MVHR ventilation for new builds

Why airtight new builds need mechanical ventilation

A modern new build is sealed to a standard an older house never reaches. That is excellent for heating bills and poor for air quality, because the draughts that used to change the air in a room have been designed out. Cooking, showering, drying washing and breathing put several litres of water into the air of an average home every day, and with nowhere to go it lands on the coldest surface and turns into mould.

MVHR deals with that by running continuously instead of in bursts. Damp, stale air is extracted from kitchens, bathrooms and utility rooms, fresh air is supplied to bedrooms and living rooms, and the two streams pass through a heat exchanger where the outgoing air warms the incoming air without ever mixing with it. A decent unit recovers most of that heat, so the house gets a full air change without the warmth going straight out through the wall.

It also filters what comes in, which matters more than people expect out in the arable country inland, where dust and pollen at harvest are a real nuisance. Windows can stay shut on a bad day and the house still breathes properly, which is not something an extract fan and a trickle vent can honestly claim.

Ductwork design decides whether it works

The unit itself is the easy part. Most disappointing MVHR systems are let down by their ductwork, and ductwork is designed at first fix or not at all. Runs want to be as short and as straight as the building allows, with radiused bends instead of tight ones, correctly sized duct throughout, and no flexible hose pressed into service as a way out of an awkward corner. Every unnecessary bend is resistance, and resistance turns into fan noise.

Duct running through an unheated loft has to be insulated or it will sweat through the winter and drip through a ceiling. Attenuators go on the supply and extract branches near the unit so it cannot be heard from a bedroom, and cross talk between rooms is designed out instead of discovered after the plasterboard goes on. It pays to have us involved before the joists are covered, because retrofitting duct into a finished house is expensive and never as good.

Commissioning, filters and when to skip MVHR

A system that has not been commissioned is not a finished system. Every valve is measured and adjusted until supply and extract flow rates match the design and the whole house is balanced, and those figures are recorded for building control. Out of balance, the property sits either under or over pressure, and it will feel draughty or smell stale depending which way it went. Neither is a fault with the unit.

After that it needs filters and very little else. Two changes a year is the usual pattern, more if you are near a busy road or a field that gets harvested, and a clogged filter costs you both airflow and efficiency long before anyone notices. We handle that servicing alongside a check of the heat exchanger, the condensate drain and the fan speeds.

If the building is not genuinely airtight, MVHR is the wrong system. In a leaky older house it cannot recover heat from air that is escaping through the fabric anyway, and the money is better spent on the fabric first, with good extract fans or a single room heat recovery unit where the moisture is produced. Fitting a whole house system into a draughty building only buys you a filter bill.