MVHR for retrofit

Can you retrofit MVHR into an existing house?

Yes, MVHR can be retrofitted, but it is more disruptive and more expensive than fitting it during a new build, because supply and extract ducting has to be routed through finished floors, ceilings and cupboards. It works best where the home is airtight, or is being made airtight as part of a wider renovation, so the heat recovery has heat to recover.

The technology is the same as in a new build; the challenge is getting the ducts in without tearing the house apart. Homes with accessible loft and floor voids are easier than solid, finished properties. Where a full duct network is impractical, single-room heat recovery units are sometimes used, though they do not match a properly ducted whole-house system.

Is MVHR worth retrofitting?

It is most worth it when the home is being deeply renovated and made airtight anyway, so the ducting goes in with the other works and there is real heat to recover. In an untouched, leaky home the case is weaker: the disruption is high and a leaky home loses much of the benefit. There, continuous extract or positive input ventilation is often more proportionate.

The key question is airtightness. MVHR pays back its disruption and cost where the home holds heat well, which usually means it has been insulated and sealed. Bolting MVHR onto a draughty house rarely makes sense: the fabric lets the recovered heat straight back out, and simpler ventilation would do the job for less.

What makes MVHR retrofit harder than a new build?

In a new build the ducts are run before the rooms are finished. In a retrofit they must be threaded through an existing structure, around joists, through cupboards and boxed-in runs, which takes more labour and more compromise. Finding space for the unit and keeping duct runs short and rigid is the main challenge, and it drives both the cost and how well the finished system works.

Compromised duct routes are the usual reason a retrofit system underperforms or is noisy: long flexible runs and tight bends add resistance and noise. A good retrofit design works out the duct routes early, ideally when other works expose the structure, rather than squeezing them in afterwards.

Does PAS 2035 apply to an MVHR retrofit?

If the MVHR is part of a wider retrofit under a government-funded scheme, the work follows PAS 2035, which requires a ventilation assessment and functional testing. Even for a privately funded retrofit, the PAS 2035 approach is good practice: assess the ventilation, make sure airtightness and ventilation are considered together, and test that the system performs.

PAS 2035 exists partly to stop the classic retrofit mistake of sealing a home without sorting its ventilation, which causes damp and mould. Whether or not your project is formally under PAS 2035, following the same logic protects the home. See our PAS 2035 guide for what the assessment involves.

Sources: Retrofit Academy

What are the alternatives to MVHR in a retrofit?

If full MVHR is too disruptive, continuous mechanical extract, either MEV or dMEV, is simpler to fit and still provides controlled ventilation, though without heat recovery. For a leakier home with condensation, positive input ventilation is a low-cost option. The right choice depends on the home airtightness and what you are trying to fix.

dMEV, with a small fan in each wet room, is often the least disruptive mechanical option in an existing home. PIV, a single loft unit, is cheaper still and suits leakier homes with damp. Our system selector and the guide to the four system types walk through which one fits which situation.