Open_erv

BTW that regenerator did the 45 cfm at 3.25 mm H2O of air pressure.

I can actually get small thermistor based hot wire anemometers for about a dollar, so I can just use those with a PID control loop to control fan speed. Then the flow is quite well regulated. However high winds will overwhelm the fan, a problem you don't have so much with centralized units, however that's because the fans are so powerful in centralized units. This implies quite high electrical energy consumption and also noise production. The noise production is not as bad because the fan is in your basement or whatever, but it's still serious and not as good for the most part as a well designed, flow compensated decentralized unit, even for the flow vs noise ratio. I added an automatic valve which only costs about $30, which closes in a storm. So as long as most of the time the wind isn't crazy everything works pretty good with the decentralized approach.

Yes, that's correct.

You can 3d print one actually, it works pretty well. The only problem is the alumina has a higher volumetric heat capacity, of about 3.5, whereas PLA has 2.25. Which is pretty high. The thermal conductivity isn't a big deal, the main impediment to heat flow is in the air channel, not the solid material, plastic or ceramic. I commented elsewhere in this thread on how to print your own exchanger relatively easily. You can also get your own fans, so this begs the question of why you would pay $600 for them! Also you need a pair of them, you have to have two of those modules operating, one blowing air in and the other out at any given moment. Then they reverse direction. 70 seconds is a bit long for reversal time, it's better to make it shorter if you can. If you are diying one you can do that. If you try to mod an aliexpress one it's only going to cost more and probably not be as good to be honest, because you can't optimize it. Not hackable!

They vary considerably, the aliexpress ones are about as good, but none of them are very good, they give more like 70% efficiency at the medium flow rate, overall. Not what they claim. They do work though.

BTW don't know if I said this, but there are some reversible fans on digikey that should work. I was going to open them up and paint them with dielectric grease for water resistance. I ordered the 136 mm diameter one, they are pretty expensive and we will see how quiet they are.

Yes, I agree :). I don't know if you saw my proposal to Kanro, an anti-pandemic charity, to develop a fan for a superior CR box. I think it will be really good if they chose to fund it, it bothers me that they take several months to approve a proposal though. I see no reason for such waste, this is an emergency...

I don't like the telescopic ducts because they can leak into the wall cavity. There is nearly no point in adding more mass. It does increase efficiency but the typical regenerators are alumina which already have a lot of thermal mass. The problem is their surface area is too low. I made a simulator in python to evaluate various designs, and the efficiency of them is pretty poor. The main reason the fan reversal time is so long is the fans suck, they take a long time to accelerate, which affects average flow rate, the shorter the period the more time it takes to accelerate.

Once the thermal mass of the "batch" of air is relatively small compared to the thermal mass of the regenerator, adding more thermal mass doesn't help.

The reason you can't go to town with high surface area is viscous drag. The fans make a lot of noise when they have to work at high speed, which they have to do to produce higher pressures. Fan blade noise increases in proportion to the eigth power of blade velocity. So really really fast. So if you make the holes in the regenerator smaller and the walls thinner to increase surface area, you don't get enough flow.

The usual units really hobble at the best of times. If you calculate how much energy (which increases linearly with flow*efficiency) they save a year, they usually actually cost more in amortized total cost of ownership than they save.

The best thing is to redesign both the fan and regenerator, that's what I did.

There is a nice fan that's produced by a german company that I'm trying to get samples of but they are super slow.

They all are capable of doing so under some circumstances with limited efficiency, because as the air exits the house and cools down at some point water condenses out of the air onto the regenerator media. Then it evaps again when flow reverses.

However if you want maximal efficiency you want to add some zeolite 3A adsorbent, which you can sort of do yourself I am pretty sure. You just put some through a coffee grinder, put the regenerator in a garbage bag with it and shake it to get the powder inside the regenerator. It's not idea but it'll certainly help. I tried this with a thermal wheel of comparable size with silica gel and measured the result with AHT10 sensors and it worked pretty good actually.

Zeolite lasts longer but you need more. Obviously you gotta try not to get the powder everywhere by getting excess off etc.

You can understand quantitatively how this goes for different areas using Heating degree days and Cooling degree days, factoring in latent heat in both cases, ideally. There are calculators for this. I made a spreadsheet on my website www.openerv.ca that shows some of how to reckon this, there are links in there to get data.

In Austin, texas, I think you got quite a bit more energy and money saved per year from avoiding cooling costs, but in Ottawa, Canada, you save about 100x as much in winter as you ever could in summer.

This is approximately correct, except for one big problem with counterflow exchangers, which is frost. Even the membrane types still frost up. What you really want is a regenerator, either a thermal wheel or a fixed matrix regenerator, if you really want maximal efficiency. And it should be what they call the total sorption type, so it has some adsorbent in it. Even then the latent efficiency could be improved with a better adsorbent, the industry standard is zeolite 3A. It's good because it lasts. Silica gel becomes slowly inactive as it adsorbs other stuff that clogs the adsorptions sites basically.

But the adsorbent only starts adsorbing at about 60% RH. So in some cases that actually leads to efficiency loss, compared to some kind of ideal adsorbent that adsorbed at any differential.

There was a period where I considered DIY for such machines impractical. I now think that to diy all the way through is impractical, the design process is too laborious even for those suitably skilled. However once the design is worked out it is actually pretty practical, with a 3d printer. You definitely have to own the printer, they charge way too much to have things printed at cloud services for some reason.

I have pubished my STL files for an earlier version of the TW4 energy recovery ventilator. It's totally practical to make your own with a printer, but it would take you a few days of labor time for sure to get everything working nicely.

Reliability is very much on offer even with diy. In reality I have looked at many commercial units and they have serious reliability problems for long term like 5 years or more, and we should be planning for 50 years wherever building tech is concerned, imo.

I'm the OpenERV guy. It's been a long road and it's not very open source any more I'll admit, however I did publish a bit Wikifactory article with all the cad etc. for a quite efficient window mount unit. I now have the know how to boost the flow through that thing by 2.5x or so so it could actually be a viable product/machine. However it's not a good business proposition.

There is a whole class of energy recovery ventilators called decentralized, ductless or sometimes push-pull which are quite amenable to DIY. All you need is a good reversible fan and the regenerator and a pipe.

I have shared elsewhere how to 3d print a regenerator. I use a 0.3 mm nozzle and it takes about 24 hours to print a regenerator that can do about 45 cfm at about 80 percent sensible efficiency. Basically just take a cylinder in Cura of the desired diameter, and use lines infill, no top layer, no bottom layer. That's basically it. Make the lines as thin as you can and the line spacing as small as you can while getting the desired airflow. About 2.7 mm on center spacing was about right for me and a typical fan with 6.5 mm stall pressure. The operating point was about 3.5 mm h2o and 45 cfm at maximal fan power.

These devices do capture some water vapor during cold weather because the water vapor condenses on the heat exchanger then re-evaporates.

I have tried sprinkling zeolite and silica gel in there to get good latent heat recover as well and it seems to work pretty well. You don't need much, only about 5 grams of silica gel, more for zeolite 3A molecular sieve.

I have built two window mount units. One for myself, the mega sized one, which there are pics of on my twitter (@open_erv) and also one for my friend Alex, whose landlord wouldn't let him use it, so I sold it for the cost of the parts.

It took forever to sell that thing even at the cost of the parts, so clearly window mount units are not a great business proposition. Also for other reasons, mostly people don't think of them as long term propositions so they aren't willing to amortize the cost of manufacturing a good machine over long periods.

There is one called the purifresh, for windows too.

The ductless models for houses include the lunos e2 for about $1800 CAD and only 25 cfm, and the blauberg Vento (there is another related one called Twinfresh), however I don't believe their noise or efficiency claims, and I have investigated.

There are many cheap chinese knockoffs and some of them aren't too bad Holtop makes one. But mostly they are very poor.

Ultimately I would like to make a hackaday or something that explains how to do all this, with a raspberry pi pico etc. Also an anemometer based on thermistors and the hot wire technique is useful to regulate flow, any extra flow in either direction hurts efficiency.