MVHR Enthalpy Exchangers, BS or SB?

[JohnMo]

We seem to drop down to about 39% worst case

[JohnMo]

A screenshot I did last year, compared outside humidity levels to yinside the house (normal MVHR), the pod is a summer house that is heated and has dMEV. The summer house almost tracks the humidity levels of the MVHR.

[Kelvin]

I’ve also noticed that the accuracy of the various sensors is quite variable, especially the Loxone ones. I bought a Ruuvi one that you can calibrate and use that to apply an offset to all the others. 

[LnP]

55 minutes ago, Nick Laslett said:

Maybe I’m wrong, but isn’t water vapour lighter than air? ChatGPT, Gemini, and Grok all seem to think so? Science is not my strong point, so easily confused by these topics. I appreciate our A.I. Overlords can’t be trusted, but I do live in hope.
 

You're right about our AI overlords, but they're not telepathic yet, so at least we need to ask them the right question 😁. Even then, they sometimes give the wrong answer.

 

The confusion here is that clouds are not water vapour, which is invisible, but small water droplets. Also it's wrong to say that water vapour in air rises because it is lighter than air. It doesn't rise. It is true to say that water vapour (molecular weight = 18) is lighter than air (average molecular weight = 28.96), but that doesn't make it rise any more than it would be true that oxygen would sink because it is more dense (MW=32).

 

Clouds float because of bulk phenomena of the surrounding air masses - convection, warm fronts rising over colder air etc - and the very low settling or terminal velocity of the tiny water droplets. When the droplets coalesce and become bigger ones, they don't float any more and it rains! The terminal velocity of the big droplets is more than the local bulk air movement.

Edited November 3 by LnP

[LnP]

2 hours ago, Nick Laslett said:

Here is some more material for anyone that is interested. 
 

A short paper by Zehnder comparing an ERV & HRV in the same house in Rotterdam over two consecutive 8 day periods during winter.  Doesn’t seem particularly rigorous or compelling, but does confirm the humidity is 10% higher with ERV with colder weather. 
 

https://www.zehnder-systems.ch/fileadmin/user_upload/CH/HUB/Mythen_und_Legenden/the_effect_of_enthalpy_recovery_ventilation_on_the_residential_indoor_climate_-_aivc2014.pdf.pdf

 

Nice. Very interesting.

 

My bottom line:

• The case is well made, that the membrane energy recovery exchangers help to achieve a more comfortable humidity level.
• The case for the membrane technology being more energy efficient is not well made, but for most people that probably doesn't matter much.

I'm still of the view that the industry have given these so called "enthalpy" exchangers the wrong name, which is possibly symptomatic that they don't understand the thermodynamics or even the energy performance. It would have been better to call them either by what they are - membrane exchanger vs aluminium exchanger (or whatever material); or by what they do - heat and moisture exchanger vs heat exchanger. Or maybe the marketers decided to dress up their new product with a name which hints at technical superiority, knowing that most people won't spot the BS.

 

Again, thanks for all the great thoughts and comments in this thread. BH has once again taught me a lot!

[Dan F]

Enthaphy exchanger also doesn't need a pre-heater to protect the heat exchanger against frost (at least not in UK temperatures).  Ours was supplied with one anyway though 

[SteamyTea]

  

On 03/11/2025 at 19:01, Nick Laslett said:

isn’t water vapour lighter than air

As is sciences want, it is not as simple as that.

There are polar forces at work also.  These can cause molecules to clump together, with H₂O molecules forming a tetrahedral shape.  This increases the effective surface area, which allows more water molecules to be attracted, it also means that smaller forces per unit of surface area can cause them to move.

But I don't think we need to worry about that in a ventillation system.

Probably a hole in the wrong place.

Edited 4 hours ago by SteamyTea

[JohnMo]

9 hours ago, Dan F said:

Enthaphy exchanger also doesn't need a pre-heater to protect the heat exchanger against frost

Don't actually think any MVHR unit needs them, they have in-built frost protection, mine slows the incoming air rate down a little, then when everything is ok goes back to normal speed. This has zero issues at -9.

 

Enthalpy HE help with prolonged cold periods as your house doesn't dry out as much.