[boomskats]

I was reading up on counter-flow heat exchangers a few weeks ago after I'd just installed a MVHR system and realised that the actual heat exchanger components themselves were, counter-intuitively, a fraction of the price of the whole unit.

I was surprised when I saw they're mostly made of thin plastic and don't depend on thermal capacity at all (unlike, say, HX espresso machines). The way they work is quite simple:

                                                               c
    w                                                          o
    a  │                                                     │ l
    r  └─────────────────────────────────────────────────────┘ d
    m                                                           
        air at 50 deg ────────────────────────► air now 5 deg  o
    i                                                          u
    d  ───┬──┬──┬──── heat exchanging surface─────┬──┬──┬───── t
    o  ── ▼  ▼  ▼ ─────────────────────────────── ▼  ▼  ▼ ──── d
    o                                                          o
    r   air now 45 deg ◄─────────────────────────air at 0 deg  o
                                                               r
    a  ┌─────────────────────────────────────────────────────┐  
    i  │                                                     │ a
    r                                                          i
                                                               r

It's just a bunch of thin parallel channels where warm and cold air flow in opposite directions, separated by thin plastic walls. Because the flows are counter to each other, there's always a temperature difference driving heat transfer across the dividing walls, even as the warm air gradually cools and the cold air gradually warms.

The lightweight plastic walls are advantageous here - while plastic isn't particularly conductive, the walls are so thin that heat transfers readily. It's how these heat exchangers can achieve 80-90% efficiency without needing any expensive materials or thermal mass. The warm exhaust air leaves only slightly warmer than the incoming cold air, having transferred most of its heat to the incoming stream.

Clever design.

[ent]

Thank you for the illustration! I was sitting here, wondering that the whole system sounds paradoxical but seeing it drawn down with the arrows really helped grasp how this works!

[turtlebits]

Not sure what you mean by the heat exchanger core being a fraction of the price, but I've seen replacement cores cost around 1/3 of the total unit.

They should be made of high thermal conductivity material like resin or ceramic.