LnP

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!

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.

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

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 🤣.

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.

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?

In a nutshell: Consideration of biodiversity is required for all developments, with the possibility that you might have to demonstrate how you will achieve a net gain. The small sites biodiversity metric tool allows the calculation of biodiversity value for the purpose of BNG for small developments. Yours is a small development, so if you do need to do an assessment, you will use the SSM. There are two exemptions: the de minimis one you mentioned and the one for self builders. The obligations required by some LPAs to successfully claim the self build exemption are onerous. If you're looking to claim the de minimis exemption, your best route is to get an ecologist to provide an assessment supporting your claim. The LPA are unlikely to challenge an assessment by a competent ecologist. It cuts short all the back and forth. Some ecologists are not only providing ecology services but also operating BNG credit banks and will sell you the credits their assessment determines you need. It's clearly a conflict of interest, so find one who isn't doing that. Perhaps your planning consultant can recommend somebody. Otherwise phone round a few until you find one who sounds sympathetic. The assessment should only cost ~£300, Do some gardening before you get the assessment done .... My planning consultant thinks the days of BNG are numbered because of the problems it's causing, especially given the government's house building targets. Maybe we do have a problem with a loss of biodiversity, but there must be better ways to tackle it. Insofar as the concern is about what's going on in our gardens, I read a study recently that the average pet cat kills 22 rodents and small birds a year ....

If you don't want to remove the skirting and don't like the look of quarter round, you can get thicker skirting which goes over the existing skirting and has a recess to sit over the old skirting.

I shared some thoughts on the fluid mechanics and heat transfer aspects of antifreeze in the attached thread. It might explain why your required pump head doubles.... and the required power required might triple... not an expert but going back to first principles.

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... and I should have added, pressure drop (required head) is proportional to the square of the flow rate, so in the example above of increasing the viscosity and hence the flow by 50% would mean pressure drop goes up by 1.52 = 2.25, i.e. more than double the head required at the pump. ... furthermore ... the power required by the pump is proportional to the head and the flow rate, so in this example, the head has gone up by a factor of 2.25 and the flow rate by a factor of 1.5, so the power required will go up by a factor of 3.38. I'm a bit rusty on my fluid mechanics, but it seems to me that adding antifreeze is not a trivial decision.

A further thought ... quite often, self build projects require an F10 notification to the HSE - more than 500 person days of work. The F10 has to declare who the principal contractor is. Is each of your contractors going to submit a new F10 as they are the new principal contractor? Or will you argue that each contract is a new project and less than 500 days? Could be an interesting discussion if an HSE inspector arrives at your site and asks why you've not submitted an F10.

Not sure what you mean by different project sections. What are your sections? It sounds like you're self managing and will engage the various trades under separate contracts, in which case, are you really going to only have one trade on site at a time? Will your trades be prepared to be nominated as principal contractors? The best guidance I've seen on this is from the Self Build Portal, which the HSE point to in their web page on CDM and self build. If you're self managing, read the paragraph which starts, "The self builder acts as their own project manager, employing individual trades at different times." Long story short, you're the principal contractor. Unfortunately this guidance predates the recent changes with the Building Safety Act. If there's good guidance out there on how that applies to self builders, I'd be happy to hear about it.