Maintenance and cleaning of heat recovery units – how often and how correctly

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Maintenance and cleaning of heat recovery cores – how often and how to do it properly

Heat recovery ventilation is now standard in low-energy and passive houses, but practical experience shows one recurring issue: owners buy the system, have it installed, and then forget about it. For several years it runs without any maintenance, fans become clogged with dust deposits, the heat recovery core gets dirty, the efficiency of heat recovery drops from the original 80–90 % to less than 50 %, and yet no one knows why it is always cold in the house and energy bills keep rising. Meanwhile, the solution is simpler, cheaper, and less time-consuming than most people think.

This article focuses on the part of the heat recovery ventilation system that determines whether the whole system works properly or just appears to be running – the heat recovery cores, their cleaning, replacement, and regular maintenance. We will go through what actually happens in the heat recovery core, why it gets dirty, how to recognize when it is time to do something, and step by step we will describe the correct cleaning procedure for different types of cores, including specific values, frequencies, and practical tips I have learned over the years not only from technical data sheets, but especially from real situations during service.

What is a heat recovery core and why it is a key component of the entire system

The heat recovery core (sometimes also called a heat exchanger block or heat exchanger core) is the heart of every heat recovery unit. Its task is to transfer heat from the exhaust (internal, used) air to the fresh air from the outside – without the two air streams physically mixing. The principle is simple: two air streams pass next to each other through a thin-walled membrane or metal lamellas, while heat transfers from the warmer stream to the cooler one.

Depending on the design, we distinguish several basic types of cores:

Counterflow plate cores – air flows in opposite directions through thin plastic or aluminum plates. Highest thermal efficiency, typically 75–92 %. The most common type in central units.
Crossflow cores – air streams cross at a right angle. Lower efficiency (60–75 %), but compact dimensions and simple construction. Typical for smaller decentralized units.
Enthalpic (hygroscopic) cores – a special membrane transfers not only heat, but also moisture. The advantage is the preservation of indoor humidity, the disadvantage is more demanding maintenance and higher price.
Rotary heat exchangers – a ceramic or metal rotor rotates and alternately absorbs and transfers heat. High efficiency even at low temperatures, but moving parts require mechanical maintenance.

For decentralized units, such as the Decentralized ventilation unit HRC E – electronic version – 05 Master; pr. 160mm, ceramic or foam storage cores are typically used, which work on the principle of alternating charging and discharging of heat – one cycle air flows out, the next in, and the core remembers the heat from the outgoing air. This type has specific cleaning requirements, which we will get to.

Why heat recovery cores get dirty – the mechanism of dirt accumulation

Air in every house contains a lot of microscopic particles – dust, pollen, fibers from textiles, kitchen vapors, microorganisms, tobacco smoke (if smoked in the house), soot deposits from candles. All these substances pass through filters, but no filter is 100 % efficient – especially when the filter itself is dirty and not replaced. What the filter does not catch goes directly into the heat recovery core.

On the cold surface of the core (where the cold outside air comes in), moisture can condense, which acts as glue for dust and oily particles. In kitchen areas the situation is even more pronounced – oily aerosols from cooking gradually form a layer that is difficult to remove and at the same time is an ideal nutrient medium for bacteria and mold.

The result of a dirty core has several manifestations:

Increased aerodynamic resistance – fans have to work with higher power at the same air flow, which increases energy consumption and noise
Drop in thermal efficiency – a thin layer of dirt acts as an insulator and reduces heat transfer
Frosting risk – a dirty core retains moisture, which at low outside temperatures increases the probability of condensation freezing
Hygienic risks – biological contamination (mold, bacteria) in a dirty core can be distributed into the interior
Shortened lifespan – long-term dirt accumulation accelerates the degradation of the core material

From practice I can say that the worst cases I have seen were in households where heat recovery was not cleaned for 4–5 years. The cores were literally sealed with a layer of dust, which in some places had turned into a hard crust. In one case the air flow through the unit was reduced to less than 30 % of the designed value – and yet the fans were running at full power.

Filters vs. core – understanding the difference is key

A common mistake many owners of heat recovery systems make is confusing filters with heat recovery cores. These are two different components with different functions, different replacement frequencies, and different cleaning procedures.

Filters are the first line of defense – they catch coarse dirt before it reaches the core. They are replaced much more frequently (every 2–4 months) and are relatively cheap. Their regular replacement is the best protection for the core itself.

Heat recovery core is the heat exchange core, which is cleaned less frequently, but the process is more demanding and requires more attention. The core is usually a more expensive component and its damage during cleaning can mean a significant investment in replacement.

How often to clean heat recovery cores – specific recommendations

There is no one-size-fits-all answer. The frequency depends on several factors, but we can establish basic rules:

Standard household without special conditions

For a typical family home or apartment, where filters are regularly replaced (every 3 months), no pets are kept indoors, candles are not burned, and excess moisture from the bathroom or kitchen without extraction is not accumulated, I recommend an annual inspection of the core and a thorough cleaning every 1.5–2 years.

Household with pets

Hair and microscopic skin particles from pets (epithelium) clog filters and cores much faster. In such households, I replace filters every 6–8 weeks and recommend cleaning the core once a year, or every 9 months if there are more pets.

Household with allergy sufferers or higher hygiene requirements

Here, a simple logic applies: the more sensitive the occupants, the stricter the regime. In such cases, I recommend cleaning the core every 6–12 months and checking the seals and internal surfaces of the unit at every opportunity.

Office and commercial spaces

Higher foot traffic, photocopiers, air conditioning, carpets – all of this accelerates clogging. In commercial spaces, core maintenance should be part of the service contract, at least every 6 months.

Older house with insufficient building envelope sealing

Leakage and thermal bridges cause more dirt to enter the system from surrounding structures. At the same time, there is a higher risk of condensation in the core. I recommend a visual inspection every 3 months and cleaning according to the actual condition, usually once a year.

Practical tip from the field: never rely solely on the calendar. Learn to regularly (every 3 months) visually inspect the condition of the core – it's enough to remove it, connect a flashlight, and check how much dust is in the channels. This way, even a layperson can tell whether it's time for cleaning.

How to recognize a clogged core – symptoms from practice

Not everyone waits for a scheduled inspection. These symptoms indicate that it's time to remove and inspect the core:

Reduced airflow – rooms are less ventilated, the air feels "heavy" despite the system running. Check the airflow through the outlet grilles, if you have a measuring device.
Increased noise from fans – fans have to work at higher performance to overcome the resistance of a clogged core. If the system seems to "hum" louder than it did months ago, it's time for an inspection.
Higher electricity consumption – a clogged core increases the power consumption of fans. With precise measurement, this can be up to 20–40% more than the nominal state.
Condensate or moisture at the outlet – a clogged core disrupts the condensate balance and can cause its leakage into the duct.
Odor when starting the system – biological contamination in the core manifests as a characteristic musty smell, especially when starting after a long period of inactivity.
Error message on the control unit – modern systems (including the electronic version HRC E) can indicate excessive resistance or reduced airflow directly on the display.

If you notice a combination of several of these symptoms at once, it is very likely that the problem is not just the core – read our article Common faults of heat recovery units and how to eliminate them, where you will find a systematic diagnostic procedure.

Core cleaning procedure – step by step

Important: before any manipulation with the unit, always turn off the entire system and disconnect it from the power supply. Never try to clean the core while the unit is running.

What you will need

Soft cloth or brush (natural fibers, not metal)
Hand vacuum cleaner with a soft nozzle (without hard plastic edges)
Bucket with warm water and a few drops of mild soap (pH 7, without aggressive chemicals)
Soft cloth or sponge
Dry air sprayer or compressor with reduced pressure (max. 2–3 bar for delicate cores)
Gloves and mask (dust)
Flashlight for checking the condition after cleaning

Step 1: Turning off and accessing the core

Turn off the heat recovery unit on the control panel and disconnect it from the power supply – it's not enough to just turn off the switch, unplug the plug or turn off the circuit breaker for the given circuit. Open the unit cover according to the manufacturer's instructions. In central units, it is usually a removable panel, while in decentralized units (such as HRC E), access to the core is provided directly from the front side.

Step 2: Removing the core

Cores are usually mounted on slides or held by clips. Proceed carefully – do not pull the core with brute force if it resists, check whether it is secured by a safety lock. Before removing, always mark the orientation (e.g., with a tape or marker) so that you can return the core in the correct position – incorrect orientation would disrupt the airflow direction. In some systems, the core is part of a modular distribution system – similar to systems compatible with the modular distribution box connector for IVAR.PROFI-AIR CLASSIC pr. 90mm; in such cases, it is necessary to first disconnect the corresponding ducts before accessing the core.

Step 3: Dry mechanical phase

Always start with dry cleaning – do not put a dusty core into water, as dust deposits press even more into the channels when wet and are harder to remove. Gently wipe the outer surfaces of the core, then carefully blow out dust deposits from the channels with a compressor at low pressure (for plastic cores max. 2 bar, for ceramic cores max. 1 bar). Always direct the air flow in the opposite direction to the standard airflow – this way you will push the deposits back out through the inlet side.

Step 4: Wet phase – washing the core

Plastic and aluminum cores are usually allowed by most manufacturers to be washed in warm water with mild soap. Never use aggressive degreasers, acids, lyes or chlorinated preparations – these can damage thin-walled lamellas, degrade seals or damage the enthalpy membrane (if it is a hygroscopic type). Submerge the core in water and gently wipe the surfaces with a soft sponge or cloth. Do not scrub forcefully – dust channels are cleaned more by repeated rinsing than by mechanical scrubbing.

Enthalpy (hygroscopic) cores are a special category: their membrane must not come into contact with chemicals and even wet cleaning is controversial – some manufacturers explicitly prohibit it. For these types, the rule is: soft cloth and compressor, and in case of heavy contamination, controlled replacement of the core.

Ceramic storage cores in decentralized units (e.g., in HRC E) are cleaned differently – a more detailed procedure can be found in the following section.

Step 5: Drying

After washing, rinse the core thoroughly with clean water to remove any soap residue and let it air dry at room temperature. Never insert a wet or damp core back into the unit – condensation and residual moisture create ideal conditions for mold growth. Drying typically takes 12–24 hours depending on the type of core and ambient humidity.

Step 6: Inspection and reinstallation

Before returning the core, visually inspect the lamellas with a light – if you see deformations, cracked lamellas or biological contamination (black spots = mold, yellow deposits = grease), consider replacing the core instead of putting it back. At the same time, check the condition of the seals on the core frame – damaged or hardened seals cause bypass (air leakage around the core) and reduce the efficiency of heat recovery. Suitable replacement seals, such as tongue-and-groove circular seal pr. 63mm; classic, are available separately and their replacement at every core cleaning is a good practice.

Special case: cleaning ceramic accumulation inserts in decentralized units

Decentralized heat recovery units, such as HRC E – electronic version – 05 Master, operate on a different principle than central systems. Instead of continuous airflow in two separate ducts, alternating cycles occur here: for 70 seconds air flows out (the insert is charged with heat), then the fan reverses and for 70 seconds fresh air flows in (the insert transfers the stored heat). This principle is called thermal regeneration.

The ceramic foam from which these inserts are made is very porous and fragile. This brings specific cleaning rules:

Never use a high-pressure compressor – pressure above 1 bar can fragment the ceramic structure
Gentle airflow from a hand pump or a compressor set to minimum is sufficient
Wet cleaning only carefully – ceramic absorbs water, so longer drying is necessary (minimum 48 hours at room temperature, never with a hot air gun)
Do not soak the insert completely – a gentle rinse with clean warm water without detergent on the surface is sufficient for normal dirt
Inspection every 6 months – recommended frequency of visual inspection for regular use

For these units, there is also a practical recommendation: if you live in an area with increased pollen levels, it is advisable to switch the system to summer bypass mode, which under suitable conditions allows air to flow without passing through the insert – the ceramic is less loaded and the cleaning interval is extended.

Cleaning surrounding components – what should not be forgotten

The insert itself is only one link in the chain. With each maintenance, I recommend checking the following parts as well:

External grilles

The external grille protects the air intake and exhaust from insects, birds and coarse dirt. A dirty grille increases resistance and reduces airflow. Clean it with a soft brush or wipe with a damp cloth – ideally every spring (after the winter period, when leaves and needles can accumulate). An aesthetic external grille for systems with a diameter of 160 mm, such as external aesthetic grille for IVAR.HRC pr. 160mm, can be disassembled and cleaned under running water.

Drip tray and condensate drain

Below the heat recovery insert is a tray for condensate drainage. This can become clogged with biological growth or sediment. Clean it with a damp cloth and rinse with clean water. Check whether the condensate drain flows freely.

Fans and blades

A layer of dust settles on the fan blades, which disrupts aerodynamics and increases vibration. Carefully clean the blades with a soft brush or compressed air. Never use wet cleaning directly on the motor.

Internal ductwork

Ducts are cleaned less frequently (every 5–7 years with a properly functioning filtration system), but in critical points – bends, joints and transitions – dust can accumulate. During reconstruction or when the system is heavily soiled, it is advisable to have the ducts cleaned by professional ventilation technicians with a rotating cleaning device.

Replacement of the heat recovery core – when cleaning is not enough

Every heat recovery core has a limited lifespan. With proper maintenance, aluminum and plastic plate cores in central units last 10–15 years. Ceramic storage cores in decentralized units typically have a lifespan of 8–12 years, depending on the intensity of operation and the quality of the surrounding air.

Replacement is necessary when:

Lamellae or channels are visibly damaged (deformations, cracks, perforations)
Biological contamination (mold) cannot be removed even by deep cleaning
The enthalpy membrane is perforated or has lost its hygroscopic properties (this is easily recognized: humidity from the exhaust air side penetrates into the fresh air stream)
The core still shows excessive pressure resistance after cleaning (measurable with an anemometer)
The thermal efficiency of the system does not return to the designed level even after complete maintenance

Always ensure the replacement core is correctly dimensioned when replacing it. Replacing it with a core of different resistance or cross-section can disrupt the hydraulic balance of the entire system. If you are unsure, consult a specialist or refer to the parameters in your ventilation system's project documentation – more on this is also covered in the article How to properly design a ventilation system with heat recovery.

Tips for extending the lifespan of a heat recovery core

Preventive measures are always cheaper than subsequent repairs. Here are a few practical tips I have recommended to customers in the field:

Do not delay filter replacement – a dirty filter allows significantly more dirt to enter the core. Set a reminder every 3 months and replace the filter on time.
Ventilate the kitchen with local extraction – the heat recovery system is not a substitute for a range hood. Grease aerosols from cooking are dangerous for cores.
Use the bypass mode in summer – if your system allows it, switch to bypassing the core when outside temperatures exceed 20–22 °C. The core will accumulate less dirt.
Turn off or reduce the system performance during long absences – dust accumulation in an inactive house is less if the air is not circulating.
Check the outside air inlet – make sure the external grille is not overgrown with plants or covered by a tarp.
If you are renovating your house, turn off the system – construction dust (gypsum, plaster, wood fibers) can clog the core extremely quickly. Do not run the system during renovation or cover it to protect from dust.

I also recommend checking the airflow booster once a year if you have one in your system – for example, airflow booster, 75mm or 90mm; classic can get clogged just like other components and affect the overall air distribution in the house.

Frequently asked questions (FAQ)

Can I wash the heat recovery core in a dishwasher?

No, not in a regular dishwasher. The water temperature in a dishwasher can reach 60–75 °C, which can deform plastic cores. In addition, dishwasher tablets contain aggressive alkaline and enzymatic components that attack the surface coatings of aluminum cores and definitely damage enthalpy membranes. The only acceptable method of wet cleaning is hand washing in warm water with a mild neutral detergent.

What happens if I put the core back before it is completely dry?

There are several risks: residual moisture in the core provides ideal conditions for mold and bacteria growth, which are then distributed into the interior with the fresh air. At low outside temperatures, a wet core can freeze, causing mechanical damage to the lamellae due to ice expansion. Minimum drying time is 12 hours at room temperature, 24–48 hours for ceramic cores.

I have a ceramic core in a decentralized unit – how do I know it is dirty?

The most reliable method is a visual inspection with a light source – when looking through the core, you should see light passing through the channels. If the channels are dusty or you see a gray or brownish layer, it is time for cleaning. Another indicator is measuring the airflow at the outlet – a drop in airflow by more than 20 % compared to the nominal value indicates clogging.

Do I need a professional company to clean the heat recovery core, or can I do it myself?

Cleaning the core itself can be done by any handy homeowner with basic tools – as long as it is a simple counterflow or crossflow core in a central unit. Cleaning ceramic cores in decentralized units with heavy biological contamination and a complete inspection of internal ductwork is more complex. For a full system inspection, including airflow measurements, thermal efficiency check, and balancing of the hydraulic system, I recommend professional service at least once every 3–4 years.

How long does the entire cleaning process take, from removal to reinstallation?

The mechanical preparation (removal, brushing, blowing) takes 15–30 minutes. Wet cleaning adds another 20–30 minutes. The longest phase is drying – 12 to 48 hours. The entire cycle thus takes 1–2 days, with active work taking less than an hour. During drying, the system is out of operation, which is not a problem in summer, but in winter it is advisable to plan the cleaning for a warm day.

Does a dirty core reduce room temperatures in winter?

Yes, and in two ways. First, reduced thermal efficiency of heat recovery means that the incoming air is colder – instead of recovered 18–20 °C, it may only have 12–14 °C with significant clogging, which significantly worsens thermal comfort. Second, reduced airflow (a consequence of higher resistance from the clogged core) worsens ventilation and heat distribution. Combined, this can lead to the boiler or heat pump trying to compensate for heat losses and consuming significantly more energy. This is confirmed by a customer service case I have encountered, where the customer reported increased heating costs – after cleaning the clogged core (3 years without maintenance), the costs decreased by about 15 %.

Conclusion – a small investment, a big return

Heat recovery ventilation is a system that deserves attention and regular care due to its effectiveness. As I have mentioned several times from my own customer experience – most of the problems reported by customers originate not in the malfunction of the components themselves, but in neglected maintenance. At the same time, cleaning the heat recovery core once a year represents an investment of less than one hour of your time, and the cost of the necessary tools is minimal.

Proper maintenance will extend the life of the core by several years, keep the thermal performance of the system at the designed level, reduce the energy consumption of fans, and ensure a hygienically clean indoor environment. If you have not yet installed a system and are considering a centralized versus a decentralized solution, I recommend reading the article Centralized vs. Decentralized Heat Recovery Units – Which is Better, where the differences are discussed in more detail, including maintenance requirements. And if you are currently in the selection and installation phase, also check out Step-by-step Installation of a Decentralized Ventilation Unit HRC – proper installation is just as important for the long-term life of the entire system as regular care.

Thorough and regular maintenance of heat recovery cores is not a luxury or unnecessary complication – it is a basic condition for the investment in heat recovery ventilation to actually work as it was designed.

Do you have a question about this topic?

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