What is the difference between MVHR, air conditioning, air cooling, and natural ventilation? (a paste)

[Fallingditch]

(Good explanation from JSHarris, 2014. My emboldening.)

 

There are differences between air conditioning, air cooling, MVHR and natural ventilation, and they are all to do with air quality.

Broadly speaking, we have three main measures of air quality (ignoring pollutants). Air temperature is the one we mainly think of, as it is immediately felt in terms of comfort level. Next is probably humidity, as this mainly determines our rate of body heat loss to the surrounding air (by decreasing our natural evaporative heat loss from perspiration when humidity levels are high). Last, but not least is CO₂ concentrations. We tend to feel that rooms are stuffy when the CO₂ level rises much above 800 to 1200 ppm (outside air will typically be a little under 400 ppm). The reason for the latter is that blood acidosis controls our resting breathing rate, and this is determined by blood CO₂ concentration. When this rises (because our lungs aren't able to get rid of CO₂ as effectively due to the increased partial pressure of CO₂ in the surrounding air)then we tend to breathe a bit deeper and faster, and to falsely feel that the air has less oxygen, so we feel it's a bit stuffy.

Proper air conditioning ensures that incoming air is both cooled and humidified, and that CO₂ levels are around the same as those outside. It's used commercially, but is uncommon in domestic and vehicle systems. Air cooling is the most common "air conditioning" system we encounter, in buildings and in cars. This both cools the air and dehumidifies it, and it's the latter effect that tends to dry the air and cause symptoms of nasal congestion, dry skin and even exacerbate conditions like asthma.

Conventional MVHR doesn't appreciably change the humidity level, nor does it change the CO₂ concentration, so is perhaps the healthiest way of ensuring good, through house, ventilation. I've been measuring the RH, temperature and CO₂concentration in our bedroom for the past year, and found that even with a window open the ventilation is grim. By 4 am the RH will often be around 65% and the CO₂ up around 1400 to 1600 ppm, way over the comfortable limits. As soon as the bedroom door is opened these levels drop. MVHR does ensure that there is a constant flow of fresh air across a room, which should keep both the RH and CO₂ level close to those outside.

Air cooling, in the form of a typical "air con" unit, or the active air to air heat pump we have, will maintain comfortable CO₂ levels, but will reduce the humidity. This is very obvious with our set up, as within a few minutes of it going into active cooling mode you can hear the condensate dripping down the waste pipe. Luckily, the air flow rate from the MVHR isn't enough to overcome the extra humidity introduced into the house from breathing, showers, running taps etc, so I've never seen it drop below about 45%.

Finally, natural ventilation means accepting whatever the air temperature and humidity is outside. CO₂ isn't an issue, as that will be a reasonable level of around 400 ppm. The main problem with natural ventilation is the lack of temperature and humidity control. We really want to keep the house in the 40 to 60% RH range, and the temperature within our personal comfort zone (and that depends on the individual, and the nature of the building). Our new build feels comfortable at 19 deg C, and too warm at 22 deg C. Our current house feels cold at 19 deg C, and just about OK at 20 to 21 deg C.

[Jeremy Harris]

Thanks for digging that out.

I have the graphical data from that bedroom air quality measurement:

 

and also a later measurement I took where I annotated the plot with some event details: