[ZeroGravitas]

There's some comments here suggesting that the incoming and outgoing air pass each other in a heat exchanger. But this is a different model that passes he two streams in turn through a heat sink:

> Recuperative types are what most people think of, consisting of a thin layer of material that separates two gas streams. Regenerative heat exchangers are different. They briefly store the energy while air flows in one direction, then release it when the air flow reverses.

I have to admit I am slightly more dubious about this type as they are new to me, though I did see a YouTube video about a commercial one recently and they seem to be a hot new thing.

Possibly this is something sensible that only becomes practical with software and wireless communication? Rather than running ducts to a central location.

Though then fitting two side by side in a window seems odd. Why not use the traditional type in that case?

[mrspuratic]

I have two equivalent regenerative commercial units (HRV, no vapour handling) fitted at opposite sides of a mostly open plan ground floor. They use a heavy ceramic core, and sync for opposite or coordinated flow (optional). They go up to 60m3/h (~35CFM) which is extractor fan level for me, 60CFM (~100m3/h) is quite a step up. They were under €200 a unit about 18 months ago.

They are rated 90% recovery at low speed. Today it's 11C 75%RH outside, 18C 65%RH inside, at low speed (15m3/h rated at 1.2W) there's barely a difference: 17.8-17.9C air intake temperature. They keep the air noticeably fresh, drier and also keep the CO2 down (<600ppm right now). I'm running them below the "recommended" 50% air-change per hour (ACH about 35%), and boost when needed.

There's a recuperative ducted type in the attic for the first floor, when I checked last month it was 4C outside, 18C at the outlet vent, and 17C at the inlet vents. That runs at 50% ACH.

The reasoning for the paired up window model isn't obvious, maybe a simple increase in capacity. The website is quite clear you need a push/pull pair to be efficient, and an immediately adjacent such pair is not going to work so well.

[open_erv2]

I have traced the air with a smoke generator, there is a youtube video of me doing this on my youtube channel. There is not significant re-inhalation (short circuiting) of the air. The reason for the window mount is that a lot of people rent or otherwise cannot punch holes in their walls. However the reason the window mount is nearly the same as the TW4 (through wall) type, is that I cannot invest the time and money to re-design a whole new machine for a window. The primary value is in the TW4, but a lot of people wanted window mount ones so I helped some people out by deving a quick window adapter and then they helped me out by testing some of the components/the fundamentals of the system in the real world. Unfortunately the window mount units nobody is willing to really pay the cost of manufacture for, so they have to use grade B parts etc. and they have to serve the purpose of testing the TW4 or they aren't worth making.

[mrspuratic]

Empirical evidence trumps conjecture :)