The disadvantages of MVHR

What are the disadvantages of MVHR?

The main disadvantages of MVHR are the upfront cost, the space and planning the ductwork needs, ongoing filter changes and servicing, the noise a poorly installed system can make, and the fact that it only pays back in a reasonably airtight home. None of these rule it out, but they are the honest trade-offs against cleaner air and lower ventilation heat loss.

Taken one at a time: cost is the big one, because MVHR means a unit, a full duct network and professional commissioning, which runs to several thousand pounds and more in a retrofit than a new build. Ductwork needs designing into the build or it ends up boxed in and intrusive. Filters need checking and changing through the year, and the heat exchanger needs an annual clean, so it is not a fit and forget system. Noise, stuffiness and weak airflow are real complaints, but they almost always trace back to bad duct design, undersized ducting or a system that was never properly commissioned, rather than to the technology. And because the whole point is to recover heat that would otherwise leak out, a leaky home undermines the payback. See our cost guide and running cost guide for the current figures.

Is MVHR worth it?

MVHR is worth it in an airtight new build or a deep retrofit, where natural ventilation is no longer enough and the heat recovery has something to recover. In a leaky older home with plenty of natural air movement it is harder to justify on cost alone. It is a ventilation decision first, and a heat saving second.

The honest test is airtightness and need. As homes are built tighter under Part L, there are fewer gaps for stale, damp air to escape through, so a mechanical system earns its place. In that setting MVHR gives constant filtered fresh air, controls condensation and mould, and recovers most of the ventilation heat. In a draughty period home the same money may be better spent on targeted extract and fixing the damp at source. Approved Document F sets the ventilation your home must have; MVHR is one compliant way to provide it, not a legal requirement in itself.

Sources: GOV.UK

What are the most common problems with MVHR systems?

The most common MVHR problems are noise, weak or uneven airflow, and condensation in the ducts. In almost every case the cause is the installation, not the unit: undersized or crushed flexible ducting, ducts run through cold lofts without insulation, or a system that was commissioned poorly or never balanced. A well specified, properly commissioned system rarely gives trouble.

Noise usually comes from cheap flexible ducting with a ribbed inner surface, or from the unit being mounted where its low hum carries into a bedroom. Weak airflow comes from long, kinked or undersized ducts that the fan cannot push against. Condensation forms where warm, moist extract air meets cold duct running through an unheated loft, which is why those runs must be insulated. The fix for all of these is design and commissioning done properly, which is why who installs the system matters as much as which unit you buy. See our guides on noise, duct design and commissioning.

Does MVHR cause overheating in summer?

MVHR does not cause overheating on its own, but it will not cool a home either, and set up wrong it can move warm air around in summer. A summer bypass solves this by letting incoming air skip the heat exchanger when the house is warmer than outside. Overheating in airtight homes is mainly a shading and glazing issue, not an MVHR one.

Every modern MVHR unit worth buying has an automatic summer bypass. In winter the incoming fresh air is warmed by the outgoing air through the heat exchanger. In summer that would be unwelcome, so the bypass routes the fresh air around the exchanger and brings in cooler night air instead. Where homes do overheat, the dominant causes are large unshaded south facing glazing and high airtightness, and the answer is shading, ventilation strategy and, on some homes, purge ventilation, not switching the MVHR off. See our summer bypass guide for how it works.

What are the advantages of MVHR?

For balance, the advantages of MVHR are constant filtered fresh air without opening windows, much lower ventilation heat loss, good control of condensation and mould, and quieter, draught free rooms in an airtight home. In the right property those benefits are hard to get any other way, which is why MVHR is the standard for new build and Passivhaus ventilation.

MVHR filters incoming air, which helps with pollen and traffic particulates in a way an open window cannot. It recovers most of the heat that ventilation would otherwise throw away, so a tight home stays fresh without a cold draught or a big heating penalty. It runs continuously and quietly in the background, and it keeps humidity down, which is the practical route to fewer mould and condensation problems. The trade for all of this is the cost, the ductwork and the upkeep set out above.