replacing both roof vents with air admittance valves

[arbitrary]

My home (built circa 1990) has two soil stacks, one serving a bath, loo and basin on the first floor and the 2nd serving a shower, loo and basin on the first floor and a cloakroom on the ground floor. As part of redoing both bathrooms I would like to use air admittance valves instead of the current 80mm soil stack extensions that go through the roof. In another thread (//www.diynot.com/forums/viewtopic.php?t=155359&start=0&highlight=) I read that the drains must have at least one vent. What constitutes a vent? There are access points in the ground to the drains at the front and rear of the house, the rain water downpipes feed into the drains and outside of the house there are what appear to be hoppers by the kitchen and utility room drains (not adjacent to the sol stacks) where water can be heard running when he kitchen or utility room sinks are draining.

So my question is: can I install 2 air admittance valves or just one?

 

[arbitrary]

4th post in //www.diynot.com/forums/viewtopic.php?p=1664604#1664604 suggests I'll be OK

 

[Hugh Jaleak]

Building regs should supersede manufacturers advice. Regs require at least one open vent, usually at the head of the drain. I'd be extremely cautious about getting rid of both vents, (quite why you want to remove both vents I dont know, apart from anything else you'll need to repair the hole in the roof), and you may find you create more problems than you solve.....

AAV's will (as the title suggests) let air in, but any positive pressure in the drain needs an escape route, AAV's dont provide this. You could find you get pressure bubbling back through appliance traps and causing smells internally.