Ducting
We decided to install Mechanical Ventilation with Heat Recovery (MHVR). This is a whole house ventilation system which will gently suck damp smelly air from the kitchen and bathrooms and push clean fresh air into the bedrooms and living room. The clever bit is that the outgoing air will be used to warm the incoming air – around 90% of the heat should be saved. Without this either lots of heat would be lost or there would not be much ventilation (typically a bit of both).
There’s a couple of building challenges associated with this system: the need to install the ducting and the need to make the house as air-tight as possible.
We we’ve pretty much ripped the house apart it’s relatively easy to install the ducting. Most retro-fit installations use semi-flexible ducting; we’re using 110mm diameter rigid plastic ducting (actually it’s underground drainage pipes with push-fittings, which can be bought at a good price from Plastic Express). These smooth-walled pipes should give us better airflow than the smaller flexible stuff, as well as being much cheaper and more serviceable (for cleaning or inspection etc).
Airtightness is important with MVHR because if the building has drafts (as most do, especially older houses) then they will provide the ventilation (and let heat escape). We have the opportunity to achieve good airtightness (we hope) as the house is reconstructed. We’ll be testing airtightness and finding leaks later on in the build process. This is achieved by using a fan (typically fitted over the front door) to push air into the house and then measuring the internal air pressure and using smoke to look for leaks. I’ll write more about this when the time comes.
We’ll probably be using a Vortice Prometeo HR 400 unit, but we won’t make a final decision for a couple of months yet – it will be installed in a loft-space which hasn’t been built yet.
Critics point out that these units are electrically powered so may not save energy. We’ll minimise electricity usage by choosing a low-power unit and running it only in the cooler months (in the summer we’ll use windows to meet most of our ventilation requirements, although we will have a cooker-hood extractor for when we’re doing a lot of smelly cooking). We’re pretty confident that the energy used when the system is running will be a lot less that the energy saved by the heat recovery, although I can’t see any easy way for us to measure it.
