4 ms·

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 w

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.
  - willyt 5y ago
    As built and as normally used measurements being the key here. Very small new build houses in the damper western parts of the uk often have problems with mould growth because of actual in everyday use ACH below building regulations design levels. It’s particularly an issue in new build social housing. People don’t open trickle vents, they turn down the extract fan timer because of fan noise and buildings are often more airtight than regulations expect and the minimum standard details are a compromise between the convenience of old conventions and prevention of cold bridging. In my experience it’s more common to fail the airtighness test by being too airtight to vent only with trickle vents than it is to come anywhere near 10@50pa. 2-3@50pa being common which drops you below the 0.4 guaranteed ACH of passive house. I’ve never seen a result over 4 @50pa. According to your calculation methodology 2-4@50pa could be anywhere between 0.07-0.4 ACH. Also, this air circulation is not designed so it just vents randomly, some rooms in the house could have very stale air while others might be quite good.
    - _benedict 5y ago
      So, firstly 88% of homes were built pre-2000[1] and the regulations that resulted in these improvements came into force in 2010. Assuming the regulations instantly had the desired impact (unlikely) we're discussing around 6% of UK houses. Those built earlier (on average) achieve significantly worse than 10m3/hr/m2 air permeability of the building envelope[2]. So the vast majority of UK homes have more ventilation than a Passivhaus building, and for the purposes of the initial discussion I think this is enough to demonstrate the inadequacy of this level of ventilation for the stated purpose, else Covid spread wouldn't be a problem in the UK within the home. It appears that for new builds latest research[3] shows an average of around 4m3/hr/m2, which is better than I expected, however the research suggests this is in part due to temporary measures by builders to achieve higher ratings than they would otherwise, and this is born out by the distribution of ratings achieved with an unlikely precipitous drop at precisely 5m3/hr/m2. However, for houses built to this standard some form of additional ventilation is a requirement of Part F[4]. It is possible to build a house with as low as 3m3/hr/m2 and provide no additional ventilation, but it must have been designed to > 5m3/hr/m2 so this should not be common. For social housing we can assume a single bedroom home will be around 40m2 with 2.6m between floors, and the required ventilation rate is 13l/s, or 47m3/hr/m2. This is around 0.45ACH. In fact the legal minimum rate per m2 is 1.08m3/hr, so for a typical home with 2.6m between floors this works out to 0.42ACH. So any house built to modern standards and used correctly should achieve the Passivhaus level of ventilation, unless it has tall ceilings. Certainly any UK home built with continuous mechanical ventilation is pretty comparable to Passivhaus standards for airflow. > People don’t open trickle vents, they turn down the extract fan timer because of fan noise People can also turn off the MVHR in a Passivhaus. I'm not sure what purpose it serves to compare correct use of a Passivhaus to incorrect use of this small fraction of UK homes. [1]https://www.statista.com/statistics/292252/age-of-housing-dwellings-in-england-uk-by-tenuree/ https://www.statista.com/statistics/292252/age-of-housing-dw... [2]https://www.researchgate.net/profile/Dominic-Miles-Shenton/publication/267304548_AIRTIGHTNESS_OF_UK_DWELLINGS_SOME_RECENT_MEASUREMENTS_AIRTIGHTNESS_OF_UK_DWELLINGS_SOME_RECENT_MEASUREMENTS/links/561f71bf08aecade1ace3adc/AIRTIGHTNESS-OF-UK-DWELLINGS-SOME-RECENT-MEASUREMENTS-AIRTIGHTNESS-OF-UK-DWELLINGS-SOME-RECENT-MEASUREMENTS.pdf https://www.researchgate.net/profile/Dominic-Miles-Shenton/p... [3]https://discovery.ucl.ac.uk/id/eprint/1573697/1/1-s2.0-S0378778817322612-main.pdf https://discovery.ucl.ac.uk/id/eprint/1573697/1/1-s2.0-S0378... [4]https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/468871/ADF_LOCKED.pdf https://assets.publishing.service.gov.uk/government/uploads/...

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.
- willyt 5y ago
  As built and as normally used measurements being the key here. Very small new build houses in the damper western parts of the uk often have problems with mould growth because of actual in everyday use ACH below building regulations design levels. It’s particularly an issue in new build social housing. People don’t open trickle vents, they turn down the extract fan timer because of fan noise and buildings are often more airtight than regulations expect and the minimum standard details are a compromise between the convenience of old conventions and prevention of cold bridging. In my experience it’s more common to fail the airtighness test by being too airtight to vent only with trickle vents than it is to come anywhere near 10@50pa. 2-3@50pa being common which drops you below the 0.4 guaranteed ACH of passive house. I’ve never seen a result over 4 @50pa. According to your calculation methodology 2-4@50pa could be anywhere between 0.07-0.4 ACH. Also, this air circulation is not designed so it just vents randomly, some rooms in the house could have very stale air while others might be quite good.
  - _benedict 5y ago
    So, firstly 88% of homes were built pre-2000[1] and the regulations that resulted in these improvements came into force in 2010. Assuming the regulations instantly had the desired impact (unlikely) we're discussing around 6% of UK houses. Those built earlier (on average) achieve significantly worse than 10m3/hr/m2 air permeability of the building envelope[2]. So the vast majority of UK homes have more ventilation than a Passivhaus building, and for the purposes of the initial discussion I think this is enough to demonstrate the inadequacy of this level of ventilation for the stated purpose, else Covid spread wouldn't be a problem in the UK within the home. It appears that for new builds latest research[3] shows an average of around 4m3/hr/m2, which is better than I expected, however the research suggests this is in part due to temporary measures by builders to achieve higher ratings than they would otherwise, and this is born out by the distribution of ratings achieved with an unlikely precipitous drop at precisely 5m3/hr/m2. However, for houses built to this standard some form of additional ventilation is a requirement of Part F[4]. It is possible to build a house with as low as 3m3/hr/m2 and provide no additional ventilation, but it must have been designed to > 5m3/hr/m2 so this should not be common. For social housing we can assume a single bedroom home will be around 40m2 with 2.6m between floors, and the required ventilation rate is 13l/s, or 47m3/hr/m2. This is around 0.45ACH. In fact the legal minimum rate per m2 is 1.08m3/hr, so for a typical home with 2.6m between floors this works out to 0.42ACH. So any house built to modern standards and used correctly should achieve the Passivhaus level of ventilation, unless it has tall ceilings. Certainly any UK home built with continuous mechanical ventilation is pretty comparable to Passivhaus standards for airflow. > People don’t open trickle vents, they turn down the extract fan timer because of fan noise People can also turn off the MVHR in a Passivhaus. I'm not sure what purpose it serves to compare correct use of a Passivhaus to incorrect use of this small fraction of UK homes. [1]https://www.statista.com/statistics/292252/age-of-housing-dwellings-in-england-uk-by-tenuree/ https://www.statista.com/statistics/292252/age-of-housing-dw... [2]https://www.researchgate.net/profile/Dominic-Miles-Shenton/publication/267304548_AIRTIGHTNESS_OF_UK_DWELLINGS_SOME_RECENT_MEASUREMENTS_AIRTIGHTNESS_OF_UK_DWELLINGS_SOME_RECENT_MEASUREMENTS/links/561f71bf08aecade1ace3adc/AIRTIGHTNESS-OF-UK-DWELLINGS-SOME-RECENT-MEASUREMENTS-AIRTIGHTNESS-OF-UK-DWELLINGS-SOME-RECENT-MEASUREMENTS.pdf https://www.researchgate.net/profile/Dominic-Miles-Shenton/p... [3]https://discovery.ucl.ac.uk/id/eprint/1573697/1/1-s2.0-S0378778817322612-main.pdf https://discovery.ucl.ac.uk/id/eprint/1573697/1/1-s2.0-S0378... [4]https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/468871/ADF_LOCKED.pdf https://assets.publishing.service.gov.uk/government/uploads/...