Interior air quality: the top priority goal of ventilation
Mechanical ventilation ensures sufficient fresh air at all times. In a household of three persons, 3 × 30 m3/h is required, equivalent to 90 m3/h of outdoor air. The same amount of extract air has to be removed from the kitchen and bathrooms. The ventilation system in Kranichstein is described in detail in Feist et al. (2016b). Here, we will only discuss a few crucial aspects and the results of IAQ field tests. Figure 13 shows the CO2 concentration profile for the upper floor Jan to Feb 2016 as measured with a recalibrated Netatmo sensor (Wörner et al. 2015). The concentrations are largely within the IDA1 range (high indoor air quality). For the averages, however, only the occupancy times are taken into consideration, producing an average CO2 value of 850 ppm during occupancy (uncertainty of this value: 150 ppm). On February 12 and 13, 2016, airtightness test were conducted and the ventilation system was shut off. While windows were opened in intervals, the monitoring clearly shows that this was insufficient to ensure good air quality.
CO2 is an indicator of indoor air quality; on its own, it does not impact on health up to a concentration of 2500 ppm. Nonetheless, other indoor air contaminants generally correlate with CO2 concentrations. In the presented case, in addition to CO2, two measurement campaigns were conducted on February 20 and 21, 2016, to detect microorganisms. Furthermore, air samples were taken on February 23, 2016, to test for volatile organic compounds (VOCs) and formaldehyde. All results show very low indoor air concentrations (Feist et al. 2016b).
Three Radon Scout units (Sumesh et al. 2013) were used to determine the average radon activity during normal operating hours, yielding 43 (± 10) Bq/m3, a level that rose to 65 (± 10) Bq/m3 on the days when the ventilation system was switched off. Here, it becomes very clear that mechanical residential ventilation considerably improves IAQ.
The duct system: clean thanks to ePM1 80% particulate filters
The ventilation system used in Kranichstein has a filter box with a F8 comb filter at the external air inlet. These filters are standardised and produced in large quantities, making them relatively inexpensive (16 to 40 €). The large filter surface also keeps pressure losses low (at 106 m3/h: 9.7 (3) Pa). High-quality filters at the front end prevent dust from accumulating inside the ductwork. In particular, respirable fine particles from partly combusted hydrocarbons, rubber particles and soot remain in the filter, thereby considerably improving IAQ. Figure 14 shows a filter after 1 year of operation (left) and a new one (right). More than 80% of particulate components > 1 μm remain in this filter. Downstream duct systems therefore remain free of these particles, as the image of the air duct branching off from the filter box shows. On February 20 and 21, 2016, images were taken of the fresh air duct and sections of the supply air duct network after 25 years of operation (never been cleaned in the meantime) as described in the following section on the hygiene inspection. There were minimal traces of dust in a few spots. The method of using high-quality filters to keep ventilation systems clean has proven worthwhile in this project. For this reason, we recommend the use of F8 or F9 (ePM1 80%) filters in the outdoor air stream.
Hygiene inspection of the ventilation system after 25 years of operation
The examination of the heat recovery ventilation system showed that, with the exception of wearing parts such as filters and fans, virtually no components had to be replaced. The longevity of the ducts (in this case galvanised spiral ducts) as well as of the ventilation unit including the heat exchanger could be proven beyond doubt. This longevity of the components raises the question of whether the hygiene over this long period could be maintained easily, especially since in this example, no cleaning of the channels occurred so far. Especially the hygiene and the question of the necessary cleaning intervals are one of the most frequent topics of discussion in the public. The following investigations and results can provide valuable insights. The extent and level of detail of the sampling allow conclusions to be drawn about the hygiene of the duct system as well as the indoor air quality.
Sampling and evaluation
The hygiene inspection, which was carried out in the western terraced house (February 20 to 21, 2016), examined the indoor air hygiene as well as the hygiene of the ventilation system. The sampling was carried out according to DIN ISO 16000-16 (2008) and DIN ISO 16000-17 (2008). Air samples were taken by means of air sampler MBASS30 (Hohlbach) in the interior as well as of the outside air on the north side of the building (suction side) for comparison purposes. With this sampling, the respective total germ density in the air (collection volume per 100 l at a flow rate of 30 l/min) was determined. The exact documentation of the incubation and evaluation of the malt extract bearing and DG18 agar plates can be found in the laboratory report no. A-2015/6127 (Kirchmair 2016, p.1). As a result, the respective colony-forming units (CFU/m3 air) were reported (Fig. 15).
In order to be able to investigate the air hygiene of the air at the supply air outlets, 200 l of air was collected on gelatin filters by means of a filtration sampler (Sartorius MD8) and evaluated in the microbiological laboratory. From this, it was possible to deduce the colony-forming units per cubic metre of air.
In addition, a sampling in the supply air duct should be done at various points before and after the device. For this purpose, the method of isokinetic sampling was used, i.e. the suction speed in the inserted glass tubes corresponded exactly to the air velocity in the channel. The samples were then also applied to malt extract and DG18 nutrient media (Fig. 16).
Moreover, the duct wall was examined with the help of selvedge samples (impression plates Merck ENVIROCHECK CONTACT TVC). This type of sampling was particularly well suited because the plates can be easily applied to the inner surface of the round channels (Fig. 17).
Results of the visual inspection of the supply air ducts
For the visual inspection, the system was opened on the supply air elements as well as on the outdoor air filter box and the central ventilation unit. Virtually, no impurities could be detected in the supply air duct network, the galvanised spiral duct and the walls of the filter boxes and internals were completely bare except for tiny dry dust deposits in areas of low flow velocity. The same results were found for the inspection of the supply air ducts with the help of the special illuminated camera (Snakecam). No significant dust accumulation and no corrosion could be detected (Fig. 18).
This excellent cleanliness in the supply air duct network is due to the high filter quality of the exterior air filter and the regular filter change. Thus, the supply air duct system can obviously be kept clean for several decades without cleaning. Prerequisite for this is both the cleanliness of the channel sections during the construction phase (covering the open channel ends of the respective sections to commissioning) and a dense filter seat (correct sealing of the filter to the filter housing, so that air bypasses are avoided). If it is not possible to guarantee the cleanliness of the duct sections during the construction phase, the duct system must already be cleaned prior to commissioning, which would actually cause unnecessary costs. In general, filter replacement is cheaper than cleaning.
In the examined building, the exhaust air outlets in the baths were not protected with prefilters. Thus, some dust deposits on the pipe walls occurred by condensing grease vapours in the first metre section of the duct after the exhaust air outlet. In this installation, this section of the canal was made of cost-effective Aluflex pipe, which can easily be replaced. With the help of the built-in grease condensation filters made of stainless steel, the grease vapours from the kitchen exhaust air can also be condensed out. The stainless steel filters can be removed from the grease in the dishwasher at regular intervals and used again.
In general, although the investigation of the exhaust air duct network revealed clearly visible dust deposits, even after 26 years of continuous operation without purification, there was still no functional impairment. Hygienically, the dust deposits in the exhaust air are irrelevant, because they are led outside with the exhaust air—which runs permanently.
After the inspection, the exhaust air outlets were also retrofitted in the bathrooms with easy-to-install prefilters on the air outlets in order to keep the dust level even lower on the exhaust air side in the future. Again, stainless steel expanded metal filters are used, which can easily be cleaned regularly in the dishwasher. The recommended cycle for this is about half a year.
Air sampling (indoor and outdoor) and evaluation results
The evaluation of the room air samples according to fungus species and yeasts as well as bacteria was carried out on a total of six air samples (living room, children’s room, guest room, working room and outdoor air before and after the indoor air measurement). The detailed results about colony-forming units (CFU/m3 air) yeasts and fungi on malt extract agar (MEA) or DG18 nutrient medium as well as the results in relation to bacteria can be found in Kirchmair (2016, pp. 2–3). In summary, Kirchmair stated: “The germ densities of mould fungi were low in all interior samples and significantly lower than in the outdoor air samples. The germ spectrum was comparable.”
Air sampling at supply air outlet and isokinetic sampling in the supply air duct
Air sampling was performed at the air outlets in the living room as well as in the children’s rooms, in the working room and in the attic. The results of the isokinetic air sampling using malt extract agar (MEA) as well as DG18 nutrient medium were summarised as follows: “The bacterial germ density was classified as low” (Kirchmair 2016, p. 3).
Swab samples at the surfaces of supply air ducts
Swab samples were taken from the surfaces in front of the heat exchanger as well as at the surface of the supply air ducts of the children’s room and the attic. The result was summarised by Kirchmair as “The plates showed only little mould growth. There is no indication of mould.” (Kirchmair 2016, p. 5).
Filter material samples
The same holds for the evaluation of the filter probes, which were diluted with saline and applied to the nutrient media: “The filter was unloaded. The dust sample had a common germ density and seed composition. There is no indication of increased mould load.” (Kirchmair 2016, p. 6).
Summary of hygiene inspection and recommendations
The results of the hygiene examination of the ventilation system and the indoor air in the Passive House Darmstadt Kranichstein building after an operating period of 25 years show no evidence of increased germ density or mould contamination. Although the system has never been cleaned during the entire operating period, both the optical findings (visual inspection and camera inspection) in the supply air duct network and the microbiological laboratory findings have a perfect hygiene status. This is due to the high efficiency of the front-end fine filter, which was changed regularly. The filter change cycle depends on the outdoor air quality; in this case, it was regularly changed once a year. In case of discontinuous operation (system out of operation in summer period, which was not the case here), the filter should be changed before restart.
The use of F8 or F9 (ePM1 80%) filters in the outdoor air stream can therefore be fully recommended for future plant design; an at least annual filter change is advisable. In general, changing the filter is cheaper than cleaning the duct system. Additionally, the correct filter seat (tightness in the filter box) should be checked to avoid air bypasses.
From today’s point of view, the use of jet nozzles and thus shorter ducts is recommended, thus cleaning and maintenance is easier. The ability to dismantle the heat exchanger is useful for hygiene reasons (possibility for cleaning) and already standard in today’s devices.
From the point of view of the hygiene, the following planning and maintenance aspects must also be taken into account in all designs (it was taken care of this in the pilot project):
The location and design of the outdoor air intake and its minimum floor clearance must ensure that the least polluted outdoor air is drawn in. Short circuits with the exhaust air and other exhaust air systems (chimneys etc.) must be avoided. An intake directly above the soil is not permitted. The air ducts must be made of abrasion-resistant smooth-walled material and must be accessible for cleaning (sufficient number of inspection openings). The free drainage of the condensate tray has to be ensured; also here, the accessibility for the cleaning is important.
Power consumption and heat recovery rate
Each dwelling has a central heat exchanger with two electronically commuted direct-current (ECM) ventilators. On average, power consumption under normal operation is 29.4 W, producing a specific demand of 0.27 Wh/m3. This value is exceptionally low for ventilation systems with heat recovery. Wireless data loggers were used to measure temperatures and humidity in the central unit’s air flows; Fig. 19 shows the profiles in a typical winter season. The ventilation unit has an effective heat recovery rate of 82 (2)%. Including passive preheating, the overall system has a heat recovery rate of about 86%, so the ventilation system consumes 257 kWhel/a, compared to a greater than 2000 kWhheat/a reduction of heating energy consumption. The effective seasonal performance is therefore 7.8 kWh/kWh.
Condition of the system
The four central units (one in each dwelling) all look as good as new. The ducts do not have any deposits. In 2002, one of the systems had its two fans replaced, but the rest of the systems are unchanged—including ventilation ducts, noise dampers and supply/extract air valves. In our assessment, such a system can be used for an additional 25 years without comprehensive maintenance (aside from the annual filter change). The ECM ventilators are expected to have a service life of 15 to 20 years.
Conclusion: The ventilation systems in the pilot project are still clean and running smoothly after 25 years of constant use. There is no reason why the basic components (ducts, cases, filter boxes and counter-flow heat exchangers) could not be used for 50 years. Only the ventilators have a service life of 15 to 20 years; replacing them costs around 500€. The F8 (ePM1 80%) front filter concept has proven to be hygienic and useful: The outdoor/supply air tract does not need cleaning. We now also recommend simple G3/G4 filters at the extract air valves. Tenants themselves can easily service these filters; with these filters installed, the extract/supply air ducts only need to be cleaned once within 10 years.