and many of the ducts would have to be installed in soffits around the corners of the rooms which would be unattractive.  I had also installed a Heat Recovery Ventilation (HRV) system in my workshop in order to bring warm fresh air into my office in the winter, so I was familiar with the benefits.  In recent years a few European companies have been selling miniature Energy Recovery Vents (ERV) in the US that are quite efficient, relatively inexpensive, and easy to install.  When I discovered that Home Depot were now carrying the TwinFresh Comfo model (shown at left) on their website for under $500, I decided to install one in the kitchen/dining area.  You can also get one from HVAC Solutions Direct for under $350.
TwinFresh Comfo functional diagramOne distinction between an HRV and and ERV is that an ERV will maintain humidity inside when it is dry outside in the dead of winter, and this is a significant issue for us here in Maine where it becomes very dry. 

The unit I purchased installs in a 6" hole in an outside wall and has a small blower fan that exhausts warm air past a ceramic heat storage element.  After 70 seconds the fan pauses briefly and then reverses bringing cool air in past the warmed up ceramic cartridge which heats the incoming air.  It can also be set for exhaust or fresh air feed continuously at 3 speeds.  To reduce humidity it will come on when the humidity is above 40%RH (low), 50% (medium), or 60% (high).   When the humidity drops below the set point, it shuts off - a feature I don't see has much value given the low CFM.  Finally there is a night mode which runs the fan at its minimum speed when the interior room is dark.

These vents are also engineered to be synchronized as pairs through a five wire cable and dual connectors on the mounting plate.  This means that when one is exhausting - the other is bringing in fresh air and the reverse.  By doing this the airflow in the house is balanced, overcoming the problem inherent in installing a single unit.  If a single ventilator is installed, when it is removing air from the house cold air has to enter the house from other locations compromising the overall performance.  Similarly when it is bringing fresh air in, warm air is leaving the house somewhere.

  TwinFresh Comfo box end graphicsTwinFresh Comfo retail boxI received the very colorful box a few days after placing my order on the Home Depot website and noted the claimed efficiency is 91% at 32F.  As an energy efficiency maven, I am also impressed that the maximum power consumption of this device is only 5.61 W with the fan running at full speed (32 cfm).
SPEED POWER CFM
1 3.8W 8
2 3.96W 16
3 5.61W 32

I took everything out of the box and inspected all of the parts, here they are all lined up.  From left to right you have the interior fan/control module, the ceramic core, air duct and exterior vent cover:
TwinFresh Comfo parts
When I opened the box I was assaulted with a powerful scent that smelled like a cheap motel.  Apparently they decided to infuse the ceramic core with a "fresh air" sent which I found extremely offensive.  So I placed the whole unit outside with the fan running full speed to clear out that smell and it took about four hours.


Looking closer, here is the hefty (3.5lb) ceramic core with filters on each side and a detail of the honeycomb core structure:
TwinFresh Comfo ceramic honeycomb coreTwinFresh Comfo ceramic core detail 
The installation was relatively simple.  I began by locating studs in the wall and marking off a spot between the studs for the 6 inch circular cutout.  There are additional cutouts for the connectors and power cord etc.  The images below show the vent and core installed in the wall and the mounting plate that secures to the wall.  I added foam weatherstrip to the back of the plate rather than squirting in expanding foam around the interior of the hole as they suggest.
TwinFresh Comfo - mounting plate installationTwinFresh Comfo - mounting plate installed
I went outside and cut a hole through the exterior wall and pushed the duct through and then secured the exterior vent mounting plate over it:
TwinFresh Comfo - exterior wall ductTwinFresh Comfo - exterior vent mounting plate
Back inside, I was briefly confused by a mention in the manual of "dowels".  I eventually figured out that these are pointed aluminum alignment pins that attach to the mounting plate and help to guide the fan assembly onto it.  The fan assembly attaches magnetically and also via two green electrical connectors that feed 120 V power to the unit via the provided black power cord.  (I decided to replace the black cord with a white one  to make it look a little more attractive and I secured the white cord to the side of the window frame with white nail-down cable clips.
TwinFresh Comfo - dowel alignment pinsTwinFresh Comfo - interior fan assembly
Back outside again, I installed the outside vent cover.  I have not decided whether that gray cover needs to be painted to match the house trim.
TwinFresh Comfo - outside vent cover
When I plugged in the unit and turned it on using the remote control I heard a long "beep" and after several seconds I noticed that  the shutters slowly opened up to allow air movement through it.  This takes almost a minute to happen which is surprisingly slow. Similarly when power is turned off the shutters in the vent slowly close up to seal off airflow.  Here it is opened up fully:
TwinFresh Comfo installed
There are control switches on the right side that offer limited functionality, but they are hard to reach:
TwinFresh Comfo - manual controls

It is much simpler to use the remote control which offers a broader range of options:
TwinFresh Comfo - remote control functions

Page 15 of the manual contains the operating instructions, but leaves out a lot of useful details so I have revised the page and you can download my revised version from here.

When I start cooking - I can press the "Air Extract" button to use it as a fume hood since I do not have one in the kitchen.  Once the smells have been eliminated, I can switch back to the "Ventilation with Energy Regeneration" mode.  I plan to leave it in "Night Mode" so that it will continue to provide fresh air but at a much reduced airflow.  Apparently the unit detects ambient light and switches to Night Mode automatically when the room is dark - this coincides with the coldest night time temperatures and a reduced need for fresh air.  it is a well conceived feature for this device.

   

 Some Improvements
When I originally bench tested the assembled vent, I noticed that there is a dim orange power light that does come on when it is activated but there is no indication of the direction of airflow.  Since I plan to use this both as an ERV to bring in fresh air and also as a kitchen ventilator to remove odors, it is important for us to know which direction the airflow is going.  So I decided to install an LED that would light up red when air is being exhausted and green when fresh air is coming in.  As an electrical engineer who designs products for a living, I had no trepidations about taking the unit apart and looking for a place to wire in a bicolor LED.

Inside the fan assembly I found a circuit board  that controls the fan speed and direction and noted that it also has sensors for ambient light, humidity and a receiver for the remote control.  By probing around with a voltmeter I determined that three of the four wires that feed the fan could be used to drive a bicolor LED.
TwinFresh Comfo - inside front cover showing circuit boardTwinFresh Comfo - fan control board with added LED
I installed the LED so that it pokes up through a hole I drilled in the housing.  For the technically inclined, here is a schematic of the connections that I made.  Click on the image to see a PDF schematic document.
TwinFresh Comfo - LED modification schematicTwinFresh Comfo - LED modification detail
I wired the LED to turn red when stale air is leaving the house and green when fresh air is coming in:
TwinFresh Comfo - fan housing with added red LEDTwinFresh Comfo - fan housing with added green LED

When I used the remote control, I was disappointed to find that it only worked within 6 feet or so and you had to point it directly at the vent to get it to work, this is unacceptable.  When I had opened up the case to add the LED above I had noticed the IR sensor for the remote and light sensors near the end of the circuit board.  So I decided to cut out the case to allow infrared light from the remote control and ambient light for the light sensor to get into the case.   To hide the visible electronics I used a piece of theatrical gel made by LEE Filters ( blue frost 221).  I tested this white diffusion filter carefully to ensure that it would pass infrared.  Many clear and milky colored plastics block infrared completely as does white paper.   Now the remote control works from over 20 feet away, but you do have to point it directly at the vent.
TwinFresh Comfo - case cutouts for IR and light sensorTwinFresh Comfo - case cutouts for IR and light sensor with gel filter
Here is a detailed overview of my modifications:
TwinFresh Comfo - case modifications
I jacketed the light sensor and the bi-color LED with black heat shrink tubing to prevent the LED from casting light onto the light sensor and giving it a false read of the ambient light.

 

Performance Testing
Anyone reading my website knows that I am a geek and I am proud of it!  Within minutes of installing the TwinFresh Comfo I deployed my HOBO data logger with four temperature sensors.  I placed one sensor on the inside of the vent and one on the outside and additional sensors to measure the ambient temperature in the room and outside.
TwinFresh Comfo interior vent  with temperature probeTwinFresh Comfo exterior event with temperature probe
Here is a chart spanning six minutes with temperature readings taken every second at 7am with the outdoor temperature hovering around freezing.  This shows the 70 second cycle as the fan reverses back and forth.  Note that the interior room temperature is 60F (thermostat set back for the night) and the air entering the room varies between 53F and 60F effectively gaining 15F as it is warmed by the ceramic core, but not dropping more than 6F below the interior room temperature. 

Interpreting the chart.
Looking at the red line that represents the temperature of the probe taped to the inside surface of the vent (photo above left).   The downward slope indicates incoming airflow as heat is released from the ceramic core.  The fan reverses, and the upward slope of the red line indicates heat building up in the ceramic core and being stored there for the next cycle.
data log chart showing temperatures recorded for six minutes
As a reminder, here is the graphic showing the energy cycle of the ventilator:
TwinFresh Comfo function cycle
And here is a thermal image time-lapse video showing the whole cycle.  The hot spot at top right
is the vent opening actuator which stays warm.  Watch the temperature reading in the center.


Since the ventilator is about 92% efficient, the room is losing a small amount of heat, but gaining an assurance of fresh, healthy air in my living environment.  Over the last 14 years I have succeeded in reducing propane consumption by about 50% thanks to the solar water heater and demand water heater I installed, so I have some wiggle room to add a little propane as necessary to make up the losses of this well engineered Energy Recovery Ventilator.

FINAL NOTE:
Another feature of this clever ventilator is that it has a connector that feeds power and signals through to a second unit that can be synchronized with the first one, but with a reverse airflow.  This means you can deploy two of these units so that one is always blowing in while the other always blows out.  This prevents heat loss by balancing the air flow.  I am seriously considering installing a second unit in the bedroom directly above the dining room and synchronizing the two units.  Stay tuned!


Night mode
According to the instructions, if you press the "Night Mode" button on the remote control, "the ventilator is switched to low speed on a signal from built-in light sensor".  The vent emits a long beep and turns off the orange indicator light briefly to indicate that night mode is active.  Pressing the night mode button again causes the vent to emit a short beep indicating that the mode has been disabled.
TwinFresh Comfo remote control close-up
I like the idea behind this feature because it gets extremely cold at night here in Maine in the winter and there is no real need to bring in slightly cooler air fresh at night when there is no one in the room.  I thought I would test this feature and turned off the lights at night and noticed that the fan slowed down at the next cycle. 
TwinFresh Comfo - night mode
Looking closer we see the transition in more detail:
TwinFresh Comfo - night mode
Watching the unit at night, I noticed that the orange indicator light goes off for several seconds when the fan changes direction and presumably the device is  detecting ambient light at this point.  The room needs to be really dark before it will switch over to night mode, the moment the sun rose in the morning it switched back to full speed:
TwinFresh Comfo - night mode

This data was sampled at one second intervals so the spikes in the charts represent the brief power surges and sags at the moment that the fan starts and stops.