What is heat recovery ventilation (MVHR)?

Heat recovery ventilation, often called MVHR, is a whole-house system that constantly swaps stale, damp indoor air for fresh outdoor air, while keeping most of the heat inside. MVHR stands for mechanical ventilation with heat recovery. It gives you fresh air in every room without opening windows and throwing your heating away.

Modern homes are built, and older homes upgraded, to be far more airtight than they used to be. That's great for keeping heat in. The catch is that moisture, cooking smells and stale air have nowhere to go. Without proper ventilation, you get condensation on windows, black mould in corners and air that feels heavy and stuffy.

A heat recovery system solves that. It's made up of a few main parts:

•     A central unit, usually in the attic, a utility room or a hot press, containing two fans and a heat exchanger.

•     Extract ducts that pull stale, moist air from kitchens, bathrooms and utility rooms.

•     Supply ducts that deliver fresh, filtered air to bedrooms and living rooms.

•     Two ducts to the outside, one bringing fresh air in and one sending stale air out.

•     Filters that catch dust and pollen before the fresh air reaches you.

•     Controls, often with a boost setting for showers and cooking.

The heat exchanger is the clever bit. Warm outgoing air passes the cold incoming air without the two mixing, so most of the warmth is handed over to the fresh air on its way in. You get fresh air at close to room temperature, rather than a cold draught.

The system runs quietly all the time, at a low speed, using little electricity. It's a natural partner for an air-to-water heat pump, because both work best in a well-insulated, airtight home, and together they keep the house warm, fresh and dry.

It's not a heating system on its own, and it's not air conditioning. It moves and refreshes air, recovering heat that would otherwise be lost. Your heating still does the heating.

If you're building in Roscommon or doing a deep retrofit and want to know whether heat recovery ventilation suits your house, ring us and we'll talk it through.

How does heat recovery ventilation actually work?

Heat recovery ventilation works by running two streams of air through a heat exchanger at the same time: stale air going out and fresh air coming in. The heat from the outgoing air warms the incoming air, so you get constant fresh air without losing most of the heat you've paid for.

Here's the process step by step:

•     Extracting. Fans pull warm, moist, stale air from the "wet" rooms, the kitchen, bathrooms, en-suites and utility room, through ducts in the ceiling or walls.

•     Recovering heat. That warm air passes through the heat exchanger in the central unit. At the same time, fresh outdoor air passes through it on the other side. The two airstreams never mix, but the heat moves from one to the other.

•     Exhausting. The stale air, now cooled, is sent outside through a duct.

•     Filtering. Fresh air is filtered as it comes in, catching dust, pollen and insects.

•     Supplying. The fresh, warmed air is delivered to the "living" rooms, bedrooms, sitting rooms and offices, through ceiling or wall vents.

•     Circulating. Air moves from the supply rooms through the house, under doors or through gaps, to the extract rooms, giving the whole home a gentle, steady flow.

The system runs all day and night at a low background speed. When you shower or cook, a boost setting speeds it up to clear moisture and smells quickly, then it drops back down.

Most units have a summer bypass. On warm days, it lets the fresh air skip the heat exchanger, so you're not warming air you'd rather keep cool. It isn't air conditioning, but it helps take the edge off a stuffy evening.

In cold weather, a frost protection setting stops ice forming in the heat exchanger. The unit also produces a small amount of condensation, which drains away through a condensate pipe.

Filters need changing regularly to keep the air clean and the fans working efficiently. It's a simple job, and the unit will usually remind you when it's due.

The whole system depends on good design. The airflow to each room must be worked out, the ducts sized correctly, and the system balanced so the air going out matches the air coming in. Pair it with underfloor heating and you have a home that's evenly warm and always fresh.

We design and install heat recovery ventilation across Longford and Roscommon. Give us a call and we'll explain how it would work in your house.

What types of heat recovery ventilation are available?

The main types of heat recovery ventilation are whole-house ducted MVHR systems and single-room decentralised units. There are also related systems, like extract-only ventilation, that don't recover heat. The right choice depends on whether you're building new, doing a big retrofit, or fixing a problem in a few rooms.

Whole-house MVHR:

This is the full system: one central unit, ducts to every room, balanced supply and extract. It gives the best results and recovers the most heat. It's the standard choice for new builds and deep retrofits, where ducts can be planned and hidden during the build.

Single-room heat recovery units:

These are small units fitted through an outside wall, each with its own little heat exchanger. They suit retrofits where running ducts through a finished house isn't practical, or where only a few rooms have condensation problems. They're less efficient overall than a whole-house system, but much less disruptive to fit.

Mechanical extract ventilation (MEV):

These systems pull air out of wet rooms continuously, with fresh air coming in through wall vents. They don't recover heat, so they're cheaper to fit but lose more warmth. They're mentioned here because they're often confused with heat recovery systems.

Duct types:

•     Rigid ducts. Traditional round or rectangular ducting, laid out like a tree from the unit.

•     Semi-rigid radial ducts. Flexible ducts running from a central distribution box to each room. They're quicker to install, easier to clean, and reduce noise travelling between rooms.

Unit position:

•     In the attic, which saves space, though the unit and ducts must be well insulated.

•     In a utility room, plant room or hot press, which keeps the unit inside the warm part of the house and makes filter changes easier.

Controls:

From a simple three-speed switch to humidity sensors and app control. Humidity sensors boost the system automatically when you shower or cook.

Summer bypass:

Most good units include one, letting cool night air in without passing through the heat exchanger.

The best type for your home depends on how airtight it is, how much space you have for ducts, and your budget. If you're also putting in an air-to-water heat pump, it makes sense to plan both together.

Not sure which fits your project in Athlone or Roscommon? Ring us and we'll walk you through the options.

What makes heat recovery ventilation different from other plumbing, heating and renewable energy services?

Heat recovery ventilation is different because it deals with air, not water or fuel. It doesn't heat your home directly. Instead, it keeps the air fresh and dry while holding on to the heat your heating system has already produced. That makes it a partner to your heating, not a replacement for it.

Here's how it compares with other work:

•     A boiler replacement or a heat pump makes heat. Heat recovery ventilation keeps that heat in while changing the air.

•     Plumbing deals with water supply and drainage. Ventilation deals with airflow, ducts and filters.

•     Extractor fans in bathrooms and kitchens remove moist air, but they throw the heat out with it and leave other rooms unventilated.

•     Opening windows gives fresh air, but in an Irish winter it also lets your heat straight out.

•     Air conditioning cools and dehumidifies by refrigeration. Heat recovery ventilation doesn't refrigerate air at all.

It's also different in how it's designed. A heating system is sized by heat loss. A ventilation system is sized by airflow: how many people live in the house, how many rooms there are, and how much moisture each room produces. Every room needs the right amount of air in or out, and the system must be balanced so the air going out matches the air coming in.

The building side matters more too. Ducts have to be planned around joists, beams and walls. In a new build, that's best done at design stage. In a retrofit, it means finding routes through a finished house, which takes careful planning.

It works best in an airtight house. In a leaky house, air escapes through gaps instead of passing through the heat exchanger, and much of the benefit is lost. That's why heat recovery ventilation usually goes hand in hand with insulation and airtightness upgrades.

Commissioning is more detailed as well. Each vent is measured and adjusted so the airflow matches the design. For new homes, the building regulations require ventilation systems to be commissioned and their performance checked.

What doesn't change is the need for honest advice. If your house isn't airtight enough to benefit, we'll tell you. If single-room units would suit you better than a whole-house system, we'll tell you that too. If you're in Roscommon and weighing it up, give us a call.

What should homeowners know before starting a heat recovery ventilation installation?

Before a heat recovery ventilation installation, you should know that it works best in an airtight home, that the ducts need careful planning, and that the system must be designed, balanced and maintained properly. Getting these right at the start makes all the difference to how well it works.

Here's what to think about.

Airtightness matters. A heat recovery system only works well when air goes through the unit, not out through gaps around windows, doors and attic hatches. If your home is draughty, airtightness work should come first, or at the same time.

Plan early. In a new build, the ducts should be designed before the roof and ceilings go in. Leaving it until later means awkward routes, boxing-in and more cost.

Think about duct routes. Ducts need space in ceilings, floors or attics. In a retrofit, finding routes without damaging finishes takes planning. Single-room units may be the better choice where ducting is impractical.

Choose a sensible spot for the unit. It needs to be accessible for filter changes and servicing. A utility room, plant room or insulated part of the attic all work.

Insulate ducts in cold spaces. Ducts running through a cold attic must be insulated, or warm air inside them can cause condensation and drips.

Check your stove or fire. Open fires and some stoves need their own air supply. In an airtight house with heat recovery, a room-sealed stove with a dedicated outside air supply is the safer choice. Mention any fire or stove at the design stage.

Remember the building regulations. Ventilation rules apply to new homes and to major renovations. For new homes, systems must be commissioned and their performance checked. You'll find the detail on the Department of Housing building regulations [INSERT-URL] pages.

Plan for maintenance. Filters need changing regularly, and the system should be checked periodically. Ask what the filters cost and how easy they are to get.

Electrical supply. The unit needs a power supply, and any new circuit should be done by a registered electrical contractor.

Think about the whole house. If you're also fitting an air-to-water heat pump or underfloor heating, plan everything together. The systems work best as a team.

If you're planning a new build or deep retrofit, talk to us early. One phone call before the plans are fixed can save a lot of rework later.