Retired building airtightness technician Trevor Clark gives his advice on how to maintain good indoor air quality.
This month, I am talking about exchanging stale for fresh air.
Humans, animals and plants perspire and within a confined space the odours can turn ‘offensive’ to our senses.
To prevent this, our forefathers invented ventilation, which is a means of exhausting the stale air and replacing it with fresh air from outside and delivering this fresh air to the rooms inside our buildings where it is most needed.
However, over the years we have found that the air ‘holds’ energy and by just exhausting the stale moist air using extract fans alone, we also throw away the energy previously used to warm the air to comfort levels.
By collecting the energy held in the outgoing air we can pre-warm the fresh air before delivery into the building and this can save a significant amount of money in heating costs.
What is indoor air quality?
Indoor air quality is known to affect the health, comfort, and wellbeing of building occupants.
Poor indoor air quality has been linked to sick building syndrome, reduced productivity, and impaired learning in schools.
This generally infers the air has an amount of pollution held within the building.
Source control, filtration, and the use of ventilation to dilute contaminants are the primary methods for improving indoor air quality in most buildings.
What are we trying to do?
Maintain indoor air quality by removing stale moist air while introducing fresh filtered air into the building. However, this presents a problem for energy saving.
Ultimately we must do two things together: exchange stale air for fresh air, while also capturing energy leaving to warm the fresh air.
Mass transfer of energy happens because stale moist air contains high levels of energy, which had been heated by the central heating system (and paid for) and now we need to expel it, and bring fresh cold air into the building that now needs heating.
As we expel warm moist stale air we need to collect the energy and use this otherwise wasted energy to warm the fresh filtered incoming air.
Move the energy from where it is no longer required, to heat the incoming fresh air, this will reduce the energy loss that will increase our energy costs if we just run extract and exhaust fans alone.
This will reduce the buildings heating costs and the size of the main heating equipment as well, making it smaller.
It uses less fuel, reduces capital cost and even lowers running costs going forward.
How much energy could we exchange?
If we were to use an energy recovery unit, we could save 75% of the energy used by the main heating system and only need to add 25% more energy from the heating to maintain comfort conditions inside.
If we were to use a small Air2Air heat pump to exchange the energy between incoming and outgoing air we would get a minimum 3kW energy output from 1kW energy input.
This would mean the main heating would not need to work as hard and have the potential to lower overall heating costs!
But this only applies for the hour that the fan/heat pump runs for and the ‘wet’ rooms we are ventilating.
To maintain comfort conditions and good indoor air quality inside the building we need to run the ventilation system for longer but at a reduced speed to reduce the air change rate for these wet rooms and spread it over the whole house over a longer period.
This would reduce the energy gain peaks and spread this gain over a longer period.
The kitchen ventilation would not be included in the ventilation system, as the ducting and filters can become ‘sticky’ from grease and oils used in cooking, which can create problems with the ventilation system as a whole.
For this reason kitchen hob ventilation is generally not joined to a whole house system. It can be done but costs increase due to special filters and additional maintenance requirements.
The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) recommends ‘0.35 air changes per hour but not less than 7.5 L/s per person’ in living areas.