Water in a heat recovery unit? Where it comes from and what you should do about it

Water in a heat recovery unit – where does it come from and how should it be handled?

A mid-winter phone call:

“Sir, water is dripping from the heat recovery unit — there’s a stain on my ceiling!”

The investor’s first thought? A unit failure. A faulty seal. Scrap.

Meanwhile, as engineers, we know that in around 90% of cases the unit itself is working perfectly, and the root cause lies with the installation. One contributing factor is also that the building is not yet fully dried out. New buildings need time to dry — walls often contain large amounts of residual moisture, which later condenses inside the heat recovery unit.

Water in a heat recovery unit is not a fault — it is proof that heat recovery is working.
The real issue is what happens to that water next.

In this article, we walk installers through a quick leakage diagnostic process and show designers a solution that can reduce condensate formation to almost zero.

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Key takeaways

• The heat exchanger cools humid air below its dew point. In winter, several litres of water per day in the condensate tray is completely normal.
  
• Even a small gap in the insulation of “cold” ducts (intake/exhaust) will cause condensation outside the unit.
  
• Lack of proper slope in the ductwork guarantees water backflow.
  
• The use of an enthalpy exchanger (with moisture recovery) drastically reduces the amount of condensate, eliminates the need for a condensate drainage system, and improves winter comfort. Supply air is pre-humidified before entering the rooms.
  

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Where does water come from in a “dry” device?

Before you start dismantling the casing, it’s important to understand the mechanism involved. The phenomenon is called the dew point.

When warm, humid air from a bathroom reaches the heat exchanger fins, which are cooled by incoming outdoor air, it rapidly releases energy. Water vapour turns into liquid.

It is exactly the same process you observe on a fogged-up bathroom mirror after a shower. The difference is that in a heat recovery unit, this process takes place 24 hours a day, potentially generating buckets of water under extreme conditions.

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Why does a heat recovery unit leak?

If water is escaping beyond the condensate tray, check these three critical points:

1. Are gravity slopes maintained?
Water will not flow uphill. The unit must be levelled with a slight fall towards the drain, and the condensate discharge pipe must have a continuous downward slope.

• Common mistake: Using flexible hoses that deform over time, creating “bellies” (natural traps) where water collects and stagnates.

2. Has the drain frozen?
If the condensate pipe runs through an uninsulated loft or exits the building envelope, a frozen blockage is only a matter of time during frost.

• Solution: Use heating cables on external sections or route the drain within the building envelope, connecting it directly to the sewer system.

3. Siphon check
Negative pressure generated by the fan can hold water in the tray if the siphon is empty or incorrectly selected. Instead of draining, water backs up until it overflows.
Make sure the siphon is primed (if it is a water trap) and fully unobstructed.

“Sweating ducts” – the silent killer of ceilings

I often receive calls saying: “The heat recovery unit is leaking at the joints!” I arrive on site and find the unit completely dry inside. So where is the water coming from?

This is secondary condensation.

Intake and exhaust ducts carry ice-cold air. If they pass through warm rooms (boiler rooms, utility rooms) and are not perfectly insulated, moisture from the surrounding air condenses on their outer surface.

Use rubber (elastomeric) or mineral wool insulation of appropriate thickness.

Seal all joints with foam sealing tape — every gap is a thermal bridge and a potential source of condensation.

If a duct is wet on the outside, the problem lies with the insulation, not with the heat recovery unit.

Requirements for the installer

• Use elastomeric rubber or mineral wool insulation of adequate thickness.
  
• Seal all joints with foam sealing tape — every gap creates a thermal bridge and a potential point of condensation.
  
• If a duct is wet on the outside, the issue lies with the insulation, not with the heat recovery unit.
  

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How can you forget about condensate drainage?

Instead of fighting the symptoms, you can eliminate the root cause. The solution we recommend at Reventon Group for modern buildings is the use of an enthalpy heat exchanger combined with a pre-heater duct heater.

This approach significantly reduces condensate formation while improving winter comfort and system stability.

A traditional heat recovery unit recovers heat only (using a cellulose heat exchanger). By contrast, enthalpy exchangers are capable of recovering both heat and moisture (via a polyethylene–graphene exchanger).

Why does this pay off?

In an enthalpy exchanger, water does not freeze in the way it can in a standard exchanger, where moisture condenses on the exchanger walls. Of course, if the supply air temperature drops below 0°C, even an enthalpy exchanger may freeze. For this reason, we always recommend duct pre-heaters for heat recovery units.
Importantly, the freezing process occurs much more slowly than in exchangers without moisture recovery.

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Summary

The presence of water in a heat recovery unit is most often proof that the heat recovery process is working correctly, not a sign of equipment failure. Condensation is a natural physical phenomenon resulting from cooling warm, humid exhaust air.

The key to avoiding stains on ceilings is not replacing the unit, but correct installation: proper duct insulation, a clear and correctly sized siphon, and adequate gravity slopes.

For those looking for low-maintenance solutions and maximum comfort, the best option is an enthalpy heat exchanger. Thanks to moisture recovery, this technology reduces condensate formation almost to zero while also preventing indoor air from becoming too dry in winter.

Remember: water in a ventilation system is a problem to be solved — not a reason to panic.

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FAQ – Technical questions

1. Can condensate be drained into a gutter?

Not recommended. In winter, water in gutters freezes, blocking condensate discharge. If condensate must be routed outdoors, use a large-diameter pipe, keep the run as short as possible, and always install a heating cable with a thermostat.

2. Why does the heat recovery unit “spit” water only at the highest fan speed?

This is caused by water droplets being carried off the condensate tray by fast-moving air. It often occurs when the unit is incorrectly levelled (water sits too close to the fan) or when the siphon is partially blocked, causing the water level to rise too high.

3. Can an enthalpy exchanger be washed with water?

Yes. Modern enthalpy exchangers used in Reventon units can be washed, but this must be done in accordance with the operating manual — typically using lukewarm water, no aggressive chemicals, and low pressure.