Heat Recovery Ventilation Explained: How ERVs Work (Without the Engineering Degree)

You’ve heard the term “heat recovery ventilation” thrown around. Maybe a builder mentioned it. Maybe you saw it in a BER report. Maybe you’ve been reading about condensation and mould solutions and it keeps coming up.

But what actually is it? How does it work? And why is everyone suddenly talking about it?

This guide explains heat recovery ventilation in plain English — no engineering jargon, no assumptions about what you already know. By the end, you’ll understand exactly what it does, how it does it, and whether it’s right for your home.

The Quick Version

The problem: Irish homes need fresh air, but opening windows or using trickle vents means losing your expensive heat.

The solution: Heat recovery ventilation extracts stale air and brings in fresh air, but passes them through a heat exchanger first — so the incoming fresh air is pre-warmed by the outgoing stale air. You get ventilation without heat loss.

The result: Fresh, filtered air in every room. No condensation. No draughts. No wasted heat. Running cost: about €10 per year.

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The Basic Principle

Here’s heat recovery ventilation in one sentence: it swaps your stale indoor air for fresh outdoor air, but keeps the heat.

Think of it like this. Imagine you could grab the warmth from the air leaving your house and hand it to the cold air coming in. The stale air leaves cold. The fresh air arrives warm. You’ve changed the air without changing the temperature.

That’s exactly what a heat exchanger does.

The three-step process

1. Extract: The unit pulls stale, moist air out of your room through one channel
2. Exchange: The outgoing warm air passes through a heat exchanger alongside the incoming cold air. Heat transfers from the warm stream to the cold stream through thin walls — the two air streams never actually mix
3. Supply: Fresh, pre-warmed, filtered air enters your room through a separate channel

The outgoing air leaves your home stripped of its heat. The incoming air arrives at near room temperature. No draught. No cold spot. No heat wasted.

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How the Heat Exchanger Works

The heat exchanger is the clever bit. It’s the component that makes the whole system worthwhile.

In a unit like Optim Vent, the heat exchanger is a ceramic regenerator. Here’s how it works:

Phase 1 (Extract cycle): The unit extracts warm air from your room. As this warm air passes through the ceramic core, it heats up the ceramic material. The warm air exits outside, now cooled.

Phase 2 (Supply cycle): The unit reverses direction. Cold fresh air from outside is drawn through the same ceramic core, which is now warm. The ceramic transfers its stored heat to the incoming air, warming it up. The fresh air enters your room at near room temperature.

This cycle alternates every 70 seconds or so, continuously warming the incoming air with the heat that would otherwise be lost.

The efficiency figure — up to 97% — means that if your room is 20°C and it’s 5°C outside (a typical Irish winter night), the fresh air entering your room will be around 19.5°C. You’d barely notice it.

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HRV vs. ERV: What’s the Difference?

You’ll see two terms used almost interchangeably: HRV (Heat Recovery Ventilator) and ERV (Energy Recovery Ventilator). They’re similar but not identical.

HRV — Heat Recovery Ventilator

• Transfers heat only between the air streams
• The outgoing moisture is fully expelled outside
• Best for very cold, dry climates where you want to keep all the heat but don’t need moisture management

ERV — Energy Recovery Ventilator

• Transfers both heat and moisture between the air streams
• Some of the moisture from the humid outgoing air is transferred to the dry incoming air (and vice versa)
• Better at maintaining comfortable humidity levels
• Better for mild, damp climates — like Ireland

For Irish homes, an ERV is generally the better choice. Our winters are mild and damp, not cold and dry. An ERV helps balance the humidity in your home rather than just exchanging heat. Optim Vent is an ERV — it manages both heat and moisture for optimal indoor comfort.

Why the distinction matters

In a pure HRV, all the moisture extracted from your home goes outside. That’s fine in winter when your home is too humid. But it can over-dry your indoor air in some conditions.

An ERV is more balanced. It recovers moisture as well as heat, so your indoor humidity stays in a comfortable range (typically 40-60% relative humidity) rather than swinging between too humid and too dry.

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Types of Heat Recovery Ventilation Systems

There are two main approaches to heat recovery ventilation in homes:

1. Whole-House MVHR (Mechanical Ventilation with Heat Recovery)

This is the centralised approach. A single large unit — usually in the attic or a utility room — serves the entire house through a network of ducts.

How it works:

• Ducts run from the central unit to every room in the house
• Supply ducts deliver fresh air to bedrooms and living areas
• Extract ducts pull stale air from kitchens, bathrooms, and utility rooms
• All air passes through a central heat exchanger

Pros:

• One system serves the whole house
• Very efficient in new builds where ductwork can be planned from the start
• Central filtration and maintenance

Cons:

• Requires extensive ductwork — difficult to retrofit in existing homes
• Installation is disruptive (ceiling voids, boxing, redecoration)
• Higher upfront cost (typically €5,000-€10,000+ installed)
• If the central unit fails, the whole house loses ventilation
• Ducts need cleaning periodically

2. Single-Room ERV (Decentralised)

This is the room-by-room approach. Individual units install directly into external walls, each serving one room independently.

How it works:

• Each unit contains its own heat exchanger, fan, and filter
• Installed through a single core hole in the external wall
• Each room is independently ventilated
• Units can be controlled individually or together via an app

Pros:

• No ductwork — installs in existing homes without disruption
• Each room is independent — if one unit needs maintenance, others keep running
• Installed in a couple of hours per room
• Much lower cost than whole-house MVHR
• Easy to add rooms over time (install one now, add more later)
• Individual room control

Cons:

• Each room needs its own unit (though most homes only need 2-3)
• Not as “invisible” as ducted system (unit is visible on the wall, though modern units like Optim Vent are compact and unobtrusive)

We compare these two approaches in detail in our guide: Single-Room ERV vs. Whole-House MVHR.

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What Heat Recovery Ventilation Actually Does for Your Home

Let’s get practical. What changes when you install heat recovery ventilation?

1. Condensation disappears

The most dramatic and immediate change. Windows that used to stream every morning stay dry. Windowsills that were constantly wet dry out. Most homeowners notice this within the first week.

Why: The unit continuously removes moisture-laden air before it can condense on cold surfaces. Read more in our guide to condensation.

2. Mould stops growing

With the moisture removed, the conditions that feed mould disappear. Existing mould stops spreading. New mould doesn’t form. (You’ll still need to clean existing mould — the unit prevents new growth, it doesn’t scrub your walls.)

Why: Mould needs sustained surface humidity above 70% to grow. Continuous ventilation keeps surface humidity well below this threshold. Read more in our guide to mould.

3. Air feels fresher

This is the one people don’t expect to notice but always do. That “stuffy” feeling when you walk into a closed-up room? Gone. CO2 levels stay low because stale air is continuously replaced. You wake up feeling more refreshed because your bedroom air was fresh all night.

4. Heating bills can decrease

Counter-intuitive, but true. Many homeowners actually spend less on heating after installing HRV. Here’s why:

• You stop opening windows “just to get some air” (which dumps heat)
• The heat recovery means ventilation isn’t fighting your heating system
• You can keep your home sealed and warm while still having fresh air
• The unit runs on 7.8W — that’s less than the LED bulb on your bedside table

5. Air is filtered

Every breath of incoming air passes through an F7-rated filter that captures 85%+ of airborne particles — dust, pollen, traffic pollution. If anyone in your household has hay fever or allergies, this is life-changing. Fresh air without opening the window to the pollen.

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The Numbers That Matter

When evaluating a heat recovery ventilation system, these are the specifications to focus on:

Specification	What It Means	Optim Vent
Heat recovery rate	How much heat is transferred from outgoing to incoming air	Up to 97%
Power consumption	How much electricity the unit uses	7.8W
Noise level	Operating volume	< 33dB (quieter than a library)
Filter rating	How effectively it filters incoming air	F7 (captures 85%+ of particles)
Room coverage	Maximum area one unit can effectively ventilate	Up to 500 sq ft
Annual running cost	What it costs to operate 24/7	~€10 per year

The numbers that don’t matter (as much as you’d think)

• Air flow rate in m³/h — important for engineers, but what matters to you is whether the unit adequately ventilates your room. Coverage area is a better guide.
• Exact heat recovery percentage — anything above 90% is excellent. The difference between 93% and 97% matters less than whether the unit actually runs 24/7 (which it should).

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Is Heat Recovery Ventilation Right for Your Home?

It’s ideal if:

• You have condensation on windows — especially in bedrooms
• You’ve noticed mould on walls, ceilings, or window reveals
• Your home feels stuffy or the air feels stale
• Anyone in your household has allergies, asthma, or hay fever
• You’ve recently had insulation installed and now have moisture problems
• You’re relying on trickle vents that you keep closed
• You want to improve your BER rating
• You’re planning an SEAI-funded energy upgrade

It’s probably not what you need if:

• Your damp is caused by rising damp or structural leaks (get a surveyor first)
• You need to extract large volumes of cooking steam (that’s what a cooker hood is for — HRV handles background ventilation)
• Your home is so leaky that you have plenty of natural ventilation already (rare in modern Irish homes)

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Common Myths About Heat Recovery Ventilation

“It’ll make my house cold.” The opposite. With 97% heat recovery, the incoming air is near room temperature. There’s no draught. It’s the opposite of opening a window.

“It’s expensive to run.” 7.8W. €10 per year. Less than a phone charger. Less than a dehumidifier by a factor of 15.

“I need a whole-house system with ducts.” Not if you choose single-room units. One unit per room, installed through the wall. No attic unit, no ductwork, no boxing.

“It’s noisy.” Under 33dB — quieter than a library. You can run it in a bedroom all night without hearing it.

“I can just open a window instead.” You can, but you’ll lose your heat, let in unfiltered air, create draughts, and need to remember to do it. Heat recovery ventilation does the same thing — continuously, silently, and without the heat loss.

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Getting Started

If you’d like to experience the difference heat recovery ventilation makes:

1. Identify your worst rooms. Where’s the condensation? Where’s the mould? Where does it feel stuffy? Those are your starting points.
2. Consider single-room ERV. For most existing Irish homes, Optim Vent single-room units are the most practical option — no ductwork, minimal disruption, installed in hours.
3. Check grant eligibility. If you’re planning other energy upgrades, ventilation may be covered. See our SEAI grants guide.
4. Start with one room. You can install one unit and experience the results before committing to additional rooms. Most people add more rooms after seeing how the first one performs.

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Heat recovery ventilation isn’t complicated. It’s just doing what you’d do naturally — swapping stale air for fresh air — but doing it cleverly enough to keep the heat. In Ireland’s damp climate, it’s one of the best investments you can make in your home’s comfort, health, and efficiency.

Get in touch for a free home assessment and find out what Optim Vent can do for your home.