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Comment by ilyagr | Hacker News Reader

I'm confused how the heat retention could be around 80-90% without expending a ton of energy.

Naively, if on average the same amount of air goes in and out, I'd expect the temperature of the heat exchanger (on average in space and time, eventually) to be the average of the outside and inside temperatures. If the outside is hotter, the air coming in would be cooler than the outside air (which is a win), but it couldn't be cooler than the average of the temperatures. So, it would still not be anywhere as cool as the inside air, which doesn't sound like 90% heat retention.

Is the heat exchanger attached to a heater or a cooler? The linked video, https://www.youtube.com/watch?v=CDCu0IbEn8Q , would suggest not, as it talks about saving the energy needed for cooling or heating. Is there another clever trick?

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What you describe is kind of like a theoretical heat exchanger that only averages temperature at a single point.

You can improve this by exchanging heat across a continuous length along opposing flows. Imagine two parallel pipes thermally bonded where fluids flow in opposite directions. Each point still averages the temperatures, but the average temperature varies across the length and approaches the interior temperature on the interior side and the exterior temperature on the exterior side.

Yeah, I think this makes sense. If you connect many of my heat exchangers in series, the temperature gradient increases; only the middle one will work at the average of the inside and outside temperature (the example of 3 in a row makes sense to me). At the limit, it becomes what you described.

Thanks!

The mechanism described is called a countercurrent exchange. One fun detail is that it's quite commonly found in biological systems in nature too!

https://en.wikipedia.org/wiki/Countercurrent_exchange

It's actually a regenerative heat exchanger: https://en.wikipedia.org/wiki/Regenerative_heat_exchanger.

The person you are replying to was discussing "exchanging heat across a continuous length along opposing flows", which is countercurrent exchange. Regenerative exchange is, at least to my understanding, more of a cyclical store and release process.

Yes, which is what OpenERV is. I can understand why they might have thought I was talking about their system, my wording was a bit ambiguous there. So it doesn't hurt that they cleared that up :)

There isn't a uniform temperature across the entire exchanger. There's a smooth gradient extending from one end to the other. If the outside is hotter, then the inbound air gradually cools as it gives up heat to the outbound air which is gradually warming.

I find the idea of reversing the air flow direction every 30s simpler to understand than two counter-flowing pipe side by side.

Imagine a pipe filled with 3 metallic grid sections (such that the air temperature in the section will equalize with the metal temperature) separated by plastic grids (such that the heat isn't conducted through the metal), and you push air alternatively from one hot side at 20°C to a cold side at 0°C for 30s and in the other direction for 30s.

For symmetry reason, the pipe will passively (we don't count the energy required to move the air) have a gradient of temperature from the hot side to the cold side. The first section will be ~15°C, the second ~ 10°C, the third ~5°C. (Each section temperature is the temporal average of the temperature of the air flowing from previous sections : so because air switch direction, it means it's the average of left and right sections.)

From the point of view of the house, you only lose energy from the first section of the pipe which will be more like 15°C rather than 0°C.

Counter-flow heat exchangers can be very efficient, without a heater or cooler attached. That said, I don't think I've seen a commercial ERV claim to be more than 80% efficient, so I'm skeptical of the 90% measurement.

(I've seen ERVs with heaters attached; but for the purpose of avoiding frost buildup when it's below freezing outside.)

Commercial HRVs often use rotating disks, not counterflow heat exchangers. Disks are freeze-proof, but they need to be powered.

In my experience 80% plus is quite common in the models sold for colder environments. E.g. Mitsubishi electric Lossnay advertises 86%.

> I'm confused how the heat retention could be around 80-90% without expending a ton of energy.

Imagine two air streams counter-flowing. They "swap places" within your heat exchanger, so you can (theoretically) get 100% heat recovery.

This principle is used by animals to minimize the heat waste, by counter-flowing warm and cold blood: https://en.wikipedia.org/wiki/Rete_mirabile

Co-current flow (both flows moving in the same direction) work the way you described.

Countercurrent-flow heat exchangers (where the two channels of the fluid/gas) move in opposite directions on both sides of the heat-transfer mechanism maintain a heat flow gradient over the entire length of the heat exchanger. This can result in an almost complete transfer of heat from one current to another.

High efficiency HRV/ERVs use counter-current flow heat exchangers.

I'm not 100% sure about this. But with ERV (opposed to HRVs), iirc, also the moist of the air is transferred to the incoming air. The moist contains a lot of the energy.

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