FAQ

There is a True HEPA filter option that can be purchased separately.   Hepa filters are the best for removing pollen and wildfire smoke during summer, equivalent to approximately a MERV-13 filter.  You must choose between the Automatic Storm Valve add-on and a filter, at any given time.  You can switch from one to the other at the right time of the season.  The filter is usually used only in warmer weather, outdoor air in winter does not generally need to be filtered. 

The filters are standard, widely available filters intended for use in car cabin air filtration systems.   You do not need to buy new filters from us at a high markup, and you do not even need exactly the right size of filter.  You will always be able to get the filters you need for optimal functionality.

For maximal flow rate and ease of use, it is recommended that you use the Filter Attachment Kit, and the filter it is designed for.  This is just two parts that hold the filter slightly away from the intake to improve airflow, and screw onto the outdoor plate, under the vent hood.  See manual for details.

Any filter reduces the air flow rate.  In Summer Mode with the heat exchanger removed, maximum net provided fresh air flow rate with the HEPA filter is 60 CFM in one direction, per module, so 120 CFM per unit (a unit is a pair of modules).  


  See the spreadsheet on the Learn More page for exact figures.  In Ottawa, the calculations show a single pair of modules (one ERV unit) saves about 166 cubic meters of natural gas per year.  That's about 150 kilograms of fuel!

   You can save a copy of the spreadsheet, and then edit it, to enter the figures for your region for electricity and fossil fuel prices, etc. and see for yourself that the rate of return on a given quantity of capital/the rate of return on investment is more than twice as high as a photovoltaic panel installation in many areas.

Heating fresh air that is brought into buildings is responsible for about 20 percent of all global carbon emissions.  The OpenERV captures about 91% of that energy.


There is a 5 year Warranty against defects in material or workmanship on all our products.  Unlike with conventionally manufactured products, this is not an indication of design life, it's a matter of the administrative overhead and complications with trying to maintain any kind of relationship over extremely long time periods.

In reality, all conventionally made products include what's called planned obsolescence - they are deliberately made to break soon after the warranty expires, and we have all come to expect this.  When they do break, they cannot be fixed.  The OpenERV is not like that.  There is no planned obsolescence.  It is made to last as long as the fundamentals of technology allow.  This improves the investment properties greatly.  We believe doing things right is better than any warranty.  We believe good engineering is better than a paper promise, especially for very long time periods, and a building element should last for 25, 50 or even 100 years.  

Extremely long warranties don't really help very much in practice as companies go out of business or don't honor their promises, proof of purchase gets lost etc.

It is worth noting that the pipe inner and outer diameters are actually the same for TW4 and most of the other industry leading ductless energy recovery ventilator units.  Also most of them run on the same 24 volts DC power supply.  As a result, you can trade in one system for another after installation to some degree, although the wiring and some other details could be different. 



If you live in an area where storms occur with enough severity to force air or water through the unit, and the unit is not installed in a sufficiently sheltered area, you should use the Automatic Storm Valve add-on module.  This will close the intake when the system detects extreme wind, and prevent the intrusion of air during a storm.  The unit is installed at a very slight angle so that in the unlikely event any water gets in, it runs back outdoors again.  This is one reason the use of a contiguous, high quality pipe is a better idea than those telescoping tube designs.  The telescoping tubes can leak water into your wall where the two telescoping sections meet. 


Yes, but we can't/it is not economical to try to honour the warranty on units that have been modified, it's just too complicated and labor intensive, unfortunately. 


Open source has major implications for the long term return on investment, and real-world reliability of the system.  You also know what your are getting before you buy a lot better.  

Any of this nature appliance must have some custom parts, but my making them 3d-printable and making the source code available, you can know that you will always be able to obtain replacement parts, unless we get into a zombie apocalypse.  Even then, it's still better!

When a system is made from common, widely available parts, and you have the blueprints, you will never be at the mercy of any one company, which will keep you waiting for long periods or charge too much for replacement parts or service.   All conventional companies engage in what's called Planned Obsolescence.   They design things to break and in such a way that you have to come back to them for parts and service - and then they eventually stop supporting a product or go out of business, leaving the product an expensive piece of trash.  Which is built into your wall.

 If you want to amortize the cost of a product over a long time period, it has to last a long time.  25 or even 50 years is sensible for a building element.

Open source also provides a certain inertia and staying power, and is more respectful of the user in many ways.  

The design can be seen and criticized by anyone, helping prevent the existence of problems at an early stage.

For a rare few of us, the ability to modify, reprogram and enhance the unit, or shimmy it into a different role, is valuable. 

Environmental friendliness also shakes out of this designed-to-last approach.  It's way less stuff in the trash, and less stuff produced means less pollution. 

We support the Right To Repair movement.  This is especially important for farm equipment and other basic infrastructure, such as housing and building technology, which we all depend on and form a major part of the cost of living.

Consider: fans never last more than a few years of continuous operation.  If you purchase a conventionally made unit, will you be able to find a replacement fan when the time comes, several years from now?  Will the company even sell you one at all?  Better ask them before buying. What if the company is out of business? What is the plan?

What happens if something gets damaged, due to a lighting strike/power surge, fire, severe storm, or just careless mechanical trauma?  Kids or pets? How much will it cost to rectify?  These are part of life for a building element.  With the OpenERV, these things can all be solved relatively quickly and cheaply.


 All the other ductless ERV and HRV units on the market are prone to getting an excess of air blowing through them, which greatly reduces real world efficiency.  This is partly due to the limitations on fan technology.  The efficiency they advertise is merely under laboratory conditions with no wind, and is not realistic.  The OpenERV automatically monitors the wind speed and compensates.

This improves comfort by helping prevent drafts, and improves the return on financial investment.  It also allows the fan to reduce it's speed during calm conditions, reducing noise levels.



 Summer mode just allows you to command the quite powerful yet very quiet fan to blow in only one direction, whichever you prefer, rather than reversing direction periodically.  You would usually remove the heat exchanger,  once per season, which only takes a couple minutes.  It still closes the valve if high winds are detected.  You can use it with the heat exchanger, but it serves no purpose in summer mode and reduces airflow.

The flow rate at maximal power is 120 CFM.  The stall pressure is 6.5 mm H2O (that's really a lot for a quiet, reversible fan).

The HEPA filter is expected to be used in Summer mode with positive pressure i.e. air going into the house.   The flow rate at maximal power is 50 CFM with the True HEPA filter in place and the regenerator removed.  

It's not warrantied for use as a bathroom fan, which has unique requirements.  Notably, water mist could get on the electronics. 

  We should ask this question of any appliance.  The TW4 requires almost no maintenance.  Dust can be blown out with compressed air, if desired.  If the unit gets dirty due to unusual conditions, you can remove the components from the tube and clean them, but that should not normally be needed or useful.  The HEPA filter may need to be cleaned or replaced, it should be good for at least one season, depending on pollution levels.

After several years, the motor bearings will need to be replaced.  This process of wear has some randomness to it and depends on usage levels. 

When this happens, you can dismantle the unit according to the instructions and replace the bearings.  You need some <1.2 mm circlip pliers or two small screwdrivers of the right size.  See the manual for details.  This only has to be done once in a very, very long time, and it sure beats trying to find a replacement fan with all the right characteristics and interface, which for any such appliance is generally impossible, unfortunately.  If we could have used a commodity fan, we would, but they don't exist, with these specs.

If it is damaged due to extreme weather, mechanical trauma, or a power surge or lighting, the relevant parts can always be sourced and replaced, either with identical 3d printed parts, or commodity, standard parts.   Even the electronics can be replaced with standard modules.  All screws are stainless steel, so they will never rust.


  Yes, it is directly compatible with AdafruitIO, and is a generic MQTT device.  This means with some configuration, it can be controlled using Google Assistant, HomeKit, Alexa and other major home automation systems.  You can also configure it to be controlled through these platforms by smart switches and knobs.   

We plan to make it Matter compatible when suitable software modules are released for that system.  The configuration is not completely trivial, but we will release a guide on how to do it.

The electronics actually have a Bluetooth module in there, but  it's not being used and there are no plans to use it, because WiFi does all that is needed.


  Ductless ERV manufacturer's frequently indicate a laundry list of supposed standards the units are compatible with.  Some even claim inapplicable standards, such as those made for fans or ducted ERV units.  They claim the units are suitable for X many square feet of floor area, which is not how ventilation actually works. 

 Fundamentally, the efficiency, flow rate and electrical efficiency of the fan in terms of watts per CFM is what determines compatibility with these standards.  The OpenERV unit is RoHS and WEEE compliant, meaning it does not contain any of a list of toxic substances, such a lead. 

 By most standards, the OpenERV is suitable for about 700 square feet of interior space.  35 CFM gives an ACH of 0.35 for 700 square feet of floor area with 9 foot high ceilings, which is "good" ventilation.  If you just need some supplemental ventilation, you can get away with a larger floor area.

Some manufacturer's claim compliance with DIN 1946-6.  This is merely a standard for the amount of ventilation air for a building.  A ventilator unit cannot itself be compliant, only a building with adequate ventilation can be compliant.  The OpenERV can be used to make a compliant building, certainly.

DIN 308 is a legitimate standard for measuring the efficiency of Energy Recovery ventilators and Heat Recovery Ventilators.  But it does not apply to ductless units, only conventional ducted units.  The testing protocol is specific to ducted units.  It also does not  factor in the impact of wind pressure on efficiency.  With ducted systems, this is reasonable as the fans are powerful and high pressure.  In ductless units, this does not make sense as wind has a very large impact on units that do not have compensation systems.  The TW4 does have a compensation system, but it is the only unit on the market that does. See FAQ #7, on the subject of the built in efficiency optimization system the TW4 features.

Basically, the standardization and regulatory framework is lagging technology, as usual.  

Also, make sure you google a standard and know what it means before taking anything a manufacturer says seriously.  There is no one going around making sure that manufacturer's tell the truth or avoid being misleading, at least not in such detail.  If the standard is not explained in detail somewhere, it means nothing. 



At very low settings, it is truly inaudible in a normal environment.  At higher setting, it is not perfectly inaudible if you are in the same room and there is zero other background noise,  and neither is any other fan in the world.    It will be drowned out by a typical computer fan, refrigerator, an open window with the typical  background roar of the city, a forced air furnace or central ventilation fan of any kind, and many other normal noise sources.    It is extremely quiet. 

Talking about and conveying an accurate idea of what exactly something sounds like is prone to error because individual perception, the type of noise, background noise, the acoustic environment (including the tendency to reflect echos, etc.) and other factors all play a role.  But we can do some quantitative measurement, which all else being equal gives you a reasonable idea.  

The standard quantitative measure for fan noise is the Sone.  For an explanation regarding how Sones work, I can recommend this page.  For now, suffice it to say that a measure in Sones is meant to correlate with perceived sound level, so 0.6 Sones sounds approximately, to a typical person, twice as loud as 0.3 Sones.  0.3 Sones is extremely quiet as fans go, but humans can hear much fainter sounds. 

We have leveraged additive manufacturing to produce the quietest fan blades that are physically possible,  and use the best quality motors and electronics, as well as many features to passively absorb sound.    This includes custom viscoelastic rubber isolators, and fiberglass.  We pull the bearings out of the motors and carefully replace them with top quality ABEC-5 Z4 bearings.   The fan is also located on the outside side, which helps reduce the radiation of sound energy into the home, compared to putting it on the indoor side, or fully inside the room.  However it increases cost because now the fan has to be rain-proof.  This provides a profoundly quiet unit, at higher flow rates and efficiency levels than other units on the market. 

We have chosen to include the acoustic cover by default, because although most people will consider it overkill, we take pride in doing really good work.  It makes no sense to cut corners when you are going to be using something for 25-50 years.

Quantitatively, see the specs on the main product page.   

It is important to realize that fan noise increases exponentially with higher flow rates.  Actually, fan noise increases very roughly with the *8th* power of flow rate.  The eighth.  That's a lot!  That also means it goes *down* really fast when you settle for lower flow rates.  The OpenERV is capable of higher flow rates than other ductless ERV units, however you don't have to turn it up.  You just have choices.  At the same flow rate, it makes less noise by far than any other ductless ERV or HRV.

We experimented with over 65 different fan prototypes, and picked the very best.  We also did experiment with Roots blowers and other interesting mechanisms that have potential to eliminate the last of the noise that a fan makes, but they are unfortunately not economical to produce. 

Important note: placing the fan on the outdoor side reduces the noise, but it could lead to frost accumulation under extreme conditions.  If this happens, you just have to put the fan on the inside.  It's theoretically possible to run the fan in such a way as to prevent frost from accumulating by using heat from indoors to periodically defrost the fan.  However detecting frost up reliably is a challenge that I haven't implemented yet, and this frost up phenomena does not seem to occur, during testing. The motion of the fan appears to prevent frost accumulation, and I have tested it down to about -20.  It is hard to test it beyond that.  Also it depends a lot on the indoor humidity levels.  But there is a backup plan; just put the fan on the indoor side and you are sure to be ok.  The fan will not be damaged by frost, it shuts down automatically if it gets stuck.


No, it is designed and tested to work all year round, including during Canadian weather at -30 degrees C. 


 Frost up and condensation issues don't generally occur with these regenerator type ERV units, because the water that gets deposited by air on the way out evaporates again into air on the way in.  However  it's theoretically possible some kind of defrosting will be required if you have a very humid home and it's very cold out.  This can be accomplished by running the unit in exhaust mode (summer mode, with egress direction i.e. negative CFM) for ~10 minutes if frost accumulates.  This can be done from a phone or computer through any browser.  However testing indicates this is not a problem.  If you have an extremely humid home you might have to put the fan on the indoor side, which leads to a slightly higher noise level but is easy to do.  This is part of the reason for beta testing. 

 If, theoretically the fan actually frosted up and got stuck, it has several safety redundant safety mechanisms that will cause it to safely and harmlessly shut down and await assistance.  The necessary work to free and defrost the fan can all be performed from the indoor side.  You just have to unscrew 4 screws, remove the heat exchanger and then pull the fan out carefully.  The temperature sensor on the fan would ideally be removed and taped to a region near the outdoor side.  This will be describe with illustrations in the manual.   This issue will be settled during beta testing.  The unfortunate reality is that most manufacturers just put the fan on the indoor side or prevent the unit from operating at all at very low temperatures;  we are trying to do a bit better than that.


 It is not warrantied for use in a bathroom, because although the fan is waterproof, the indoor electronics are not sealed against water droplets produced during a shower, and they could cause problems.  


 Small scale manufacturing is not common, but it is seen reasonably often in the HVAC and building industry.  Additive manufacturing is coming into it's own now, but it's true it is not conventional.  However, it has been proven to be capable of producing quality, reliable components.

 All units are extensively tested before being released from Quality Control.  The sub-modules we use are standard, and used in many other quality products.   The power supplies, for instance, are made by a reputable manufacturer and are CE marked and UL listed.  We don't make the motors or the electronic modules, we just get quality components from respectable manufacturers, assemble them with custom mechanical and acoustic components, then test and quality assure the result.  The system runs on low voltage DC and is low power and inherently safe.   There is a main fuse and 4 redundant over heating protection systems for the motor.

 We believe that quality engineering and manufacturing is essential, open source or not.

The warranty helps, if a unit works for 5 years it is probably pretty sound, but above all, the maintainable, open source nature gives the best promise that you won't be stuck with trash.  

It is worth noting that the pipe inner and outer diameters are actually the same for TW4 and most of the other industry leading ductless energy recovery ventilator units.  Also most of them run on the same 24 volts DC power supply.  As a result, you can trade in one system for another after installation to some degree, although the wiring and some other details could be different. 

 We present a part of a new paradigm for getting good things done, in the near and long term, locally and globally, and invite you to be a part of it. 



The OpenERV is an Energy Recovery Ventilator, which means it recovers at least some of the water vapor. Under testing for Toronto weather, it recovered an average of 45% of water vapor from the air over the season.  This is better than an HRV, which does not recover water vapor, but is otherwise the same.  This occurs because when the outgoing air gets below the dew point, water vapor condenses onto the regenerator media.   When flow is reversed, it evaporates back into the airstream.   The efficiency of latent heat recovery changes when the temperature differential and humidity changes, while sensible efficiency stays about the same, even when it is extremely cold out.  

This recovery of water vapor is valuable because when water evaporates, it absorbs heat energy.  If you bring low-humidity air into a home, it causes accelerated evaporation of water, which cools the house down.  Then that heat energy has to be provided, to maintain the temperature of the dwelling, by burning fuel.


There is a bug mesh that clamps on under the storm valve add on module.  You can clamp any sheet of bug mesh under the clamp.   However you can filter out no-see-ums, blackflies, or whatever you wish.  Mosquitos generally can't get past the fan and regenerator anyway, especially when it's on.  Most people won't need the bug mesh,  but it's an option if you do.   The HEPA filter of course also prevents bugs from getting in.


The unit is not warrantied for use in areas that may get above 40 degrees C right now, because combined with direct sunlight this could lead to damage to some of the components.  For now, it's primarily to save on heating energy.  In the future we may employ a different polymer that allows use in hot climates.


The maintenance cost per year plus the yearly payments on the capital cost amortized over the life of the unit is far superior to a conventionally produced unit.   See spreadsheet for details.  

The unit is designed to last much longer than any other on the market.   A conventionally made unit can't reasonably be expected to last much longer than it's warranty period.  If it does, it's a fluke and, arguably a mistake of the producing company, because they forfeit that ability to charge for labor, parts, or a new unit after the warranty expires.  Reputation doesn't really change much, in reality. 

 When the fan will wear out is an important question, and how to get another one.  No axial fan lasts longer than a couple years in constant operation, that is beyond current cost-effective technology for this application.   Many of us think of the fans in our forced air heating systems or similar, which do last for a very long time before the bearings wear out.  Such fans rotate at relatively low speed.  The type of fan used in a ductless energy recovery ventilator is fundamentally different, they don't last as long.

 Mean Time Before Failure ratings for fans are statistical measures that do not represent average usable life span, for several reasons such as the exclusion of some causes of failure, and unrealistic criteria for failure.   Normal wear is actually excluded as a "valid" cause by most companies.   Secondly, the test is only done for a limited time period.  Analogously, if the same is applied to a human, the MTBF of a 25 year old human comes out as about 800 years.  Clearly, this does not mean what we thought it meant.

Secondly, failure is usually considered as only when the fan actually stops blowing air.  In the context of a ductless ERV, it's when it starts making too much noise that it becomes useless, which happens far sooner.  

With the TW4, you can just open the unit and replace the bearings, we even include a free set of replacement bearings.  They cost about 25 cents each from the right source, about 2 dollars each at a typical hobby supply store. The lifespan limitations of the electronics is almost negligible, but not quite.  They can get fried by power surges and so forth.  The design is therefore as suited as possible to replacing them.   The outdoor hood might look crummy after being exposed to the sun for a few decades, in which case you can have a new one printed for about $50 at today's prices.

With an open source unit made from standard parts, the incentives are inherently aligned differently.  Because you don't depend on us for parts or labor, there is no incentive to try to rig the design of the unit to drive you back to us, before or after the warranty period expires.

When a design is  open source, you don't have to take the company's word when they say things are done in this or that way - nor do you have to have the skill to  look at the design and check, even.  People across the globe with the right skills can easily look in and tell others in the community what is up.   This helps to prevent design flaws both in the long term as oversights are caught, and also in the near term as the engineers know that other skilled people are looking over their shoulder and will notice if they cut corners. 


 Taken together, the incentives are better aligned this way, and that paves the way for a more promising financial proposition - when the cost of a unit can be amortized over 40 years instead of 5, that's a factor of 8 more service life - for the same capital cost, and for lower running costs, because maintenance is less costly.

  Open source is no mere philosophical proposition, it has very tangible benefits. 

 The thing is, any equipment like this needs a few custom components.  To arrange things so that you do not have to buy those parts from us, we design them with additive manufacturing and provide the source code - the STL files.  Now, any company can supply you with parts.  Voila, no more incentive to make things break.


The unit is actually designed so the pipe can just stick out the exterior wall.   This is a unique design feature among ductless ERV units.  The seal around the wall and pipe is accomplished with caulk and foam.  If you wish, for cosmetic reasons to add some kind of box to enclose the small segment of visible pipe, that is something the installer can do for you, in the same way as for other ductless ERV units.  In the case of the TW4, the box does not have to have significant strength, because the pipe itself is actually quite strong.  The exterior components actually clamp to the pipe with a radial clamp, tightened by a screw.  This also makes (rarely required) types of maintenance and repair much more practical.  Making the pipe and seal against the wall independent is generally advisable.

Ventilator units of any type, whether they have heat recovery or not, can be tested to ascertain the amount of outdoor sound that they allow through/block.  This matters in a city because there are traffic noises and things that can be annoying. 

Clearly, most units do not provide the same level of sound attenuation as a solid wall.  But they do provide more than an open window.

It is measured in Decibels, A-weighted, usually  (dBA).  The test is that they install the unit in a wall in a laboratory, and then produce sound on one side of the wall, then measure the reduction in sound volume that arises after the sound waves pass through the unit.  So there is one microphone on the outside, one on the inside, and you measure the difference between the sound levels that they detect.  This process is repeated at a range of frequencies, and then an algorithm is used to summarize the data points into a single number.  The higher the number, the better.

It's helpful to think in terms of earplugs.  Those yellow or orange foam earplugs give a sound reduction of about 30 decibels, if you use them right.  The so called NRR, the Noise Reduction Rating, is included with the earplugs and should be stated on the package.  So 30 decibels is a pretty large reduction in sound level.

A typical ventilator of this type gives about 30 decibels of sound reduction.  They come with various accessories sometimes to improve this figure.  Usually the accessories are in the form of indoor or outdoor hoods with a small amount of sound absorbing foam or fiberglass.

 The TW4 already includes a fiberglass panel build into the interior cover, by default, no extra charge.  This reduces the sound from the fan a lot, and also helps significantly with traffic noise.  It 's possible you could put some sound absorbing felt, foam or fiberglass (felt is the best for sound, but it could get wet) under the outer hood, but there isn't really space if you also use the storm valve.

The storm valve blocks a lot of noise when it is closed, too.  The storm valve closes automatically when the unit is turned off.

Low frequency noises are particularly hard to block.  The reality is that any ventilator will allow a small amount of noise in.  However it is far less than a window that is open a small crack.  That allows you to see some idea of what it will be like.  Very few people will even notice the difference.

It helps a lot to install it in a wall that does not face the street.

It's actually possible to further increase sound attenuation by replacing the fiberglass with felt.  Felt works better but it's much too expensive to include by default.  You can do this yourself easily, you just undo some screws, cut the felt with scissors, put it in place and screw everything shut.

The TW4 has not been tested in a laboratory yet, so we cannot give exact figures.  However, given the physics and the geometries and materials used etc.  it is expected to be better than other units which do not include any fiberglass.