Humidity control is the answer

Today’s energy efficient air-tight homes require a ventilation system that can guarantee the best possible Indoor Air Quality (IAQ) at all times.

According to the publication ‘Energy efficient ventilation in dwellings – a guide for specifiers’ which is published by the Energy Saving Trust: “All dwellings need a supply of fresh air, not just for the health and comfort of the occupants, but also to control condensation, remove pollutants, and to ensure the safe and efficient operation of some combustion appliances. The amount of fresh air should match the needs of the dwelling and the people living within it.”

It continues by saying: “Moisture is probably the most significant element because of the high rates generated by activities such as cooking and bathing, and because of the problems associated with condensation and mould growth. Research has shown that if relative humidity levels exceed 70% for prolonged periods, there is a high probability that the condensation occurring on cold surfaces will lead to mould growth. A ventilation rate of between 0.5 and 1.5 air changes per hour (ach) for the whole dwelling will usually be sufficient to control condensation.”

According to Aereco, the pioneer of humidity sensitive Demand Controlled Ventilation (DCV), people are most comfortable at a relative humidity level of between 40% and 50%. The human respiratory process produces CO2 along with water vapour which, unless extracted and replaced with fresh air, will linger indoors with detrimental effects on the health of inhabitants and on the fabric of the building itself. DCV helps to eliminate these as well as other airborne pollutants such as carbon dioxide, carbon monoxide, oxides of nitrogen, tobacco smoke, allergens and other bio-contaminants.

Recent trends

The recent trend in the UK has been veering in the direction of adopting Mechanical Heat Recovery Ventilation (MHRV) despite fears that use of these systems may actually increase the risk of health and respiratory problems in tenants and householders if they are not installed and maintained properly.

The logical, and already proven, alternative is to adopt DCV systems that include humidity sensors on the grilles that automatically react to occupancy -so if the adults have gone to work and the kids are at school, the system operates on minimum, producing a trickle of air. When the house is occupied the special humidity sensitive grilles sense that and boost ventilation.

There are no filters to change, they are whisper quiet in operation and the system is virtually maintenance free. Serious concern has been expressed that tenants and homeowners might be tempted to cut corners with MHRV systems and isolate fans to save money and to cut down on noise levels. One of the big downsides with MHRV ventilation systems is that the filters must be changed annually and there are fears that tenants and homeowners will not invest the time and money to fit new filters. If there is no ventilation in wet rooms and no clean air circulating around these new air tight dwellings, these factors highlight a potential health hazard of airborne hazards and pollutants.

A report commissioned by the previous Government with NHBC and BRE ‘Indoor air quality in highly energy efficient homes – a review’ gives credence to worries about MHRV systems. It also asks whether the filters will be cleaned regularly and points out that: “The requirement for cleaning fan units and ductwork needs to be thoroughly investigated. Evidence from the USA suggests that a market has been created which is based on fear, with cleaning costs being high. The rate of build-up of dust and dirt within fan units and ducts in UK dwellings needs evaluating and guidance produced on good practice maintenance.”

The study also found widespread failure to maintain filters. “There is no market for replacement filters, with several manufacturers reporting no filter sales at all. Even basic maintenance is not undertaken,” says the report. It goes on to say: “The efficiency of fans and thermal heat exchangers has reached a level where there are only marginal gains still to be made. The next drive by the industry will be for advanced controls and in particular for demand controlled DCV. If the energy savings resulting from the potential reduction in fan operation and heat loss are to be realised, the ‘building empty’ and ‘room empty’ minimum ventilation rates must be determined.”

Lower energy use

Humidity sensitive DCV is proven to have much lower energy usage than MHRV systems whilst providing greater levels of comfort in living areas and eliminating the problems of mould and condensation in wet rooms. A 2008 study by the Fraunhofer Institut Bauphysik in Germany shows that DCV is a far more cost effective and reliable method of providing the optimum indoor environment. The study, (reference IBPBericht RKB-12-2008), comes down heavily in favour of DCV.

In exhaustive tests, the Institut found that a humidity sensitive DCV system offers an excellent method of staying below 1,200 ppm CO2, which guarantees optimal indoor air quality in the dwelling.

On test

In France, where DCV is major player in new build, another test bed, the Performance project, allowed testing in-situ of mechanical ventilation systems from two manufacturers, including Aereco. The Performance project monitored demand controlled MEV on two large apartment blocks. The project, supported by French ADEME, concentrated on two buildings erected in 2007 – one in Paris and one in Lyons – and the efficiency of humidity controlled MEV in general and the Aereco DCV system in particular.

A total of 30 occupied dwellings were monitored over two years from November 2007 to measure representative parameters for energy consumption and IAQ.

Measurements of CO2 concentrations over that time show indoor air quality is ensured in a low occupied bedroom with one adult present as well as for one with high occupancy (four adults). The peak of CO2 concentration shifts from 700 ppm in the low occupancy bedroom, to 950 ppm in the latter, but even there the 1500 ppm level was not exceeded for more than just a few hours in the heating season.

The fan was stopped for maintenance for one month, and CO2 concentrations were compared with the other months in the heating season when the fan was on. When the fan was stopped, a strong rise in CO2 concentrations (above 1900 ppm most of the time) could be observed, however, there was no particular reaction from the occupants to compensate for the lack of air renewal which confirms the major role of ventilation on IAQ and demonstrates that occupants are unaware of poor ventilation and don’t compensate, for instance, by opening windows. Importantly, the project illustrates the high performance of DCV systems in providing high levels of indoor air quality.

Summing up, the potential for DCV is high and will become higher as the density of occupancy is lowering on a global trend for years in all countries.

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