Common faults of heat recovery units and how to eliminate them

Heat recovery ventilation units are reliable devices, but like any technology, they occasionally require attention. From practice, I know that most faults are not dramatic – they are predictable situations that usually arise from the same causes: neglected maintenance, improper installation, or simply the wear of one of the components after years of operation. In this article, I systematically go through the most common faults that appear on both centralized and decentralized units, and for each of them I provide a specific procedure on how to detect and eliminate them – without unnecessary delays.

If you are new to heat recovery, I recommend reading the topic Centralized vs. decentralized heat recovery units – what is better before reading this text, where the basic differences between the two types of systems are explained. Some faults are specific to one type, others occur in both.

Why do faults occur in heat recovery units at all?

A heat recovery unit works continuously – most systems run 24 hours a day, 365 days a year. The air passing through the heat exchanger brings with it dust, moisture, greasy aerosols from the kitchen, pollen, mold spores and a whole range of micro particles. These deposits gradually accumulate on the filters, in the heat exchanger, in the ducts and on the fan blades. To this must be added mechanical wear of bearings, degradation of seals and – in winter – the risk of condensate freezing.

According to my experience, there is a simple relationship: the better the device is regularly serviced, the fewer faults occur. Units that receive regular care every 3–6 months have significantly fewer serious problems than those that someone remembers once every three years.

Fault No. 1 – Reduced air flow or weak exchange

Symptoms

The most common complaint I encounter is the feeling that the heat recovery "isn't working as before". People describe stale air in the bedroom in the morning, condensation on windows, odors in the bathroom that don't go away. All these phenomena may indicate a significant reduction in the actual air flow compared to the designed value.

Causes and solutions

The first step is always to check the filters. Clogged filters are responsible for at least 70 % of cases of reduced flow. On G4 filters (coarse filter), larger particles – dust, pet hair, textile fibers – are trapped. These filters should be cleaned every 1–3 months and replaced every 6–12 months depending on the load. F7 filters (fine filter) trap pollen and fine particles and have a shorter lifespan.

If the filters are clean and the flow is still low, the heat exchanger should be checked. A clogged heat exchanger significantly increases the system's pressure resistance. On aluminum foil heat exchangers, fine deposits can be removed with a soft brush or light blowing with compressed air (max. 2 bar). Plastic heat exchangers can be carefully rinsed with warm water – always according to the manufacturer's instructions.

The third possible cause is a mechanical obstacle in the duct. A piece of insulation that has broken off, a tool left behind during installation, a bird's nest in the external outlet – I have seen all of this in practice. For systems with distribution via modular ducts, it is important to check whether all connectors and elbows are properly inserted and airtight. For example, the connector of the modular distribution box for IVAR.PROFI-AIR CLASSIC (e.g. 90 mm) must be properly inserted and latched – if it becomes loose, air escapes directly into the floor or ceiling cavity and only a fraction of the planned amount reaches the rooms.

The fourth cause may be a problem with the fan – dirty blades or beginning bearing wear reduce the speed and thus the flow. I write more about this issue further below.

Fault No. 2 – Noise and vibrations

Symptoms and their interpretation

Noise is one of the most common reasons why people turn off the heat recovery – and then wonder why the house has problems with humidity. It is important to distinguish what type of noise it is, because each has a different cause and solution.

• Low-frequency humming (hum...) – typically vibrations of the motor or fan. Cause: unbalanced blades (dust deposits), loose installation of the unit, worn bearings.
• Squeaky or whistling sound – most often a rubber seal that needs to be replaced, or air leaking through a narrow place due to a clogged filter (underpressure causes "hissing").
• Knocking or clicking – a loose object in the duct or in the fan, or cracked fan blades.
• Noise from the duct (thundering) – air flowing too fast through a duct with a small cross-section. The designed flow speed should not exceed 3–4 m/s in the main ducts and 2–2.5 m/s at the outlets.

The topic of noise in heat recovery units is covered in a separate article Noise of heat recovery units – causes and solutions, where you will find a more detailed analysis and acoustic parameters of common units.

Practical example from practice

Recently I dealt with the case of a family house, where after two years of operation, a pronounced humming from the unit installed in the technical room began to appear. The sound was clearly audible even through the wall in the adjacent bedroom. After removing the cover, we found that the blades of both fans were covered with a layer of dust and grease about 3 mm thick – unevenly, which caused dynamic imbalance. After thorough cleaning with a solvent and restarting, the noise was practically imperceptible. The whole operation took about 40 minutes.

Another case: a new installation of a decentralized unit in an apartment, where the unit was mounted directly into the wall. The noise was transferred directly into the wall and was enormous. The solution was vibration dampers (rubber pads) between the unit body and the wall – a standard part of proper installation that the installer omitted. More about the correct procedure can be found in the article Installation of a decentralized ventilation unit HRC step by step.

Failure No. 3 – Frozen heat exchanger in winter

This is a seasonal failure that occurs during prolonged frosty weather – typically when the outside temperature drops below –5 °C and the operation is continuous without properly set anti-frost protection.

How heat exchanger freezing works

When warm, moist indoor air passes through the heat exchanger, it transfers heat to the cold outside air. At low outside temperatures, the temperature on the exhaust air side drops below 0 °C and the condensate begins to freeze. Ice accumulates on the heat exchanger fins and gradually clogs them. Result: air flow drops dramatically, recovery performance drops to a minimum, and in the worst case, freezing can damage the heat exchanger (membrane heat exchangers with a membrane risk rupture).

Anti-frost protection – types and settings

Well-designed systems have anti-frost protection. There are several methods:

• Bypass and defrosting – when the temperature drops, the intake of outside air is reduced or interrupted, the exhaust fan continues to run and melts the ice with residual heat.
• Preheating of incoming air – electric or water preheater (preheats air before it enters the heat exchanger to a minimum of +3 °C).
• Ground heat exchanger – air passes through underground piping before the unit, where it is preheated in winter. The topic of solution selection is covered in the article How to choose a mechanical ventilation system with heat recovery for a family home.
• Reducing airflow – in extreme frost, some units automatically reduce the speed of the intake fan, thus reducing the amount of very cold air entering the heat exchanger.

If the unit has actually frozen, the procedure is simple: turn off the intake fan (or the entire unit), let the exhaust fan run at low speed – it will melt the ice from the inside. Never turn on full power in this condition! A sudden change from extreme cold to full airflow can cause thermal shock to the heat exchanger and cracks in the membrane. Defrosting can take 30–90 minutes, depending on the amount of ice.

Failure No. 4 – Condensate does not drain or drains to the wrong place

Condensate is formed during heat recovery on the cold side of the heat exchanger. Most devices have a drain outlet and a trap through which the condensate drains into the sewer. Problems arise in several situations:

• Clogged trap – the most common problem. The trap is clogged with biological deposits, algae, and dust. Result: water accumulates in the trap, overflows into the unit and can damage the heat exchanger, insulation, even the electronics. The trap must be cleaned at least once a year – remove, rinse, disinfect.
• Incorrect slope of the trap or drain pipe – the installer laid the drain pipe horizontally or even with an upward slope. Water accumulates. The drain pipe must have a minimum slope of 2 % (2 cm per meter).
• Frozen drain in winter – if the drain pipe runs through an unheated space or exits outside without thermal insulation, it will freeze in frost. Solution: thermal insulation of the drain, or possibly a resistance heating cable.
• Condensate on the pipe – if the cold air supply pipe is in a warm space without insulation, it will condense. Solution: proper insulation of the supply pipe.

Failure No. 5 – Seals and air leaks

Air leaks in the duct system are insidious – they are not visible, but significantly reduce the efficiency of the entire system. Yet their detection and repair are usually simple, if you know where to look.

Where to look for leaks

Critical points are always at the joints – whether between individual pipe sections, at the connection to outlets, at the inlet/outlet to the unit, or at the door of inspection hatches. On flexible ducting, leaks appear due to improper installation of clamps or rodent damage (yes, this can happen in an attic).

Detection: the simplest method is to move your hand along the suspicious joint while the unit is running – you will feel the air leak. For greater precision, a smoke stick or pressure test is used. If you want to be really precise, a Blower Door test of the entire ventilation system can be performed.

Replacement of gaskets in modular systems

In modular ducting systems (such as the IVAR.PROFI-AIR CLASSIC system), rubber gaskets are part of the joints. These gaskets have a long service life when used properly, but they can crack due to mechanical stress (repeated disassembly, extreme temperatures in the attic) or simply dry out and harden after years. Replacement is simple and inexpensive – tongue-and-groove circular gasket (e.g. 63 mm) for the Classic system is exactly this type of spare part that I recommend keeping at home as a reserve for every modular ducting system. Replacing the gasket takes literally minutes: disconnect the joint, remove the old gasket, install the new one, and reconnect – done.

Problem No. 6 – Fan issues

Bearing wear

Fan bearings are designed for 30,000–50,000 hours of operation, which at continuous operation corresponds to 3.5 to 6 years. In practice, this means that after 4–8 years (depending on the quality of the equipment and conditions), characteristic sounds begin to appear: first a faint humming, later a squeaking. If the bearings are ignored, motor seizure is a risk.

Replacing bearings is technically possible, but it requires fan disassembly and professional tools. In the case of simpler units, it is sometimes more economical to replace the entire fan or motor-reducer. In the case of decentralized units, such as Decentralized ventilation unit HRC E – electronic version 05 Master (e.g. 160 mm), fans are accessible after removing the front cover and their replacement is designed so that an experienced DIY enthusiast can handle it.

Dirt on fan blades

Dust and grease settle on the blades and the rotor becomes dynamically unbalanced. Consequences: increased noise, higher energy consumption, mechanical stress on the bearings. Cleaning the fan is part of regular maintenance, and I recommend doing it at least once a year. Clean the blades with a soft cloth or damp cloth (without aggressive chemicals that could damage plastic or aluminum). Never use a high-pressure cleaner directly on the fan – you can damage the bearings or motor.

Fan failure – electronics

Modern fans are controlled by EC motors (electronically commutated – brushless), which are efficient, but their control electronics can be sensitive to voltage surges. If the unit stops responding or the fan does not start, the first step is to reset: turn off the power for 30–60 seconds, then turn it back on. If that doesn't help, check the fuses (many units have an accessible fuse strip). In electronically controlled units, the control panel displays an error code – always note the code before calling service, it will save you time and money.

Problem No. 7 – Issues with the external grid and intake opening

The external grid is the first line of defense – it captures insects, leaves, bird feathers, and coarse dirt. At the same time, it is the place that is most often neglected, because it is not in a visible location (on the façade) or is hard to access (at the roof opening).

A clogged external grid dramatically increases the pressure resistance of the system. The symptoms are the same as with clogged filters: low flow, increased noise, higher energy consumption. Cleaning the grid should be part of the seasonal inspection – at least in spring (after winter – leaves, ice) and in autumn (after summer – insects, spider webs).

In decentralized systems mounted in the wall, the external grid also has an aesthetic function – external aesthetic grid for IVAR.HRC (e.g. 160 mm) is a type of grid used in systems built directly into the perimeter wall. When replacing or cleaning it, it is enough to unscrew the grid, clean it under running water, and return it to its place – the whole operation takes 5 minutes.

Important: never block the external grid with decorations, curtains, stacked materials, or covers. I have seen cases where owners themselves clogged the opening with garden fences or wood stacked against the wall – and then wondered why the heat recovery is not working.

Problem No. 8 – Signal drop and losses in long duct runs

In long duct runs, pressure losses occur, which can be so significant that air does not flow sufficiently into distant rooms. The designed flow for a living room on the ground floor may be fine, but a bedroom in the attic at the end of the duct receives only a fraction of the designed amount.

Solution: air flow booster

For these cases, there is a solution – an inline booster, i.e., an auxiliary fan installed directly into the duct. Booster for diameters 75 mm or 90 mm (Classic system) is installed in the duct before the distant outlet and locally increases the pressure, thus compensating for losses on a long run. Installation is simple – the booster is inserted into the duct like any other segment and connected to the power supply (usually 12V or 24V DC, depending on the system). It is important to match its performance with the overall system so that the flow at other outlets does not change – ideally consult with the designer or follow the manufacturer's instructions for the system.

More about the correct design of ducting can be found in the article How to correctly design ventilation ducting with heat recovery and about pipe sizing in the text What pipe diameter do I need for heat recovery ventilation.

Failure No. 9 – Problems with electronics and control

Error codes and reset procedures

Electronically controlled units (with their own controller, or connected to KNX or BMS) display error codes in case of faults. Each manufacturer has its own code system – without the unit's manual, you won't make sense of them. The most common codes relate to: clogged filters (notifying for replacement), problems with temperature sensors, anti-frost protection, and fan failure.

The first step in any code alarm: read the manual, find out what the specific code means. The second step: remove the cause (e.g., replace the filter). The third step: confirm the alarm on the panel or restart the unit. If the alarm repeats, the problem is not resolved.

Malfunction of sensors

Modern units include multiple sensors: temperature (inlet, outlet, exterior), humidity (sometimes), CO₂ (in demand-controlled ventilation systems), and filter contamination (differential pressure). Sensors may show incorrect values after contamination or failure. CO₂ sensors are sensitive to chemicals from cleaning products – if placed near air intake from a kitchen or a cleaning supplies storage area, they may give false readings.

Power outages and backup batteries

Frequent power outages can damage the control electronics. I recommend installing an UPS (uninterruptible power supply) for systems connected to a house with an unstable grid. Units with volatile memory (without setting backup) reset to factory settings after a power outage – it's annoying, but solvable: write down or photograph the settings before installation.

Failure No. 10 – Odor from the ventilation system

An odor from the vents is always a warning sign. The causes can vary:

• Biological odor – moldy filters or heat exchanger. If the air smells musty, the filters are almost certainly wet and moldy. Humidity arises from insufficient condensate removal or if the air entering the unit was already humid (e.g., during renovation of wet areas). Solution: replace filters, disinfect the heat exchanger.
• Odor from an adjacent room or from the exterior – the outdoor intake is too close to a source of odor (chimney, waste outlet, parking lot). The topic of vent placement is covered in the article Rooftop vs. façade exhaust for heat recovery – what to choose and when.
• Chemical odor after installation – new plastic ducts or insulation may slightly smell when first started. It usually goes away after 1–2 weeks of operation. You can speed it up by intensive ventilation (maximum performance for several hours).
• Smoke or combustion odor – leakage from the boiler room or chimney through an improperly located intake. This is a safety issue, not just a comfort issue – the intake must be immediately relocated.

Preventive maintenance as the best way to prevent failures

Most of the described failures can be prevented by systematic preventive maintenance. From practice, I recommend the following schedule:

• Every month: visual inspection of filters (view through transparent cover or after opening the door). Replace or clean if visibly dirty.
• Every 3 months: cleaning or replacement of filters (under normal conditions); check the condensate tray and trap; check the exterior mesh.
• Every 6 months: cleaning of the heat exchanger; check the condition of seals at pipe connections; measure airflow at vents (at least roughly by hand).
• Once a year: full inspection of the fan (blades, bearings); check condensate drainage; disinfection of the heat exchanger and inside the unit; check electronics and sensors; read the error log from the control unit.

A detailed cleaning procedure for heat recovery cores can be found in a separate article Maintenance and cleaning of heat recovery cores – how often and how to do it correctly.

When to call a service technician and when you can do it yourself?

This is a question I get very often. The answer depends on the nature of the problem and your technical skills. In general:

You can do it yourself: filter replacement, cleaning of the exterior mesh, cleaning of the condensate trap, replacing the gasket in a modular duct system, resetting the control unit after a power outage, visual inspection of the duct system.

Consider help from a technician: cleaning of the heat exchanger (for more complex types), setting the airflow balance (system balancing after layout change), fan or bearing replacement, repair of electronics and control cables.

Always call a technician: if there is a suspicion of combustion or carbon monoxide odor, if there is a water leak into the electrical part of the unit, if there are recurring alarms you cannot identify the cause of, for any interventions in the pressure system of a ground heat exchanger.

Most frequently asked questions (FAQ)

My heat recovery stopped working overnight, nothing works in the morning. What happened?

The most likely cause is the anti-frost protection – in heavy frost, the unit may automatically switch to a defrost cycle or completely block due to freezing of the heat exchanger. Check the display for an error code. If the code is related to temperature or frost protection, turn off the supply fan, let the exhaust run for 30–60 minutes, then try to restart. If that doesn't help, check the condensate drain and make sure the exterior mesh is not iced over.

Warm air from the heat recovery is cold even in winter. Is it a fault?

Not necessarily. Heat recovery brings in outside air, which is heated by the heat exchanger. The temperature of the supplied air is usually slightly lower than the interior temperature (typically 16–19 °C at an outside temperature of about 0 °C and good heat exchanger efficiency). This temperature should not feel like a "cold draft," provided the vents are correctly oriented (not directly at people). If the air is really cold (below 14 °C) or if you feel a noticeable draft, check the efficiency of the heat exchanger – it may be dirty or there may be a bypass (bypassing the heat exchanger).

Heat recovery is noisy only at higher speeds, it's quiet at low speeds. Should I fix it?

It depends on the intensity. A slight increase in noise at higher speeds is normal – more air, higher speed, more noise. If the noise at full power is really disturbing, first check the filters (clogged filters increase air speed through a narrowed cross-section and thus noise), then the condition of the fan blades, and finally the unit mounting (anti-vibration pads, loose screws). If the noise appeared suddenly where it wasn't before, it indicates a change in condition – the cause must be identified.

We have significantly less air at some vents than at others. What to do?

This is a problem of an unbalanced system. It can occur during the original installation (poor setting of control dampers), after pipe reconstruction, or after adding a new room. Solution: measure the actual airflow at each vent (with an anemometer) and adjust the control dampers to achieve the designed airflow according to the ventilation calculation. For long runs, consider adding a booster, as described above.

After replacing the filters, my heat recovery still shows an "filter" alarm. How to reset it?

Most units have a timer or filter life indicator that must be manually reset – just replacing the filter is not enough, you also need to confirm the alarm. The procedure varies by manufacturer: for some, it's enough to press a button on the panel, for others you need to go through the menu. Find the "reset filter alarm" or "filter reset" section in the manual – it's usually holding the button for 3–5 seconds. If you don't have the manual, search for it by the unit's model number on the manufacturer's website.

Is it normal for heat recovery to consume more electricity in winter?

Yes, it is normal. In winter, when outside temperatures are low, the anti-frost protection activates more frequently (or the air preheater), which increases consumption. In addition, the greater temperature difference between the interior and exterior places higher demands on the fan. An increase in consumption by 20–40 % compared to the summer period is standard. If the increase is significantly higher (more than double the summer consumption), check the anti-frost protection settings and the condition of the heat exchanger.

Conclusion: heat recovery is reliable – when you take care of it

Heat recovery ventilation systems are technically advanced, reliable, and, when installed correctly, function for years without major problems. Most of the faults I have encountered in practice had a simple cause: neglected filter maintenance, improperly performed installation, or unresolved condensation path. It is usually enough to detect them in time, and the solution is typically simple and inexpensive.

Key message: do not underestimate preventive maintenance. A unit that receives regular care will last you 15–20 years with minimal costs. A neglected unit will trouble you every winter. If you have noticed any of the described symptoms on your system while reading this article, start with the simplest solution: check the filters. In nine out of ten cases, this is exactly the issue.

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