As I have discussed the design of the system with the community, and found discussion of the design of other systems, several issues have been raised. Most people don't care about most of these, but in the interest of transparency, to aid further design efforts, and aid decision making for potential buyers, I'm making this page to discuss the various issues.
Dust can accumulate in the heat exchanger. How do you remove it?
Yes, this applies to any ERV or HRV, however most larger units and conventionally produced units have filters on them, at least on one side. The OpenERV 5.2.4 currently has no filters. It is meant to be one step up from an open window, which of course has no filter, and in winter there is no pollen or anything anyway, so there is little reason to filter the air. It's expensive enough, and I didn't want to increase the price any further. The rumors about dust accumulating in regenerators is mostlydue to fibrous bed type regenerators, like the Recoupaerator (which looks like a good design). They can be washed, it's just a bother. However the OpenERV uses parallel flow channels, not fibers, so it is easier to clean, and dust doesn't get stuck in there very well, it tends to just pass through and out the other side. Dust may get into the regenerator core over a very long period, such as years, same as the ducts in a house. However, it can easily be removed by blowing compressed air in there, I assume, same as a duct. You can use canned air, which is easy to purchase. If the dust somehow got glommed on there, you can remove the core and do whatever you want to clean it, soak it in soapy water, or whatever. It's not hard to remove and put back again. The 5.2.4 model used glue to hold the actual rotating part with the straws in it shut, so you can't take the straws out practically, but you can take the rotating component out of the housing and do whatever with it.
In a thermal wheel type design, some of the outgoing air gets stored in the heat exchanger and comes back into the building on each rotation of the wheel.
Yes, this is true, however it's not a practical problem for us in single-room ventilation. For the 5.2.4 model, at maximum flow rate, roughly 6 percent of the outgoing air actually just comes back into the room, however this does not hurt efficiency. It is slightly unfortunate, as the de facto fresh air you are actually getting is less than it appears. This is factored in behind the scenes when I compiled the specs on the specs page. However, it cannot possibly cause any kind of odor problem, because it's exactly the same air that was in the room a second ago. Nor does it cause any heat loss. So it's a minor idiosyncrasy of the design. The idea that this is a problem comes from centralized ERV systems, which may collect exhaust from smelly areas of a house like the bathroom, and contaminating the fresh air with that air could lead to an odor in the fresh air. That doesn't apply to us, though, because the outgoing air is coming from the same room that the fresh air is going back to. There is no contamination or odor issues that could occur.
Thermal wheels have more moving parts and therefore wear out.
The parts move very slowly and therefore last an extremely long time. The problem is the fans, not the thermal wheel component. Slow moving bearings last an extremely long time, because it's not time that causes the wear of a bearing, it's the motion. Same applies to the drive belt. The wear rate of a rubber belt is roughly proportional to the square of the belt speed. This implies that the wear rate goes down really fast as you slow the belt down. The belt is moving quite slowly. I did an accelerated wear test and took pictures of the belt, compared with a brand new belt. After 2 weeks of going 4 times the normal speed, which implies about 16 times the wear rate, there was literally no observable wear. The belt can wear significantly and still work fine. I estimated from this accelerated test and projected acceptable belt wear, that the belt will last for at least 30 years, before it needs replacing. I would expect the bearings on the axle of the thermal wheel to last similarly long.
However, the fans turn much faster and will not last this long, for any unit, any fan. These particular fans have a rated Mean Time Between Failure of 50,000 hours, which is pretty good, about 5.7 years of continuous operation, and if used for only 8 months of the year, that's 8.5 seasons. However, it is important to remember they will wear out, which is why it's a good idea to go open source, because you will be able to replace them inexpensively when they do. These fans are about $25 each. Also, that's an average, remember, they could go belly up before or after then.
Wind can overpower the fans in these decentralized units, and cause problems.
This applies to some units, but not the OpenERV, except in a storm, which does not affect average year round energy savings significantly. It uses high pressure centrifugal fans, with a static pressure of 25mm H2O. That means the pressure they can provide, if you blocked the output side entirely, would raise a column of water by 25 millimeters. It might not seem like much, but a typical quiet axial fan is more like 3 mm. The wind blowing against the side of a house creates a static pressure, and the fans have to fight this pressure. This table shows the relationship between the wind speed and the pressure against the side of a building, and therefore against the fan. It's in pascals, not mm of H20. One mm of H2O is 9.8 pascals. So the fans the OpenERV use can handle about 245 pascals before they were overwhelmed completely. Now, in reality the fans become incapable of giving an effective ERV functionality at about half that pressure, very roughly, and flow of fresh air is reduced before then if automatic flow balancing is employed (The OpenERV will use auto flow balancing as soon as I get the firmware programmed to do it, it has the sensors it needs built in already). If auto flow balancing is not employed, the situation is even worse as efficiency drops rapidly as the flow in either direction becomes imbalanced (there is a spreadsheet that I used to estimate real world impact of this floating around somewhere). So about 15 pascals for a typical axial fan, and 122 for the OpenERV fans. That corresponds to 5 meters per second and 14.5, or about 18 kph vs. 52.2 kph. You would have to look at the weather data for your area to see how common such wind speeds are, but 18 kph is actually pretty common . IIRC the average for ottawa is 12 kph! 52.2 kph is definitely not common, that's a storm, let's say that. I did make a spreadsheet that estimated based on hourly weather data for Toronto what the impact of different fan characteristics were, but there were a lot of approximations.
Yes, I put a lot of work into making the unit quieter, and was successful to some degree, but the fans still make noise, especially at high flow rates. However, you can' t buy quiet fans, and I think this is because people don't care that much. As mentioned above, the fans have to be high pressure capable fans in order to get good efficiency, so you have to pick super quiet or efficient, with existing fan designs. All the appliances in our homes make more noise than an OpenERV does at 20 CFM, even a computer or most fridges. I would like to do a crowdfunding campaign to design a very quiet centrifugal fan, which I believe is possible, but that's a whole project of it's own. The thermal wheel makes almost no noise. The secret is to make everything move slowly, that helps a lot. Also I tried several different motor types and makes and models, as well as different motor drive electronics, and picked the quietest combo. The stepper motor didn't work out, and not all brushless motors worked, the best was the brushless gimbal motor. It has to be a non-cogging motor. It has to be a brushless motor so it lasts a very long time. You can't use gears, but belts are ok. I added rubber washers to isolate the small amount of vibration from the motor. FOC drive electronics would be nice but there are none available ready to use unless I design my own pcb and have it custom made, except the Mosquito, which is not easy to get ahold of, and is kind of expensive. None of the electronics make any noise (some of the motor drivers and power supplies actually made significant noise).