How An Engineer Designs A DIY Energy Recovery Ventilator

We have no idea whether [Nick Goodey] is a trained engineer or not. But given the detailed design of this DIY energy recovery ventilator for his home HVAC system, we’re going to go out on a limb and say he probably knows what he’s doing.

For those not in the know, an energy recovery ventilator (ERV) is an increasingly common piece of equipment in modern residential and commercial construction. As buildings have become progressively “tighter” to decrease heating and cooling energy losses to the environment, the air inside them has gotten increasingly stale. ERVs solve the problem by bringing fresh, unconditioned air in from the outside while venting stale but conditioned air to the outside. The two streams pass each other in a heat exchanger so that much of the energy put into the conditioned air is transferred to the incoming unconditioned air.

While ERV systems are readily available commercially, [Nick] decided to roll his own after a few experiments with Coroplast and some extensive calculations convinced him it would be a viable idea. One may scoff at the idea of corrugated plastic for the heat exchanger, but the smooth channels through the material make it a great choice. He built up a block of Coroplast squares with the channels in alternate layers oriented orthogonally, letting stale inside air pass very close to fresh outside air to exchange heat without ever mixing directly. The entire system, including fans, an Arduino for control, sensors galore, and the Hubitat home automation hub, is powered by DC, so no electrician was needed. [Nick] has a ton of detail in his build log, including all the tools and calculators he used to design the system.

Given the expense of ERV systems, we’re surprised we haven’t seen more stories about DIY versions. We have talked about HVAC systems a lot, though — after all, HVAC techs are hackers who make housecalls.

80 thoughts on “How An Engineer Designs A DIY Energy Recovery Ventilator

  1. I can’t even stand the ‘fart and smell it twice’ (recirculate) on a car, let alone on mah house. Luckily, all of my houses have been old and have plenty of fresh air coming in and out… also, I tend to open doors and windows whenever possible. I like living outdoors. I can see how this might be the opposite for people in cities… but realistically speaking, you could just crack one or two windows in your house and utilize convection. But a better solution is to design buildings and homes in such a way where hvac units are not as necessary. Such as putting most of your windows on the south side and having an overhang that leaves direct sunlight out during the summer and lets it in during the winter among various other intelligent designs and material choices. To be perfectly honest, we should be building most of our biggest structures UNDERGROUND, and leaving more real estate above ground for green spaces. Underground dwellings are just better for a plethora of reasons, one of which is that they never get destroyed by natural disaster. I mean, you could flood one and kill everyone inside, but that can also be overcome by engineering. Holes in the ground, green space on top. constant 55 degrees @6ft means no air conditioning or heating required to sustain life. Thermal mass of rocks overcomes the heat change of the day/night cycle, especially if you have enough plants and mycellium around. Over the years, archaeologists have found many underground dwellings, still perfectly in tact. Very few above ground dwellings survive for more than 100 years or so unless they are made of the stones we pull out of the earth, and even at that, they have to be large enough to not be swept away by the water (think pyramids) that we don’t have the technology to reproduce. The only real downside to building underground is cost at this point… but i feel like that could partially be negated by the fact that we have concrete 3d printers now that are structurally sound. It sure would be a start to solve a lot of our ecosystem issues.

    1. I grew up in a sub-grade house. Not great for mental health.

      You are kind of missing the main point with your comment in that the only reason these systems exist is to drive overall efficiency up. Opening the window or having a drafty house on purpose completely negates what an ERV is trying to accomplish.

      1. In modern bottle houses the problem is that they no longer let any air flow through the walls, by adding plastic tarps to stop the air leaks. This is to stop cold air from seeping in/out, but what it also does is make moisture not leave the walls.

        If it’s a humid day, it stays in the wall for longer. All your cooking, laundry, things that raise the humidity indoors are absorbed into the walls and the longer the humidity stays in, the greater the chances you develop mold and bad smells etc. This then requires extra ventilation to stop wet spots from accumulating in the corners an places where air moves the least, but more ventilation means more heat lost that way – so an ERV is practically required to make a modern house work at all.

        If you have a forced air heating system, then the ERV can save you money by pre-heating the furnace air, so that works as well.

    2. It doesn’t recirculate the same air. Inside air goes out, outside air comes in, but the heat/cool from the outgoing air is transfered to the incoming air. Air in/out. Temperature stays in. Same theory as stork legs. They can stand in cold water without the their overall blood being cooled even though blood is being circulated to their feet. The design of their leg blood vessels is a heat exchanger. Like what we build into homes now.

      1. How about using the a/c unit’s external fan pull the stale inside air across the exchanger? That way be it the cold or heat is successfully extracted and then released to the inside air. You can crack the window, from occasional door opening or natural building leaks…. you will get fresh air in the latter case warmed by the masonry’s thermal mass. No fancy exchangers required. And btw ther is such a thing as aluminium version of coreflute…. perfect for this project.

    3. I’ve been lately wondering about a solution to recycling plastic is to use for lining underground structures and more underground use. Same goes for above ground also structures more. Seems more higher volume demand potential. I’d like to use an old silo reconditioned to run the ducting up with the intake and exhaust being at two different layers to lower cost exchange air and tap the dryer source.

  2. I love it! I was wondering, how is he going to force the flow into the edge of a piece of coroplast? packing it tightly with coropast pointing the other way! delightfully simple!

    of course at my house we achieve plenty of air flow through the gaps around the windows

    1. The fire will protect him from the cold.

      Never heard of this being called energy recovery ventilator before. Heat recovery ventilation or heat exchange perhaps.

      This may be splitting hairs, but I have to as am quite bald, and a split hair is 2 hairs…

      1. In my climate they mostly recover heat from inside the house by exchanging the heat from the stale inside air to the fresh outside air, but in a normally hot climate these actually recover the hard earned cold in the house (exchange the AC’s cold from the inside air to the incoming hot but fresh outside air).

      2. There are Heat Recovery Ventilators and Energy Recovery Ventilators. I don’t recall specifically what the difference is, but it boils down to “use a HRV in cold climates and an ERV in hot and mild climates”

        1. An HRV has a plastic heat exchanger core like the one in the article above. An ERV has a core that is made of paper/plastic membrane, which moisture can pass through, in addition to heat. It helps keep from bringing in a bunch of humidity in the summer. Keep in mind that neither of these units is 100% efficient. I believe it’s somewhere in the 55%-65% range for heat, and moisture transfer is about 50%. It helps provide fresh air, but at a cost.

  3. According to the numbers in the data dashboard picture, the system is recovering 0.137 kg/min * 1.8 C = ~4.1 watts of heat. Good job! Assuming 24x7x365 operation and the fuel rates where I live, that’s $1.30 per year in your pocket! (assuming it costs nothing to run it or build it or maintain it.)

    Bonus: as well as reducing carbon footprint by reducing fuel use for heating, it’s also sequestering carbon in its plastic!

    1. Perhaps this setup is better than simply exchanging the air without exchanging any heat. If the drag of the heat exchange channels adds more than 4 watts of additional load on the fans, though, it seems like it’s a net loss in terms of energy usage. Notably, 4 watts of electricity generally costs a lot more than 4 watts worth of natural gas, so from a cost perspective, it could be more expensive to operate even if it’s a net gain in energy conservation. All the instrumentation likely adds up to more than 4 watts on its own, too; not that it isn’t worth doing purely for the enjoyment of it.

      Regarding carbon sequestration via constructing stuff out of plastic, it seems like a lot of carbon must have been released into the environment when the plastic was manufactured. If the plastic was bought new, the manufacturer has an incentive to manufacture more, and so more carbon is released into the environment than otherwise. If the plastic was re-purposed from a previous project, though, then yeah it’s a net benefit.

      1. Corrugated aluminum maybe? Get’s me thinking about like with plastic recycling… recycling aluminum cans first by processing to cut the sheets, then weld the sheets into larger sheets, then stamp into the corrugated sheet and finally laminate most cost and energy effectively.

        Maybe old political signs can be used if they still use corrugated plastic? Tis the season for a lot of free material if available.

      2. Correx (Polypropylene) has a thermal conductivity of 0.17 – 0.22 W/m °K, which is basically the same as a brick wall. Comparing that to aluminium with a thermal conductivity 100 times greater at around 210 W/m °K, you can see why we use it for heat exchanges in the industry. With a delta of 7°C available (Exhaust – Intake) the most the system will recover is ~140w with a proper core (I suspect you wont even see a net gain with that – due to having 7 fans in the system)

        If you look at the data for the supply side (I’m assuming the airflow is the same in and out – it’s not specified on the dashboard picture) the pickup is around 6W. Without knowing where the temp sensors are it’s hard to say, but I’d suspect that is the heat load of the supply fan/s.

        1. That’s literally what this contraption is doing though: exchanging inside air with outside air. The purpose of the heat exchanger is to warm the incoming outside air with the cooler outgoing inside air, so that the new air doesn’t need to be heated as much. The claim is that the heat exchanger only exchanges 4W of power, though, and the assumption is that the fans and electronics consume more than 4W of power to operate. Therefore, from a power consumption perspective, the system is indeed worse than simply opening a window.

          Depending on where the fans and electronics are, their power consumption might help heat the incoming air; that’s better than heating the outgoing air, but it’s still heating air via electricity, and that’s a lot more expensive than heating air via natural gas, and likely worse for the environment depending on where the electricity came from.

  4. This kinda seems nuts. First, what is stale air? I know air outside “feels” cleaner or fresher, but without knowing the cause, there’s no reason to assume dumping air from the outside is better. It might contain worse toxins than your house does. Second, this is a heat wasting device. Assuming you have 21C air in the house, and 27C outside, then when the two mix, at perfect hear transfer would give 24C air coming in, so you’d still have to cool it 3C. Lastly, how much of your house air needs to be replaced before you achieve maximal “freshness”? This is the problem with bunk science… It’s not science.

      1. It’s a dumb comment, but I really needed a laugh, so thanks for that.

        OP has a valid point. What actually IS stale air? We all know it when we breathe it or smell it but I don’t know way of quantifying what it is.

        I’m quite serious I would really like to know what measurement system is used for this.

        There is a scale of phenols for “peatyness” in Scotch whiskey, something you would often wonder how to quantify- so how do we quantify “staleness”?


        1. “Stale” is pretty subjective and qualitative, and I’d guess have as much to do with perceived smell as more quantitative measures.

          There are several *outdoor* air quality metrics used by different countries, typically focusing on the usual outdoor pollutants. Canada, for example, computes a Air Quality Index from the concentration of ground-level ozone, nitrogen dioxide and particulate matter (PM2.5 and PM10). Other administrations include more pollutants like sulfur dioxide and carbon monoxide.

          Indoors, air quality pollutant limit guidelines in Canada for VOCs are described in

          Carbon dioxide is separate:

          In that document, the proposed guideline is a 1000ppm (0.1%) CO2 limit. This implies about a cubic meter per minute per person air exchange, or roughly a roomful per hour. An HVAC tech would call this 400 fpm through a 4″ duct.

          And then there is radon, a different can of worms to measure.

    1. Except it’s well documented that indoor air quality can be 10-100 times as bad as ‘fresh’ air from outside, and CO2 levels can get high enough in uncirculated rooms to cause health impacts in humans.

      And yes, you still have to cool your incoming air, but not as much as you’d have to cool incoming air from an open window or intake vent.

      What is your argument again?

    2. I can’t speak to your point about defining stale, but I can clarify the heat transfer part. The ideal system in your example would heat the internal air to 27°C and exhaust it outside at the same temperature as outside air. Meanwhile the warm outside air is cooled to 21°C by the outgoing cool air. It won’t be perfect, and you will lose energy, but an equilibrium of 24°C is far from what is achieved.

    3. Written by someone clearly ignorant of the concept of the counterflow heat exchanger.

      Also apparently unaware that humans breathe and exhale 5% CO2, but find inhaling even 0.5% CO2 “stale” or “stuffy”. If you don’t dilute your own breath by a factor of 10 or more (that’s around 15 cubic meters per hour per person), the average person will be looking for fresher air.

      Seal up your house in the name of efficiency and you’ll need artificial means to keep it livable.

      1. Hmmm, ignorance?

        The reason you need to ventilate your house in temperate climates is to get rid of moisture – which will destroy your home and encourage fungi that will make you sick. CO2 is only going to be a problem if you live in a plastic bag (but in this case, humidity will be a noticeably worse problem.)

    4. Talk about Dunning-Kruger.

      Andra, air is 79% N, 21% O2, and lingering gases. Exhaled breath is 79% N, 16% O2, 5% CO2, and .5L of air exchange every breath. The average male breathes 20 breathes per minute, or 720L per day. Five percent of that is 36L. Median house size is 2301 sqft, with a median ceiling height of 9ft, which works out to 586420.655L. Therefore, in a hermetically sealed house, the average male would change the CO2 composition by 0.0061%. CO2 levels that have observable effects on humans is 0.1%. Therefore, the average male would take 1624 days, in a perfectly hermetically sealed house, to cause CO2 to have a discernible effect.

      James, your incorrect. Q=mc∆T. The only two gases are the same composition, and therefore have the same specific heat, and mass, then only variable is therefore ∆T, so the equilibrium temperate would be the effect.

      Paul, you are the Dunning-Kruger effect in walking form. See above.

      Hackaday’s user base has really sunk…

        1. Lots of things affect indoor air quality, from what I’ve picked up probably the single biggest issue is moisture; from showers, cooking, and vent-less gas heaters (they put out a lot of water vapor), excess moisture leads to mold, fungus, and insects.
          And then there are the billion different types of VOCs being put off by everything from cooking dinner to newly installed wallpaper.
          I think that, undisturbed, asbestos might literally be a safer thing to have in the house than some types of modern manufactured goods.

        2. Indeed very common. Here in the Netherlands you need to comply with a points system when building a new home. A WTW (warmth recovery) ventilation system counts for a certain amount of points. It has to be well balanced (balanced throughout the house) and have good filters. If done right it will bring you a very pleasant living atmosphere. I have my second home with two of these units:

        3. Same here in Canada, now part of the building code. I’ve added one to both my houses and both use the coroplast heat exchanger. And for those trying to do the math, the usual delta between indoor and outdoor temps in winter is 30deg C.

      1. Your numbers appear to be based on human breathing at rest, which is pretty much the minimum.

        Calculating by food consumption, the average male should put out close to a kilogram of CO2 per day, which is roughly half a cubic meter of CO2 gas, and in a house with a volume of 600 cubic meters this would increase the CO2 content by roughly 800 ppm.

        With the CO2 concentration starting from 300-400 ppm already, this puts the concentration well above 1,000 ppm which is the limit where people start to feel unwell. Slight cognitive effects are measurable at 800 ppm already.


          Resting or low activity work: 0.02 m3/h or 0.48 cubic meters per 24 hours per person.
          Normal work: 0.08 – 0.13 m3/h or around 2.5 cubic meters per 24 hours.

          Assume a person doing normal activities and rest for 20 hours and working normally for 4 hours, they would add 0.82 cubic meters of CO2 into the house, which in a 586 cubic meter house would be 1,400 ppm + 350 ppm = 1,750 ppm after one day without any air exchange.

          The reason why you have to exchange the air in a house faster than once a day is because it is not simply replacing the air, it is mixing and diluting it. By doubling the ventilation rate, you halve the difference to the ambient concentration, so by exchanging the air twice a day, you go from 1,750 ppm to 700 ppm. This ventilation rate for the average house would be 49 cubic meters per hour, or 814 liters per minute (28 CFM) which is quite typical.

          1. This is like the only sensible comment thus far. It is also true that there are many other indoor air pollutants, and also that the air from outside man contain desirable components like the smell of flowers and stuff. I don’t know about that last bit, but I honestly don’t see how some people have reached adulthood without some grasp of why fresh air is desireable.

        2. No one mentioned the methane+hydrogen sulfide output of the average male. Even though you are nowhere near the flammable or explosive limits of methane in a standard atmosphere, I guarantee the air quality is affected. That alone is worth ventilating.

    5. Huh? You can add a filter like even a MERV17 or higher rated HEPA filter. Also you can add a carbon filter or in fact any other filter you desire. I spray my HVAC filters with nanosilver and glycerin as another method to process the air.

      I think a geothermal circuit can be handy with these types of ventilation heat exchange systems to extract or exchange heat too.

  5. There have been several new developments around weather and air sealing. Air sealing is not super mainstream now, but it’s becoming more common.

    Matt Resinger on youtube has a lot of videos about the subject. He’s a custom home builder, so he does favor the more expensive materials. But he’s also very focused on making sure things last a long time. I find a lot of his stuff interesting.

    Here’s one where Matt talks ERV vs HRV

    Here are a couple This Old House videos on the ERV subject:
    Seeing it work with smoke bombs
    A discussion on indoor air quality

  6. HRV heat recovery ventilation, not ERV, energy recovery ventilation.
    HRV can only transfer sensible heat and heat exchanger can be made of lots of non permeable materials.
    ERV can transfer latent heat, water vapors, and requires heat exchangers made of special permeable materials or rotating wheels with a air washing system.

    1. Yes. Heat, wintertime. Furnaces! There can be a large temperature difference in the winter whereas in summer it’s much less. That’s what this is mostly about.

      I saw somewhere about a rotating wheel shape of corrugated cardboard where warm air going out heats up the disc of all parallel channels whilst half a diameter away cold incoming air is heated by the heated air portion of the constant rotation. Instead of having to transmit heat thru it just gets heated up. Yes it’s cardboard and replaceable for cheap and dirty cardboard can be recycled. Intake screens of metal and what ever filtration you want, even if in fire country.

      The plastic channels will get dirty on both sides, mold is of concern.

  7. Interesting. Would like to see some long term data. But you’d really need a before/after set.

    I would have opted for a “swirl” type diffuser – these are much better at mixing the air in a room.

    Cold air at low speeds will just sink to the floor – a swirl diffuser creates vortexes that mix with the incumbent air better.

  8. I think swapping air when outdoor conditions are most favorable might be a better way to extract the most efficiency out of a system. If it happens to be -10 out you would NOT run it at all that day, but if outside temps are close to your ideal run a big fan that swaps as much air as possible.

  9. Very awesome. The Build Show noted one recently and there are some interesting designs online I seen over the years. Awesome to see the article and will read into. I’ve been thinking about recently… especially when I was in the attic recently. That and a layer of closed cell spray foam lining the roof would be awesome. I’ve also thought a geothermal ductwork for exchanging air coming in from the basement up would be useful. Very awesome timing!

      1. NZ$ is a funny unit for energy consumption :-)
        Can you convert that to something the other 99.94% of the world can relate to, like total kWh?
        My electric bill last month was 0.46% of gross income. Is that any more relatable?

  10. “we’re going to go out on a limb and say he probably knows what he’s doing”

    If he was an engineer he wouldn’t be home brewing and using un-certified electrical appliance duct taped into a flammable plastic storage container, sticking it in an un-insulated attic space, using un-insulated high friction flex ducts, and weaving spaghetti wiring and ductwork throughout his attic. Unless he went to the Trump School of Engineering, then its all good.

    Sorry I’m not impressed with half witted implementations especially when high efficiency HRV/ERV’s are commercially available at very reasonable prices. Buddy would be better off leaving his window open a crack and avoiding a house fire and the insurance telling him to take a hike. Looks to me more like another home grow operation waiting to get caught by the cops because the neighbours are complaining of strange smells from the house with un-cut lawns and tin foil on all the windows.

  11. This topic leads me to Radon Remediation Systems. Here in Minnesota, where we have basements, Radon is a bad problem in some areas. Many homes have living quarters (bedrooms, family rooms, etc) in basements and there are building codes to have Radon Remediation Systems installed. So after measuring for Radon and determining there is indeed a high level, they drill several holes in the basement floor (concrete) and take more measurements. Eventually they settle on a location to install a PVC pipe into that hole and draw out air from below the concrete, exhausting it out through the roof or through an outside ‘chimney’. Even though the air is drawn from below the concrete, some air is drawn from the basement itself. That means 24/7 there is air being drawn out of the house and exhausted outside. Good for Radon removal but not so good for heating and cooling costs. The motor also runs 24/7. Realize that Radon is really bad and causes cancer, but this is sort of like the ERV topic of removing air from the house that needs to be replaced somehow.

    Anyone know more about this topic from an HVAC perspective? Is Radon removal systems and ERV systems in a position where health is of utmost priority VS energy waste and costs?

    1. An ERV design would reduce the negative impacts of that radon removal system.

      Also of note, and I’m surprised that this hasn’t been brought up yet:

      Bringing in outside air is highly beneficial for building HVAC systems if you are trying to reduce transmission of respiratory viruses…

  12. Imho is the shown efficency / heat recovery rate not correct unless you measure the air humidity on the inlet and exhaust air. The water vapor in the air contains a lot of “latent” energy and this has to be taken into account.

  13. Radon mitigation is entirely different. Much depends on if it is a new build or if it is a retrofit to address a known or assumed problem. Usually air isn’t drawn from below the concrete slab, but it depends on what the soil-gasses are doing in that area. Usually the idea is to provide an air path to above the roof, like a plumbing stack, with the goal of equalizing pressure to reduce soil-gas entering the home (cracks, seams, holes, sumps, etc.).

    ERV (or HRV in the example above) should not be relied upon to mitigate radon, but certainly helps to dilute it. Radon is one reason why someone shouldn’t cobble together their own HRV. They may be creating a negative envelope pressure that encourages soil-gas ingress, which may include Radon.

  14. As a kid in the 70’s in Sweden, I helped installing commercial variants of heat recovery systems.
    Turns out small kids are great at crawling under pipes in glasswool insulated atticts to rivet and seal the pipes.
    I don’t really remember the deal I got, but I made plenty of money that summer
    It all started with my aunts house, where I was curious and helpfull with the install, the installer asked me if I wanted to earn some money, and I helped him installing in all houses on a large block.

    I remember my aunt telling me they cut a third of the heating bill after installing it and adjusting it properly.

    You need to get the exhaust fan speeds correct to harvest as much heat as possible before venting it, and same with the intake fans, and this was before computers in everything.

    They had a pretty big airflow before installing it, and after installing it and measuring/feeling the air quality they lowered the flow significantly without decreasing the air quality.

    The construction looked like stacked aluminium heatsinks, with the flows crossed diagonally. (inflow N-S, outflow E-W)
    You could open the lid of the box and lift them out and clean them.

    Best part of the job was that I got a box full of smokebombs used to test them for leakages, they might have ended up in the schools air intake a few years later somehow…

  15. Can any one help me understand this

    I need to understand this properly I think maybe it will explain whats happening here

    I don’t understand how I get a smaller delta T stale – exhaust than between inlet and fresh, if the temperature boost is a result of heat being exchanged?

    Checked for leaks and instrumentation. I saw similar phenomenon with the small scale experiment, strange. I need to know why.

    Given: Ql = hwe ρ q Δx / 3600

    Is it Δx = difference in humidity ratio (kgh2o/kgdry_air) causing this reading? Seemingly double the ΔT inlet to fresh than stale to exhaust.

    Curtesy of
    Latent Heat Flow
    Latent heat is the heat, when supplied to or removed from air, results in a change in moisture content – the temperature of the air is not changed

    Latent Heat Flow – SI-Units
    The latent heat flow due to moisture in air can be expressed in SI-units (metric) as

    Ql = hwe ρ q Δx / 3600 (2)


    Ql = latent heat flow (kW)

    hwe = latent heat of vaporization of water (2454 kJ/kg – in air at atmospheric pressure and 20oC)

    ρ = air density at standard conditions (1.202* kg/m3)

    q = air flow (m3/hr)

    Δx = difference in humidity ratio (kgh2o/kgdry_air)

    Sensible Heat Flow
    Sensible heat is dry heat causing change in temperature but not in moisture content

    Sensible Heat Flow – SI-Units
    The sensible heat flow can be expressed in SI-units (metric) as

    Qs = cp ρ q Δt / 3600 (1)


    Qs = sensible heat flow (kW)

    cp = 1.005* – specific heat air (kJ/kg K)

    ρ = 1.202* – air density at standard conditions (kg/m3)

    q = air flow (m3/hr)

    Δt = temperature (oC)

    * Note that air properties changes with temperature. Interpolate values if necessary.

    Mollier Diagram
    The Mollier diagram is a graphic representation of the relationship between air temperature, moisture content and enthalpy – and is a basic design tool for building engineers and designers

    1. Since there’s no condensation happening, it’s obvious your humidity sensors (and possibly temperature sensors) are a bit out of calibration: the reported numbers are inconsistent with each other according to Mollier.

      It’s unlikely you’re changing the pressure in the airflow enough to change the temperature significantly, but it’s worth running the numbers if you have the pressure measurements.

      All that said: how much heat are you putting in with the power dissipated by the fans, and where? Do your temperature sensors correctly measure the actual heat exchange, including the fan power input?

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