7 ms·

> Pumping in adequate amounts of fresh outside air, however engineered, will challenge running costs as well as carbon status. This is wrong. Mechanical Heat R

by willyt 5y ago

> Pumping in adequate amounts of fresh outside air, however engineered, will challenge running costs as well as carbon status. This is wrong. Mechanical Heat Recovery Ventilation (MVHR) is a well established technology which can recycle 96% of the heat from the waste air in a house. It costs about the same to fit as a central heating system. Passive houses have very high Air Changes per Hour (ACH) compared to conventional houses, at 0.4 ACH all the air in the house is replaced approximately every 2.5 hours. All while having a whole house heat load measuring in the hundreds of watts to maintain 21ºC inside when it’s -1ºC outside. There is no reason not to put MVHR in normal houses, it’s just not possible to supply all the heat demand using air supply alone so you need a conventional heating system as well. EDIT: Also this just goes to show what happens when people make uninformed comments on fields they don’t understand, this sentence could make a whole lot of work on reducing energy consumption in buildings get thrown out the window. I would have expected better from people that have just had 1.5 years first hand experience of the effects of misinformation on their own field.

Eelongate 5y ago
In much of the world for much of the year, it doesn't get below freezing for at least part of the day. The solution to the heating problem is simple then: simply don't. Open windows when it's above 50F / 10C (to give a healthy safety margin to the water pipes) and tell people to wear sweaters. Do this where it's possible and you'll offset the energy expenditure of heating in places were it actually earnestly gets cold. And for that matter, limit their thermostats to 50F / 10C too. Being able to wear t-shirts in the winter when there's a blizzard outside is an absurd luxury.
- antoniuschan99 5y ago
  I'm surprised the opening the window part is still not been mentioned much. It's been several months since Hong Kong made all establishments to maintain air flow of 25 times per hour (something like that). It's definitely more difficult to tell colder climate countries to open windows, but really just wear a sweater or a jacket.
  - 5e92cb50239222b 5y ago
    If your definition of "colder climate" is around 0°C, ugh-huh. It's −15°C where I live, and it's not even winter. In December-January I expect the usual −30-40°C, with down to −50°C for short periods of time. Open your windows for more than two minutes once every few hours and die.
    - antoniuschan99 5y ago
      I live in Toronto. You can still open the windows just wear a sweater, sweatpants, and socks. When the polar vortex happened a few years ago we had no power for a week but the -40C is usually just in Jan-Feb.
      - tsimionescu 5y ago
        Toronto is the same latitude as Cannes in southern France. Sure, it's colder because of wind and ocean patterns, but it's not exactly the ultimate argument in "I know what it's like to live in a cold climate".
        antoniuschan99 5y ago
        Cannes weather is more like Vancouver. Toronto might not be as Cold as Winnipeg, Edmonton, or Montreal but it's in Canada. Other than if they lived in Vancouver, to say someone who lives in Canada doesn't know what it's like to live in a cold climate is strange.
  - bluGill 5y ago
    > just wear a sweater or a jacket. Where I live my pipes will freeze and burst sometime in late December, but I won't find out until late February when the pipes start to thaw again. I have a few HEPA air filters in my house. They work with whatever HVAC you have and make a difference.
- chiefalchemist 5y ago
  This article on Covid in Africa supports your theory / case. https://apnews.com/article/coronavirus-pandemic-science-health-pandemics-united-nations-fcf28a83c9352a67e50aa2172eb01a2f https://apnews.com/article/coronavirus-pandemic-science-heal...
- Macha 5y ago
  > Open windows when it's above 50F / 10C https://www.rte.ie/weather/22259-dublin/ https://www.rte.ie/weather/22259-dublin/ I live in Ireland, it's not typically considered a cold climate. But it won't be above 10C this week, and probably the case for most weeks until mid-March so this advice is less globally accurate than you might imagine.
  - Eelongate 5y ago
    The forecasts for Dublin I'm seeing predict 10C or higher for most of Tuesday. Regardless, my advice is to keep the windows open where possible and it will offset the energy needed where it isn't. And the other component of my advice, turning down the thermostat in places where it gets cold, is certainly applicable in Ireland. You don't need 25C indoors in the winter.
- viraptor 5y ago
  > Open windows when it's above 50F / 10C (to give a healthy safety margin to the water pipes) and tell people to wear sweaters. Yeah, please don't. There's a comfortable level somewhere above that, but at 10C I wouldn't be able to function in context of work / school. With worse circulation you just end up with your hands / feet feeling like they're constantly freezing. Some teachers thought that's a good idea, but I can tell you writing in gloves just gets annoying.
  - Eelongate 5y ago
    Dress appropriately for the season and you'll be just fine. If you wear warm clothes that cover your arms, your hands will be warmer too. Poor bloodflow to your hands is a consequence of your core loosing too much heat; your body compensates for that by reducing bloodflow to the extremities. Warm clothing makes gloves utterly unnecessary at 10C.
    - lotsofpulp 5y ago
      Is it possible other humans are different than you? Is it also possible the person wearing gloves at 10C was also wearing warmer clothing considering they bothered to use gloves in the first place? I also do not think it is possible to describe what one should feel at 10C without specifying the humidity.
    - viraptor 5y ago
      Did you really try to explain dressing appropriately to someone dealing with this issue for years? Did you expect me to go "sure, I wear winter thermals sub 10C, but this random person may have a point about dressing warmer?"
    - 3np 5y ago
      I’d say something similar but no, I think there are very few individuals who will not suffer in 10C, whatever the clothing. Being physically active makes a huge difference too. With the same clothes I can be perfectly comfortable on a promenade at the same temperature that makes my fingers hurt typing at my desk. My house is currently ~10C if I don’t activate heating. That’s miserable. My fingers get stiff and hurt a bit when at the keyboard. Even with 3 warm layers legs/4 warm layers upper. And I’m born and raised just a few miles from the Arctic circle. People around me see me as “that guy who can handle any cold” and my breaking point for comfortable sedentary is around 15C. Oh, and humidity does make a huge difference for how could absolute temperatures are experienced.
- andi999 5y ago
  Do you live in a non tropical region?
  - Eelongate 5y ago
    I live in Seattle. It's 12C in my apartment right now, I have the windows open and I'm certainly not wearing any gloves. Hell, I'm not even wearing any socks. I'm wearing long pants and a thickish shirt with long sleeves, and I feel totally toasty.
    - andi999 5y ago
      Next holiday try Shanghai in winter.
    - lotsofpulp 5y ago
      West coast of North America is less humid than many other places, and hence 12C in Seattle is not 12C in the Midwest or East coast. Less humidity means less heat transfer, and so 12C in Seattle can feel like 17C in NYC. And 12C in NYC can feel like 6C in Seattle.
neilwilson 5y ago
"There is no reason not to put MVHR in normal houses" There's no reason in new normal houses, but it is very often impossible to do it sensibly in the existing housing stock. But then there is no reason houses shouldn't be built in factories, shipped to site and assembled. After all we already do that for hotels and the like. Yet that hasn't happened yet either.
- mdekkers 5y ago
  The fine folks at Hufhaus would like a word. As well as quite a few others….
  - neilwilson 5y ago
    Yes, but they are more like a Rolls Royce than a Model T Ford. There's no mass production, and we really need to ask why.
- lotsofpulp 5y ago
  I have experience with construction of many hotels in the US, and I have never heard of a hotel built in a factory here. It comes up in theory, but in practice everyone always goes with the usual construction methods. Even the publicly listed tract home builders that put up developments of hundreds and thousands of homes in a couple years do not use factory built homes. It is safe to assume they know how to optimize for construction costs, and evidently, hiring local construction crews and subcontractors is still the best way.
- anon9001 5y ago
  I had never heard of MVHR before, but it seems like a very good idea. I wonder if it would work during wildfires.
- willyt 5y ago
  You can get self contained through wall units and do it room by room, less efficient but cheaper to retrofit. Hotels prefab the bathrooms typically, but not the rest of the room.
jacquesm 5y ago
> It costs about the same to fit as a central heating system. Far less actually, if you use natural convection to carry the air from the inlet to the pickup points. A central heating system is a lot more expensive than your average heat exchanger+fan and a couple of pickup points and a central outlet. New buildings in NL have these installed as standard now, mostly to combat the fungi and other humidity related problems associated with insulating a house completely.
sillysaurusx 5y ago
It sounds like they were referring to cooling air, not heating it. Is there an energy-efficient strategy for that? If not, then it doesn't seem like they were mistaken, just imprecise.
- BoorishBears 5y ago
  It's the same system. Heat recovery works both ways, maintaining heat out the outlets or maintaining heat at the inlets
  - sillysaurusx 5y ago
    That's interesting. It's tempting to think that if we build houses in slightly different ways, we could have energy efficient heating and cooling systems. It seems like humidity might be a problem.
    - Danieru 5y ago
      Most high end houses in Japan are already built this way. Lots of insulation with a air exchanger. They are not as low energy as passive houses, but sell brand new at prices regular families can afford.
  - _benedict 5y ago
    This isn't true. Most MVHRs have a summer bypass mode in which they admit the exterior air without retaining any interior heat, but they do not cool the air entering the house.
    - BoorishBears 5y ago
      The system in my apartment specifically calls out cooling recovery, but maybe that's not standard
      - _benedict 5y ago
        Do you happen to know what make and model? The only cooling functions I have found besides summer bypass involve a separate cooling loop that ultimately relies on a heat pump. I'd be interested to read about an MVHR with this facility.
helsinkiandrew 5y ago
The article is about transmission of SARS-CoV-2 in our existing environment. Not about how to build cool/heat buildings. How many buildings have MHRV now? or could install MHRV without replacing or adding to their current heating/cooling system before it needs to be replaced - which would "challenge running costs as well as carbon status" for the next few years or more.
3np 5y ago
I think it’s not wrong when considered in the context of already existing large buildings (office complexes, public transit stations, department stores, restaurants, hotels, etc) as opposed to newly built individual houses (where, arguably, increased ventilation may not have that much effect on infection spread) Or are you saying it’s still cost-efficient to retrofit in a high-rise from the day 1980s?
- willyt 5y ago
  Most of those places are already very well ventilated. You’re not catching covid in the train station, you’re catching it while sharing 1m² with 4 other people on the train. Or because you’re in a crowded restaurant and you’re between the air extract point and the person on the table next to you who has covid. I can’t see how it would be different if the air circulation is because of open windows or mechanical ventilation in these situations. Most high rise offices since the 50/60’s will have a mechanical ventilation system of some kind already but it may be quite energy inefficient. Any building with an existing air con system is already doing much more air changes than a building without ducted air. Offices need these systems for active cooling and heat rejection for fairly large parts of the year because the concentration of people and computers inside tends to overheat the building. Hotels are an interesting one, I’m not an expert on hotel design but it’s possible that some hotels use positive pressure ventilation to supply air to the rooms and passive extraction through the en-suite bathrooms. In that case when you open the room door you let a blob of Covid air out into the corridor? But the corridor should probably also be at positive pressure to stop smoke from a fire in a room from blocking the escape route. Not sure about this one. I would hypothesise that people mostly catch covid in Pubs, Clubs, Restaurants, Schools, each other’s houses and other specific hard to solve confined situations where we can’t avoid breathing each other’s exhaled air like lift cars and underground trains. If money were to be spent retrofitting heat recovery ventilation anywhere. Schools would be the best place to start.
  - mattzito 5y ago
    FWIW, my office building was quite transparent about the fact that pre-pandemic, there were 1.5-2 ACH, well below the recommended 3-6 for reduction of Covid spread. They have since upgraded/reconfigured the HVAC for 4 ACH. Two of my previous offices didn’t have building-wide HVAC at all, relying on local units without configurable access to fresh air. This doesn’t invalidate your point, just highlighting that perhaps many office buildings are not up to the levels of air turnover needed without real investment.
_benedict 5y ago
Many non-passive house buildings have 0.4ACH or more without mechanical ventilation. The purpose of an MVHR is to provide similar trickle rates of ventilation without the high energy cost, but the goal of "opening a window" is to provide much higher airflow rates, in which case the cost would be much higher with mechanical ventilation. It looks like 3-6ACH[1] or more is desirable to control the spread of infection. This would require very expensive MVHR systems to be installed. Realistically much larger units, probably more than one unit, as well as large ducting would need to be installed. Such high flow rates would come with other problems: These flow rates would lead to unacceptably dry interior environments with typical heat exchangers, though there exist heat exchangers that retain some moisture. Higher flow rates also achieve lower heat exchanger efficiencies, and the power required to move the air would not be insignificant either. [1] https://qz.com/1907977/how-to-check-air-ventilation-to-prevent-covid-19-spread/ https://qz.com/1907977/how-to-check-air-ventilation-to-preve...
- willyt 5y ago
  Depends on construction standards. I can’t find my source, but my understanding is that passive house is 3-4x better ACH than a typical recently built house in the UK when you take as built measurements of typical usuage patterns. Heat recovery ventilation is an available technology and it’s not practical in the winter to sit in a house with the windows wide open, even with the heating on full so no-one will actually do it.
  - _benedict 5y ago
    > my understanding is that passive house is 3-4x better ACH than a typical recently built house in the UK Part L1A of UK building regulations stipulate an air permeability of no more than 10m3/hr/m2 of building envelope at 50pa of pressure. My home has approximately a 1:1 ratio of building envelope to volume, though this will vary a great deal. But for such a home this works out to 10ACH at 50pa. There are various studies on how to convert these measurements to natural ventilation rates, but the conversion rate is around 10-30, with the typical house being adjusted by a factor of 20, i.e. 0.5ACH, however this can be as high as 1ACH. Some homes will be constructed to higher standards, but will not likely achieve less than half this rating. This also ignores the presence of trickle vents inside windows in homes without mechanical ventilation, and that the majority of homes in the UK were built to lower standards - typical values for homes built prior to 2010 are 15-20m3/hr/m2. Therefore, 0.5-1ACH is a pretty standard background ventilation rate for homes in the UK. Your typical Passivhaus home with 0.4ACH from mechanical ventilation has less ventilation than a typical home, not more. Only very large homes built to strict modern standards and with a high interior volume to building envelope ratio could possibly have 3-4x less ventilation.
nicolaslem 5y ago
On this topic, HVAC often come with an "Air scavenging" feature. After reading manuals and searching online I cannot figure out what this feature actually does. Is it another name for MVHR?

by willyt 5y ago

Eelongate 5y ago

In much of the world for much of the year, it doesn't get below freezing for at least part of the day. The solution to the heating problem is simple then: simply don't. Open windows when it's above 50F / 10C (to give a healthy safety margin to the water pipes) and tell people to wear sweaters. Do this where it's possible and you'll offset the energy expenditure of heating in places were it actually earnestly gets cold. And for that matter, limit their thermostats to 50F / 10C too. Being able to wear t-shirts in the winter when there's a blizzard outside is an absurd luxury.

antoniuschan99 5y ago
I'm surprised the opening the window part is still not been mentioned much. It's been several months since Hong Kong made all establishments to maintain air flow of 25 times per hour (something like that). It's definitely more difficult to tell colder climate countries to open windows, but really just wear a sweater or a jacket.
- 5e92cb50239222b 5y ago
  If your definition of "colder climate" is around 0°C, ugh-huh. It's −15°C where I live, and it's not even winter. In December-January I expect the usual −30-40°C, with down to −50°C for short periods of time. Open your windows for more than two minutes once every few hours and die.
  - antoniuschan99 5y ago
    I live in Toronto. You can still open the windows just wear a sweater, sweatpants, and socks. When the polar vortex happened a few years ago we had no power for a week but the -40C is usually just in Jan-Feb.
    - tsimionescu 5y ago
      Toronto is the same latitude as Cannes in southern France. Sure, it's colder because of wind and ocean patterns, but it's not exactly the ultimate argument in "I know what it's like to live in a cold climate".
      - antoniuschan99 5y ago
        Cannes weather is more like Vancouver. Toronto might not be as Cold as Winnipeg, Edmonton, or Montreal but it's in Canada. Other than if they lived in Vancouver, to say someone who lives in Canada doesn't know what it's like to live in a cold climate is strange.
- bluGill 5y ago
  > just wear a sweater or a jacket. Where I live my pipes will freeze and burst sometime in late December, but I won't find out until late February when the pipes start to thaw again. I have a few HEPA air filters in my house. They work with whatever HVAC you have and make a difference.
chiefalchemist 5y ago
This article on Covid in Africa supports your theory / case. https://apnews.com/article/coronavirus-pandemic-science-health-pandemics-united-nations-fcf28a83c9352a67e50aa2172eb01a2f https://apnews.com/article/coronavirus-pandemic-science-heal...
Macha 5y ago
> Open windows when it's above 50F / 10C https://www.rte.ie/weather/22259-dublin/ https://www.rte.ie/weather/22259-dublin/ I live in Ireland, it's not typically considered a cold climate. But it won't be above 10C this week, and probably the case for most weeks until mid-March so this advice is less globally accurate than you might imagine.
- Eelongate 5y ago
  The forecasts for Dublin I'm seeing predict 10C or higher for most of Tuesday. Regardless, my advice is to keep the windows open where possible and it will offset the energy needed where it isn't. And the other component of my advice, turning down the thermostat in places where it gets cold, is certainly applicable in Ireland. You don't need 25C indoors in the winter.
viraptor 5y ago
> Open windows when it's above 50F / 10C (to give a healthy safety margin to the water pipes) and tell people to wear sweaters. Yeah, please don't. There's a comfortable level somewhere above that, but at 10C I wouldn't be able to function in context of work / school. With worse circulation you just end up with your hands / feet feeling like they're constantly freezing. Some teachers thought that's a good idea, but I can tell you writing in gloves just gets annoying.
- Eelongate 5y ago
  Dress appropriately for the season and you'll be just fine. If you wear warm clothes that cover your arms, your hands will be warmer too. Poor bloodflow to your hands is a consequence of your core loosing too much heat; your body compensates for that by reducing bloodflow to the extremities. Warm clothing makes gloves utterly unnecessary at 10C.
  - lotsofpulp 5y ago
    Is it possible other humans are different than you? Is it also possible the person wearing gloves at 10C was also wearing warmer clothing considering they bothered to use gloves in the first place? I also do not think it is possible to describe what one should feel at 10C without specifying the humidity.
  - viraptor 5y ago
    Did you really try to explain dressing appropriately to someone dealing with this issue for years? Did you expect me to go "sure, I wear winter thermals sub 10C, but this random person may have a point about dressing warmer?"
  - 3np 5y ago
    I’d say something similar but no, I think there are very few individuals who will not suffer in 10C, whatever the clothing. Being physically active makes a huge difference too. With the same clothes I can be perfectly comfortable on a promenade at the same temperature that makes my fingers hurt typing at my desk. My house is currently ~10C if I don’t activate heating. That’s miserable. My fingers get stiff and hurt a bit when at the keyboard. Even with 3 warm layers legs/4 warm layers upper. And I’m born and raised just a few miles from the Arctic circle. People around me see me as “that guy who can handle any cold” and my breaking point for comfortable sedentary is around 15C. Oh, and humidity does make a huge difference for how could absolute temperatures are experienced.
andi999 5y ago
Do you live in a non tropical region?
- Eelongate 5y ago
  I live in Seattle. It's 12C in my apartment right now, I have the windows open and I'm certainly not wearing any gloves. Hell, I'm not even wearing any socks. I'm wearing long pants and a thickish shirt with long sleeves, and I feel totally toasty.
  - andi999 5y ago
    Next holiday try Shanghai in winter.
  - lotsofpulp 5y ago
    West coast of North America is less humid than many other places, and hence 12C in Seattle is not 12C in the Midwest or East coast. Less humidity means less heat transfer, and so 12C in Seattle can feel like 17C in NYC. And 12C in NYC can feel like 6C in Seattle.

neilwilson 5y ago

"There is no reason not to put MVHR in normal houses" There's no reason in new normal houses, but it is very often impossible to do it sensibly in the existing housing stock. But then there is no reason houses shouldn't be built in factories, shipped to site and assembled. After all we already do that for hotels and the like. Yet that hasn't happened yet either.

mdekkers 5y ago
The fine folks at Hufhaus would like a word. As well as quite a few others….
- neilwilson 5y ago
  Yes, but they are more like a Rolls Royce than a Model T Ford. There's no mass production, and we really need to ask why.
lotsofpulp 5y ago
I have experience with construction of many hotels in the US, and I have never heard of a hotel built in a factory here. It comes up in theory, but in practice everyone always goes with the usual construction methods. Even the publicly listed tract home builders that put up developments of hundreds and thousands of homes in a couple years do not use factory built homes. It is safe to assume they know how to optimize for construction costs, and evidently, hiring local construction crews and subcontractors is still the best way.
anon9001 5y ago
I had never heard of MVHR before, but it seems like a very good idea. I wonder if it would work during wildfires.
willyt 5y ago
You can get self contained through wall units and do it room by room, less efficient but cheaper to retrofit. Hotels prefab the bathrooms typically, but not the rest of the room.

jacquesm 5y ago

> It costs about the same to fit as a central heating system. Far less actually, if you use natural convection to carry the air from the inlet to the pickup points. A central heating system is a lot more expensive than your average heat exchanger+fan and a couple of pickup points and a central outlet. New buildings in NL have these installed as standard now, mostly to combat the fungi and other humidity related problems associated with insulating a house completely.

sillysaurusx 5y ago

It sounds like they were referring to cooling air, not heating it. Is there an energy-efficient strategy for that? If not, then it doesn't seem like they were mistaken, just imprecise.

BoorishBears 5y ago
It's the same system. Heat recovery works both ways, maintaining heat out the outlets or maintaining heat at the inlets
- sillysaurusx 5y ago
  That's interesting. It's tempting to think that if we build houses in slightly different ways, we could have energy efficient heating and cooling systems. It seems like humidity might be a problem.
  - Danieru 5y ago
    Most high end houses in Japan are already built this way. Lots of insulation with a air exchanger. They are not as low energy as passive houses, but sell brand new at prices regular families can afford.
- _benedict 5y ago
  This isn't true. Most MVHRs have a summer bypass mode in which they admit the exterior air without retaining any interior heat, but they do not cool the air entering the house.
  - BoorishBears 5y ago
    The system in my apartment specifically calls out cooling recovery, but maybe that's not standard
    - _benedict 5y ago
      Do you happen to know what make and model? The only cooling functions I have found besides summer bypass involve a separate cooling loop that ultimately relies on a heat pump. I'd be interested to read about an MVHR with this facility.

helsinkiandrew 5y ago

The article is about transmission of SARS-CoV-2 in our existing environment. Not about how to build cool/heat buildings. How many buildings have MHRV now? or could install MHRV without replacing or adding to their current heating/cooling system before it needs to be replaced - which would "challenge running costs as well as carbon status" for the next few years or more.

3np 5y ago

I think it’s not wrong when considered in the context of already existing large buildings (office complexes, public transit stations, department stores, restaurants, hotels, etc) as opposed to newly built individual houses (where, arguably, increased ventilation may not have that much effect on infection spread) Or are you saying it’s still cost-efficient to retrofit in a high-rise from the day 1980s?

willyt 5y ago
Most of those places are already very well ventilated. You’re not catching covid in the train station, you’re catching it while sharing 1m² with 4 other people on the train. Or because you’re in a crowded restaurant and you’re between the air extract point and the person on the table next to you who has covid. I can’t see how it would be different if the air circulation is because of open windows or mechanical ventilation in these situations. Most high rise offices since the 50/60’s will have a mechanical ventilation system of some kind already but it may be quite energy inefficient. Any building with an existing air con system is already doing much more air changes than a building without ducted air. Offices need these systems for active cooling and heat rejection for fairly large parts of the year because the concentration of people and computers inside tends to overheat the building. Hotels are an interesting one, I’m not an expert on hotel design but it’s possible that some hotels use positive pressure ventilation to supply air to the rooms and passive extraction through the en-suite bathrooms. In that case when you open the room door you let a blob of Covid air out into the corridor? But the corridor should probably also be at positive pressure to stop smoke from a fire in a room from blocking the escape route. Not sure about this one. I would hypothesise that people mostly catch covid in Pubs, Clubs, Restaurants, Schools, each other’s houses and other specific hard to solve confined situations where we can’t avoid breathing each other’s exhaled air like lift cars and underground trains. If money were to be spent retrofitting heat recovery ventilation anywhere. Schools would be the best place to start.
- mattzito 5y ago
  FWIW, my office building was quite transparent about the fact that pre-pandemic, there were 1.5-2 ACH, well below the recommended 3-6 for reduction of Covid spread. They have since upgraded/reconfigured the HVAC for 4 ACH. Two of my previous offices didn’t have building-wide HVAC at all, relying on local units without configurable access to fresh air. This doesn’t invalidate your point, just highlighting that perhaps many office buildings are not up to the levels of air turnover needed without real investment.

_benedict 5y ago

Many non-passive house buildings have 0.4ACH or more without mechanical ventilation. The purpose of an MVHR is to provide similar trickle rates of ventilation without the high energy cost, but the goal of "opening a window" is to provide much higher airflow rates, in which case the cost would be much higher with mechanical ventilation. It looks like 3-6ACH[1] or more is desirable to control the spread of infection. This would require very expensive MVHR systems to be installed. Realistically much larger units, probably more than one unit, as well as large ducting would need to be installed. Such high flow rates would come with other problems: These flow rates would lead to unacceptably dry interior environments with typical heat exchangers, though there exist heat exchangers that retain some moisture. Higher flow rates also achieve lower heat exchanger efficiencies, and the power required to move the air would not be insignificant either. [1] https://qz.com/1907977/how-to-check-air-ventilation-to-prevent-covid-19-spread/ https://qz.com/1907977/how-to-check-air-ventilation-to-preve...

willyt 5y ago
Depends on construction standards. I can’t find my source, but my understanding is that passive house is 3-4x better ACH than a typical recently built house in the UK when you take as built measurements of typical usuage patterns. Heat recovery ventilation is an available technology and it’s not practical in the winter to sit in a house with the windows wide open, even with the heating on full so no-one will actually do it.
- _benedict 5y ago
  > my understanding is that passive house is 3-4x better ACH than a typical recently built house in the UK Part L1A of UK building regulations stipulate an air permeability of no more than 10m3/hr/m2 of building envelope at 50pa of pressure. My home has approximately a 1:1 ratio of building envelope to volume, though this will vary a great deal. But for such a home this works out to 10ACH at 50pa. There are various studies on how to convert these measurements to natural ventilation rates, but the conversion rate is around 10-30, with the typical house being adjusted by a factor of 20, i.e. 0.5ACH, however this can be as high as 1ACH. Some homes will be constructed to higher standards, but will not likely achieve less than half this rating. This also ignores the presence of trickle vents inside windows in homes without mechanical ventilation, and that the majority of homes in the UK were built to lower standards - typical values for homes built prior to 2010 are 15-20m3/hr/m2. Therefore, 0.5-1ACH is a pretty standard background ventilation rate for homes in the UK. Your typical Passivhaus home with 0.4ACH from mechanical ventilation has less ventilation than a typical home, not more. Only very large homes built to strict modern standards and with a high interior volume to building envelope ratio could possibly have 3-4x less ventilation.

nicolaslem 5y ago

On this topic, HVAC often come with an "Air scavenging" feature. After reading manuals and searching online I cannot figure out what this feature actually does. Is it another name for MVHR?