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Can You Retrofit MVHR?

Can You Retrofit MVHR?
A typical home that could use MVHR

Can you retrofit MVHR?

Can you retrofit MVHR? Perhaps the question should really be, Can I Add MVHR to My Property? The answer will really depend on the fabric of the property, or the layout. 

What is MVHR?

Mechanical Ventilation with Heat Recovery (MVHR) is a whole home ventilation system. It works by extracting the stale, moist, and polluted air from the inside of your home or other property. It then resupplies with fresh, filtered air. 

At the same time, the air travels through a heat exchanger. This then transfers a large percentage of the heat energy from the extracted air to the incoming air, resulting in a more energy-efficient building when installed correctly.

This creates a more comfortable, condensation and mould-free environment year-round.

Read more about this in our previous article! What is MVHR?

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Build with MVHR In Mind

In an ideal situation, you would want to build a property with MVHR in mind, planning for ducting work, electrical cables and the site of the Unit at the very least. Even better, MVHR would be installed as you build. 

Read more about why in our previous article, When in the Building Process Should I Design My MVHR System?

Retrofitting does come with its drawbacks, especially in some older properties with no wall/ceiling cavities, as this would make it difficult or impossible to hide the ductwork.

This does not make it impossible though, to install MVHR in such a property.

A typical building site

Retrofitting MVHR into Older Buildings

Older buildings are often quite leaky, but ventilation in old buildings is still important. This may seem odd as usually, we try to make older buildings more airtight, sealing holes and gaps. While this is important to stop cold air from getting in, airtight buildings have no air circulation which can create condensation and mould. This is why mechanical ventilation is important.

It is often said that older buildings do not benefit from MVHR, however, this is not the case. It may not be as efficient as it is in newer buildings but there are still benefits, such as:

  • Improved indoor air quality (IAQ)
  • Lower humidity
  • No need for bathroom fans or window vents
  • Reduced noise and dust

Read more about this, and installing MVHR in new builds in our article, Is MVHR Suitable for My Property?

There will be occurrences where not all rooms in an older property can have MVHR ducting run directly to them due to their construction. However, it will be possible to install it on the top floor, allowing for bedrooms and bathrooms to be condensation and mould-free.

Read more about it in our previous article Are MVHR Systems Worth It?

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How Airtight Should a Property Be For MVHR?

The more airtight a property starts before MVHR, the better. However, this does not mean that MVHR does not benefit “leakier” properties. 

MVHR will still improve indoor air quality in a leakier property for example.

The efficiency however is much greater with an actual airtightness level of 3 m3/(hr.m2) or lower.

Generally speaking, newer, building regulation-meeting properties will be more airtight, and older buildings will be “leakier”. Renovated buildings have their own level of regulations to meet, and will generally be quite airtight. 

Read more about MVHR efficiency in our previous article How Efficient are MVHR Systems?

Can You Retrofit MVHR?
A village road full of older homes

If you Insulate, you must Ventilate

In the days before loft insulation, wall insulation, or even double glazing, this vapour-filled, humid, stale air would have escaped through background ventilation. Ill-fitting doors and windows for example would have allowed this air out, and replaced it with fresher, colder air. If you have ever felt a draught, you’ve experienced this process.

In new builds or renovations that meet building regulations, however, this air cannot escape so easily. With the addition of energy-saving measures such as double glazing, cavity wall insulation and draught-proofing, there is little to no natural ventilation. This leads to a build-up of vapour-filled air being trapped in the property without the addition of proper ventilation.

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Daily Tasks that Cause Condensation

We all bathe or shower regularly, but other things we do regularly also cause condensation. Some examples are:

  • Boiling the kettle
  • Cooking
  • Using a tumble dryer
  • Drying clothes indoors
  • Breathing

The average family of 4 will contribute around 4 pints of water per person each day just by doing these tasks, which equals around 100 pints of water vapour every week!  With poor ventilation, this vapour will likely turn into condensation.

Read more about the problems condensation can cause, in our article What Causes Condensation?

Condensation on a window

What Is The Lifespan of MVHR?

On average, the lifespan of an MVHR unit will be around 15 years. This does not however mean that the whole unit will need replacing after 15 years. Many individual part defects can be solved by replacing that part. 

People sometimes choose to replace their unit much sooner than that 15-year time frame. This is simply because newer units are often more energy efficient than older units, and provide better performance

So, Can You Retrofit MVHR?

Hopefully, by now you understand a few more of the ins and outs surrounding this question.

MVHR is suitable for retrofit, but as always you should work with an expert to make sure it is the right choice for you.

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