There are several ways of making sure a house is adequately ventilated - by far most dwellings being built today still use the tried and tested method of installing intermittent extraction fans to all wet rooms and using trickle vents above windows – this is known as ‘natural ventilation’.
It is the cheapest method to install in a newly built home, there’s no maintenance (apart from the occasional replacement fan) and there are no usable controls; that is to say it ventilates without being told to by the occupants of the house.
This is fine for most homes; however Part F recommends you only use natural ventilation with an air permeability higher than 3 – otherwise you run the risk of not having enough ventilation in your home.
So at the high end of the market we have whole house mechanical ventilation heat recovery systems (MVHR) which operate at their best in very tightly built houses. MVHR usually involves a box of fans and valves which sits in the loft of a building; connected to numerous ducts and vents. It pulls warm, damp air from bathrooms and kitchens, removes the condensation and pushes the warm, filtered air into other rooms in the house, reducing heating bills.
The theory behind this type of ventilation is excellent when it comes to energy efficiency, and there are many homeowners who swear by them, but there are also a number of negative points to overcome.
The big issue is usually down to the people who live in the house. Leaving windows open, even on the warmest of days, will seriously affect the efficiency of the ventilation system and could make it very expensive to run. Also, we’ve heard of homeowners switching the systems off completely because they mistake the breeze of the filtered air as a draught.
Poorly fitted ductwork can also be a problem, and many manufacturers suggest their products are only used in houses with an air permeability rating better than 5. MVHR systems are also more expensive to buy and install compared to typical intermittent fans.
We find mechanical ventilation systems are used more commonly on larger, more expensive houses, and on houses with very low emission targets. Typical new homes and conversions of existing building are much more likely to stick with the old fashioned, intermittent fans. So what if you have a very good air test (less than 3) and natural ventilation? In extreme circumstances, your house could be considered as being at risk from condensation; and you may need to install a whole house ventilation system retrospectively.
On the flip side, what if you have an air test higher than 5 and mechanical ventilation? You may find the house is a bit too leaky for your system to run properly, which could in turn lead to higher fuel bills.
So the balancing act of choosing the correct ventilation system for your home is more important now than it has ever been.