MVHR Enthalpy Exchangers, BS or SB?

[LnP]

I saw @Jolo's post about smells transmitting across to the MVHR  incoming air.

 

I was curious about the term enthalpy exchanger, since any MVHR heat exchanger is by definition intended to increase the enthalpy of the incoming air by decreasing that of the outgoing one. So I did some Googling. The Zehnder web site says, "Choosing the enthalpy exchanger, rather than the standard heat exchanger, means that the unit becomes an Energy Recovery Ventilation (ERV) system, as opposed to solely a Heat Recovery Ventilation (HRV) system."

 

Since this is thermodynamic BS, I was tempted to conclude the whole concept is BS. I read their web site further to try and understand how these things differ from a conventional MVHR unit.

 

Is it a correct understanding that a conventional MVHR has an aluminium heat exchanger while an "enthalpy" one has a heat exchanger made from a membrane which is supposed to transmit not only heat but also be permeable to humidity i.e. water molecules?

 

The Brink website is a bit clearer in that it only makes claims about humidity, but calling it an enthalpy exchanger is still thermodynamic BS.

 

I suppose that if you want to increase the humidity of the incoming air, there is an energy efficiency advantage of doing that by recycling the water vapour from the extract air into the incoming air, rather than using additional heat to humidify the incoming air by evaporating water. But I'd be curious to know what Zehnder think that energy saving would be, i.e the latent heat of evaporation of the water required to achieve the required humidity.

 

Compared to an aluminium heat exchanger, there would seem to be questions about a semi permeable, aka "enthalpy" one - is the membrane more fragile, does it let smells through as well as water molecules, does the membrane conduct heat as well as aluminium, to what extent will it transfer humidity, how much energy will it save?

 

So I'm asking myself the question, even if they are mis-sold as "enthalpy" exchangers, are they the best thing since sliced bread or just BS?

[-rick-]

I'm going to try and put my thoughts down here. The physics of enthalpy is a bit much for me so this is just how I understand it. But my understanding was that enthalpy is not a bad way of talking about what these things are doing.

 

Air with 0% humidity at 30oC is in a much lower energy state than air with 100% humidity at 30oC. If you live in a hot climate with high humidity. Say Missisippi, then you really want to have an ERV vs HRV because the incoming air will be saturated with moisture even after the heat has been exchanged, which you will then need to use AC/dehumidifier to extract. In many parts of the world the dehumidification demands more power than cooling, so these ERVs are doing what they say they are.

 

In this country where we don't generally have a lot of high humidity high temperature days and we also don't tend to air condition our homes then ERVs are not as useful. They will provide some benefit, possibly by preventing moisture leaving the house during cold winters*, but at least to my understanding, it was generally considered unnecessary to spend the extra for an ERV vs HRV in this country.

 

As to your other questions, I'll leave that to others as I don't have the personal experience of one.

 

* Thinking aloud but this may be seen as a negative in some homes with no other way of dehumidification, as you are trading the extract of humid air with dry air most of the year (which we often want) with preventing the air getting very dry in very cold conditions.

[JohnMo]

1 hour ago, LnP said:

aluminium heat exchanger

More like a few hundred plastic straws in matrix.

 

Real advantage of enthalpy units is in prolonged sub zero temperatures where internal air can become increasingly dry. I monitored our air for 2 years with a normal exchanger, the air never gets to the really dry stage and we are in Scotland.  So not sure I would bother getting one for the aded cost and benefits in the UK

 

[Mike]

5 minutes ago, JohnMo said:

I would bother getting one for the aded cost and benefits in the UK

+1. I seem to recall that they're more commonly used in Nordic countries.

[Nick Laslett]

I have the Zehnder enthalpy exchanger in my Q600 unit. 
 

This decision was based on my understanding that in a large volume house with low occupancy, there was potential for the air to get too dry. 

See thread linked below with post from @Dan F

 

Quote

MVHR is fantastic, but a standard (non-enthalpy) exchanger can mean the house can be drier than without MVHR which can be a problem in some cases.  When I discussed this with our MVHR designer his view was that this was more of a problem in a large houses with low-occupancy than in smaller houses with high occupancy.  This, I assume, is due to the amount of humidy generated (by occupation) vs. amount of air moved.  We're using a enthalpy exchanger with occupancy of 4, there house isn't huge but it is quite big.

 

 

@lizzie changed her MVHR to an Enthalpy core, due to excessive dry air. 

 

 

Zehnder webinar, with is quite interesting, has Q&A at the end. 

 

 

[Nick Laslett]

Another useful source for why you might want to specify an Enthalpy exchange core. 
 

https://www.heatspaceandlight.com/whats-difference-between-mvhr-heat-exchanger-enthalpy-moisture/

[SteamyTea]

I have often wondered about all this.

Never got around to looking at it closely.

While there is energy to be scavenged when water vapour changes phase to liquid, is there really enough grams of it to make a real difference.

If the outgoing air is at 60% RH and 21°C, there is about 0.011 kg/m³.

That is about 7 Wh.

If the incoming air is at 30% RH and 5°C then 0.002 kg/m³.  Or about 4 Wh.

So at very best 3 Wh to be reclaimed for every m³ shifted, but probably half that.

 

(it is late and have changed my phone that is my hotspot, so had to find out why all my monitoring kit had stopped working, why do manufacturers change IP addresses for no advantage, so my mathematics may be a bit wonky tonight)

[ProDave]

I have an Enthalpy mvhr unit.  It was more a case that one was available at the right time at the right price than any technical reason for choosing it.

 

One advantage, not mentioned yet is an Enthalpy mvhr unit does not need a condensate drain, so installation is slightly easier.

[Mike]

Just pulled up the standard text on the topic from the PassivHaus certificate, which provides some guidance:

 

Indoor air humidity can be increased by using a system with moisture recovery in a cool, temperate climate, especially during the winter. These higher humidity levels will reduce evaporation from building elements and furniture during the heating period and thus have a positive effect on the building’s heating demand. In order to account for this effect, the heat recovery efficiency is increased by a certain percentage, depending on the achieved level of moisture recovery. In case the unit’s moisture recovery rate is larger than 60 % its airflow rate must be controlled based
on the indoor air humidity, in order to prevent temporarily elevated humidity levels.

 

Application of humidity recovery:
■ In cool temperate climates, heat exchangers with moisture recovery should generally only be used if the moisture load inside the building is comparatively low (e.g. in a residential building with an occupancy rate significantly below the average).
■ If moisture recovery > 60 % is to be used in a building with an average occupancy rate and typical use, the energy balance of the building is to be calculated with an increased airflow rate.

 

While the heat recovery efficiency may be increased, enthalpy units have no advantage in that respect compared to standard units, when looking at their database as a whole (though Efficiency Ratio, not heat recovery rate, is the key figure).

[LnP]

Thanks for all the informative replies. I understand this much better now. The Zehnder webinar shared by @Nick Laslett was especially informative, and the Mollier diagram explanation in that was particularly helpful. Here are my conclusions regarding heating in cold weather:

• It is indeed thermodynamic nonsense to say, as Zehnder did, that an "enthalpy exchanger" becomes an Energy Recovery Ventilation (ERV) system, as opposed to solely a Heat Recovery Ventilation (HRV) system. It's marketing BS, which puts me off considering buying one, but there are reasons why you might. It was telling that the expert from the membrane company described the units more scientifically as "membrane energy recovery ventilation".
• The energy efficiency possibilities are not well explained and might not materialise for many customers.
• At 5:18 in the video, Zehnder share data showing that the conventional HRV unit has an efficiency of 90% compared to the 86% or the ERV. The "enthalpy exchanger" is less energy efficient.
• As @Mike pointed out with the quote from PassivHaus, you get some of that back. Without the membrane technology, the supply air is lower in humidity and therefore there is more water evaporation from the building elements, which will take latent heat out of the structure. However, perhaps that is just a transient effect because once the water content of the building elements reaches equilibrium with the dryer air, there won't be any more evaporation.
• Also while it's true to say that the membrane technology recycles the warm water vapour back into the supply air, some or all of that heat would have been recovered by condensing the water out of the exhaust air.
• Where there would undoubtedly be energy savings with the ERV vs the HRV, is if you want to control the humidity to get into the comfort zone (Zehnder video 24:20). In their example, with a conventional HRV you'll need to additionally run a humidifier to get the water content from 3 g/kg up to 5 or 6 g/kg to get into the comfort zone. But if you're not sensitive to humidity, and would be happy with whatever comes out of the HRV, the ERV might well be overall less energy efficient. If you're really sensitive to humidity, you might want to anyway have a humidifier even with an ERV, but you won't need to add as much water, so it should use less energy.
• There are the additional benefits of the enthalpy exchanger that you don't need a water drain and you don't need to protect against frost until -6 ^oC whereas for an HRV it's 0 ^oC.

Bottom line for me is that, despite the enthalpy and energy marketing BS,  the decision which type to get is not so much about energy, and more about comfort and humidity.

 

And back to @Jolo's post I'm wondering whether his smells cross talk is due to a dodgy membrane. The membrane is only 1.5 microns thick (Zehnder video 43.02). He mentioned it can't be the exchanger because the supplier had put a replacement in and the problem had persisted ... maybe try putting in an aluminium exchanger instead of a membrane one and see if that changes anything.

Edited yesterday at 00:24 by LnP
typos

[JohnMo]

11 hours ago, Nick Laslett said:

Another useful source for why you might want to specify an Enthalpy exchange core. 
 

https://www.heatspaceandlight.com/whats-difference-between-mvhr-heat-exchanger-enthalpy-moisture/

Just read the article - this bit jumps out

 

"A problem can occur in homes which are very large but under-occupied, and for this reason an enthalpy exchanger could be a better solution"

 

A different solution is to set the MVHR flow to an appropriate flow rate to suit house and occupancy levels.

[JohnMo]

8 hours ago, LnP said:

 don't need to protect against frost until -6 ^oC whereas for an HRV it's 0 ^oC.

Most if not all MVHR units have in-built frost protection. If you have no preheater, the system just monitors temperature across the heat exchanger and either stops or modulates down the in coming air to keep unit away from freeze zone. As temp recover the unit increases speed to normal and repeats this until normal inlet temperature resumes. I don't see any ill effects of this down to -9 degs, and have two units operating this way.

[LnP]

54 minutes ago, JohnMo said:

Just read the article - this bit jumps out

 

"A problem can occur in homes which are very large but under-occupied, and for this reason an enthalpy exchanger could be a better solution"

 

A different solution is to set the MVHR flow to an appropriate flow rate to suit house and occupancy levels.

I stopped reading at the point where Patrick, describing how the membrane works, said that water vapour is lighter than air and this is why clouds float in the sky 🤣.

[LnP]

52 minutes ago, JohnMo said:

Most if not all MVHR units have in-built frost protection. If you have no preheater, the system just monitors temperature across the heat exchanger and either stops or modulates down the in coming air to keep unit away from freeze zone. As temp recover the unit increases speed to normal and repeats this until normal inlet temperature resumes. I don't see any ill effects of this down to -9 degs, and have two units operating this way.

So claiming that a membrane, aka enthalpy, exchanger has frost protection benefits is more BS.

[JohnMo]

3 minutes ago, LnP said:

So claiming that a membrane, aka enthalpy, exchanger has frost protection benefits is more BS.

Just remember most the sites quoting this stuff, are also selling it, bigger price for item equals more profit. Most claims in the UK are marketing BS, same as MVHR based cooling systems - people still fall it. I like reading technical manuals and understand what I am getting, before parting with my money.

 

If I lived in the centre of Europe I would buy an Enthalpy exchanger every time, but I am located near to the coast in Scotland, so weather changes frequently - we do not gets weeks and months below zero. I also tweaked down flow rates to suit house and number of people in it.

[Jolo]

11 hours ago, ProDave said:

One advantage, not mentioned yet is an Enthalpy mvhr unit does not need a condensate drain, so installation is slightly easier.

This is one reason I chose the ERV exchanger -- it cost an extra 500 euros or so, but if I'd chosen the regular exchanger then I'd have probably spent more than that getting a plumber out to install a drain line, which would have been a pain as there's no suitable point near the unit. It would have involved drilling through two brick/concrete floors and adding a connection directly to the main soil pipe leaving the house.

 

Plus I'd read (I think on here) how some people had uncomfortably dry houses in the winter with a regular heat exchanger, and figured that that, plus the plumbing challenges, made the extra unit cost worth it.

 

9 hours ago, LnP said:

Bottom line for me is that, despite the enthalpy and energy marketing BS,  the decision which type to get is not so much about energy, and more about comfort and humidity.

For us, the decision about whether to buy any MVHR at all was mostly about comfort! We use so little gas in this house that savings on heating bills never really came into consideration, it was always about living comfort (including indoor air quality).

 

10 hours ago, LnP said:

And back to @Jolo's post I'm wondering whether his smells cross talk is due to a dodgy membrane. The membrane is only 1.5 microns thick (Zehnder video 43.02). He mentioned it can't be the exchanger because the supplier had put a replacement in and the problem had persisted ... maybe try putting in an aluminium exchanger instead of a membrane one and see if that changes anything.

Wow that really is a thin exchanger material! I had no idea there was such a futuristic thing in our cupboard  

 

I guess it's true that there could be manufacturing issues with the exchangers, but we've also tested the problem with bypass mode on, and the problem persists.

 

I'm not sure how other units do it, but on the Brink Flair 200/225 left-hand model, when bypass mode is on the extract air is routed directly to the exhaust port, so it doesn't go through the exchanger at all. The incoming air still does pass through the exchanger, though.

 

Although thinking about it, there must be a point at which the extract->bypass channel and the extract->exchanger channels meet just before the exhaust port, so I guess there's a possibility for some back-flow into the exchanger from there..? Anyway, just thinking out loud, that's something for the other thread really!

[ProDave]

4 minutes ago, Jolo said:

I'm not sure how other units do it, but on the Brink Flair 200/225 left-hand model, when bypass mode is on the extract air is routed directly to the exhaust port, so it doesn't go through the exchanger at all. The incoming air still does pass through the exchanger, though.

 

Although thinking about it, there must be a point at which the extract->bypass channel and the extract->exchanger channels meet just before the exhaust port, so I guess there's a possibility for some back-flow into the exchanger from there..? Anyway, just thinking out loud, that's something for the other thread really!

I have a different unit but the operation is similar, the bypass flap only diverts one of the flows away from the heat exchanger.

 

But this brings the discussion to looking closely at that mechanism.  If the bypass flap is not closing properly when not in bypass mode, it could perhaps be the route of the "leak"?

[Jolo]

2 hours ago, ProDave said:

I have a different unit but the operation is similar, the bypass flap only diverts one of the flows away from the heat exchanger.

 

But this brings the discussion to looking closely at that mechanism.  If the bypass flap is not closing properly when not in bypass mode, it could perhaps be the route of the "leak"?

I've checked the bypass by watching it through the port, as it's right below the top of the unit. It's working correctly, but even if it wasn't I don't think it would account for the leak, as the both routes just diverge then meet again anyway -- one route goes through the exchanger, and the other one goes around the back. They're both parallel routes, which converge again before the extract/exhaust fan.

 

So I suspect the leak is either before the bypass valve (although I can see in there via the port on the top, and it all looks fully sealed), or after the two routes converge again, somewhere before or after the extract/exhaust fan. I think it must be the latter, but I can't see much of anything in this area without taking stuff apart, which I'd rather leave to Brink for now as it's all still under guarantee!

[JohnMo]

2 hours ago, Jolo said:

checked the bypass

Is this really a real issue, the bypass is on one side of the system and simply provides a bypass around the heat exchanger.  The bypass doesn't cross between the flow and return paths.

 

To get smell transfer,

1. Either piping/ducts are mixed up and giving cross over in the system on the house side. Get two sheets of toiler paper, on wet rooms the paper should be sucked up and stay in place, on dry rooms it should be blown away - check every terminal.

2. The outside terminals are allowing exit air, to be reintroduced into the suction stream. Do you have the minimum distance between terminals outside (see manufacturers instructions) or do you have combined inlet outlet terminal?

3. Are the outside and inside MVHR nozzles connected to the right parts of the system - easy to check, left and right hand units easy to mix up, so be careful check manufacturers name plate and instructions. Does the outside terminals suck and blow as you would expect, same for the house side terminals?

3. The heat exchanger in not right, you are on your second one, so unlikely unless a major manufacturing issue, you could ask supplier to fit a normal HE for test purposes?

4. A seal within the MVHR unit is comprised allowing flow and return air to mix. Would require full strip to find out and unless done by supplier your warranty could be void - so do last2. A seal within the MVHR unit is comprised allow flow and return air to mix. Would require full strip to find out and unless done by supplier your warranty could be void - so do last

 

Test as much as possible with unit on and nothing removed - then its not your fault or becomes easy to blame you.

 

 

[Jolo]

@JohnMo Thanks, and yep we crossed off 1, 2 and 3 already, this was pretty much my thinking process when the problem occurred. 

 

I'll let the supplier decide whether to try a regular exchanger, but as I mentioned I'm sure it's not that, as the odours transfer even when the extract air isn't flowing through the exchanger.

 

So it can only be number 4 (which is also my conclusion) and as you say, not something I can fix myself, it's for Brink or the supplier to do. 

[Mike]

9 hours ago, JohnMo said:

Most if not all MVHR units have in-built frost protection. If you have no preheater, the system just monitors temperature across the heat exchanger and either stops or modulates down the in coming air to keep unit away from freeze zone.

Yes, I'm sure they must (in Europe, at least) to avoid damaging the heat exchanger. Here in France we get extended sub-zero temperatures from time to time and, since I have an apartment, it doesn't take long for the air quality to deteriorate without the MVHR air supply or an open window. As that's the worst time of year to open a window, it was definitely worth the the extra €300 for the preheater. But, in a larger home and/or somewhere with milder winters, that would be a lesser concern.

[ProDave]

It is common here to get a week or 2 at -10 in a winter and we have had no issues with that temperature bothering the MVHR

[Nick Laslett]

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

 

Edited 16 hours ago by Nick Laslett

[Nick Laslett]

9 hours ago, LnP said:

I stopped reading at the point where Patrick, describing how the membrane works, said that water vapour is lighter than air and this is why clouds float in the sky 🤣.

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.
 

Edited 16 hours ago by Nick Laslett

[Kelvin]

Our house  got  quite dry last winter for a couple of weeks in December (dropped to 34%) Don’t how that compares to others on here. The only thing that seemed bothered by it is the large Fern we have in the living room but I have a small humidifier beside it that comes on automatically.