Why a waterless trap makes sense for MVHR condensate drainage

Saturday 5 September 2026

With more MVHR units being installed in attic spaces, Wavin explains why a waterless valve like HepᵥO offers a practical advantage over conventional traps when managing condensate drainage.

Why does MVHR condensate drainage need particular consideration?

MVHR systems generate condensate when warm, moisture-laden extract air is cooled as it passes through the heat exchanger, and that condensate needs to be safely drained away.

Technical Guidance Document F – Ventilation (TGD F), which provides guidance on complying with Part F of the Irish Building Regulations, states that installed heat-recovery units should have condensate drains fitted and the pipe insulated.

The particular challenge with MVHR is that condensate production is not constant. During warmer summer periods, when internal and external air temperatures are closer, significantly less condensate may be generated. At the same time, attic temperatures can rise considerably, increasing the risk of the water held in a conventional trap evaporating.

Why can a traditional U or P trap be a weak point?

Traditional traps depend on standing water to maintain the seal between the foul drainage system and the MVHR unit.

Many MVHR units are located in attic spaces, which can become particularly warm during summer. A combination of higher attic temperatures and periods of low condensate flow can mean that the water held within a conventional trap gradually evaporates.

Once that water seal is lost, foul or contaminated air can travel back through the condensate connection towards the MVHR unit and into the living space.

There is also a practical maintenance issue. An MVHR unit located out of sight in an attic can easily be forgotten, so a traditional trap that requires checking and refilling may not receive regular attention.

What makes HepvO such a strong solution?

Wavin HepᵥO removes the reliance on standing water. Its self-sealing elastomeric membrane opens to allow condensate to discharge and then closes to create an airtight seal.

Wavin HepᵥO with membrane.

There is therefore no water seal to evaporate and no requirement to keep a conventional trap topped up simply to maintain that barrier. For an MVHR unit installed out of sight in an attic, that removes a potential maintenance issue that might otherwise be easily overlooked.

HepᵥO has also been performance tested under negative-pressure conditions and demonstrated suitability as an air lock for heat-recovery condensate drainage at negative pressures up to -4 kPa.

What should installers consider?

Attic installations bring winter considerations too. Any condensate pipe passing through an unheated void or area that could fall below 4°C should be insulated to prevent freezing. Wavin angled adaptors feature a 2.5° angle to help maintain the required fall.

Wavin HepᵥO can also be installed horizontally or vertically. Where space is restricted, horizontal installation provides a compact option and is usually preferred for maximum space saving.

Wavin also recommend flushing the valve through before installation. The valve requires a minimum of 50-100mm head of water to open the valve on its first flush, after this first flush, only 15mm of water is required to open the valve, in standard operating conditions.