Popped up on Hacker News today (well, yesterday, but I always catch up in digest form): https://www.openerv.ca/

Might be worth following.

Some kind of dMVHR. Original story - https://news.ycombinator.com/item?id=42427888.

At least he wants to get it tested by the PassiveHaus institute.

Ah, thanks. I can see those photos today, dunno what was wrong yesterday. There's something strange about the way the website behaves. Not sure what the idea of two units side-by-side is?

Posted By: Doubting_ThomasTo make this approach work and to keep things balanced, you need a pair of units working in opposite directions.

Yes, but NOT adjacent to one another, otherwise they'll short-circuit the airflow! You need one at one place and another somewhere towards the other side of the building AIUI.

We're at cross purposes, Peter. You're talking about how a single unit operates alternately in suck-blow mode. I'm happy about how that works, although very confused about how many fans this particular design has in a single unit since it talks about two fans and one fan reversing direction.

However what is clear is that each unit has a single heat store, and that heat flows through it in both directions alternately. It's easiest to understand if first air flows outwards thus warming the heat store to near room temperature. Then the flow reverses and the incoming air is warmed by the heat store (which cools the heat store of course).

The discussion that Tom and I are having is about the photos that show two units adjacent to each other. If these were operating in a pair then one would be sucking air into the building whilst the adjacent unit blew essentially the same air out again! After thirty seconds, they'd swap roles, but still not ventilate the building. Unless I've missed something.

You can operate a single unit by itself, which results in temporary pressure increases and reductions in the building, but another approach is to put a second unit some distance away from the first unit with the two units synchronised in push-pull fashion so the fresh air has to pass through at least part of the building before it is exhausted elsewhere.

Conventional MVHR systems using regenerative techniques typically have a large rotating wheel heat store, so they can push exhaust air through on one side of the wheel whilst passing incoming air through the other side of the wheel all at the same time.

This is in development stage, and the photos of two units seem to be of a test in his home workshop. They're not supposed to be next to each other in real life. Explained here https://www.openerv.ca/learn-more

There are lots of push-pull heat recovery units coming on the market now, the only way this one is different is it's a one-person open source project.

Ductless linked MHRV can be more effective than traditional ducted systems, the enabler has been the wireless comms revolution for home devices to talk to each other. Traditional fan manufacturers didn't have the software/comms skills, so there's space for new entrants and hobbyists.

Next step will be multiple (3-6+) units in different rooms, coordinating such that each room gets just right ventilation depending on how it is occupied at that moment, while keeping the overall building pressure in balance.

Will
The unit is in the development stage as you say - to me the write up is also in development. I still have some confusion. The https://www.openerv.ca/learn-more says

"Each OpenERV 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."
2 fans and 1 heat exchanger

then says

"Then, the fan reverses direction, moving clean air from outdoors to the indoors"
So thats 1 reversible fan.

Then later it says

"The OpenERV TW4 modules are made to always work in pairs. One always sucks air while the other blows air, synchronized over WiFi. This should be done, or hot air would be pushed out from the building through the walls during the ingress phase, causing heat loss........

The different modules of a pair can be located anywhere in a home, even a long distance away from each other."

If the pairs of fans, one sucking one blowing are quote " even a long distance away from each other." where is the single heat exchanger?

Can someone explain - or do we need much more detail than provided?

The thing I don't understand with push-pull HRV units is the high-efficiency claims. Looking at the BSK unit for example. At the time where it switches from push-out to pull-in assume the heat store is at room temperature. Then over the next 70 seconds it transfers heat from the store to the incoming air and by the end the heat store is at external temperature. So towards the end of the period the incoming air isn't being heated to near room temperature; it's hardly being heated at all. So how can it be efficient? Except by redefining 'efficient' to mean something that doesn't reflect the real-world reality!

If anybody can suggest some other temperature profile that leads to better heat recovery, I'd be glad to hear it. Or otherwise explain how it works.

This is interesting and answers some of the issues I have with MVHR.

Although the article says this is an open-source project it's not in the conventional sense of the term. The heat exchanger and housings are proprietary, but the, (Computer) fans and particulate filters are aftermarket and can come from various, (cheap sources).

The WiFi controls are commercially available items, but with proprietary code that can presumably be ported to replacement controllers if the need arises. [I have idly wondered if this could be achieved using some of the G3 mobile phones that will become effectively worthless next year but have fantastic computing and wireless communication built in]

The storm cowl is an excellent addition which I'd need to have permanently fitted for my location, although it sounds like you cannot use this with filters.

It sounds like he has a novel heat exchanger design - at least in terms of the materials and printing process. If the housing parts can be produced as 3D printer files and the other components can be sourced locally, the heat exchanger might be the only part that needs shipping around the world.

As a troglodyte, I would like to see a hardwired interconnection option rather than adding more WiFi pollution - after years of setting up PV WiFi I wouldn't trust it long-term.

Sorry. It shows how the temperature fluctuates as the DMVHR goes from running in and then out.

Like this chap did from around 15 mins in https://www.youtube.com/watch?v=U-hVUczzlL4&t=922s

Posted By: Peter_in_HungaryIf the pairs of fans, one sucking one blowing are quote " even a long distance away from each other." where is the single heat exchanger?

I don't really understand the confusion. At least the way I read it:

- you have two holes in your envelope as close/far as you can practically put get them (further = better)
- each hole has a single TW4 module through it
- each TW4 module has two fans and a single heat exchanger (so 4 fans and two HE's total)
- Module 1 starts to expel stale warm air, which heats up its heat exchanger
- Module 2 in the other pipe simultaneously starts to intake fresh cold air, which cools its own (already warmed HE)
- After 30 seconds the flow in both modules reverses
- Module 1 is now intake and uses the heat that it has just harvested from the expel cycle
- Module 2 is now exhaust and starts to reheat its depleted Heat Exchanger

Rinse & repeat

Posted By: sgt_wouldssome of the G3 mobile phones that will become effectively worthless next year

Ah yes, damn, just found that the solution to tolerable phone reception at home (for tethered internet when the landlines are down awaiting road closure, like last 3wks) is to force the thing to use 3G, not 'flexibly' switch between 3G/4G, let alone non existent 5G.