Probably the best known is the very expensive unit unit that's marketed in the UK as the Odour Buster (but which is very clearly either a re-badged or just imported far-eastern unit - they've been on sale there for years, despite them being on Dragon's Den). This costs around £200, which is a crazy price for what it is. It works by drawing air from around the pan when it's in use and pumping it into the foul drain, via a one-way valve and a very low power fan. OK, but a bit complex, in my view.
Next are the many and varied units that connect a fan to the cistern and utilise the internal overflow pipe that connects directly to the back of the toilet flush pipe. Some of these have electric fans and cartridge filters that sit inside the cistern and which turn on via a proximity sensor, some connect to the air space above the cistern and have a small external fan that exhausts to the outside of the house, via a small duct (typically around 1.5" to 2"). These only need a small fan because the volume they are extracting from is small, perhaps 20 litres at most (it's just the internal volume of the toilet bowl with either the lid down or someone sat on the seat). A fan that can extract at just a litre per second would be more than adequate to pull any odour out of the toilet basin. These units are fairly cheap, the one that has a fan and filter cartridge inside the cistern is around £20. The snag is you need to replace the cartridge every year, as it relies on something like activated charcoal as an odour filter.
Whilst looking at these I had an idea. If you have a house with MVHR, then you already have a continuous extract system. Simply connecting a bit of small bore duct from the top of the cistern to the nearest extract duct would ensure that air was extracted (via the cistern internal overflow pipe) to the MVHR exhaust.
As we're fitting built in units, with internal cisterns, it should be easy to drill holes in the plastic cistern lids and connect them up to a bit of waste pipe to act as a duct. In our house I have a space in the eaves right behind both bathrooms that can take these ducts and feed them to the MVHR extract manifold pretty easily.
My guess is that I can probably ventilate both toilet basins via the MVHR for less than £20 in parts. I shall report back when the house is finished as to how well this idea works!
Edited by jsharris, 26 January 2014 - 02:56 PM.