HRV settings during winter – reliable baseline for MVHR

Use these as your default winter settings for MVHR/HRV in an Irish home:

1. Run MVHR 24/7 on normal background speed

• Keep the MVHR unit running continuously.
• Do not switch it off at night or when leaving the house – stopping ventilation allows moisture and pollutants to build up.

2. Keep heat recovery ON

• In winter, you want to recover as much useful heat as possible.
• Ensure heat recovery is enabled and MVHR summer bypass is disabled.

3. Use boost for moisture spikes

• Use boost during showers and for 10–20 minutes afterwards.
• Use boost during heavy cooking and immediately after.
• Then always return to normal speed.

4. Maintain transfer air routes

• Keep internal doors generally open for good transfer airflow,
• except bathroom doors immediately after showers – closing them helps contain moisture so extraction is more effective.

This simple combination gives a solid, energy-efficient winter baseline for most Irish homes.

HRV summer settings – what to change for warmer months

Summer requires a different logic. The goal is to maintain good indoor air quality while avoiding overheating and not dragging in unnecessary humidity.

1. Keep MVHR running 24/7 on low or normal

• Do not turn your MVHR off in summer.
• Set the unit to low/normal background speed, depending on design airflow and occupancy.

Fresh air, odour control and continuous moisture dilution are just as important in July as in January.

2. Enable MVHR summer bypass

• Activate MVHR summer bypass or auto bypass mode according to your user manual.
• The idea is to reduce unwanted heat transfer when outdoor air is cooler than indoor air (especially evenings and nights).

Remember:

MVHR summer bypass helps with temperature and comfort, not with active dehumidification.

It does not dry the air; it simply avoids adding extra heat through the heat exchanger.

3. Use boost only for spikes

• Keep using boost for showers, cooking and indoor laundry,
• but avoid leaving boost on all day; that can pull large volumes of warm, humid outdoor air through the house for little benefit.

4. Use windows strategically for night cooling

• Open windows for cross-ventilation (night purge) when outdoor air is cooler than indoor air.
• Keep MVHR running, ideally with heat recovery summer bypass active, so you still get filtered, balanced ventilation.

If you expect MVHR to behave like AC or a dehumidifier in humid weather, you will be disappointed. The system manages air quality and heat recovery; it does not change the basic moisture content of outdoor air.

Heat recovery summer bypass – what it actually does

Inside the MVHR unit, the heat recovery summer bypass:

• uses an internal damper or valve,
• diverts air around the heat exchanger instead of through it,
• reduces or stops heat transfer between extract and supply.

What bypass can do

• Help prevent overheating when outdoor air is cooler than indoor air.
• Allow the system to supply cooler evening/night air directly into rooms.
• Improve comfort without needing to switch off the MVHR.

What bypass cannot do

• It does not remove water from the air.
• It does not function as a dehumidifier or air conditioner.

Think of MVHR summer bypass as a temperature tool, not a humidity gadget. Use it to manage overheating and comfort, while continuous ventilation deals with indoor air quality and moisture dilution.

MEV winter settings – typical best practice

MEV (Mechanical Extract Ventilation) systems work differently from MVHR but still need seasonal thinking.

In winter:

• Keep MEV running continuously on low/normal speed.
• Use boost in wet rooms during showers and cooking.
• Keep trickle vents and designed air inlets open.
• Maintain transfer routes (door undercuts, grilles, open internal doors where appropriate).

Avoid blocking vents to “reduce draughts”. If comfort is an issue, the root cause is usually:

• incorrect MEV fan speed,
• poor balancing of extract points, or
• cold surfaces, not the fact that vents are open.

MEV summer settings – avoid over-boosting in humid weather

In summer:

• Keep MEV on low/normal background speed.
• Use boost only for clear moisture spikes (showers, cooking, indoor laundry).
• Control solar gains with blinds, curtains and shading.
• Use windows for cooling only when outdoor air feels cooler than indoors.

Over-boosting MEV on hot, humid days can simply pull more moist outdoor air through random leakage paths. That increases fan energy use without delivering real comfort gains.

Common mistakes that reduce performance

Avoid the following behaviours; all of them fight against good HRV settings:

• Turning off MVHR or MEV for long periods “to save energy”.
• Running boost all day in summer.
• Leaving bathroom doors wide open immediately after showers.
• Blocking trickle vents or wall inlets in MEV homes.
• Ignoring filters and assuming the unit “still runs fine”.

Instead:

• keep background ventilation stable (24/7),
• target moisture spikes with short boost,
• maintain filters and valves as recommended by the manufacturer.

Compliance note for Ireland

Irish Building Regulations (Technical Guidance Document Part F) expect ventilation systems to:

• provide adequate fresh air, and
• control moisture to reduce the risk of condensation and mould.

At the same time, Part L focuses on energy efficiency. Correct MVHR and MEV design, commissioning and seasonal HRV settings help you:

• meet the intent of Part F (health, moisture control), and
• align with Part L (reasonable energy use).

Good seasonal setup is therefore not just about comfort; it also supports regulatory compliance in modern airtight homes.

FAQ – MVHR Summer Bypass & HRV Settings

When should I turn on HRV summer bypass?

Use HRV summer bypass when:

• outdoor air is cooler than indoor air, and
• you want to reduce heat gain and gently cool the house.

Do not expect summer bypass to dehumidify the home – it mainly improves temperature comfort.

Should I increase airflow in summer to reduce humidity?

Usually not. Increase airflow only for short spikes (showers, cooking, laundry). On very humid days, high continuous airflow may simply bring more moisture in from outside without solving the problem.

Should I ever turn my MVHR off in summer?

In most Irish homes, you should not turn your MVHR off. You still need continuous extraction of moist, stale air and a balanced supply of fresh air. If you feel the system is too strong, review fan speeds and balancing rather than switching it off.

How do I know if my MVHR summer bypass is working?

Signs that MVHR summer bypass is working include:

• supply air feels closer to outdoor temperature than to indoor extract temperature in the evening,
• the unit’s display or app indicates “bypass” or “summer mode” is active,
• the home cools more effectively at night with the MVHR running than with it off.

If in doubt, a professional service can test the unit and confirm bypass operation.