Why is my MVHR not working properly?

What this usually means

An MVHR system continuously extracts stale, humid air from wet rooms (kitchens, bathrooms) and supplies fresh, filtered air to living rooms and bedrooms, recovering heat from the outgoing air to warm the incoming. For it to work, it has to be commissioned — set up so the airflow to and from each room matches the design, and the supply and extract are balanced — and kept maintained, with clean filters and clear ducts. A great many MVHR systems that 'do not work' were simply never commissioned: installed and switched on but never adjusted to the correct flow rates, so they move far too little air, or are out of balance, and cannot do their job.

Beyond commissioning, several faults degrade performance. Filters that are never changed clog and choke the airflow; ducts that are crushed, kinked, too long, poorly routed or full of debris add resistance and cut the flow; and poor installation — undersized or leaky ductwork, the unit sited in a cold loft without insulated ducts, or condensate drainage not provided — undermines the system. The symptoms map onto these causes: stuffiness and humidity mean too little fresh air is being delivered or extracted; condensation can indicate poor airflow or cold, uninsulated ducts; noise often points to ducts or settings; and draughts or a cold feel can mean the balance or the supply temperature is wrong. The unit itself is usually sound.

Because the causes are specific and different, the fix begins with diagnosis rather than replacing the unit. Measuring the actual airflow at each supply and extract terminal reveals whether the system is moving the designed volumes and is in balance; inspecting the filters and ducts finds blockages and installation faults; and checking the commissioning records (or their absence) and the unit settings shows whether it was ever set up. From that, the remedy follows — commissioning or re-balancing the system, replacing filters and clearing or correcting ducts, insulating ducts in cold spaces, or remedying an installation defect. An assessment that establishes which fault is present gets the MVHR delivering the air quality and heat recovery it was installed for, rather than leaving an expensive system running ineffectively or, as often happens, switched off in frustration.

Common causes

Signs and symptoms

An MVHR system is designed to deliver specified supply and extract airflow rates to each room, balanced so the whole-house ventilation rate and the room-by-room distribution meet the design, with the heat exchanger recovering warmth from the balanced extract stream. Its effectiveness depends entirely on achieving those flow rates: commissioning adjusts the terminals and the fan speed so the measured flows match the design and the supply and extract are in balance. Without commissioning, the as-installed flows are essentially arbitrary — typically far below design — so the system under-ventilates, humidity and pollutants are not removed, and the occupant experiences stuffiness and condensation despite a running system.

Airflow is also degraded by resistance and leakage in the distribution. Duct systems are sized for a given pressure drop; crushed, kinked, over-long or poorly routed ducts, and clogged filters, raise the resistance so the fan delivers less than its design flow, while leaky ductwork loses conditioned air into voids. Heat-recovery performance and condensation control depend further on the unit and ducts being correctly sited and insulated: an MVHR or its ducts in a cold loft without insulation cools the air and can cause condensation within the ducts, and the unit's condensate must be drained. These are installation and maintenance properties, not faults of the heat-exchange principle, and each suppresses performance in a characteristic way.

Diagnosis therefore rests on measuring the system's actual behaviour against its design. Airflow measurement at each supply and extract terminal establishes whether the design rates and balance are achieved; inspection of filters, ducts and the unit's siting and drainage identifies resistance, leakage and installation defects; and the commissioning documentation (or its absence) and the control settings reveal whether the system was ever set up. The remedy follows directly — commissioning or re-balancing, filter replacement and duct cleaning or correction, duct insulation in cold spaces, or rectification of an installation defect. This measured approach restores the designed ventilation and heat recovery, in contrast to assuming a faulty unit or, as commonly happens, abandoning a sound but uncommissioned system because it never appeared to work.

How to get an MVHR system working properly

Diagnose before replacing: measure the airflow and balance, inspect the filters, ducts and installation, and confirm commissioning — then put right whichever fault is suppressing the system.

1. 01

   Measure the airflow and balance

   Check the actual supply and extract rates at each terminal against the design.

2. 02

   Check the commissioning

   Establish whether the system was ever set up to the design flow rates and balanced.

3. 03

   Inspect filters and ducts

   Find clogged filters and crushed, kinked or dirty ducts choking the airflow.

4. 04

   Check the installation

   Confirm the ducts are insulated in cold spaces and the condensate is drained.

5. 05

   Commission or re-balance

   Set the system to the design flow rates and balance, replacing filters and clearing ducts.

6. 06

   Verify the result

   Confirm the air quality, humidity and comfort have improved as intended.

How to prevent it coming back

• Insist on proper commissioning records when MVHR is installed.
• Change filters and check ducts on a regular schedule.
• Insulate ducts and the unit in cold spaces.
• Diagnose the airflow before assuming the unit is faulty.