Cleaning and maintenance of HVAC systems

HVAC systems have come under greater scrutiny as a result of the coronavirus pandemic. The Gesa Hygiene Group advocates a holistic approach to HVAC systems – encompassing qualified hygienic inspection, assessment, and treatment, as well as the airtightness of air distribution systems.

"It may be difficult to take anything positive away from the SARS-CoV-2 pandemic. What we can view positively, however, is the fact that HVAC systems are now finally receiving the attention they should have been given over the past decades," explains Jörg Schönfelder, owner and managing director of the Gesa Hygiene Group. The topic of indoor air hygiene and the associated hygiene requirements for HVAC systems is more complex than many assume. The expert adds: "Various regulatory frameworks, specific requirements for cleaning and maintenance, and lesser-known parameters such as leakage rates all need to be taken into account when hygienically evaluating an HVAC system." The first step, he says, is to determine which technical regulations apply to the individual case, as several different standards and guidelines may be relevant. In his view, every HVAC system requires an individualized and well-thought-out hygiene concept "that comprehensively covers all structural, hygienic, and operational aspects."

Most Important Guideline: VDI 6022

The VDI 6022-2018:01 guideline applies to all rooms in which the same person spends either more than 30 days per year or regularly more than two hours per day. According to Schönfelder, who is himself a qualified VDI 6022 Category A expert, this guideline can rightly be considered one of the most important for everyday use. "Unfortunately – despite the removal of some ambiguities – a wide range of interpretations has accumulated on the market," he notes. To ensure hygienic operation, VDI 6022 defines various inspection intervals depending on how the system is operated: systems without humidifiers must be inspected every three years, systems with humidifiers every two years. If special circumstances require it – for example, heavily polluted outdoor air – any Category A expert may shorten this inspection cycle. The inspection itself consists of a visual check of the system, a microbiological examination, a structural assessment, and comprehensive documentation of the inspection including laboratory results.

Sampling with "Dip-Slides"

Microbiological examinations serve to determine total colony counts, legionella, pseudomonads, molds, and yeasts. The aim is to establish the extent to which the HVAC system is microbiologically contaminated and the quality of the incoming outdoor air is affected. The expert explains: "Both the air flowing through the system and the reference air (e.g. outdoor air), the circulating water from humidifier systems, and the recooling water are examined." Sampling must be carried out using "dip-slides" (contact plates) for taking surface samples. Schönfelder adds: "Whereas it was previously standard practice to take samples exclusively using casein soy peptone agar (CASO) plates, VDI 6022 now explicitly requires that surface germ and airborne germ tests be carried out using selective culture media for the evaluation of total germ counts and molds."

Further details to be observed: an initial hygiene inspection must be carried out before commissioning, following structural modifications, or after prolonged shutdown of the system. "This inspection is more extensive in its scope, as it focuses in particular on the structural design of the system and its influence on air quality." Based on this inspection, appropriate measures must be initiated and implemented where necessary. An initial hygiene inspection in accordance with VDI 6022 is also required for small HVAC systems – i.e. systems whose sole function is filtering and/or heating air. This means that air purifiers also fall under this guideline. Schönfelder: "Pollutant measurements in accordance with VDI 6022 may only be carried out by an RLQ engineer."

ISO 14644 for Cleanrooms

Another standard may also be relevant for HVAC systems: ISO 14644 governs the requirements for cleanrooms and describes the necessary measurement procedures for acceptance qualification as well as for the requalification of a cleanroom. According to Schönfelder, it is recognized worldwide as the current state of the art and replaces withdrawn standards such as US Federal Standard 209E. "Unfortunately, it must be noted that many providers on the market still qualify cleanrooms on the basis of US Federal Standard 209E, even though it was withdrawn in 2001 in favor of DIN 14644," he says. In addition to defining maximum values for the various ISO classes with regard to particulate contamination, DIN 14644 also describes the measurement procedures for verifying filter integrity (checking HEPA filters for leaks) and the number of measuring points required to determine particulate contamination.

While the guidelines and standards set out clear qualification requirements for those involved in hygiene inspections, equivalent requirements are lacking for the personnel who carry out the actual treatment of the system. A well-thought-out hygiene plan is also essential. "The importance of a hygienically functioning HVAC system has shown that a 'one size fits all' cleaning and hygiene plan is unlikely to deliver the desired results," says the Gesa managing director. Only specialists, he argues, are able to evaluate all relevant factors – such as environmental conditions, supplied rooms, duct networks, and the structural characteristics of the HVAC system – from both a technical and hygienic perspective. The creation of an individualized hygiene plan is also required by VDI 6022.

Recognized Method: Scrub-and-Wipe Disinfection

"It should generally be noted that disinfection without prior cleaning is ineffective or only partially effective," explains the Gesa owner. Every disinfectant, he says, has a so-called protein error – a term that describes the degree to which its effectiveness is reduced by proteins. This error occurs when the disinfectant binds to proteins before it can reach the actual microorganisms. Schönfelder also emphasizes that the scrub-and-wipe method is the only disinfection procedure recognized by the Robert Koch Institute. Spray disinfection is only permitted in exceptional cases – for example, where access for conventional disinfection is not possible. Hard-to-reach components such as ventilation ducts must at least be pre-cleaned to ensure effective disinfection along the entire airflow path.

Airtight Ducts for Hygienic Operation

In addition, airtight air ducts are, in the expert's view, an important factor in the hygienic operation of an HVAC system. If the airtightness class of the air distribution system is unknown – meaning no airtightness test was carried out at commissioning – leakage rates of up to 15 percent must be expected. With airtightness class ATC 5 (formerly Class A, the lowest class), the figure is still 6 percent, while the second-highest class, ATC 3 (formerly Class C), reduces leakage to 0.67 percent.

Schönfelder: "In addition to a reduced airflow rate and the associated reduction in air exchange, leaky air ducts also carry the risk of air escaping at points where it is not wanted." This is particularly problematic when ventilation ducts run through suspended ceilings, which – especially in inner-city areas – are often contaminated with mouse and rat droppings. Dust particles can then carry viruses uncontrolled and unfiltered into rooms. A similar problem arises on the exhaust air side: "Leaky ducts can lose contaminated air along the way before it reaches the filter units of the exhaust air unit."

Source: Jörg Schönfelder: "Why indoor air hygiene and technology must be viewed holistically."