MVHR (Mechanical Ventilation with Heat Recovery) systems do produce condensation, especially in colder weather. Warm, humid indoor air passes through the heat exchanger and cools down, causing water vapour to condense into liquid water. This condensate collects at the bottom of the unit (often in a drain pan or tray) and must be removed to prevent water build-up, which could damage the unit, reduce efficiency, cause mould or lead to overflows.
The condensate drain is the outlet pipe (typically 21-22mm in diameter) from the MVHR unit that carries this water away. It usually connects to a suitable internal drainage point, such as a soil stack, bathroom/kitchen waste pipe (eg from a sink, bath, shower, or basin overflow) or another foul water drain. The pipe should run with a sufficient fall (slope) to allow gravity drainage and be insulated if it passes through unheated spaces to prevent freezing.
The condensate trap (often a P-trap, U-trap, or sometimes a dry trap / waterless valve in MVHR contexts) is fitted in the condensate drain line. Its main purposes are:
- To prevent air movement through the drain pipe. MVHR units create a slight negative pressure (suction) in parts of the system, so without a trap, the fan could suck air (or sewer gases) back through the drain into the unit, or disrupt proper drainage.
- To stop sewer gases, odours, or pests from entering the home via the drain connection (especially important when connecting to a waste pipe).
- To ensure smooth condensate flow while maintaining a seal. The water in the trap acts as a barrier, but allows condensate to push through and drain when it accumulates.
Fitting the condensate trap to an internal pipe drain
- The MVHR unit has a condensate outlet (usually at the base). Attach a short length of compatible pipe (e.g., 21.5-22mm overflow or waste pipe) to this outlet.
- Install the trap immediately after (or as part of) this pipe run. Typically, a U-shaped or P-shaped trap is formed using standard plumbing fittings (eg swept bends and a straight section to create the water-holding loop). Some manufacturers supply a pre-formed trap or kit (like an in-line drain trap).
- Ensure the trap is primed (filled with a small amount of water during setup) or uses a dry trap design if specified.
- Run the pipe from the trap outlet (with at least a 5° downward slope) to the internal drain point. Connect it properly - often via a T-fitting, strap-on boss, or direct insertion into a waste pipe/socket (eg, into a bath overflow or kitchen sink waste). Use solvent weld or push-fit fittings as appropriate, and seal connections to prevent leaks.
- Avoid kinks, upward loops, or blockages. If the pipe runs through cold areas, insulate it.
Always follow the specific manufacturer's instructions for your MVHR model, as trap design and pipe sizing can vary slightly (some recommend a "dry trap" or specific kit to minimise maintenance). Proper installation ensures reliable drainage, prevents issues like gurgling or blockages and complies with building regulations for ventilation and condensate disposal. If in doubt, please consult a qualified plumber or the unit's manual.
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