Overview
The TW4 is a ductless Energy Recovery Ventilator, also called a decentralized or push-pull type. The above video explains the general concepts of ERVs, but does not talk about ductless units.
A ductless unit just goes in a hole in the wall to access the outdoors directly. There is no need to install or interface to ductwork in your building.
For simplicity I'll talk about a winter scenario, but the TW4 also works in reverse i.e. to keep heat out of a building during hot weather.
Each TW4 module has a very quiet pair of fans, pointed in opposite directions, and a heat exchanger in a 6 inch pipe, that goes through a wall. The hot, polluted air from inside goes out for about 30 seconds, and the heat from it is stored in the heat exchanger.
Then, one fan powers down and the other powers up, moving clean air from outdoors to the indoors. On it's way in, it picks up that heat from the heat exchanger. This type of heat exchanger is called a regenerative heat exchanger. The kind shown in the video is a recuperative type, not regenerative.
The OpenERV TW4 modules are made to always work in pairs. One always sucks air while the other blows air, synchronized over WiFi. This must be done, or hot air would be pushed out from the building through the walls during the ingress phase, and sucked in from the outdoors during egress, causing heat loss.
The different modules of a pair can be located anywhere in a home, even a long distance away from each other.
You can have many pairs per home to get the total ventilation required, but 60 CFM is actually considered enough by some metrics, such as ASHRAE 62.2, for a small house.
These ductless units do not tend to work well in areas of a building with a strong chimney effect, i.e. the top or bottom of buildings more than 6 stories, because they have difficulty fighting the static pressure that results from the buoyancy of air. However some tall buildings incorporate measures to largely eliminate the chimney/buoyancy effect.
Main features:
Far superior return on investment. Return on investment for the TW4 is many, many times better than any other ductless ERV unit and comparable green energy or ERV/HRV systems. This is due to the multiplicative combination of many factors and is demonstrated by the spreadsheet below.
Very long life span/lack of planned obsolescence.
Commodity parts and Open Source design. Both ensure maintainability and remove the incentive and capacity to use planned obsolescence.
Wind compensation. There is only one other ductless ERV I know of that uses wind compensation, the Lunos e60.
A better heat exchanger, enabled by additive manufacturing.
Top quality fans. The bearings are standard and replaceable and so is the whole fan, so it will last a very long time. It is water proof (IP55, epoxy potted electronics).
Wireless (WiFi) synchronization and power level control.
The efficiency and flow rate determines how much energy is saved each season. Both are higher than the next best units available. This is due to the combination of better fan and heat exchanger. Also the low noise of the system allows it to actually be used at a higher flow rate more often.
Smart home connectivity, as a generic, highly compatible WiFi-connected MQTT device. This allows for any combination of remote switches, central control, humidity and CO2 level controls, time of day scheduling, etc. through AdafruitIO Google Nest, Amazon Alexa, and compatible devices and similar platforms. This allows it to be controlled through a phone with a app such as MyMQTT.
Modular, durable design using quality materials and best in class approaches.
Also works as a quiet, powerful supply or extractor fan (a unidirectional fan, not reversing direction periodically). The hardware is present but this feature is not yet built into the beta firmware as of Feb 16, 2025.
The approach with the solid, high quality pipe through the wall is a very good, high quality approach, superior to the telescoping tubes some systems use.
Good attenuation of sound from outdoors (sound pressure differential). There is a heavy duty attenuator option that increases the sound pressure differential greatly, making it much better than any other comparable unit.
These are/will be these main options for the TW4:
The storm valve kit, which prevents the intrusion of air when closed. This is not always needed, depending on where it is installed, which side/area of the house, and the typical weather patterns. The functionality is, as of feb 16 2025, not yet built into the firmware, to use this.
The HEPA (H13) filter kit. HEPA H13 filters remove greater than 99.97% of all particulates from the air, including smoke, smog and pollen. Great for allergies or areas with problems from wildfire smoke or smog. You must chose between the storm valve or a HEPA filter at any given time. Custom adapters for a variety of filters can be produced relatively easily.
Bug mesh kit. Almost no bugs can get past the fan and heat exchanger anyway. You can use any type of bug mesh, including very fine mesh to exclude very small bugs such as no-see-ums. You just clamp some bug mesh under the base of the storm valve.
Heavy duty sound attenuator. This is an assembly made of heavy pipe which blocks and absorbs sound energy from outdoor sources, and also the fans. See pictures below.
Sorbent or no sorbent for the heat exchanger. The sorbent increases the latent heat capture efficiency i.e. the amount of water vapor that is kept in or out of the dwelling.
Please note:
The unit is not currently designed for very hot areas like the desert. To ensure it works well in this context will require extensive testing, which has not been performed yet. The initial roll-out is for cold climate use.
Comparable equipment/units
There are quite a number and I have no problem talking about them, they are interesting but usually used only in European areas, where thermal energy is more expensive than it is in North America. I like the Blauberg Vento the most, there are many versions of it. The Lunos e2 is perhaps more well known, they have a variation called the e60 which has wind compensation, it is the only unit aside from the TW4 that I know of which has wind compensation.
The Vento is about $1600 CAD for a pair, plus shipping and taxes. The e2 is 1800, the e60 is even more and does not appear to be for sale anywhere right now anyway. they do not include provisions for window mount, of course, that would be extra. The Vento gets about 30-35 CFM according to the manufacturer, on max. Unfortunately the efficiency claims are based on the very low flow end of the regime, and only at the beginning of the cycle, apparently. Thus they do not reflect the actual seasonal average.
The figures I use for the TW4 are for the average over time, which is what you want to know when you do your calculations.
Noise figures tend to be extremely unreliable when reported by a manufacturer and I have not measured the levels of this or other units, so I cannot comment on their noise level except that in my experience they are louder and more annoying quality of noise than the manufacturer provided dBa levels indicate.
Other decentralized type units also tend to frost up in very cold weather, because the efficiency is low and there is no sorbent, this document (Improving Indoor Air Quality for Small Alaska Homes) is an interesting read on the subject. The summary is that they were kind of useless when it gets really cold because they frosted up so severely, and didn't provide very much clean air at the best of times. But when it gets really cold is when an ERV is the most valuable, because there is the most thermal energy to be had.
The TW4, especially with silica gel, does not have this problem, it has now been tested at -22 degrees C and showed no signs at all of frost accumulation. Without sorbent, the machine needs to be adjusted for slightly less than optimal efficiency, but frost up can still be prevented (slightly more ingress than egress air is programmed). If any frost up does occur, it does not cause jamming or clogging, it takes the form of a slight ticking noise as frost gets on the tips of the fan blades. This merely alerts you to the need to adjust it, it is soluble. After adjustment the frost will sublimate and the noise disappears.
For window mount units there is only one available on the market, the Purifresh, which I have obtained and inspected. It's fine except it's quite loud with a real bothersome grinding type of noise, and not built to be a long term proposition.
Installation of a TW4
The unit can be installed in a wall of any thickness, but if the wall is thinner than 170 mm, the unit will stick out slightly more, indoors or outdoors, your choice.
The pipe it uses is standard 6 inch SDR-35 PVC pipe. This is available across the globe, but there are many similar products that are also the same size or close enough. Dimensions of the SDR-35 pipe can be found here.
Although made to be as DIY friendly as possible, in reality it still takes skill and equipment to install. Having a contractor install it is advisable, if you can find a reasonably good one, otherwise you can, ultimately, DIY.
Each half of a pair needs power, 12 volts DC, 1.2 amps instantaneous peak (during fan acceleration) >10 watts average capability (includes safety margin).
You can chose when you buy the unit which option you want for the power supply: the type that is hard wired into your wall, or the type that uses a so-called wall-wart, which you can just plug into a nearby outlet, eliminating the need for an electrician, which tends to come with a lot of complications like a code inspection.
To hard wire the power supply, you need to tap into a nearby electrical wire, put a junction box around the junction, and you need to install a switch and the power supply module in the junction box. Then, low voltage DC wires transmit the power from the power converter module to the unit. Or you can run the AC power wire somewhere else to another junction box and put the switch and power supply there.
The wires, junction box and switches are not included.
For the pipe, you need to bore a hole with a standard 162 mm core drill, or you can cut it out through other means. A core drill is recommended for a clean hole. Standard technique for penetrations in a building envelope should be used, including rubber grommets and/or sealing foam, caulk and tape, depending on the type of wall. Complete instructions would be impractical because there are so many types of wall and unique circumstances. Basically, you need to take responsibility for installing the pipe and electricity supply, and I take it from there.
The interior cosmetic cover is available with or without sanding and a white primer that matches typical white semi-gloss interior walls (the most common gloss for interior white wall paint). You can use it as-is or paint it with any interior paint to match or complement the wall.
Configuration: Each pair is programmed as a Leader and Follower from the factory. You turn one on, it will start a wifi network, and the follower will connect to it automatically. They synchronize, and power is controlled by the knob on the Leader. If you want the device to connect to your home wifi network, you have to access the device using a micro-usb cable and a program called Thonny, right now. In the future there will be a web configurator.
They can be configured to connect to an MQTT broker and receive power level commands. In the future a firmware update will make it possible to issue mode commands (heat recovery mode, ingress (supply) mode and egress (exhaust) mode). This scheme allow commands directly from a three position MQTT switch and smart knobs which give percentage outputs.
If you lock yourself out by e.g. putting the wrong password in or otherwise need to access it at a low level, you can always connect to it over USB and change or reset things.
Power level/ventilation flow rate is controlled by the knob on the side of the units, or over the Wifi network with MQTT or an MQTT app. Google home and Alexa can also interface to the device, but they require some configuration (there are bridges from those systems to MQTT such as If This Then That, and some "bridge" modules ("skills" in alexa) for some systems that do not require extra services). Matter will be supported when it's more developed.