I think they reduced the heat exchanger warranty to two years because the risk of damage is higher in an O/V situation. I would still be confident myself that it would outlast an ally exchanger though I believe they brought it out to sell in social housing where you can't or don't want a sealed system. I have fitted two so far and they are just as good as the regular A series. Idiots could operate them.The A200S OV only has a two year parts and labour warranty. All other A series have five years.If you want O/V have a look at the Atag A200S OV.
The atag also modulates down to 4.4kw which is what you should be looking at in a property with a low heat loss or where you want the heat ticking over all day. There is no excuse for boilers only modulating down to '9kw' anymore.