Ventilation systems for the home - your guide
To keep a house warm you need to block up any gaps and keep out draughts. We’ve known that for hundreds of years.
A house left like this stays in good order, provided that there are no animals inside. Dogs and cats are bad enough, but it’s homo sapiens I have in mind. Not only do animals breathe, but when they do so they exhale water vapour. Worse still we increase humidity further by cooking and taking hot showers. Aagh!
Water vapour condenses on cooler surfaces where it provides a rich micro-environment for fungi and all manner of invertebrates that start living off it. Eventually these organisms start consuming the very fabric of the building itself: dry rot, wet rot, woodworm, carpet moths and mildew all thrive in unventilated space.
For the last half-century we’ve ‘solved’ that problem by installing bathroom and kitchen fans. They suck warm humid air from the house and expel it outside, along with all the heat energy which we paid to have in our house over the previous few hours.
In bedrooms and living spaces we’ve installed windows with trickle vents above them. These re-introduce the small draughts which have carefully been eliminated elsewhere. There is an alternative.
We can expel the air but extract much of the heat from it when we do so. That’s the basic idea behind Mechanical Ventilation with Heat Recovery (MVHR).
At the heart of an MVHR unit is a heat-exchanger comprising a series of thin plates where heat is passed to incoming fresh air without the two air-streams actually mixing.
Small MVHR units are now available sub-£300 for use in just 2 or 3 rooms of a house. Here’s the HR100 from Vent Axia:
It has 5 ports:
- fresh-air input from outside
- warmed fresh air to living area(s)
- stale/humid air from bathroom & kitchen
- stale-air to outside after heat has been extracted
- 19mm condensate drain to outside
Here’s one I fitted in a garden-office about 6 years ago. This is positioned above a doorway to the en-suite toilet area. It is therefore the “bottom opening” version so that the heat-exchanger element can be removed from below and cleaned out once a year:
The Vent Axia HR100 has standard 100mm diameter connections. I used a combination of rigid and flexible pipes to squeeze everything into the small space above a suspended ceiling.
Houses with MVHR units can operate on a lower setting than the usual 15-litres/sec air extraction rate because they work continuously. See Building Regulations Part-F for information.
A whole-house installation is more common, using a larger heat-exchanger to feed fresh air into all the living spaces.
You will need to decide whether to connect the cooker-hood to a whole-house system. My preference is to keep it separate, thereby not allowing oils and fat to enter the ducting where fluff could stick to it.
The heat-exchanger unit for a “whole-house” installation would typically cost around £1100-£1600 depending on the quality of the fan motors and the exchanger plates.
I’ve added an additional radiator within my unit with water fed from the same thermal store outlet that supplies my underfloor heating.
The 150x150mm mini radiator is normally used by PC gamers for their water-cooled processor chips and graphics cards. I screwed it inside the heat-exchanger case immediately before the outlet supplying fresh air to the house. The input/output ports take ½-inch flexible hose which run through a couple of rubber grommets in the side-wall of the enclosure.
The main cost of a whole-house ventilation system is the running of the pipes themselves. Without taking up floorboards to work out how the pipes might be inserted it’s not really possible to give a price estimate.
Beware that many firms leave the pipe-runs to be installed by the most junior member of the team. That means he/she may not appreciate the need for:
- sufficiently air-tight seals between joints
- rigid (not flexible) pipe where possible to maintain pressure
- adequate insulation around all pipes, especially at bends
- the use of round pipe rather than rectangular to reduce noise
- no pipe joins within 4 pipe-diameters of an outlet vent
- not sawing through the odd joist(!) to reach a tricky location
And that’s why I’ve installed everything myself so I know it’s optimal!
The ducting components themselves are cheap and readily available online from suppliers such as TLC Direct and BES. Most of my installation uses 100mm rigid pipe which I’ve encased in aluminiumised bubble-wrap (400mm wide strip).
I hope this is helpful in letting you know your options for home ventilation!