Negative, Positive, Neither?
A live study of specific ventilation systems.
Background
My passion is creating happy, warm, dry, well ventilated, resilient, healthy homes for all people to live in.
I’m working on a multi-year renovation of 3 St Albans townhouses. This has slowed down due to interest rate increases and the time taken to obtain the Code of Compliance Certificate for work already completed. (I have CCC now!) I have convinced myself well and truly now that ventilation cannot be competently controlled by just opening windows. Without very good ventilation, no home can be called a #healthyhome.
The next stage is to upgrade the ventilation from being intermittent negative pressure (bathroom fans & range hood) to continuous mechanical ventilation with heat recovery.In the meantime, I thought I could conduct an experiment with a continuous negative pressure system.
Discussion
I’ve previously written for Abode Magazine in plain language about the types of ventilation systems https://www.abodemagazine.co.nz/home/2021/10/28/the-air-we-breathe
It’s pretty much universally agreed (except by a few slow suppliers) that positive pressure systems do not work. This from the Healthy Home Standards.Recirculating systems (products like HRV and DVS systems), or fans that do not extract to the outdoors are not suitable to meet the ventilation standard.
The Healthy Home Standards specifically require negative pressure systems, with a continuous option. https://www.tenancy.govt.nz/assets/Uploads/files/healthy-homes-standards-ventilation.pdf
Others, like the Green Building Council are requiring extraction systems to meet their voluntary standards.
But do these systems actually work in the real world?
Aim
The aim is to see what effect installing continuous negative pressure ventilation in the bathroom will have on relative humidity and CO2 levels across the first floor of two almost identical units in a real-world application.
Following the work done to the units to make them a lot more airtight, ventilation has become an issue. Specifically, relative humidity and CO2 levels are higher than optimum levels, I ideally want to achieve.
Unit 3 is owner occupied and Unit 2 is rented.
Method
I have installed the same extraction fan in the main upstairs bathroom of two near identical units. Both fans were installed by a qualified electrician.The fan as pictured below, has been supplied by Cam Kilgour of Fantech.
Fig. 1 – Extraction fan to be installed.
Fig. 2 – First floor layout showing location of fan installation in Units 2 & 3.
Unit 3
The fan in Unit 3 was set to run at 40bBa as a way of initially measuring flow rates. Later this was measured by Cam at 105m3/hr. This translates to 29.16l/s. This is well in excess of the 10l/s that is required by the Healthy Home Standards.
No doors have been undercut. All doors stay open the majority of the time.
No specific makeup air has been provided. The reason for this is just to establish a baseline and to see whether makeup air is necessary.
Windows are opened and closed as needed for ventilation purposes.
The latest blower door test for this unit indicated an airtightness rating of 3.79 air changes per hour at 50 pascals of pressure.
Unit 2
The fan in Unit 3 was set to run at 50dBa as a way of initially measuring flow rates. Later this was measured by Cam at 150m3/hr. This translates to 41.7l/s. This is quadruple the 10l/s that is required by the Healthy Home Standards. No doors have been undercut. The tenants have been told to leave the bathroom door open, when not in use, but operate as normal in the bedrooms.
No specific makeup air has been provided. The reason for this is just to establish a baseline and to see whether makeup air is necessary.
Windows are opened and closed as needed for ventilation purposes.
A blower door test hasn’t been carried out, but this unit is likely to be slightly more airtight than Unit 3, due to less external walls and openings. This was the case with the initial baseline testing.
Monitoring
Humidity and CO2 levels are being monitored by sensors that have been installed in each unit for several years now. Sensors are located in the bedrooms and in the open plan kitchen / dining / lounge downstairs.
Interim Results
The extraction fans were installed on the 25th July 2023.
The first data capture was undertaken on the 21st August 2023.
This intervention is shown as a vertical dotted line on the following graphs.
Unit 3
The fan in this unit is almost silent. It can only just be heard from the master bedroom during the dead of night.
Often when getting out of the shower, the first thought is that the extraction fan is not running!
The first very noticeable observation is how steam free the bathroom is after two showers in the morning and how weird it is not hearing the old fan running.
Humidity Levels
Relative Humidity is the amount of water vapour in the air, relative to the temperature outside. To get a better understanding of relative humidity, there is a short clip near the bottom of the Mātauranga page on this website.
The first range of measurements were taken from the 9th July to the 21st August to capture the before and after effects of installation.
Before the installation of the fan it can be seen that the relative humidity levels upstairs were higher than the optimum range (green band). After installation, relative humidity drops to almost always being in the 40-60% optimum range.
Carbon Dioxide Levels
CO2 levels are used as a trigger proxy for indoor air quality. Low CO2 levels indicate fresher air or better airflow through a home.
Indoor CO2 concentration has an impact on comfort, health and wellbeing. CO2 concentration levels around 1500 ppm can cause discomfort and above this limit can start to have health impacts such as headaches or tiredness. Even higher levels can cause dizziness, sleepiness or even trigger attacks for people with asthma.
It is quite noticeable, when you are aware of what is causing your drowsiness in the evening or when you are not feeling refreshed upon waking.
The first range of measurements were taken from the 9th July to the 21st August to capture the before and after effects of this installation.
Before the installation of the fan it can be seen that the CO2 levels, especially in the master bedroom were well above the optimum limit of less than 1000 parts per million, with the door open at all times.
After the installation, it seemed there was an initial reduction, but then normal service resumed.
The negative pressure ventilation system is not working to reduce CO2 levels.
Temperature
Just for completeness, temperature monitoring over the monitored timespan has been included to show that this was consistent, and within the ideal healthy range during the period
Unit 2
This fan is audible, but not annoyingly so. At least, there have been no complaints about it, to date.
Humidity
Relative Humidity is the amount of water vapour in the air, relative to the temperature outside. To get a better understanding of relative humidity, there is a short clip near the bottom of the Mātauranga page on this website.
The first range of measurements were taken from the 9th July to the 21st August to capture the before and after effects of installation.
Before the installation of the fan on the 25th July it can be seen that the relative humidity levels upstairs were often sitting in the 75% range, much higher than the optimum range (green band). After installation, relative humidity drops, but not significantly. The largest relative humidity drop appears to be the open plan downstairs area, such that this is now generally always within the optimum 40-60% range. Humidity generally rises when the bedroom doors are shut evening and falls, when the doors are open in the morning.
Note that beforehand Bedrooms 2 & 3 were monitored, but due to a change of tenant occupation in mid-July, a sensor was moved from Bedroom 3 to the Master bedroom; Pink to orange graph plot.
Carbon Dioxide Levels
CO2 levels are used as a trigger proxy for indoor air quality. Low CO2 levels indicate fresher air or better airflow through a home.
Indoor CO2 concentration has an impact on comfort, health and wellbeing. CO2 concentration levels around 1500 ppm can cause discomfort and above this limit can start to have health impacts such as headaches or tiredness. Even higher levels can cause dizziness, sleepiness or even trigger attacks for people with asthma.
It is quite noticeable, when you are aware of what is causing your drowsiness in the evening or when you are not feeling refreshed upon waking.
The first range of measurements were taken from the 9th July to the 21st August to capture the before and after effects of this installation.Before the installation of the fan it can be seen that the CO2 levels, especially in Bedroom 3 were well above the optimum limit of less than 1000 parts per million, spiking to nearly 4000ppm, just before the bedroom door was opened in the morning.
After the installation, possibly due to an occasional change of occupation in the Master bedroom,CO2 levels are actually higher peaking at over 5000ppm.
The negative pressure ventilation is not working to reduce CO2 levels, especially while bedroom doors are closed.
Even in the open plan downstairs, while CO2 levels average under 1000ppm, they occasionally peak over the 1500ppm level.
Note the sensors are capable of recording CO2 in the master bedroom and downstairs, only. Bedroom 2 does not have a CO2 monitor.
Temperature
Just for completeness, temperature monitoring over the monitored timespan has been included to show that if anything, more consistent temperature were achieved after the intervention and were more often within the ideal healthy range during the period of the monitoring. This is due to the change of tenants and some education around the use of the heat pump.
The heat pump is located downstairs in the open plan living area.
Again, the temperature falls away in the bedrooms, when the bedroom doors are closed.
Interim Conclusions
– The installation of the continuous extract fan in the bathroom has reduced relative humidity to acceptable levels in Unit 3
– The installation of the continuous extract fan in the bathroom has reduced relative humidity to acceptable levels in the open plan downstairs area of Unit 2
– Occupant behaviours have meant that relative humidity has not been reduced to acceptable levels in the upstairs bedrooms of Unit 2, even with a greater extraction rate.
– There has been no improvement of carbon dioxide levels due to the installation of the continuous extract fan in Unit 3.
– There has been no improvement of carbon dioxide levels due to the installation of the continuous extract fan in Unit 2.
– Occupant behaviour makes a considerable difference to the relative humidity levels inside the home.
– Under cutting of doors, would be worth trialing to see whether this makes a difference.
– Make up air doesn’t seem to be required at these extraction rates, although there is no way of telling where the air is getting in from, in what are fairly airtight units compared with standard New Zealand new build construction.
– There has been no noticeable effect on the power bill having these units running continuously, 24/7.
October 2023 updates
Monitoring for Unit 2 & 3 are provided below from the 21st August to the 9th October.
Unit 3
Humidity
Relative humidity has generally remained in the healthy range.
Carbon Dioxide Levels
Carbon dioxide levels in the master bedroom at night have not improved.
Unit 2
Humidity
Relative humidity levels have not changed significantly since the last review. The higher levels of relative humidity in the open plan living area is likely due to the occupants drying their washing in the lounge in front of the heat pump, despite being advised not to. (Drying the washing in the bathroom under the fan is the preferred method.)
Carbon Dioxide Levels
It can be clearly seen that there has been no improvement to carbon dioxide levels, over the further period of examination.
Conclusions
There has now been three and a half months of monitoring across two units. Two things are crystal clear
– Those recommending that continuous extract ventilation is the answer to improving internal air quality are not correct. Air quality has not improved with the addition of continuous extract fans.
– Occupant behaviour has a significant impact on relative humidity levels. The Unit 3 relative humidity levels are within the acceptable determined range, but Unit 2 levels are not.
Further Study
Next winter there will be an opportunity to resume the testing (the monitors will remain in place and working over the summer), with some changes to the methodology.
– Building design may have an effect. Next winter, doors can be undercut in Unit 2 to see if relative humidity levels change.
– The extraction fan in Unit 2 can be dialled back to match the fan speed and airflow in Unit 3. The extra suction is not making a difference in this Unit.
– The extraction rate in Unit 3 can be reduced to see whether a reduced airflow has an effect on the relative humidity levels.
– In summer, the ensuite window can be left open providing makeup air to see what effect that has on the overall performance.
Recommendations
Those selling and promoting continuous extract (negative) ventilation as a way to improve internal air quality (IAQ), need to reassess their position. Yes, it appears to reduce humidity where the building occupants change their behaviours to assist, but it clearly does not improve carbon dioxide levels.
Human behaviours, especially doors being closed at night, have a significant effect on IAG and comfort. Making ventilation and heating systems occupant proof, needs to be part of any home improvement strategy.
A healthy home has carbon dioxide levels that are consistently under 800-1000ppm. It appears that this can only be provided by a balanced ventilation system. In due course, testing will be carried out to test that hypothesis. It’s probably already been done elsewhere, but it may be different in St Albans :)
Organisations providing voluntary standards aiming to achieve better health outcomes, need data to prove their rating systems, do what they say on the tin.
HEALTHY HOME COOPERATION
Healthy Homes for all Kiwis. Housing is about people. People working together in cooperation to provide Healthy Homes for people to live in.
Damien McGill: Engineering Happy Healthy Homes for People & Planet
