Managing Ventilation with CO2 Monitoring

In 2003, a healthcare worker infected with SARS went to a wedding in Hong Kong and checked into the Metropole Hotel. He fell ill the next day and went to hospital – but had already infected 16 other guests with rooms on the same floor, probably largely through their ventilation systems.

Move forward 17 years and managing ventilation is now a hot topic in reducing COVID risk. A person with COVID releases particles in droplets and aerosols whenever they speak, sneeze or cough. Droplets tend to fall to ground quickly, but finer sprays (aerosols) remain suspended in the air for long periods of time if the air is not removed from the room. Breathing in these aerosols puts other people in the room at high risk of developing a COVID infection, even once the infected person has left.

Fighting this means bringing in fresh air and removing stale air to quickly remove virus particles from the environment. It is an important piece of any plan to reduce COVID risks in a public building, along with keeping distance, hand hygiene and mask wearing, but it can be difficult to monitor how effective ventilation strategies are. CO2 monitoring can give reassurance that ventilation is working to help you get your building back to work safely.

How monitoring CO2 helps

CO2 levels provide a very useful signal for any facility manager trying to assess COVID risk and ventilation rates. Every person in a space breathes out CO2 at a constant rate, at a concentration around 40,000 parts per million (ppm) meaning it can give a good indication of aerosol load in the air, if someone in the room has COVID.

Over time this CO2 builds up in enclosed spaces, so that levels will exceed the natural background rate (around 400 ppm). An effective ventilation system will be able to expel stale air and bring in fresh at a rate that keeps the CO2 levels from building up to uncomfortable levels: and by diluting any infected air it will also reduce the risk of contagion to the people in the space.

Understanding CO2 levels across the whole building helps facilities managers identify what strategies are working, reduce high risk practises (such as holding meetings in unventilated spaces) and demonstrate to occupiers that the space is working for them in the fight against COVID.

What should CO2 levels be?

If you’re thinking about using CO2 to control COVID risks, it important to recognise that while absolute levels do matter, rate of change is important as well. A rapid build up of CO2 in a room is a clear sign of over occupation, and a slow reduction can be a sign of ineffective ventilation.

Background (atmospheric) CO2 levels vary slightly over time and location but are typically around 400 -420 ppm; in a mechanically ventilated buildings 800ppm – 1000ppm is a commonly used target for normal operation. Even before COVID, research shows levels above 1200 ppm have measurable impact on productivity and decision making (see CO2 and workplace productivity below).

The chances of catching COVID are affected by a range of factors as well as ventilation (including age, activity level and distance), and these effects are still being researched, so providing a single CO2 level as a target is misleading, but current consensus amongst authorities such as ASHRAE and the FEA suggests anything above 1000ppm should be avoided.

Prof Cath Noakes, expert in ventilation and infection transmission, goes a little further: “You should be looking for CO2 below 1000ppm, and ideally around 800ppm BUT there’s a bit more to it…to understand if the ventilation is adequate, you need to measure with the normal number of people in the space. If you measure with less people you will get a lower reading which could give a false impression that the ventilation is OK.”

What else can CO2 levels tell me?

As important as the absolute levels of CO2 is the rate of change. As Prof Noakes explains: “Transient effects also matter. The CO2 builds up and decays quickly when a room has a high airflow rate, but much more slowly in a poorly ventilated space. A single low reading doesn’t tell you the whole picture.”

Watching the rate of change gives a clear picture of which parts of the building are extracting waste air effectively. The graph below shows the decay of CO2 in a meeting room with good ventilation, vs ineffective ventilation.

CO2 decay in well ventilated (left) vs poorly ventilated (right) meeting rooms

Using decay curves like this it is possible to produce an estimation of air changes per hour and map these over the whole building to identify dead areas where ventilation is less effective. Sometimes there can also be ‘dead times’ – in one building monitored by Purrmetrix system last year meeting rooms that were effectively ventilated during operating hours were being used after working hours when ventilation systems were reduced. CO2 build up was rapid and dispersal very slow. The monitoring highlighted this risky behaviour which was discontinued.

CO2 and productivity

Improvements in ventilation have value beyond reducing COVID risk – many studies have demonstrated the link between high CO2 levels and reduced productivity in office workers. Most recently the Whole Life Performance study, from Oxford Brookes and LCMB, used Purrmetrix CO2 monitoring to demonstrate the relationship; finding subjects completed sample tasks 60% faster in environments with lower CO2 levels.

Decline in performance across a range of thinking tasks when exposed to high levels of CO2

In schools, high CO2 levels also a cause for concern, and have been associated with declines in cognitive function scores in at least one Harvard study.

How do I put CO2 monitoring in place?

With all this in mind there are several key things to think about when designing a CO2 monitoring project:

How accurate are my CO2 sensors – how do they calibrate?

Before getting stuck on accuracy, it’s important to understand when it’s important and when it isn’t. Sensor manufacturers list accuracy as ± ppm ± a percentage of the reading – it’s common to find variation of 50 ppm and 3%, meaning at the lower limit of 400 a sensor can measure between 358 and 442 ppm. By sampling rapidly (up to 20 times a second) and taking an average they improve the accuracy of the read and produce a result close to the actual target. Accuracy measured this way will improve at higher CO2 levels (measuring 1000ppm the same error could produce a result of 920 – 1080 ppm, before averaging) and it has little effect on rate of change measurements.
As important as accuracy for long term monitoring is how the devices are calibrated. Low cost devices can drift in performance over time, and manual recalibration is very labour intensive. The best solutions have sensors that can self calibrate and reset back to a base level of background CO2 on a regular cycle.

How easy is it to deploy and maintain the sensors.

Sensors need to be robust, easy to fit, and – if you are working with a large site – easy to identify. Once fitted, the best can be left with no further visits for calibration or battery replacement.

Where am I putting the CO2 sensors?

This is important as you want to get the most representative figure for a gas that will vary across the space. Measurements should be taken from every room where people regularly gather and ideally from the same number of points across an open plan office as points where the air is extracted. The best locations are fairly central, not too low (CO2 sinks so this will raise your result), not directly in front of a person or a vent. We favour underside of desks or meeting rooms, if power leads will extend to those locations.

How much data do I get? How do I make sense of it?

For professional ventilation management, transient effects matter, meaning it is important to be able to see data over long periods of time, and ideally understand quickly which area this data relates to. On a large site all this data must be quickly turned into actionable information, such as heatmaps, alerts and ventilation measurement.

As facilities managers plan for re-opening sites, ventilation provides an effective way to reduce COVID risk and CO2 monitoring is an important tool in managing ventilation strategies. With hundreds of CO2 sensors monitoring thousands of hours of CO2 data, the Purrmetrix solution is a proven and powerful system for measuring and analysing ventilation rates. If you are working on ventilation strategies in your estate and have questions, get in touch and we can help.

Energy efficiency in housing – the business benefits

The benefits of energy efficiency in housing are, quite rightly, often framed in terms of the impact on climate change and the lives of tenants. But this can mean landlords overlook the fact that there is ample evidence that more energy efficient homes repay the investment needed to upgrade them.

Benefits come from a range of cost centres, and data for some of these is not commonly held by many social housing landlords. So we have assembled some of the most relevant studies and data as a brief reference to set expectations about what can be achieved from improving the energy efficiency in housing.

The largest data set available on the impact of energy efficiency comes from a Rockwool survey of social housing landlords, which provides some striking findings:

  • a clear relationship between energy efficiency and number/duration of void periods.
  • properties with lower energy efficiency generated more rent arrears. An average of two weeks more arrears for a home rated F compared to a home rated C.
  • for every 10 point improvement in the SAP score a reduction in average annual costs of responsive repairs of £90
Month of tenant rent arrears per SAP rating

Additional work done for Orbit Housing identifies an opportunity to reduce customer contacts by 75000 over 20 years if their 27,000 homes are improved.

What you need to know before you start condensation monitoring (with examples!)

It looks like a bumper year for condensation claims. As the second lock down and social restrictions increase the number of hours families spend at home, humidity levels in housing are soaring.

As any school kid can tell you, human beings are 60% water, and the spaces we occupy have to be able to dispose of the water we give off. Getting this done effectively is helped by a warm environment, but the COVID crisis means many tenants are finding their incomes reduced, putting pressure on their budgets for heating. Colder homes, occupied for longer, are a recipe for condensation and mould growth.

Condensation is a classic example of the sort of problem that if caught early and treated correctly will cost a lot less than if left undetected. It is also the problem that most frequently causes a breakdown in landlord tenant relationships, as tenant’s behaviour is often a significant contributing factor. And no-one takes kindly to being told they are part of the problem. So early detection of a condensation problem, before mould gets into the fabric, is important to trigger an action plan and keep everyone happy.

Many RSLs have been looking at pilot projects based on RH measurements to help pick up on early warning signs of condensation. If you’re in this situation, we’ve dug back through our data to give you a short guide on what you need to look for to make a success of condensation monitoring. What is best practise to get the most accurate results? What are the metrics you might look for? What can you do with this data?

Some basic physics

Wikipedia tells us that relative humidity (RH) is the ratio of the partial pressure of water vapor to the equilibrium vapor pressure of water at a given temperature. Well, thanks Wikipedia. 

In practise what this means is that RH is an expression of the air’s capacity to hold water vapour at a given temperature. Air can hold a lot more water vapour at a high temperature than at a low temperature, which is why we end up with water condensing when warm air hits a cold surface.

This means you can have a lot less water floating around in the air of a room that is 17°c with an RH of 70% than in a room which is 22°c with an RH of 50%. So the first thing to note is that simple RH %ages can be a bit misleading when it comes to measuring condensation risk.

If you take your 24°c room and keep the same amount of water, then as you reduce the temperature the RH will rise, and the point at which it becomes 100% (ie the point at which condensation occurs) is the dew point. In this case, with 50% humidity the dew point would be about 11°c.

The third metric worth knowing is the actual vapour density, that is the weight of water in the air. If you can get it, this is one of the most useful metrics to watch in condensation analysis because it’s the measure of how much water occupants are putting into the home.

RH is not the only game in town.

We’re focussing on the physics here because it is our belief that many landlords are missing valuable information by focussing on RH alone. Instead, running analysis on dew point and weight of water uncovers information that is richer and more accurate, allowing interventions to be better targeted. For example, here is a side by side comparison of two homes both of which have a serious RH problem, with measurements regularly in the 80% plus zone:

Take that data and turn it into dew point and you can really see which house has the problem. Here the black line is the real temperature, and the blue line is the dew point (when condensation occurs). House A is spending significantly more time at or below dew point, and is undoubtedly wringing wet. The other house has occasional incidents.

How to gather effective condensation data.

For any monitoring project two key questions are: where do you measure and how long do you measure.

I probably don’t need to spell this out, but if you’re looking for condensation problems using RH data, take measurements where you expect problems to appear. External walls, close to corners, in heavily occupied rooms are generally a good bet. 

However, to gather enough data to think about all the root causes, you are very likely to need more than one sensor. Measuring conditions in bathroom and kitchen will help confirm if ventilation is working correctly, a sensor closer to the most used heater will confirm patterns of heating use. 

Bear in mind that you need to look at the behaviour of the home with a variety of weather conditions and occupant behaviours – so plan for 3-4 weeks of monitoring.

How do I make sense of all this data?

If you’ve confirmed using dew point analysis that you’ve got a serious condensation problem, the obvious next question is what is causing that?

Condensation problems are generally the result of a combination of problems. To get condensation you need 1) a source of water vapour (hello humans!) 2) a cold surface and 3) air that isn’t able to circulate effectively to remove the water vapour, either because it is too cold or because it’s not being removed from the building.

Most condensation problems can be placed somewhere on this space:

Deciding where you are on this triangle will help define the action you need to take. By looking at weight of water in the house and mapping it over a week’s use it is easy to pick up the contribution from lifestyle.

For example here we have footprints from two homes – each row is 24 hours of data. One house has a significant high base level of humidity and the other has a pattern that clearly shows morning showers. Of course, neither of these are a problem in themselves, unless the dew point vs temperature analysis shows high risk of condensation.

More heating is often mentioned as a solution to condensation. This works by 1) raising the temperature of the cold surfaces in the home and 2) allowing the air to hold more water vapour so it can be removed from the home better, if the ventilation is working.

Condensation is commonest on external walls, so it’s important to consider, before asking tenants to run their heating for longer, whether the fabric of the home is losing heat too fast. A home with walls or ceilings cold from heatloss will be very difficult to heat sufficiently to avoid condensation (as an easy example, it’s nearly impossible to heat a bedroom sufficiently to avoid condensation on single glazed windows on a cold morning). We will be writing more on how to gather information on heatloss shortly, so keep checking back in.

If heating is adequate to mobilise the water vapour then the final part of the jigsaw to look at is ventilation. The simplest approach is to look at the time taken to reduce water vapour to normal levels after an event like a shower. The chart below gives a few examples of ‘natural’ and ‘artificial’ ventilation in an older house, showing what happens after cooking and showering.

Analysed correctly, RH readings are a rich source of information that can not only confirm the extent of condensation, but also predict where problems are likely to occur and demonstrate how to tackle them. Purrmetrix provides easy to use, powerful tools for measuring and analysing condensation and RH problems in any home – if you have a problem with condensation that would benefit from diagnostic monitoring, contact us for a demo or more information.